Chapter 4 Nanostructured Thin Film Schottky Diode ...

Investigation of Nanostructured Thin Film Based Schottky Diodes for Gas Sensing Applications

A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy

Mahnaz Shafiei BSc. (Electrical Engineering, Electronics) AmirKabir University of Technology (Tehran Polytechnic), Iran, Tehran

School of Electrical and Computer Engineering Science, Engineering and Health Portfolio RMIT University, Melbourne, Australia September 2010

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Author’s Declaration September, 2010

I certify that except where due acknowledgement has been made, the work is that of the author alone; the work has not been submitted previously, in whole or in part, to qualify for any other academic award; the content of the thesis is the result of work, which has been carried out since the official commencement date of the approved research program; any editorial work, paid or unpaid, carried out by a third party is acknowledged.

________________________ Mahnaz Shafiei

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Acknowledgements As the author of this thesis, I would like firstly to thank my senior supervisor, Professor Wojtek Wlodarski for granting me the opportunity to undertake my PhD research program in sensor technology laboratory at RMIT University, Australia. His invaluable support, encouragement, intellectual input and friendship for me to conduct this research were most significant to its success. I am also grateful to my second supervisor, Associate Professor Kourosh Kalantar-zadeh for his ever positive attitude, support and enthusiasm during my PhD studies.

I would like to pass my special thanks to Mr. Jerry Yu for his invaluable assistance and support throughout my PhD research program, especially critical reading of the drafts of this thesis. I wish to thank my colleagues Dr. Rashidah Arsat, Dr. Sasikaran Kandasamy, Dr. Micheal Breedon, Mr. Mohd. Hanif Yaacob, Mr. Chen Zhang, Mr. Laith Al-Mashat, Mr. Rick Zheng, Mr. Ali Moafi, Dr. Abu Z. Sadek, Mr. Muhammad Ahmad, Ms. Joy Tan, Mr. Jian Ou, Dr. Xaiofeng Yu, Dr. Samuel Ippolito and Dr. Glenn Matthews for their continuous support, encouragement and providing a friendly and energetic environment for conducting this research.

I would like to take this opportunity to thank several people, without whom this research would not have been possible. My appreciation extends to the academic and administrative members of the School of Electrical and Computer Engineering at RMIT University. I am especially grateful to the technical and MMTC staff of the school, especially Mr. Yuxun Cao, Mr. Paul Jones and Ms. Chin-Ping Wu for their assistance in device fabrication, measurements and general technical advice. Additionally, I would like to thank Associate Professor Johan du Plesis, Mr. Phil Francis and all microscopists of the Department of Applied Physics, RMIT University for their input and help in organising the material characterisations. Also thanks to Dr. Kay Latham of the Department of Chemistry, RMIT University and Dr. Paul Spizziri of School of Physics, The University of Melbourne for their assistance in material characterisations.

Throughout my PhD research, I have formed many international collaborative links and networks. I am very grateful to Dr. Elisabetta Comini and Dr. Matteo Ferroni of the

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University of Brescia, Brescia, Italy; Professor Saulius Kaciulis of the CNR, Rome, Italy; Professor Richard Kaner and Mr. Sergey Dubin of the Department of Chemistry and Biochemistry, UCLA, Los Angeles for their fruitful collaboration.

I acknowledge the School of Electrical and Computer Engineering, RMIT University for providing me with financial support through their PhD scholarship program and also providing me financial support to present my papers at different international conferences: International Society for Optical Engineering (SPIE) 2008 in Melbourne, Australia, 1st Nano Today conference 2009 in Singapore, IEEE Sensors conference 2009 in Christchurch, New Zealand, International Conference on Nanoscience and Nanotechnology (ICONN) 2010 in Sydney, Australia and 13th International Meeting on Chemical Sensors (IMCS-13) 2010 in Perth, Australia. I also would like to acknowledge the contribution by the Australian Research Council Nanotechnology Network (ARCNN) for their financial support in registering my work at ICONN 2010 and IMCS 2010 conferences.

I also would like to take this opportunity to express my sincerest love to my husband, Mohsen Mirzaei and my daughter, Nicki for their love, understanding, support, encouragement and patience throughout my PhD research. I am also grateful to my parents, Mr. Esmaeil Shafiei and Mrs. Shahnaz Fardi for their endless love, continuing enthusiasm, support and being there for me when needed most.

My special thank to my mother and mother-in-law for their help in babysitting my daughter during my PhD research. I would also like to thank my extended family and friends who supported and encouraged me; especially to my sister, Dr. Touran Shafiei. Their love, support, understanding and encouragement in undertaking a PhD are an enormous contribution to the success of this work herein.

Mahnaz Shafiei

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Contents Author‟s Declaration

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Acknowledgements

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Contents

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List of Figures

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List of Tables

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Abstract

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CHAPTER 1: Introduction 1.1. Motivation 1.1.1. Gas Sensors

1 1 3

1.1.1.1. Gas Sensors Applications

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1.1.1.2. Solid State Gas Sensors

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1.1.2. Nanotechnology Enabled Gas Sensors

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1.2. Objectives

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1.3. Outcomes and Author‟s Achievements

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1.4. Thesis Organisation

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References

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CHAPTER 2: Literature Review and Research Rationale

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2.1. Introduction

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2.2. Schottky Diode Based Gas Sensors

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2.2.1. Si Based Gas Sensors

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2.2.2. GaAs Based Gas Sensors

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2.2.3. InP Based Gas Sensors

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2.2.4. GaN and AlGaN Based Gas Sensors

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2.2.5. SiC Based Gas Sensors

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2.3. Review on Nanostructured Materials Pertaining to This Research 2.3.1. Nanostructured Semiconducting Metal-Oxide Based Gas Sensors 2.3.1.1. TiO2 Nanostructured Based Gas Sensors

37 39 39

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2.3.1.2. SnO2 Nanostructured Based Gas Sensors

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2.3.1.3. ZnO Nanostructured Based Gas Sensors

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2.3.1.4. RuO2 Nanostructured Based Gas Sensors

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2.3.1.5. MoO3 Nanostructured Based Gas Sensors

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2.3.1.6. WO3 Nanostructured Based Gas Sensors

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2.3.2. Graphene Nano-Sheets Based Gas Sensors 2.4. Summary References

45 47 48

CHAPTER 3: Theoretical Study of Nanostructured Thin Film Schottky Diode Based Gas Sensors

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3.1. Introduction

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3.2. Fundamentals of Schottky Diodes

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3.2.1. Conventional Schottky Diodes

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3.2.1.1. Formation of Barrier

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3.2.1.1.1. Ideal Condition

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3.2.1.1.2. Interface States

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3.2.1.1.3. Image-Force Lowering

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3.2.1.2. Current-Voltage Characteristics

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3.2.2. Nanostructured Thin Film Schottky Diodes

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3.2.2.1. The Effect of Electric Field Enhancement Factor

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3.2.2.2. The Effect of Carrier Density

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3.2.2.3. The Effect of Barrier Height

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3.2.2.4. The Effect of Temperature

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3.3. Conventional and Nanostructured Schottky Diode Based Gas Sensors

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3.4. Pt/Graphene/SiC Based Gas Sensors

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3.5. Summary

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References

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CHAPTER 4: Nanostructured Thin Film Schottky Diode Based Gas Sensors Fabrication

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4.1. Introduction

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4.2. Wafer Cleaning Process

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4.3. Substrate Preparation: Ohmic Contact Formation and Dicing

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4.4. Nanostructured Material Synthesis and Deposition

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4.4.1. Synthesis of TiO2 Nano-Dimensional Grains: Anodization Method

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4.4.2. Deposition of SnO2 Nanowires: Thermal Evaporation Method

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4.4.3. Synthesis/Deposition of ZnO Nanostructures

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4.4.3.1. Synthesis of ZnO Nanostructured Arrays: Hydrothermal Growth Method

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4.4.3.2. Deposition of ZnO Nanowires-Nanoplatelets: Thermal Evaporation Method

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4.4.4. Deposition of RuO2 Nanostructures: RF Sputtering Method

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4.4.5. Deposition of MoO3 Nanostructures: Thermal Evaporation Method

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4.4.6. Synthesis of WO3 Nanoplatelets: Acid-Etching Method

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4.4.7. Synthesis of Graphene-Like Nano-Sheets

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4.5. Schottky Contact Formation

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4.6. Summary

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References

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CHAPTER 5: Nanostructured Materials Characterisation

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5.1. Introduction

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5.2. Characterisation Techniques

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5.2.1. Scanning Electron Microscopy (SEM)

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5.2.2. Transmission Electron Microscopy (TEM)

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5.2.3. X-ray Diffraction (XRD)

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5.2.4. Energy Dispersive X-ray (EDX)

134

5.2.5. X-ray Photoelectron Spectroscopy (XPS)

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5.2.6. Atomic Force Microscopy (AFM)

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5.2.7. Raman Spectroscopy

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5.3. Nanostructured Materials Characterisation Results

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5.3.1. TiO2 Nano-Dimensional Grains

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5.3.2. SnO2 Nanowires

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5.3.3. ZnO Nanostructures

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5.3.3.1. ZnO Nanostructured Arrays

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5.3.3.2. ZnO Nanowires-Nanoplatelets

149

5.3.4. RuO2 Nanostructures

153

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5.3.5. MoO3 Nanostructures

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5.3.6. WO3 Nanoplatelets

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5.3.7. Graphene-Like Nano-Sheets

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5.4. Summary References

173 179

CHAPTER 6: Nanostructured Thin Film Schottky Diode Based Gas Sensors Characterisation

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6.1. Introduction

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6.2. Experimental Set-Up and Gas Testing System

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6.3. Electrical and Gas Sensing Results

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6.3.1. TiO2 Nano-Dimensional Grains Based Sensor

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6.3.2. SnO2 Nanowires Based Sensor

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6.3.3. ZnO Nanostructured Based Sensor

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6.3.3.1. ZnO Nanostructured Arrays Based Sensor

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6.3.3.2. ZnO Nanowires-Nanoplatelets Based Sensor

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6.3.4. RuO2 Nanostructured Based Sensor

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6.3.5. MoO3 Nanostructured Based Sensors

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6.3.5.1. MoO3 Nanoplatelets Based Sensor

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6.3.5.2. MoO3 Nanoplatelets-Nanowires Based Sensor

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6.3.5.3. MoO3 Nano-Flowers Based Sensor

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6.3.6. WO3 Nanoplatelets Based Sensors

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6.3.7. Graphene-Like Nano-Sheets Based Sensor

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6.4. Summary

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References

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CHAPTER 7: Conclusions and Future Work

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7.1. Conclusions

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7.2. Future Work

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Appendix A: Silicon Carbide (SiC) and Gallium Nitride (GaN) Wafers Specifications

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A.1. SiC

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A.2. GaN

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Appendix B: Author’s Publication List

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B.1. Refereed Journal Articles

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B.2. Reviewed Conference Proceedings

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List of Figures Figure 3.1. Energy-band diagrams of a metal and n-type semiconductor (conventional): (a) neutral and isolated, (b) connected as one system and (c) in intimate contact..........................................................66 Figure 3.2. Energy-band diagram of a metal and n-type semiconductor (conventional) contact under forward, zero and reverse bias. Bn0 is the intrinsic barrier height..........................................................70 Figure 3.3. Typical I-V characteristics of conventional and nanostructured Schottky diodes................72 Figure 3.4. Energy-band diagram of a metal and n-type nanostructured semiconductor contact under forward, zero and reverse bias with the effect of the enhanced localized electric fields........................75 Figure 3.5. Typical structure of a conventional thin film Schottky diode based gas sensor and the voltage shift in forward I-V characteristics of the sensor towards (a) reducing and (b) oxidizing gases........................................................................................................................................................77 Figure 3.6. Schematic of nanostructured Schottky diodes fabricated on SiC and epitaxial-GaN/Al2O3 substrates……………………………………………………………………………………………….80 Figure 3.7. Theoretical energy-band diagram of Pt/graphene contact. WM is the Pt work function; WG is the graphene work function; W is the Pt coated graphene work function; d is the equilibrium separation distance; ∆EF is the Fermi-level shift and ∆V is the potential change generated by the Pt-graphene interaction……………………………………………………………………………………………...81 Figure 4.1. Metal layers deposition at the backside of the SiC wafer using electron beam evaporation system for ohmic contact formation…………………………………………………………………...93 Figure 4.2. Metal layers deposition process for ohmic contact formation. (a) A shadow mask was placed in front of the backside of SiC substrate during deposition of Ti and Pt. The mask was made of a thin sheet of Al with a hole of approximately 1 mm in diameter. (b) Fabricated device after the removal of the shadow mask…………………………………………………………………………..94 Figure 4.3. Metal layers deposition process for ohmic contact formation. (a) A shadow mask was used in contact with the top surface of epitaxial-GaN layers during deposition of Al/Cr/Au. The contacts were deposited at the corner of the surface using a mask with a hole of approximately 1×1 mm2 and (b) fabricated device after the removal of the shadow mask…………………………………………..95 Figure 4.4. Schematic of the filtered cathodic vacuum arc (FCVA) deposition system………………99 Figure 4.5. Schematic representation of the anodization set-up……………………………………...100 Figure 4.6. The schematic illustration of principle steps for the VLS growth technique: from initial nucleation to continual growth……………………………………………………………………….102 Figure 4.7. Nanowies/rod growth process using a catalyst metal. (a) Root growth, the catalyst metal stays at the bottom of the nanowires/rods. (b) Float growth, the catalyst metal remains at the top of the nanowires/rods………………………………………………………………………………………..103

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Figure 4.8. Schematic diagram of experimental set-up for growth of SnO2 nanowires via thermal evaporation method…………………………………………………………………………………..104 Figure 4.9. (a) Planar RF magnetron sputterer and (b) schematic diagram of the system…………...108 Figure 4.10. ZnO nanostructured arrays fabrication via hydrothermal growth method. (a) Depostion of ZnO nucleation layer by RF sputtering. A mask was used to cover the contacts. (b) Sample holder and reaction vessel in hydrothermal method for growth of ZnO nanostructured arrays………………….109 Figure 4.11. Schematic diagram of the experimental set-up for growth of nanostructured MoO3 via thermal evaporation method………………………………………………………………………….113 Figure 4.12. Process steps for Schottky contact formation: A shadow mask was used for the deposition of the nanomaterials onto (a) SiC and (b) epitaxial-GaN/Al2O3 substrates. The mask was made of a thin sheet of Al with a hole of approximately 1 mm in diameter…………………………………….119 Figure 5.1. The diffraction of X-rays by lattice planes in a crystal using XRD technique…………..134 Figure 5.2. Representation of Raman scattering from particles……………………………………...137 Figure 5.3. AFM 3-D image of the FCVA deposited Ti films……………………………………….139 Figure 5.4. SEM images of TiO2 nano-dimensional grains as deposited on SiC substrate. Inset: 45° rotation......................................................................................................................…………....140 Figure 5.5. EDX spectrum of the TiO2 nano-dimensional grains deposited on SiC substrate……….140 Figure 5.6. SEM images of the TiO2 nano-dimensional grains after testing………………………....141 Figure 5.7. SEM images of SnO2 nanowires as grown on SiC substrate…………………………….142 Figure 5.8. EDX spectrum of the SnO2 nanowires deposited on SiC substrate………………………142 Figure 5.9. Low-magnification TEM image of a long SnO2 nanowire (main picture), high-resolution TEM image of the nanowire (upper inset), and indexed digital diffractogram of the high-resolution image (lower inset)………………………………………………………………………………..….144 Figure 5.10. XPS spectrum of SnO 2 nanowires. Detailed VB spectrum is shown in the inset: grey peaks – SnO, white peaks – SnO2…………………………………………………………………….144 Figure 5.11. SEM images of SnO2 nanowires after testing…………………………………………...145 Figure 5.12. SEM images (a) bridged ZnO nanorods, with fine nanowires emanating from the bridges (visible as intense white dots) and (b) ZnO nanostructured arrays 25° rotation; (inset: higher magnification)………………………………………………………………………………………...146 Figure 5.13. XTEM image; left: the overall architecture of the sample along the [ 1120 ] GaN/ZnO direction. The ZnO nucleation layer (ZnO-nl) has reduced from 1.2 μm to 0.3 μm; right: depicting a ZnO nanowire emanating from the bridged crystalline area between two large ZnO nanorods……..147 Figure 5.14. XRD pattern of (a) epitaxial-GaN on sapphire, (b) ZnO nucleation layer on epitaxial-GaN and (c) ZnO nanostructures on epitaxial-GaN………………………………………………………..148 Figure 5.15. Left: SEM image of Pt/ZnO nanostructures after testing highlighting the different stages of nanowire degradation: ranging from ideal nanowires (intact), and nanowires which have “fallen” or are in the process of “falling”; right: Pt/ZnO nanostructures as deposited and after testing…………149

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Figure 5.16. SEM image of ZnO nanowires-nanoplatelets as grown on SiC substrate. Inset: a panoramic TEM image of the nanowires and platelets…………………………………………….…150 Figure 5.17. Cross sectional SEM image of the ZnO nanowires-nanoplatelets layer on SiC substrate………………………………………………………………………………………………150 Figure 5.18. (a) Image of ZnO nanoplatelets and (b) full electron diffraction pattern from focused beam (at the marker +)………………………………………………………………………………..151 Figure 5.19. (a) Image of a ZnO nanowire and (b) the corresponding electron diffraction pattern from focused beam (at the marker +)………………………………………………………………………152 Figure 5.20. EDX spectrum of the ZnO nanowires-nanoplatelets deposited on SiC substrate………152 Figure 5.21. SEM images of the ZnO nanowires-nanoplatelets after testing (Two different magnifications)……………………………………………………………………………………….153 Figure 5.22. SEM images of the as-deposited RuO2 nano-cubular structures on SiC substrate……155 Figure 5.23. TEM view of the Ru-O nanocrystals……………………………………………………155 Figure 5.24. XRD pattern of the RuO2 nano-cubular structures……………………………………...155 Figure 5.25. SEM image of the RuO2 nano-cubular structures after testing…………………………156 Figure 5.26. Surface morphology of the deposited MoO3 based on the deposition conditions summarised in Table 5.2……………………………………………………………………………...158 Figure 5.27. XRD spectrum of MoO3 nanoplatelets; (α-β denotes crystallographic planes grow in their respective α-β phases)....................................................................…........................................160 Figure 5.28. SEM images of MoO3 nanoplatelets; (a) as-deposited and (b) after testing……………160 Figure 5.29. SEM images of as-deposited MoO3 (a) nanoplatelets-nanowires and (b) nano-flowers..161 Figure 5.30. XRD spectra of MoO3 (a) nanoplatelets-nanowires and (b) nano-flowers; (α-β denotes crystallographic planes grow in their respective α-β phases).................................…………...…..162 Figure 5.31. SEM images of MoO3 (a) nanoplatelets-nanowires and (b) nano-flowers after testing...163 Figure 5.32. Surface morphology of (a) RF sputtered tungsten on SiC substrate and annealed nanostructured WO3 samples acid-etched for (b) 1 hr (sample A), (c) 2 hrs (sample B) and (d) 3 hrs (sample C). Insets: cross sectional SEM (45° tilted view)…………………………………………...164 Figure 5.33. XRD patterns for (a) RF sputtered tungsten and annealed nanostructured WO3 samples, acid-etched for (b) 1 hr (sample A), (c) 2 hrs (sample B) and (d) 3 hrs (sample C)…………………165 Figure 5.34. SEM images of WO3 nanoplatelets samples acid-etched for (a) 1 hr (sample A), (b) 2 hrs (sample B) and (c) 3 hrs (sample C) after testing…………………………………………………….166 Figure 5.35. (a) SEM image of the graphene-like nano-sheets deposited on SiC substrate and (b) AFM image of graphene-like nano-sheets deposited on Si substrate. …................................................…...167

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Figure 5.36. A representative low frequency Raman spectrum of the SiC device with deposited graphene-like material. The labelled peaks are characteristic of first order scattering for the 6H polytype of SiC……………………………………………………………………………………….168 Figure 5.37. A Raman spectrum of the deposited grapheme-like material on a SiC device measured over the first and second-order scattering range for graphitic carbon. Scattering from the 6H polytype of SiC as well as characteristic D, G, 2D, D+G and 2G bands associated with the graphene-like material are indicated………………………………………………………………………………...169 Figure 5.38. (a) Raman spectra depicting second-order scattering from the 6H polytype of SiC as well as characteristic first-order D and G (graphitic) bands associated with the applied graphene-like layer and (b) a second-order Raman spectrum arising from the scattering of sp 2-bonded carbon phases with fitted profiles for the 2D, D+G and 2G bands depicted. Note that SiC does not make any spectral contribution to (b)…………………………………………………………………………………….171 Figure 5.39. XPS analysis of graphene-like nano-sheets……………………………………………..172 Figure 5.40. SEM image of the graphene-like nano-sheets after testing……………………………..173 Figure 6.1. Block of gas testing system set-up for the nanostructured Schottky diode based sensors………………………………………………………………………………………………...185 Figure 6.2. Illustration of a nanostructured Schottky diode based sensor with electrical connections…………………………………………………………………………………………...186 Figure 6.3. I-V characteristics of Pt/TiO2 nano-dimensional grain/SiC Schottky diode based sensor measured at temperatures ranging from 25 to 620°C towards 1% hydrogen………………………...188 Figure 6.4. Plot of voltage shift of Pt/TiO2 nano-dimensional grain/SiC Schottky diode based sensor as a function of temperature towards 1% hydrogen and 1% propene at a constant reverse bias current of 10 µA…………………………………………………………………………………………………189 Figure 6.5. I-V characteristics of Pt/TiO2 nano-dimensional grain/SiC Schottky diode based sensor towards (a) 1% hydrogen at 420°C and (b) 1% propene at 620°C…………………………………..189 Figure 6.6. Dynamic responses of Pt/TiO2 nano-dimensional grain/SiC Schottky diode based sensor towards (a) 1% hydrogen at 420°C and (b) 1% propene at 620°C with a constant reverse bias current of 10 µA………………………………………………………………………………………………190 Figure 6.7. I-V characteristics of Pt/SnO2 nanowire/SiC Schottky diode based sensor measured at temperatures ranging from 25 to 530°C towards 1% hydrogen……………………………………...191 Figure 6.8. Plots of voltage shift of Pt/SnO2 nanowire/SiC Schottky diode based sensor measured as a function of temperature towards 0.125%, 0.25% and 0.5% hydrogen at a constant reverse bias current of 1 µA………………………………………………………………………………………………..192 Figure 6.9. Dynamic responses of Pt/SnO2 nanowire/SiC Schottky diode based sensor towards different hydrogen concentrations at (a) 420°C and (b) 530°C at a constant reverse bias current of 1 µA…………………………………………………………………………………………………..192 Figure 6.10. Plots of voltage shift as a function of hydrogen concentration for Pt/SnO2 nanowire/SiC Schottky diode based sensor at 420°C and 530°C with a constant reverse bias current of 1 µA…….193 Figure 6.11. I-V characteristics of Pt/ZnO nanostructured array/epitaxial-GaN and Pt/epitaxial-GaN Schottky diodes at 25ºC………………………………………………………………………………194

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Figure 6.12. I-V characteristics of Pt/ZnO nanostructured array/epitaxial-GaN Schottky diode based sensor measured at temperatures ranging from 25 to 350°C towards 9.9 ppm NO2…………………195 Figure 6.13. Plots of voltage shift of Pt/ZnO nanostructured array/epitaxial-GaN Schottky diode based sensor as a function of temperature towards 0.6 ppm and 9.9 ppm NO2 at a constant reverse bias current of 300 µA……………………………………………………………………………………..196 Figure 6.14. Dynamic response of Pt/ZnO nanostructured array/epitaxial-GaN Schottky diode based sensor towards different NO2 concentrations at 270°C with a constant reverse bias current of 300 µA……………………………………………………………………………………………..196 Figure 6.15. Plot of voltage shift as a function of NO2 concentration for Pt/ZnO nanostructured array/epitaxial-GaN Schottky diode based sensor at 270°C with a constant reverse bias current of 300 µA………………………………………………………………………………………………..197 Figure 6.16. I-V characteristics of Pt/ZnO nanowire-nanoplatelet/SiC Schottky diode based sensor measured at temperatures ranging from 25 to 620°C towards 1% hydrogen………………………...198 Figure 6.17. Reverse I-V characteristics of Pt/ZnO nanowire-nanoplatelet/SiC Schottky diode based sensor towards different hydrogen concentrations at 620°C…………………………………………198 Figure 6.18. Dynamic response of Pt/ZnO nanowire-nanoplatelet/SiC Schottky diode based sensor towards different hydrogen concentrations at 620°C with a constant reverse bias current of 1 µA…199 Figure 6.19. Plot of voltage shift as a function of hydrogen concentration for Pt/ZnO nanowirenanoplatelet/SiC Schottky diode based sensor at 620°C with a constant reverse bias current of 1 µA………………………………………………………………………………………………..200 Figure 6.20. Dynamic responses of Pt/ZnO nanowire-nanoplatelet/SiC Schottky diode based sensor towards different propene concentrations at 620°C with a constant reverse bias current of 1 µA…..201 Figure 6.21. Plot of voltage shift as a function of propene concentration for Pt/ZnO nanowirenanoplatelet/SiC Schottky diode based sensor at 620°C with a constant reverse bias current of 1 µA……………………………………………………………………………………………….201 Figure 6.22. I-V characteristics of Pt/RuO2 nanostructure/SiC Schottky diode based sensor measured at temperatures ranging from 25 to 240°C towards 1% hydrogen……………………………………...202 Figure 6.23. Reverse I-V characteristics of Pt/RuO2 nanostrucutre/SiC Schottky diode based sensor towards different hydrogen concentrations at 240°C………………………………………………..203 Figure 6.24. Dynamic response of Pt/RuO2 nanostructure/SiC Schottky diode based sensor towards different hydrogen concentrations at 240°C with a constant reverse bias current of 1 mA…………204 Figure 6.25. Plot of voltage shift as a function of hydrogen concentration for Pt/RuO2 nanostructure/SiC Schottky diode based sensor at 240°C with a constant reverse bias current of 1 mA……………………………………………………………………………………………….204 Figure 6.26. I-V characteristics of Pt/MoO3 nanoplatelet/SiC Schottky diode based sensor measured at temperatures ranging from 25 to 360°C towards 1% hydrogen………....…………………………...205 Figure 6.27. Reverse I-V characteristics of Pt/MoO3 nanoplatelet/SiC Schottky diode based sensor towards different hydrogen concentrations at 180°C………………………………………………...206 Figure 6.28. Dynamic response of Pt/MoO3 nanoplatelet/SiC Schottky diode based sensor towards different hydrogen concentrations at 180°C with a constant reverse bias current of 100 µA………..206

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Figure 6.29. Plot of voltage shift as a function of hydrogen concentration for Pt/MoO3 nanoplatelet/SiC Schottky diode based sensor at 180°C with a constant reverse bias current of 100 µA…………………………………………………………………………………………….207 Figure 6.30. Illustration of localized electric field lines emanated from MoO 3 nanoplatelets (a) asdeposited and (b) after testing………………………………………………………………………..208 Figure 6.31. I-V characteristics of Pt/MoO3 nanoplatelet-nanowire/SiC Schottky diode based sensor measured at temperatures ranging from 25 to 250°C towards 1% hydrogen…....…………………..209 Figure 6.32. Reverse I-V characteristics of Pt/MoO3 nanoplatelet-nanowire/SiC Schottky diode based sensor towards different hydrogen concentrations at 170°C…………………………………………209 Figure 6.33. Dynamic response of the Pt/MoO3 nanoplatelet-nanowire/SiC Schottky diode based sensor towards different hydrogen concentrations at 170°C with a constant reverse bias current of 100 µA………………………………………………………………………………………………..210 Figure 6.34. Plot of voltage shift as a function of hydrogen concentration for Pt/MoO 3 nanoplateletnanowire/SiC Schottky diode based sensor at 170°C with a constant reverse bias current of 100 µA…………………………………………………………………………………………….210 Figure 6.35. I-V characteristics of Pt/MoO3 nano-flower/SiC Schottky diode based sensor measured at temperatures ranging from 25 to 250°C towards 1% hydrogen……………………………………..211 Figure 6.36. Reverse I-V characteristics of Pt/MoO3 nano-flower/SiC Schottky diode based sensor towards different hydrogen concentrations at (a) 170°C and (b) 250°C…………………………….212 Figure 6.37. Dynamic responses of Pt/MoO3 nano-flower/SiC Schottky diode based sensor towards different hydrogen concentrations at (a) 170°C and (b) 250°C with a constant reverse bias current of 10 µA…………………………………………………………………………………………………213 Figure 6.38. Plot of voltage shift as a function of hydrogen concentration for Pt/MoO 3 nanoflower/SiC Schottky diode based sensor at 250°C with a constant reverse bias current of 10 µA….213 Figure 6.39. I-V characteristics of Pt/WO3 nanoplatelet/SiC Schottky diode based sensors (acid-etched for 1 hr (sample A), 2 hrs (sample B) and 3 hrs (sample C)) measured at temperatures ranging from 25 to 200°C towards 1% hydrogen………………………………………………………………………215 Figure 6.40. Forward I-V characteristics of the Pt/WO3 nanoplatelet/SiC Schottky diode based sensors (acid-etched for 1 hr (sample A), 2 hrs (sample B) and 3 hrs (sample C)) towards 1% hydrogen at 200°C…………………………………………………………………………………………………216 Figure 6.41. Dynamic responses of Pt/WO3 nanoplatelet/SiC Schottky diode based sensors (acidetched for 1 hr (sample A), 2 hrs (sample B) and 3 hrs (sample C)) towards 1% hydrogen at 200°C with a constant forward bias current of 500 µA……………………………………………………..217 Figure 6.42. Forward I-V characteristics of Pt/WO3 nanoplatelets-3 hrs/SiC Schottky diode based sensor (sample C) towards different hydrogen concentrations at 200°C……………………………218 Figure 6.43. Dynamic response of Pt/WO3 nanoplatelets-3 hrs/SiC Schottky diode based sensor (sample C) towards different hydrogen concentrations at 200°C with a constant forward bias current of 500 µA……………………………………………………………………………………………….218 Figure 6.44. Plot of voltage shift as a function of hydrogen concentration for Pt/WO 3 nanoplatelets3 hrs/SiC Schottky diode based sensor (sample C) at 200°C with a constant forward bias current of 500 µA……………………………………………………………………………………………….219

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Figure 6.45. I-V characteristics of Pt/graphene-like nano-sheet/SiC and Pt/SiC devices in synthetic air at room temperature………………………………………………………………………………….220 Figure 6.46. I-V characteristics of Pt/graphene-like nano-sheet/SiC based sensor measured at temperatures ranging from 25 to 100°C towards 1% hydrogen……….....…………………………..220 Figure 6.47. Reverse I-V characteristics of Pt/graphene-like nano-sheet/SiC based sensor towards different hydrogen concentrations at 100°C………………………………………………………….221 Figure 6.48. Dynamic response of the Pt/graphene-like nano-sheet/SiC based sensor towards different hydrogen concentrations at 100°C with different constant reverse bias currents of 100 µA and 1 mA……………………………………………………………………………………………..222 Figure 6.49. Plot of voltage shift as a function of hydrogen concentration for Pt/graphene-like nanosheet/SiC based sensor at 100°C with constant reverse bias currents of 100 µA and 1 mA…………222 Figure 6.50. I-V characteristics of Pt/graphene-like nano-sheet/SiC based sensor measured at temperatures ranging from 25 to 100°C towards 9.9 ppm NO2………………………………………223 Figure 6.51. Dynamic response of the Pt/graphene-like nano-sheet/SiC based sensor towards different NO2 concentrations at 70°C with a constant reverse bias current of 1 mA…………………………..224 Figure A.1. Schematic representation of epitaxial-GaN layer as grown on sapphire (Al2O3)………..240

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List of Tables

Table 2.1. Gas sensing results of the developed SiC based Schottky diodes using metal-oxide thin films as a gas sensing layer....................................................................................................................38 Table 3.1. Binding energy and distance of physisorbed hydrogen on graphene layer..........................83 Table 3.2. Binding energy and distance of physisorbed NO2 on graphene layer..................................84 Table 4.1. MoO3 nanostructures deposition parameters using thermal evaporation method...............114 Table 4.2. The developed nanostructured Schottky diodes..................................................................120 Table 5.1. Surface chemical composition of SnO2 nanowires: atomic concentration (%) of the elements and their binding energy (BE)..............................................................................................................144 Table 5.2. Surface morphology of the MoO 3 nanostructures as a function of deposition parameters using thermal evaporation method.......................................................................................................157 Table 6.1. Summary of the experimental gas sensing results for the developed MoO 3 nanostructured Schottky diode based sensors...............................................................................................................214 Table 6.2. Summary of the experimental gas sensing results for the developed nanostructured based sensors...................................................................................................................................................225 Table A.1. Properties of SiC wafers purchased from different companies..........................................239

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Abstract In this PhD research, novel nanostructured thin film based Schottky diodes for gas sensing applications were developed and investigated. The author explored the sensing potential of these devices and comprehensively studied a large group of nanostructured thin films, as the gas sensing layers, with aspirations of enhancing sensor‟ performance compared to the conventional based sensors. To the best of author‟s knowledge, the author developed several nanostructured metal-oxide based Schottky diodes with morphologies that have not been reported previously and presented the first scientific investigation of Pt/graphene-like nanosheets/SiC based gas sensors.

The nanostructured materials were deposited on SiC and GaN substrates and characterised. These materials were then applied in the fabrications of Schottky diode based sensors and their gas sensing performance was investigated towards different gaseous species. Hydrogen gas was assessed below explosive level. Propene and NO2 gases were also sensed at above hazardous concentrations for human‟s health. Two different types of sensors were investigated: (1) Pt/nanostructured semiconducting metal-oxide and (2) Pt/graphene-like nano-sheets devices. The operational principles of the developed devices as well as their gas sensing mechanisms are discussed in detail.

These sensors comprised of thin Pt metal layers (~30 nm thick to form the Schottky contact, which also acts as a catalyst) and nanostructured thin films (as the gas sensing layers), which are deposited on substrates with ohmic contacts. SiC and GaN were the selected substrates, as they are wide band semiconducting materials, making them suitable for high temperature applications. To enhance these devices‟ sensitivity, the author used nanostructured forms of the semiconducting metal-oxides and graphene-like nano-sheets. The metal-oxides investigated include: TiO2 nano-dimensional grains, SnO2 nanowires, ZnO nanostructures (nanostructured

arrays

and

nanowires-nanoplatelets),

RuO2

nanostructures,

nanostructures (nanoplatelets, nanoplatelets-nanowires and nano-flowers) and

MoO3 WO3

nanoplatelets. These nanomaterials were successfully deposited onto SiC and epitaxial-GaN substrates employing different deposition techniques such as anodization, thermal evaporation, radio frequency (RF) sputtering, acid-etching and hydrothermal growth method.

xix

To the best of the author‟s knowledge, none of these metal-oxide nanostructured Schottky diodes have been reported previously in the body of literature.

Graphene-like nano-sheets were also used as a gas sensing layer, which were deposited on SiC substrates via the chemical reduction of graphite oxide. The author chose this material considering its unique properties such as ballistic electronic transport at room temperature, strong hydrogen affinity, metal-like characteristics and its large surface area, which makes graphene well-suited for gas sensing applications.

Scanning electron microscopy (SEM) of the as-deposited nanostructured materials was performed to investigate their morphological features in terms of shapes, dimensions, orientations and porosity to target parameters that can significantly affect the performance of the sensors. The SEM observations revealed sharp edges and corners in most of the nanostructures morphology, which the author ascribed them to a significant role in increasing the sensing performance of the developed nanostructured Schottky diodes. Other characterisation techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectroscopy were employed to obtain complete structural properties of the nanostructured materials in order to fundamentally understand their functionalities with respect to the sensors‟ performance.

The electrical characteristics (current-voltage, I-V) and gas sensing performance of the sensors were obtained in the presence and absence of target gas species (hydrogen, propene and NO2 in synthetic air) at different temperatures ranging from 25 to 620°C. The Pt/nanostructured metal-oxide based Schottky diodes exhibited a significantly lower breakdown voltage, and a larger lateral voltage shift upon exposure to the target gas species in reverse bias mode of operation than that in forward bias as compared to conventional Schottky diodes. This was associated to the enhanced localized electric fields originating from the sharp edges and corners of the high aspect ratio nanostructures. To the best of author‟s knowledge, for the first time it was shown that these enhanced localized electric fields lowered the Schottky barrier height energy at the reverse bias condition, allowing greater charge carrier flow and hence enhanced sensitivity.

It was found that amongst the sensors investigated, the devices based on MoO3 nanostructures exhibited the highest sensitivity with excellent baseline stability and a fast response (t90%) of

xx

40 s. At 250°C, a voltage shift of 5.7 V was recorded for the Pt/MoO3 nano-flower/SiC sensor upon exposure to 1% hydrogen at a constant reverse bias current of 10 µA. To the best of the author‟s knowledge, such a large voltage shift has not been reported previously in literature.

For the Pt/graphene-like nano-sheet/SiC devices, a unique theoretical approach was developed. To the best of author‟s knowledge, this theoretical explanation was the first description of the behaviour of the Pt/graphene-like nano-sheet/SiC junctions. The author attributed the unusual behaviour of such junction to the semi-metal like properties of the graphene-like layer, which was placed between Pt and SiC substrate. The sensors showed response to hydrogen gas; while the response towards NO2 was not significant. Therefore, these devices could sense selectively hydrogen at the presence of NO 2.

Chapter 1 Introduction This chapter presents a general overview of the research performed throughout this PhD candidature. Herein, the author discusses the motivation, objectives, her achievements and the thesis organisation.

1.1. Motivation In recent years, tremendous research effort has been directed into the development of gas sensors as increasing concerns with global warming, pollution, safety and security have prompted the need for such devices. Gas sensors are currently widely utilized in different applications such as domestic gas alarms, medical diagnostics, health care, safety, defence and security, automotive, aerospace and environmental monitoring [1].

There is an essential need to develop gas sensors, which encompass the necessary industrial requirements such as high sensitivity, selectivity, stability and reproducibility [2-4]. Many researchers have made progress in the development of new gas sensors to meet these requirements alongside logical needs such as small size, low power consumption and

Chapter 1: Introduction

2

production costs. To improve the sensors‟ performance, several approaches such as research on novel sensing materials, using catalysts, surface modification, multi-sensor array systems, fabrication techniques and nanotechnology have been employed. Recent advances in nanotechnology has provided tools to enhance the performance of gas sensors to produce higher sensitivity, lower production cost, reduced power consumption as well as improved stability as compared to the conventional sensors [5]. Nanostructured materials have been expected to demonstrate a unique mechanical, optical, electronic and magnetic properties substantially different from those observed with bulk materials as high surface to volume ratio can be achieved by controlling the surface morphology [5-7]. Interest in designing and synthesizing nanostructured materials for gas sensing applications has been grown exponentially worldwide in the last few decades as shown by the growth of investigations in literature. The nanostructured materials such as metal oxides in the form of nanoparticles, nanowires, nanorods, nanoplatelets and nanobelts [8-17], carbon nanotubes [18-20] and graphene [20-25] have shown potential to increase sensors‟ performance.

In this PhD research, the author aims to develop highly sensitive Schottky diode gas sensors based on nanostructured metal-oxides and graphene films. Based on the literature review, which will be presented in chapter 2, the author selected an informed decision to develop novel TiO2 nano-dimensional grains, SnO2 nanowires, ZnO nanostructures (nanostructured arrays

and

nanowires-nanoplatelets),

RuO2

nanostructures,

MoO3

nanostructures

(nanoplatelets, nanoplatelets-nanowires and nano-flowers), WO3 nanoplatelets based Schottky diodes and graphene-like nano-sheets based devices. The electrical and gas sensing performance of the developed sensors were investigated towards hydrogen (H2), propene (C3H6) and nitrogen dioxide (NO2) gases at elevated temperatures. The theory and gas sensing mechanisms, which explains how these sensors operate, using the most recent theory


3

available in literature was also investigated. To the best of the author‟s knowledge, the Schottky diode sensors based on the abovementioned nanostructured thin films have been investigated for the first time for gas sensing applications in this thesis.

1.1.1. Gas Sensors A gas sensor is defined as a device, which one or more of its physical properties (e.g. mass, electrical conductivity or capacitance) changes upon its exposure towards a gas species [26]. A change in these properties can be measured and quantified directly or indirectly. A typical gas sensor comprises of a sensing layer integrated with a transducing platform, which is in direct contact with the environment (gas). Gas molecules interact chemically with the sensing layer, which result in a change of the sensor‟s physical/chemical properties. The transducer then measures these changes and produces an electrical output signal [27].

Gas sensors have vast applications such as the ones, which will be presented in the next section [28]. These different applications require various expectations regarding the sensor‟s performance with respect to the sensitivity, selectivity, reliability and other parameters [28].

1.1.1.1. Gas Sensors Applications Gas sensors are increasingly needed for different applications in domestic and industrial health and safety, environmental monitoring and process control. These applications require precise real time control and monitoring in order to increase productivity, maintain health and safety as well as to measure environmental pollutions. Gas sensors are increasingly used in the growing markets of aerospace, automotive and logistic applications [28]. In these applications, gas sensors play a significant role in providing safety or in enabling process


4

control. Currently, there is an essential need for the monitoring and controlling of gas emissions from many industrial and household processes. For instance, the use of NH3 gas in refrigeration applications, hydrogen as synthetic fuel in transportation industry, H2S and NH3 (toxic or odorous gases) in industrial processes [29] have increased the need for gas sensors. Furthermore, the use of flammable gases such as liquid petroleum gas (LPG), hydrogen and CH4 in housing in addition to the emission of CO gas from burning charcoal or wood in houses have revealed that the gas sensors are very important in everyday life. Another major application of gas sensors is in oil and gas fired boilers in power plants and automobile engines. Other applications for gas sensors are in medical diagnostics, health care, agriculture, food packaging and aroma detection (e.g. in coffee). Each application places various requirements on the sensor. The ultimate aim is to achieve an accurate and stable monitoring of the analyte down to sensitivities in the range of parts per million (ppm) to parts per billion (ppb) concentrations [30].

Three gas species: H2, C3H6 and NO2 have been investigated in this research. It is well known that these gases are affecting the environment, and also more importantly, human health if over exposure occurs. The following subsections will introduce the fundamentals of these gases. The major applications for sensing them will be presented below to rationalise the author‟s choice for sensing them by developing the nanostructured based sensors as per the objectives of this thesis.

Hydrogen (H2) Hydrogen, a colourless and odourless gas, has attracted a great deal of attention due to the fact that it can be used as a clean and renewable source of energy especially in fuel cells. The primary physical hazards associated with hydrogen gas are its flammability and potential for


5

explosions. Hydrogen gas is explosive when mixed with air at concentrations as low as 4% and very low energy is needed to ignite hydrogen-air mixture [31]. Furthermore, there are safety concerns in hydrogen use, storage in a large amounts, handling and transport. Manipulation and storage of hydrogen are associated with danger of leakage, which can lead to explosion. Therefore, sensors are needed to detect hydrogen leaks to warn of explosion hazards.

Hydrogen is a major cause of metal-corrosion whereby it weakens metals internally. This is especially significant at elevated temperatures. Due to its small size, hydrogen molecules can penetrate into metals and affect their mechanical properties such as strength and durability. Hydrogen sensors are also widely used in different applications such as monitoring of process control systems in industries such as glass, aerospace, chemical, metallurgy (steel) and petroleum and even in biomedical applications.

Propene (C3H6) Hydrocarbons are serious pollutants and are largely included in exhaust gases emissions from automobiles due to incomplete combustion [32]. Human exposure to hydrocarbons, even for short-duration, can be dangerous resulting in dizziness, disorientation, intoxication, and narcotic effects. Furthermore, hydrocarbons react in the atmosphere to form ground-level ozone causing health hazards such as lung damage and reduced cardiovascular functioning [32]. Propylene (propene) is a major product in the petrochemical industry. It is widely used by chemical industry for manufacturing of polypropylene, isopropanol, propylene oxide and acrylonitrile. It is also used in petrochemical industry and industrial hygiene monitoring [33]. Therefore, monitoring hydrocarbons is necessary, as there are needs to detect hazardous concentrations of them.


6

Nitrogen Dioxide (NO2) Nitrogen dioxide (NO2) is a highly poisonous gas. It is also well known for its role in acid rain generation, as the primary cause of photochemical smog, a respiratory irritant and concerningly an olfactory paralysis agent [34, 35]. NO2 is produced as a byproduct in industrial applications such as internal combustion engines, food processing, petrol, metal refining and thermal power stations. There is a need for the detection of this gas in the interest of health and safety practices. This gas is dangerous at concentrations as low as 4 ppm at which can anesthetize a human‟s sense of smell, and therefore creating a potential for overexposure. Long term exposure to NO2 at concentrations above 40-100 µg/m3 causes adverse health effects [36]. Therefore, it is important to monitor and control the emission of NO2 with strict legal limits in confined spaces and industrial environments.

1.1.1.2. Solid State Gas Sensors There are many different types of sensors using various technologies and principles, which can analyse the gas species quantitatively and qualitatively. These sensors should be able to provide accurate, stable, high resolution and low cost sensing. Different environmental factors such as: temperature, humidity, shock and vibrations can influence the sensors‟ performance [37]. Therefore, it is essential to account for these parameters when selecting an approach.

In gas sensors, the sensitivity is usually the primary concern, whilst selectivity is just as important, it is also much harder to achieve [28]. The selection of a suitable transducing platform, materials and sensing structures to address these requirements are of critical value. Usually the selectivity is obtained at the cost of an increased complexity of the system [28]. Chemical sensing has been traditionally dominated by expensive and complex laboratory


7

analytical instruments. These analytical instruments offer the most precise form of detecting chemical concentrations or species, but they are not useful for practical or portable gas monitoring. These systems are generally located in a laboratory, where the samples are brought for analysis. Hence, they are not suitable for on-site or in-situ applications. Furthermore, these approaches involve sample preparation prior to analysis, which make the real-time analysis unfeasible. Solid state chemical sensors have been widely used as a practical solution for such problems and have shown ability to sense both low ppm levels of gases (toxic) and high combustible levels (explosive limitis). These gas sensors are based on a wide variety of technologies such as resistive-type metal-oxide, pyroelectric, piezoelectric, fibre optic, calorimetric and surface acoustic [38, 39]. Among these solid state gas sensors, semiconducting metal-oxide sensors have shown the most promising results especially in harsh and high temperature environments. They have found widespread commercial applications [28, 40] and recent advances in nanotechnology has resulted in novel classes of materials with enhanced gas sensing performance. These have provided the opportunity to dramatically increase the performances of semiconducting metal-oxide gas sensors.

In this PhD research, the author has developed nanostructured metal-oxide semiconducting Schottky diodes for gas sensing applications at elevated temperatures. In addition, gas sensing performance of Pt/graphene-like nano-sheet/SiC devices has also been investigated. Unique properties of graphene such as ballistic electronic transport at room temperature, metal-like characteristics and its large surface area has made it well-suited for gas sensing applications.

1.1.2. Nanotechnology Enabled Gas Sensors Nanotechnology incorporates the design, fabrication and application of nanostructured materials. It also includes the fundamental understanding of physical and chemical properties


8

of the nanostructures. Nanostructured materials are those with at least one dimension in the nano scale range (100 nm or less) [5, 6]. Within this nano range, the physical, chemical, optical, mechanical, electronic and biological properties of materials are unique and substantially different from those observed for the bulk materials [5, 6]. Nanostructured materials have also shown enhanced performance as compared to the bulk materials, when used in similar applications.

Nanotechnology has had significant impacts on sensor technology [5]. It enables the development of small, inexpensive and highly efficient sensors with broad applications. Improved sensitivity is a major attraction for developing nanotechnology enabled sensors [5]. Nanostructured materials have a high surface to volume ratio. Therefore, there is a potential to detect a single molecule or atom resulting in enhancement in the sensors‟ sensitivity in comparison with the conventional material thin film based devices [5, 41]. It is possible to obtain better sensitivity by engineering the structural and morphological properties of nanomaterials. Additionally, the nanostructured materials minimize the time required for analytes to diffuse into or out of the volume [5]. This definitely improves the time required for a sensor to raise an alarm preventing a potential disaster. As a result, nanotechnology offers sensors with higher sensitivity and reliability than the conventional based devices [5, 28, 42].

In this PhD research, the author investigated the gas sensing applications of nanostructured semiconducting metal-oxide and graphene-like nano-sheets at elevated temperatures. These nanostuctured materials were deposited as gas sensing layers in Schottky diode structure. PhD theses previously written by Trinchi and Kandasamy [30, 43] have shown that Schottky diodes based on conventional semiconducting metal-oxide thin films are promising candidates


9

for gas sensing applications at high temperatures. The author strongly believes that nanostructured forms of semiconducting metal-oxide thin films can enhance the gas sensing performance of the Schottky diodes.

To develop the nanostructured thin film Schottky diode based sensors, the author has chosen SiC and epitaxial-GaN layer substrates. The SiC and GaN are wide band gap semiconducting materials and their semiconducting properties are advantageous for the deposition of nanomaterials at elevated temperatures as well as allowing the operation of sensors at high temperatures. Based on the review of the current literature available on Schottky diode based gas sensors, the author investigated several nanostructured materials. These include: TiO2 nano-dimensional grains, SnO2 nanowires, ZnO nanostructures (nanostructured arrays and nanowires-nanoplatelets),

RuO2

nanostructures,

MoO3

nanostructures

(nanoplatelets,

nanoplatelets-nanowires and nano-flowers), WO3 nanoplatelets and graphene-like nanosheets. A transition metal Pt was employed both as the Schottky contact and as the catalyst for gas molecules.

The structural and material properties of the deposited nanomaterials were characterised and analysed using different techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectroscopy. The material characterisations combined with the electrical properties and gas sensing performance assisted with a fundamental understanding of the sensing mechanisms.

Upon exposure of the sensor to target gas (reducing/oxidizing), the gas molecules are adsorbed on the catalytic metal surface and dissociate into atoms, which diffuse through the


10

metal changing the carrier concentration and also forming a dipole layer at the metal/semiconductor interface. This change of carrier concentration and dipole layer formation alter the effective Schottky barrier height energy at the metal-semiconductor interface [44-48], which causes a shift in the current-voltage (I-V) characteristics of the sensor. The gas response is generally measured as a change in the voltage output upon exposure to the target gas, when the sensor is biased at a constant current or a change in current output, when the sensor is biased at a constant voltage.

1.2. Objectives The aim of this research program is to investigate novel nanostructured thin film Schottky diodes for gas sensing applications. It is understood that nanostructured based gas sensors have the potential to enhance sensitivity compared to the conventional based gas sensors due to high surface to volume ratio, allowing rapid diffusion of target gases and owning dimensions comparable to the Deby length [5-7]. To investigate the feasibility of the proposed research, the following objectives outline the aspects considered in this thesis:



Investigation of novel nanostructured thin films as gas sensing layers to increase the gas sensitivity of Schottky diode based sensors.



A comprehensive studying of properties of nanostructured materials (semiconducting metal-oxides and graphene sheets) to understand the sensors‟ performance.



Investigation of electrical properties and gas sensing performance of the fabricated nanostructured thin film Schottky diodes as a function of gas concentration and temperature.




11

Understanding and explaining of the theory of the nanostructured thin film Schottky diodes as well as their gas sensing mechanisms.

1.3. Outcomes and Author‟s Achievements This PhD research has led to many novel outcomes and contributions to the body of knowledge in the field of nanostructured semiconducting metal-oxide and graphene based gas sensors. A comprehensive theoretical and experimental investigation of these nanostructured metal-oxide Schottky diodes and graphene-like nano-sheets devices for sensing of different gas species (hydrogen, propene and NO2) at elevated temperatures is presented in this thesis. The major outcomes of this research program are summarised as follows:



To the best of the author‟s knowledge, for the first time Pt/TiO2 nano-dimensional grains/SiC, Pt/SnO2 nanowires/SiC, Pt/ZnO nanostructured arrays/epitaxial-GaN, Pt/ZnO

nanowires-nanoplatelets/SiC,

Pt/RuO2

nanostructures/SiC,

Pt/MoO3

nanoplatelets/SiC, Pt/MoO3 nanoplatelets-nanowires/SiC, Pt/MoO3 nano-flowers/SiC, Pt/WO3 nanoplatelets/SiC and Pt/graphene-like nano-sheet/SiC based devices were successfully developed and investigated for gas sensing applications. 

Structural and material characterisations of the nanostructures (deposited as gas sensing layers) were performed to study and understand their physical and chemical properties, which influence their gas sensing performance. In addition, the morphological investigation of the nanostructures after testing process was also conducted.




12

The electrical characteristics (I-V) of sensors were measured in the presence and absence of target gas species (hydrogen, propene and NO2 in synthetic air) at different temperatures ranging between 25 and 620°C.



The sensitivity (a change in the voltage at a constant bias current) of these sensors was studied as a function of different concentrations of hydrogen and propene (0.06 to 1% in synthetic air) and NO2 (0.6 to 9.9 ppm in synthetic air) gas.



A theoretical understanding of the nanostructured metal-oxide Schottky diodes and Pt/graphene/SiC devices as well as their gas sensing mechanisms was also investigated.

The author successfully fulfilled the research objectives by developing novel nanostructured semiconducting metal-oxide based Schottky diodes and graphene based devices for gas sensing applications. The results have been published in refereed high impact factor journals and proceedings of prestigious international conferences. These include:



12 publications in referred journals such as Journal of Physical Chemistry C [49], Sensor Letters [50-52], Sensors & Actuators B: Chemical [53], Applied Physics Letters [54], Journal of Physics D: Applied Physics [55], Thin Solid Films [56], Surface Interface Analysis [57], Chemical Physics Letters [21], Sensors & Transducers Journal [58] and International Journal on Smart Sensing & Intelligent Systems [59].



13 publications in proceedings of IEEE Sensors, American Institute of Physics, International Society for Optical Engineering (SPIE), Procedia Chemistry, Procedia Engineering and Transducers conferences.


13

A full list of the author‟s publications is presented in Appendix B. The author also was invited to referee for the Journal of Sensors and Sensors and Actuators B during the course of this PhD program.

The author‟s work has been presented both personally and on her behalf at several international conferences, with the author being fortunate to personally attend the following conferences to present her work:



The International Society for Optical Engineering (SPIE), Smart Structures, Devices and Systems, Melbourne, Australia, December 9-12, 2008.



1st Nano Today conference, Singapore, August 2-5, 2009.



Institute of Electrical and Electronics Engineers (IEEE) Sensors conference, Christchurch, New Zealand, October 25-28, 2009.



International Conference on Nanoscience and Nanotechnology (ICONN), Sydney, Australia, February 22-26, 2010.



13th International Meeting on Chemical Sensors (IMCS-13), Perth, Australia, July 1114, 2010.

1.4. Thesis Organisation This thesis consists of seven chapters and two appendices and is presented as follows:



Chapter 1 is an overview of the author‟s motivation for performing this research, the objectives, outcomes and the achievements of this study.



Chapter 2 presents the literature review of the gas sensors based on Schottky diode structures employing different gas sensing materials (nanostructured semiconducting


14

metal-oxides as well as graphene). The justification for the author‟s research rational is also presented in this chapter. In addition, nanostructured TiO2, SnO2, ZnO, RuO2, MoO3, WO3 and graphene for gas sensing applications are reviewed. 

Chapter 3 describes the fundamental theories of conventional and nanostructured Schottky diodes as well as their gas sensing mechanisms.



Chapter 4 outlines the processes and procedures involved in the fabrication of the nanostructured thin film Schottky diodes studied in this research. The techniques employed for the deposition and synthesis of the nanostructued materials and formation of Schottky and ohmic contacts are described.



Chapter 5 focuses on the structural and material characterisations of the nanomaterials, which were used as gas sensing layers. The characterisation outcomes using SEM, XRD, AFM, TEM, DEX, XPS and Raman spectroscopy techniques, which were used for the characterisation of the nanomaterials, are presented.



Chapter 6 presents the experimental results obtained from the electrical properties and gas sensing performance of the developed nanaostructured Schottky diodes. The effect of nanostructured materials‟ morphology, operating temperature, target gas concentrations on the sensors‟ performance is discussed.



Chapter 7 presents the thesis‟ conclusions and suggestions for possible future work.


15

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M. B. Rahmani, S. H. Keshmiri, M. Shafiei, K. Latham, W. Wlodarski, J. du Plessis, and K. Kalantar-zadeh, "Transition from n- to p-type of spray pyrolysis deposited Cu doped ZnO thin films for NO2 sensing," Sensor Letters, vol. 7, pp. 621-628, 2009.

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Chapter 2 Literature Review and Research Rationale 2.1. Introduction In previous chapter, the motivation, objectives and goals of this PhD research was outlined. This chapter will present the rationale behind the fabrication of novel nanostructured thin film Schottky diodes for gas sensing applications. Although the sensors investigated in this thesis are the Schottky diodes based on nanostructured metal-oxides and graphene-like nano-sheets using SiC and epitaxial-GaN/sapphire (Al2O3) substrates, this chapter will present literature review of the most recent Schottky diode sensors based on different substrates such as: silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), gallium nitride (GaN) and silicon carbide (SiC). The advantages and previous uses of the nanomaterials considered and utilized in this research will be revised in detail.

2.2. Schottky Diode Based Gas Sensors Extensive research works have been devoted to the development of gas sensors based on metal-oxide semiconductor field-effect transistors (MOSFET), MOS capacitors and Schottky diodes since the first reports of hydrogen sensitive Pd/MOS based structures developed by

Chapter 2: Literature Review and Research Rationale

20

Lundström et al. [1, 2] in 1975. Among them, Schottky diodes have been reported to be very promising candidates for gas sensing applications [3]. The fabrication process is very simple, which facilities the research and development of these types of sensors. Schottky diode based sensors have been fabricated employing a number of different inorganic/organic semiconductors together with thin catalytic metal layers (generally group VIII transition metals such as Pt or Pd) that acts as both a Schottky contact and a catalyst for gas adsorption [3]. The catalytic metal and semiconductor form a Schottky barrier, which varies as the device is exposed to different gas species and determines how sensitive the device can be towards target gas. The type of semiconducting material, its structure and the formation of the junction barrier between the metal and the semiconductor can be controlled and varied to sense different types of particular gases.

To the best of the author‟s knowledge, this PhD research is the first investigation on the gas sensing performance of Schottky diodes employing nanostructured semiconducting metaloxides and graphene-like nano-sheets as gas sensing layers. In most of the author‟s reported work, Pt metal was employed as Schottky contact and catalyst and the SiC and GaN substrates were chosen for the fabrication of the Schottky diodes. The SiC and GaN substrates have been selected as they are semiconducting materials with wide band gap (3.02 eV and 3.4 eV, respectively), which make them suitable for high temperature applications [4, 5].

The following sections will present literature review of Schottky diodes employing different types of semiconducting materials such as Si, GaAs, InP, InGaP, InAlAs, GaN, AlGaN and SiC substrates. It will be shown that gas sensitive materials can be incorporated to such substrates in order to form the gas sensitive Schottky diodes.


21

2.2.1. Si Based Gas Sensors Silicon based Schottky diodes have been widely reported in literature for sensing different gases such as hydrogen [6-8], hydrocarbons [8], and NO2/NO [9]. Schottky diodes based on Si are known to be simple, inexpensive devices, which are compatible with Si integrated circuit technology. LundstrÖm et al. [2] reported the sensitivity of the Pd MOS based structure towards hydrogen for the first time. The sensor was fabricated with a thin (10 nm) SiO2 layer deposited between the Pd metal and the Si substrate. They attributed the change in threshold voltage with the change in the metal-semiconductor work function.

Amongst the earliest reports, Tongson et al. [10] showed that the formation of palladium silicides at the Pd/Si Schottky interface leads to the degradation of the sensors sensitivity. A thin oxide layer was introduced into the Pd/Si interface prevented the formation of palladium silicides [10, 11]. In this case, a metal-insulator-semiconductor (MIS) was formed, increasing the fabrication complexity and cost. Shortly after, other researchers began to realise that the presence of the interfacial oxide between the catalytic metal and Si substrate was necessary for the diode to be sensitive to hydrogen gas [6, 12, 13]. One type of sensors (Pd/SiO2/Si) exhibited sensitivity towards hydrogen gas under forward bias operation and the sensitivity was attributed to the change in the Schottky barrier in the presence of hydrogen as well as an increase in the interface state density.

In response to these findings, Poteat et al. [8] presented a comprehensive study on MOS based Schottky diodes using different catalytic metals and n- and p-type silicon substrates. Their investigation covered gate metals and dielectric materials such as: Pd-SiO2, Pd-Ti-SiO2, PtSiO2, Ni-SiO2, Pd-Si3N4, Pd-Si3N4-SiO2 and Pt-Si3N4. These devices were exposed to hydrogen and hydrocarbons gases under forward biased operation and verified that all these


22

MOS devices exhibited a similar gas sensing mechanism, which related directly to the change in metal-semiconductor work function.

A further investigation expanded the application of the aforementioned Si based Schottky diodes towards NOx gas [14]. Interestingly, Au/Si based Schottky diode exhibited a higher sensitivity towards NOx gas in the reverse bias operation than in the forwards bias at room temperature. The diode‟s sensitivity was measured as a change in the current upon exposure to NOx gas, while as the sensor was biased at a constant reverse voltage of 1 V. A reversible, stable and fast response towards NOx concentrations ranging from about 10 ppb to 10 ppm was observed. The diode exhibited an increase in the reverse current of about three orders of magnitude, recording a response time of 134 s and recovery time of 45 s upon exposure to 0.75 ppm NOx.

These findings prompted vast interest in the development of sensors with improved performance towards gaseous species. Polishchuk et al. [15] reported hydrogen sensitivity of the catalytic metal/Si based Schottky diodes using porous Si. Thereafter, Zhang et al. [9] investigated the NO and NO2 gas sensing performance of Pt-Pd/polished Si/Al and PtPd/porous Si/Al Schottky diodes at room temperature. It was found that the I-V characteristics of the Pt-Pd/polished Si/Al diode were very similar to that of the conventional Schottky diodes, whereas larger current was observed for the Pt-Pd/porous Si/Al diode. The diode fabricated on porous Si surface showed a significant higher sensitivity towards NO 2 than the Si based sensor under reverse biased condition. This finding was attributed to the larger available surface area from the porous silicon for gas interaction. Furthermore, hydrogen gas sensors based on PtSi/porous Si Schottky junctions were investigated and tested under reverse bias operation at room temperature [16, 17]. These diodes exhibited a breakdown behaviour in


23

their current-voltage (I-V) characteristics at reverse bias voltages of approximately between 5 to 15 V, as very large electric fringing fields (105 -106 V/cm) existed at the sharp edges and the bottom of the pores. Very fast response and recovery of about 6 and 60 s, respectively, were recorded for these sensors.

In 2007, Gorbanyuk et al. [18] investigated the effect of hydrogen sulphide (H2S) on the I-V characteristics of MIS structures based on copper (Cu) doped nanoporous Si. It was shown that the doping of Cu into nanoporous Si caused an enhanced H2S sensitivity of MIS structures even without the presence of catalytic metals (for example, Pd or Pt) at room temperature. They observed higher lateral voltage shift in reverse bias operation than in the forward bias. They associated the gas sensitivity of the device to a chemical interaction of H2S adsorbed molecules with the Cu in the Si pores. The Cu sulphide was found in the pores, which directly caused a decrease in the potential barrier and therefore, a subsequent an increase in the reverse current.

A hydrogen gas sensor based on Pd nanoparticles/porous Si/Si was investigated at room temperature by Rahimi et al. [19]. They showed that the distribution of Pd nanoparticles over the porous Si produces high gas sensitivity towards hydrogen in the forward biased operation. Dzhafarov [20-22] performed measurements of the I-V characteristics of Schottky-type Ag/porous Si/Si and Au/porous Si/Si structures in atmospheric ambient and with the presence of different hydrogen concentrations. The experimental results suggested that these Schottky diodes based on metal/porous Si/Si structures were promising candidates for both the gas sensors as well as hydrogen cells.

The experimental results indicated that employing porous Si could improve the sensors‟ sensitivity, however long term stability was still subject for concern. Razi et al. [23] studied


24

the hydrogen sensing properties of Pd nanoparticles/porous Si/Si sensors at room temperature for the duration of nine months. They found that after 45 days, the sensitivity decreased and response became significantly slower due to increase in the amounts of oxides, carbon contamination, silicides layer formation at the interface (Pd xSi) and defects in Pd film, some of which already was present in the as-deposited samples.

Chou and Chiang et al. [24-26] reported that hydrogen sensing performance of a Pd/n-LTPS (low temperature polysilicon)/glass Schottky diode, which its performance was significantly improved with use of a TiO2 interface layer (under conditions of room temperature and a constant voltage bias of 2 V). They measured an increase in the relative sensitivity ratio from 290.4% to 1539.6% towards 50 ppm hydrogen and a reduction in response time from 61 to 40 s The Pd/TiO2/n-LTPS/glass. These results showed a new approach to produce the next generation of sensors with high sensitivity towards hydrogen at a low cost. The author believes that employing semiconducting metal-oxide other than TiO2, especially in nanostructured forms, will enhance the gas sensitivity of the Si based Schottky diodes even further.

Despite extensive research efforts, one of the major concerns of the Schottky diode gas sensors based on Si substrates is its limited operating temperature, below 200°C due to the small band gap of Si (1.12 eV) [4], which limits their use to relatively low temperatures. There is certainly a need for high temperature operating gas sensors in industries such as nuclear plant instrumentation, aerospace (emission monitoring, fuel leak detection and fire detection) and exhaust monitoring in automobiles [4]. This issue is also important for sensing hydrocarbons. High temperatures environments are needed for the dehydrogenation of these gases at catalytic metal surfaces [5].


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2.2.2. GaAs Based Gas Sensors The Si based sensors have the advantage of low cost and highly matured techniques, while the compound semiconductor based devices exhibited the benefit of higher sensitivity, faster response and higher operational temperatures [5, 27-30]. Amongst these compound semiconductors, GaAs with band gap of 1.43 eV [31] has been reported by many authors for gas sensing applications.

In a metal-semiconductor contact, the Schottky barrier height is dependent on the difference in work function between the metal and the semiconductor [32]. However, for many group III-V semiconductors, the barrier height does not always follow the theory due to the high density of surface states, which causes Fermi level pinning [3]. Freeouf et al. [33] proposed an effective work function model, which emphasizes the role of the intermediate layer formed by the reaction between the metal and GaAs to determine the barrier height. However, some researchers have found that the pinning of the Fermi level does not occur, when the metalGaAs interface fabricated on clean (100) GaAs surfaces [34, 35].

There have been many reports on performance of catalytic metals (Pt and Pd)/GaAs based Schottky diodes for sensing different gases such as hydrogen [8, 34, 36-38], hydrocarbons [8] and ammonia [36-38]. Salehi at al. [39] successfully fabricated ITO/GaAs Schottky diode sensors for carbon monoxide (CO) sensing applications. These sensors were operated in the forward biased condition and the responses towards the reducing gases were attributed to the lowering of the metal work function and subsequently lowering the Schottky barrier in the metal and GaAs interface.


26

In order to improve the performance of sensors based on GaAs (in terms of sensitivity and response/recovery time) two approaches have been investigated: (1) employing porous GaAs semiconductor and (2) employing a gas sensitive metal-oxide layer between the catalytic metal and GaAs substrates. Salehi at al. [40-43] fabricated Pd/porous GaAs and Au/porous GaAs Schottky diodes and compared the devices‟ sensing performance towards hydrogen, humidity, CO and NO gases with Pd/GaAs and Au/GaAs Schottky diodes. They demonstrated that the porous diode exhibited a lower turn-on voltage and a much lower breakdown voltage than the non-porous diode. The significant decrease in the breakdown voltage was linked to highly porous structure of the metal-porous GaAs interface, which creates a large electric field at the top of pores. The Pd/porous GaAs Schottky diode sensors showed sensitivity towards hydrogen gas under both the forward and reverse biased operations. They exhibited a response time of 1 s and sensitivity of more than three times higher than that of the Pd/GaAs diode towards 500 ppm hydrogen at room temperature in the forward biased condition. The increase in the sensitivity of the porous sensor was attributed to the morphology of the surface and was the function of two parameters: the surface area and the density of cavities between pore walls (number of pores, which is important in catalysts) [44]. The characteristics of humidity sensing of Pd/porous GaAs and Au/porous GaAs Schottky diodes were also investigated and compared with Pd/GaAs and Au/GaAs sensors, respectively. The porous GaAs based sensors exhibited a much higher sensitivity towards humidity and faster response than the non-porous GaAs sensors due to the existence of a higher effective adsorption area [40, 41]. There are other reports on gas sensing performance of Au/GaAs and Au/porous GaAs Schottky diodes towards CO and NO gases and their comparison under reverse biased operation at different temperatures [43]. It was found that the Au/porous GaAs sensor was highly sensitive to CO and NO gases, while the Au/GaAs sensor responded negligibly to


27

these gases. The significant decrease in the breakdown voltage of the Au/porous GaAs sensor was due to the highly porous structure of Au/porous GaAs diode in producing the large electric fields at the top of the pillars in the porous material.

Lee et al. [45] fabricated NO2 gas sensors based on Au/rough GaAs Schottky diodes. They grew n-GaAs epitaxial layer on n-GaAs substrates. To increase the sensor‟s response, the surface of the n-GaAs epitaxial layer was roughened using a wet etchant consisting of H2SO4:HNO3: DI solution. The sensitivity of the sensors towards 100 ppm NO2 was measured at different reverse bias voltages of 5, 10 and 30 V at room temperature. It was found that the response of the sensor could be controlled by the applied voltage. The highest sensitivity of 110% was recorded at 30 V.

The second approach to increase the GaAs based sensors‟ sensitivity was employing a metaloxide layer. Lin et al. [28] reported that the fabrication of a catalytic Pd MOS Schottky diode based on GaAs weakens the Fermi level pinning and hence, improves the barrier height and sensitivity of the diode towards hydrogen. Weichsel et al. [46] demonstrated hydrogen sensing performance of a Pd/ZnO/GaAs Schottky diode. The ZnO thin film layer with 2 µm thickness was deposited between Pd and GaAs substrate using metal organic chemical vapour deposition technique. They measured the sensor‟s response as a change in the reverse bias current at a constant voltage bias of 0.5 V immediately and 17 hrs after exposure to hydrogen at room temperature. The sensor showed an increase in current three times higher compared with the value before exposure, while the sensor did not show response after long term (17 hrs) exposure. In addition to exposure to hydrogen, annealing the sensor at temperatures above 423 K in hydrogen ambient led to degradation of the sensor‟s performance, which was expressed as a strong irreversible increase of the reverse bias current.


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They ascribed this behaviour to the chemical reaction between the Pd and ZnO or the diffusion of Pd into the ZnO layer. Tasaltin et al. [47] measured the I-V and capacitancevoltage (C-V) characteristics of a Pd/native nitride/GaAs device towards hydrogen gas. Native nitride films with different thicknesses were grown on n-GaAs epitaxial crystals by anodic nitridation. The devices showed behaviour like a Schottky diode and their responses were reversible with response and recovery time of 35 and 40 s, respectively. They showed that the device with the thickest native nitride film exhibited better sensitivity.

2.2.3. InP Based Gas Sensors InP is a semiconducting material with a wide band gap of 1.94-1.96 eV [48] and InP based Schottky diodes have been shown as promising devices for gas sensing applications [49-53]. It was shown that the Pd/InP Schottky diode exhibits high sensitivity towards hydrogen at both forward and reverse current density-voltage (J-V) characteristics at room temperature [49]. Tian [50] demonstrated that a Pd/InP Schottky diode has a good sensitivity to both hydrogen and oxygen. It was found that the barrier height decreased when the sensor was exposed to hydrogen-containing nitrogen and the barrier height increased upon exposure to oxygen-containing nitrogen.

In practical applications, a low barrier height associated with the high defect state density at the Pd/InP interface can severely limits the allowable change in the barrier height [3]. Chen et al. [54, 55] demostrated that by using electrochemical techniques to fabricate the Pd/InP Schottky diodes, they could enhance the sensing performance by eliminating the Fermi level pinning effect due to low-energy fabrication process. Although adhesion and uniformity problems must still be addressed as compared with traditional vacuum deposition techniques,


29

the electrochemical methods exhibit other advantages such as simple processes, low cost and easy operation.

Different modifications in the InP based sensors‟ structure have been performed in order to enhance the sensors sensitivity. Talazac et al. [51, 52, 56] reported improvement in the InP sensors‟ performance by using a thin shallow n-type InP layer between the Pd contact and the p-type bulk InP substrates. The sensors showed a significant increase in the reverse current at a fixed reverse voltage bias of 1 V upon exposure to NO2 and ozone (O3). This result was attributed to lowering of the barrier height at the Pd/n-InP interface upon exposure to the target gas. The sensor showed reproducible results only towards NO2. However, after each period of exposure to O3, the degradation of gas sensor characterisitcs (sensitivity and response/recovery time) were significantly noticeable, due to O 3 oxidation of Pd. Different treatments (hydrogen reduction, CO reduction and high vacuum) were investigated to avoid degradation of the O3 poisonous effect [57-59]. It was shown that initial response and recovery time of the sensor could be reduced by hydrogen reduction and high vacuum treatment of metallization layer after exposure to O3. Furthermore, the investigations also showed that these treatments increase the sensor sensitivity. CO reduction permitted reproducible measurements of low NO2 and O3 concentrations (20-100 ppb). Varenne et al. [60] showed that the Au/n-InP/p-InP Schottky diodes are also sensitive to NO2 gas. Kimura et al. [53] reported that the deposition of Pt nanoparticles on InP Schottky diodes could improve the sensing properties towards hydrogen. The sensor showed high sensitivity in reverse biased operation with sharp on-off transients upon exposure to hydrogen.

Semiconducting InGaP, InAlAs and InAlP based Schottky diodes have also been reported for gas sensing applications [30, 61-65]. They are compound semiconductor with band gaps of


30

1.92 eV [30], 1.456 eV [62] and 2.3 eV [65], respectively. By forming these compounds, the barrier height can be altered and tuned for specific gas sensing applications. A Pt/oxide/In0.49Ga0.51P Schottky diode was fabricated and tested towards hydrogen gas at different temperatures up to 600 K [30]. The sensor showed high sensitivity and fast response in forward biased operation. At room temperature, a very high sensitivity of 561% and response time of less than 1 s at applied forward voltage of 0.7 V towards 9090 ppm hydrogen in air were recorded.

Tsai et al. [61] conducted a comparative hydrogen sensing performance of Pd/InGaP and Pt/InGaP Schottky diodes and found that the Pd/InGaP sensor showed higher sensitivity than the Pt/InGaP sensor under low hydrogen concentrations. Lin et al. [66] reported employing oxide layer in Pd/InGaP structure improved the hydrogen sensing performance of the Schottky diode. The higher sensitivity was attributed to the presence of the interfacial oxide layer, which reduced the surface state density and hence reduced the influence of pinning effect.

2.2.4. GaN and AlGaN Based Gas Sensors III-nitride materials have attracted great interest due to their inherent properties such as wide band gap, resistance to chemical corrosion and high temperature durability [67]. GaN and AlGaN are wide bang gap semiconductors (3.4 eV [5] and 4.12 eV [68] , respectively), which have shown great promise in gas sensing applications at high temperatures [5, 69]. As mentioned previously, for many III-V semiconductors, the barrier height of a metalsemiconductor contact does not follow the theory due to the high density of surface states, which causes Fermi level pinning. The pinning effect would limit the Schottky barrier height


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change within a small range and hence would affect the sensor performance. Advantageously, GaN based materials exhibited less Fermi level pinning effect than others [70].

In 1999, Luther et al. [5] reported for the first time gas sensing properties of Pt/GaN Schottky diodes towards hydrogen and propane. They measured forward I-V characteristics and dynamic response of the diodes at high temperatures between 200 and 400°C. The results revealed a response to hydrogen at all studied temperatures, while a response to propane was obtained at 300°C and above. Later, Schalwig et al. [69, 71] tested Pt/GaN Schottky diodes towards different gases including: hydrogen, hydrocarbons (C3H8, C2H2, and C4H10), CO, and NOx at high temperatures up to 600°C. They found that these devices are mainly sensitive to hydrogen and unsaturated hydrocarbons with a sizable cross-sensitivity to CO and NO2. Hydrogen and hydrocarbons sensing performance of the Pt/GaN Schottky diodes have also been examined by some other researchers [72-79].

There were several attempts to improve the GaN based sensors performance by employing different catalytic metals, different thickness of catalytic metals, employing metal-oxide layers between the metal and GaN substrates and employing porous GaN.

Kim, Kouche and co-workers [80, 81] investigated hydrogen sensitivity of the Pt/GaN and Pd/GaN Schottky diodes. The Pt/GaN diodes showed larger response (changes in current at a constant forward bias voltage) than the Pd/GaN diodes due to more effective catalytic dissociation of hydrogen on the Pt surface. Confirming these results, Voss et al. [82] also reported larger response for the Pt/GaN sensors towards hydrogen and hydrocarbons gases than that for the Pd/GaN based sensors.


32

Tilak et al. [83] investigated the influence of metal thickness to hydrogen sensitivity of Pt/GaN Schottky diodes. Three sets of diodes were fabricated with 80 Å, 240 Å and 400 Å of Pt for the Schottky contacts. The sensors were tested towards up to 1% hydrogen in air, while the sensors were kept at a constant forward bias current. The responses (change in voltage) increased as the thickness of the Schottky Pt contact decreased. Reduction of catalytic Pt metal thickness resulted in the discontinuity of film and higher grain boundary densities. Hence, film porosity enabled more Pt metal surface area to be exposed to the hydrogen resulting in higher diffusion rates [84]. Cho et al. [85] also reported improvement of CO sensitivity in Pt/GaN Schottky diodes by decreasing the Pt metal thickness.

Tsai et al. [86-88] reported that using metal-oxide layer between Pt and GaN substrates enhance the sensors‟ sensitivity towards hydrogen at both forward and reverse biased operation. Hydrogen sensors based on Pt/Ga2O3/GaN Schottky diodes were fabricated and tested by Yan and Lee [89, 90]. The experimental results demonstrated an improvement in hydrogen forward response due to the presence of the Ga2O3. As the Ga2O3 layer could provide more trap sites at the Pt/Ga2O3 interface for the hydrogen atoms, a large barrier height change was observed. Besides, the associated series resistance was decreased as the sensor exposed to hydrogen due to the electrical changes of the Ga2O3 layer.

There have been several reports in the literature regarding the improvement of the GaN Schottky diodes performance using porous GaN. Yam et al. [91, 92] reported that hydrogen sensitivity of Pt/GaN and Pd/GaN Schottky diodes can be increased using porous GaN substrates. The porous and non-porous GaN based sensors were tested and compared towards hydrogen at 25 and 100°C. Both sensors exhibited response to hydrogen at forward and reverse biased I-V characteristics. The porous GaN based sensor showed a significantly larger


33

change of current upon exposure to 2% hydrogen in N 2 as compared to non-porous GaN sensor. This result was attributed to the unique microstructure at Pt/porous GaN interface, which allowed higher accumulation of hydrogen atoms, leading to a significant change in the electrical characteristics. Chiu et al. [93, 94] also showed that using porous-like mixture of Pd and SiO2 in GaN based Schottky diode structure resulted in a highly efficient dissociation of hydrogen molecules, and subsequently an increase in barrier height change of the reverse biased diode resulting in a high sensitivity. The performance improvement was attributed to an enhanced catalytic activity of the porous Pd and SiO2 mixture.

Das and Pal [95] fabricated and tested Pd/nanocrystalline GaN Schottky diodes towards hydrogen. Nanocrytalline GaN (with an average crystallite size of ~100 nm) was deposited onto the fused silica substrates by high pressure dc sputtering of Si (1 at %) doped GaN target. The developed sensors showed response to hydrogen at both forward and reverse biased operation. However, it was observed a decrease in the sensitivity with increasing temperature for both forward and reverse operation modes, which was attributed to desorption of hydrogen as the temperature was increased.

Gas sensing applications of AlGaN based Schottky diodes have also been widely reported. Song et al. [70] characterised hydrogen sensing properties of Schottky diodes based on AlGaN/GaN heterostructures using Pt, IrPt and PdAg catalytic metals at high temperatures from 200 to 800°C. Over wide large range of temperature, the forward current of all the diodes increased with exposure to hydrogen. With the increase in the temperature, the hydrogen sensitivity of the Pt and IrPt diodes improved due to a more effective hydrogen dissociation, while the sensitivity of PdAg diodes degraded due to thermal instability of PdAg.


34

Matsuo et al. [96, 97] reported higher sensitivity for Pt/AlGaN/GaN Schottky diodes than Pt/GaN towards hydrogen gas. The diodes showed increase of forward and reverse currents in their I-V characteristics upon exposure to hydrogen at 100°C. The Pt/AlGaN/GaN sensors were operated under reverse biased operation and fast response of 10 s was recorded. Hudeish et al. [98] also reported that the hydrogen response of Ni/AlGaN/Sapphire Schottky diodes in forward biased operation was much higher than that obtained for Ni/GaN or Ni/Si Schottky diodes. Miyoshi et al. [99] successfully demonstrated that Pt/AlGaN/GaN Schottky diodes are sensitive towards to low hydrogen concentration of 50 ppm under reverse biased operation.

Pt/AlGaN/AlN/GaN Schottky diodes were successfully fabricated for hydrogen and CO sensing applications [100-102]. The diodes showed remarkable sensitivity towards hydrogen and CO gases at 50 and 100°C under forward biased operation. Tsai et al. [67, 103] observed a significant large response of about 6 V under a constant reverse bias of 20 µA upon exposure the Pt/AlGaN/GaN Schottky diodes towards 9660 ppm hydrogen in air at 300°C. This was attributed to the catalytic capability of Pd metal with contributions from a highdensity 2-D electron gas induced from spontaneous and piezoelectric polarizations of the AlGaN/GaN structure.

2.2.5. SiC Based Gas Sensors The development of SiC as a high operational temperature electronic material [104] allow the fabrication of sensors, which can function in conditions where Si based technology is inoperable. The properties of SiC that make it favourable for high temperature gas sensing applications include wide band gap (3.02 eV), low intrinsic carrier concentration, superior mechanical strength and high thermal conductivity [4].


35

The gas sensing performance of catalytic metal/SiC devices have been extensively investigated by different groups. First field effect based sensors based on SiC with a Pt or Pd gate were developed in 1992 [105, 106]. Soon after, research papers on SiC based MOS devices were published by Arbab together with Lundström, LIoyd Spetz and co-workers [107110]. They employed SiC as the semiconductor substrate, a 50 nm SiO2 layer and Pt as the catalytic metal. The developed sensors showed sensitivity towards hydrogen and hydrocarbons gas at temperatures between 400 and 500°C. Later, they investigated the presence of TaSix buffer layer between two Pt layers deposited on the metal-oxide (SiO2) surface to increase the long term stability [111-113] at temperatures up to 700°C [114-116]. In addition to hydrogen and hydrocarbons, the sensors were tested towards exhaust gases [117-121] and CO [122-124]. Filippov et al. [125] showed that the sensing properties of Pt/SiC Schottky diodes degraded at high temperatures (500°C) due to the formation of platinum silicides. They suggested that sensitivity and stability of the sensors would be improved by enhancing the homogeneity of the metal-semiconductor interface and introducing a buffer layer between the Pt and SiC to prevent the formation of silicides.

Zubkans et al. [126] reported the sensitivity of Pt catalytic MOS devices towards NOx. They showed that the devices‟ sensitivity could increase via ammonia treatment. Khan et al. [127] also demonstrated NO sensing performance of Pt/SiC Schottky diodes at high temperatures up to 400°C for concentrations as small al 10 ppm.

Kim et al. [128-130] investigated I-V characteristics of both Pd/SiC and Pt/SiC Schottky diodes in the presence of hydrogen and hydrocarbon gases. They found that Pd based sensors had higher adsorption rates and much shorter adsorption time than the Pt based sensors. The result was attributed to the inherent catalytic properties of both materials.


36

In most cases, the SiC based sensors at high temperature applications showed higher sensitivity compared with those based on Si. In addition, these devices were able to monitor combustion gases, which were not possible with the Si based sensors.

There were several attempts to increase such devices‟ sensitivity by modification of their structures. Firstly, modification in thickness of the catalytic metal layer was investigated [120]. It was shown that the thicker layers result in response saturation at higher gas concentrations. It was also reported that porous or discontinuous metal layers produced an increase in the sensor‟s sensitivity [115, 131, 132]. Although the modifications resulted in improvements in the sensor‟s dynamic performance, the operating temperatures and maximum voltage shifts were remained similar.

The next approach was to change the material of the oxide layer, which resulted from the investigation on high temperature annealing and long term stability of Pd/SiC and Pd/SiO2/SiC Schottky diodes by Chen, Hunter and co-workers [133]. They observed that hydrogen sensitivity of the diodes significantly decreased by annealing the sensors at 425°C for 140 hrs. In addition, they found drift in their response when the sensors operated for long periods [134]. This was attributed to reaction between Pd and SiC. Later, they showed that using an alloy of Pd/Cr as well as SnO2 instead of SiO2 as an interfacial layer increased the hydrogen and hydrocarbons sensitivity and long term stability [135, 136]. They suggested that employing a gas sensing semiconducting metal-oxide layer instead of insulating oxide (SiO2) would result in higher sensitivity due to reaction of the target gas with the catalytic metal as well as the oxide layer. This would also increase the stability, because the oxide layer can passivate the semiconductor surface, acting as a barrier between the metal and the SiC.


37

Furthermore, there would be possibility to tailor the selectivity of the sensor by selecting proper oxide layers.

Since the team at the NASA Glen Research Centre reported employing other metal-oxide layers than SiO2 offer greater sensing possibilities in MOSiC based devices, numerous research have been conducted using other materials. These materials include CeO 2 [137], TiO2 [138, 139], Ga2O3-ZnO [140, 141], Ga2O3 [142-145], WO3 [146, 147], CoOx [148, 149], MoO3 [150], In2O3 [151], TiWO [152], Pt catalysed TiO2 [153] and ZnO [154]. These metaloxide thin films were deposited between the catalytic metal (Pt or Pd) and SiC substrates to form Schottky diodes for gas sensing applications. The sensors responses were measured towards hydrogen and hydrocarbons gases under forward biased conditions. Table 2.1 shows the summary of the gas sensing results obtained for the developed SiC Schottky diode based on metal-oxide thin films reported in the literature [155].

2.3. Review on Nanostructured Materials Pertaining to This Research Literature has shown that employing nanostructured materials as gas sensing layers within the Schottky diodes‟ structure increases their sensitivity. To date, there have been several reviews on gas sensing applications of nanostructured Schottky diodes [9, 15, 17-19, 21] [40-43] [91, 92]. However, most of these studies focus on the forward biased condition. The reverse biased response of the nanostructured Schottky diodes have been reported by some research groups [17-19, 43, 45, 93, 94, 103, 156], but the theoretical basis behind their behaviour has not been presented.


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Table 2.1. Gas sensing results of the developed SiC based Schottky diodes using metal-oxide thin films as a gas sensing layer.

Sensor

T (°C)

Analyte Gas

Voltage Shift (ΔV)

Ref.

Pt/SiO2/SiC Pd/SnO2/SiC Pt/CeO2/SiC Pt/TiOx/SiC

400 350 500 300 520

0.8 V 0.3 V at + 0.1 mA 3 V at + 0.3 mA 3 V at + 50 µA 0.425 V at + 9 µA 4.5 V at + 0.5 mA 0.74 V at + 90 µA

[109] [135] [137]

Pt/TiO2/SiC

Pt/WO3/SiC

300

2.5 V at + 0.1 mA

[146]

Pt/Ga2O3/SiC Pt/Ga2O3-ZnO/SiC Pt/MoO3/SiC

525 525 310 420

Pt/TiWO/SiC

200 420 250 420 420 330

0.3 V at + 0.1 mA 0.16 V at + 0.1 mA 0.3 V at + 0.5 mA 2.3 V at + 0.5 mA 0.5 V at + 10 µA 2.3 V at + 10 µA 1.15 V at + 10 µA 2.52 V at + 10 µA 2.8 V at + 10 µA 3.2 V at + 10 µA 0.39V at + 0.5 mA 111.87 mV at + 0.5 mA

[142, 157]

Pt/CoOx/SiC

5000 ppm H2 in air 400 ppm H2 in N2 3% H2 in 1%O2/N2 20% H2/N2 20%O2/N2 1% H2 in air 1% H2/N2-1%O2/N2 1% C3H6 in air 10% H2/N2 10%O2/N2 1% H2 in air 1900 ppm C3H6 1% H2 in air 1%H2/N2-1%O2/N2 1% H2 in air 1% C3H6 in air 1% H2 in air 1% C3H6 in air 1% H2 in air 1% C3H6 in air 1% H2 in air 1% H2 in air

Pt/catalysed TiO2/SiC Pt/In2O3/SiC Pt/ZnO/SiC

[138, 139]

[141]

[150] [148] [152] [153] [151] [154]

The nanostructured metal-oxides, which were employed by the author of this thesis as gas sensing layers in the Schottky diode structure, include TiO2 nano-dimensional grains, SnO2 nanowires, ZnO nanostructured arrays, ZnO nanowires-nanopletlets, RuO2 nanostructures, MoO3 nanostructures (nanoplatelets, nano-flowers and nanoplatelets-nanowires), and WO3 nanoplatelets.

In this PhD research, graphene-like nano-sheets were also employed as a gas sensing layer. Unique properties of graphene such as ballistic electronic transport at room temperature [158], metal-like characteristics and its large surface area has made it well-suited for gas sensing


39

applications. The detail review of these nanostructured metal-oxides and graphene-like nanosheets will be presented in this section.

2.3.1. Nanostructured Semiconducting Metal-Oxide Based Gas Sensors

2.3.1.1. TiO2 Nanostructured Based Gas Sensors In recent years, much research effort has been devoted to investigation of nanostructured forms of TiO2 as a gas sensing material due to their highly active surfaces with a large surface to volume ratio and unique properties [156, 159-163]. Grimes and co-workers [164-167] presented several promising results on TiO2 nanotube based sensors for hydrogen gas sensing applications. In their last report, they developed and tested TiO 2 nanotube arrays based sensors towards hydrogen at room temperature [167]. The nanotube arrays were fabricated by anodizing Ti foil in an aqueous electrolyte solution containing acetic and hydrofluoric acid. Pd metal was evaporated onto the surface of the nanotube films to increase the sensitivity of the sensor towards hydrogen. The sensors showed over a four order of magnitude change in electrical resistance upon exposure to 100 ppm hydrogen at room temperature.

In literature, there are several reports on Schottky diode type sensors based on nanostructured TiO2. Shimizu, Iwananga, Miyazaki and co-workers [156, 168-170] fabricated Pd/nanoporous anodized TiO2 film/Ti Schottky diodes and tested towards hydrogen gas in air and N 2 atmospheres. Nanoporous TiO2 films were prepared by anodic oxidation of a Ti plate in a H2SO4 solution. Their sensors exhibited sensitivity towards hydrogen both in air and N 2 under forward and reverse biased conditions, whereas the sensitivity in N 2 was lower than in the air. Upon exposure the sensors to hydrogen in air, higher sensitivity was observed under reverse


40

biased condition than in the forward biased condition. Interestingly, the I-V characteristics of the sensor showed ohmic behaviour upon exposure to 1% hydrogen in air at 250°C.

In this PhD research, to the best of the author‟s knowledge, for the first time Pt/TiO2 nanodimensional grain/SiC Schottky diodes were fabricated and tested towards hydrogen and propene gases at elevated temperatures up to 620°C. The nanostructured TiO 2 thin films were prepared via the anodization of deposited Ti films on SiC substrates. Anodization is a low energetic and low cost process compatible with the microelectronics industry standards. The author will show that the devices, with anodized TiO 2, show higher sensitivity towards hydrogen and propene gases in reverse bias than in the forward bias operation, which will be ascribed to the localized electric field enhancement. Later in this thesis, it will also be shown that the sensors exhibit non-linear Schottky I-V characteristics unlike the sensors reported by Shimizu, Iwananga, Miyazaki and co-workers [156, 168-170].

2.3.1.2. SnO2 Nanostructured Based Gas Sensors Gas sensing properties of one dimensional (1D) SnO 2 nanobelts and nanowires have been studied by a number of researchers [171-176] since Comini et al. [171] first reported gas sensitivity of conductometric SnO2 nanobelt based sensors. To date, only one report regarding Schottky diode type sensors based on nanostructured SnO 2 films has been published [177]. Recently, Lu et al. [177] presented a hydrogen sensitive Schottky diode based on an SnO2 film consisting of nanoscale particles with Pt electrodes on top. The SnO2 films were prepared by thermal oxidation of metallic Sn films with thicknesses of 5, 10, 20, 50 and 100 nm. The Sn films were deposited on SiO2/Si substrates using electron beam evaporation. It was observed that the average size of the nanoparticles shrank as the thickness of the original Sn films decreased. At 300°C, the sensors showed sensitivity towards 100 ppm hydrogen


41

balanced in N2 in both forward and reverse biased operations. However, ohmic behaviour in hydrogen ambient was observed. Highest sensitivity was recorded for the device based on the SnO2 film converted from 20 nm thick as-deposited Sn upon exposure to 100 ppm hydrogen under a forward bias voltage of 0.5 V at 300°C. This sensor showed very fast response of less than 10 s.

In this PhD research, the author has developed Pt/SnO 2 nanowire/SiC Schottky diodes for the first time to the best of the author‟s knowledge. The SnO 2 nanowires were grown onto SiC substrates using a thermal evaporation technique producing a dense forest of nanowires. Later in this thesis, it will be shown that highly crystalline SnO 2 nanowires, with perfect lattice structures, were obtained as the result of an optimized deposition process. It will also be shown that the device can operate reliably at elevated temperatures of up to 620°C.

2.3.1.3. ZnO Nanostructured Based Gas Sensors Nano-dimentional ZnO structures such as nanorods, nanowires and nanobelts have been identified as promising candidates for gas sensing [178-184]. ZnO nanostructures have been widely employed as a gas sensing layer in conductometic [178-182] and FET [183-185] based gas sensors. Recently, some reports have been published on electrical and gas sensing characteristics of Schottky diodes based on nanostructured ZnO [186-190]. In one of these reports, electrical measurements of an individual single crystalline ZnO nanobelt, which was placed between two Pd contacts were performed in air and NH 3 (1% and 3%) balanced in ambient air at room temperature [186]. The I-V curves of device showed rectifying behaviour in both ambient air and NH3. Upon exposure to NH3 gas, a voltage shift in the I-V curves was observed in both forward and reverse bias operations. Zhang et al. [187] also reported rectifying behaviour in the I-V characteristics of Schottky diode based on a single ZnO


42

nanorod aligned across paired Ag contacts. The device was tested towards NH3 gas (50 and 1000 ppm) at room temperature and exhibited an increase in the forward and reverse currents, indicating a high sensitivity to NH3 gas.

Another investigation of gas sensing performance of ZnO nanostructured Schottky diodes was presented by Yu et al. [188, 189]. They reported the fabrication of a Pt/ZnO nanorod/SiC Schottky diode and tested it towards hydrogen gas over a temperature range of 280 to 430°C. The sensor exhibited larger lateral voltage shift in the I-V curves upon exposure to hydrogen under reverse bias operation than in the forward bias. They showed that upon exposure to hydrogen, the effective change in free carrier concentration at the Pt/ZnO nanorod interface was amplified by an enhancement factor, effectively lowering the reverse barrier, producing a large voltage shift. Hydrogen gas sensing performance of Pt/ZnO single nanowire Schottky diodes were studied by Das et al. [190]. The sensor exhibited good sensing characteristics (sensitivity ≈ 90%) at room temperature with response time of approximately 55 s.

In this PhD research, to the best of the author‟s knowledge, for the first time, ZnO thin films with different morphologies including nanostructured arrays and nanowires-nanoplatelets were used as the gas sensing layer in Schottky structures. It will be later shown that the ZnO nanostructured arrays were grown on epitaxial-GaN layer. Metal-organic chemical vapour deposition (MOCVD) grown epitaxial-GaN layer was chosen as the media for its high lattice compatibility with ZnO [191]. Shottky diodes were also developed via the deposition of ZnO nanowires-nanoplatelets onto 6H-SiC substrates using a thermal evaporation technique. The author will show that choice of SiC was advantageous as it can tolerate high temperatures required in the ZnO thermal evaporation process. Another valuable point regarding choosing SiC was that it has a hexagonal crystal structure, which is similar to ZnO, facilitating perfect


43

ZnO crystal growth on the SiC surface. The developed Pt/ZnO nanostructured array/epitaxialGaN Schottky diodes were tested towards NO2 at high temperatures up to 350°C. The Pt/ZnO nanowire-nanoplatelet/SiC Schottky diodes were tested towards hydrogen and propene at elevated temperatures up to 620°C. Their gas sensing performance will be presented later in this thesis.

2.3.1.4. RuO2 Nanostructured Based Gas Sensors RuO2 is a wide band gap (approximately 2.4 eV [192]) semiconducting metal-oxide. The dielectric constant of RuO2 has been reported as 9.72 when deposited on SiC, which is known to be suitable and advantageous for high-k dielectric applications and Schottky contacts [193]. Most of the studies on RuO2 nanostructures were limited to its deposition and characterisation [194-196].

In this PhD research, the author aimed to investigate the hydrogen sensing potential of nanostructured RuO2 for use in Schottky diodes. To the best of the author‟s knowledge, RuO2 is not the most popular gas sensing material and the author hopes that the outcomes of this PhD investigation shed light on the capabilities of this material as a high performing gas sensitive element.

2.3.1.5. MoO3 Nanostructured Based Gas Sensors Different MoO3 nanostructures including nanowires, nanorods, nanoribbons and nanobelts have been fabricated [197-201], however a very limited number of MoO3 nanostructures have been used for gas sensing applications [202-204]. These reports are based on nanostructured


44

MoO3 conductometric based sensors and to the best of author‟s knowledge; there is no report on nanostructured MoO3 Schottky diode based sensors.

This PhD research will present hydrogen gas sensing performance of Pt/nanostructured MoO3/SiC Schottky diodes for the first time to the best of the author‟s knowledge. MoO3 nanostructured thin films with different morphologies such as nanoplatelets, nanoplateletsnanowires and nano-flowers were deposited on SiC substrates as the gas sensing layer. SiC substrate was chosen for thermal “deposition” of MoO 3 at elevated temperatures due to the preservation of its semiconducting properties at high temperatures.

MoO3 cannot tolerate high temperature “operations” as it has a low sublimation point. However, the author chose to investigate this material in Schottky diode structures due to its immensely versatile nature. Just by using a simple low cost thermal evaporation process, a wide range of MoO3 morphologies can be obtained. Later in this thesis, the author will show how sharp edged nanostructured morphologies from MoO 3 synthesis can enhance the localized electric field to investigate the performance of model nanostructured Schottky contacts in reverse bias condition.

2.3.1.6. WO3 Nanostructured Based Gas Sensors The gas sensing performance of WO3 nanostructures in conductometric structure has been reported by some researchers [205-207], but application of WO3 nanostructures in Schottky diode based sensors yet to be fully investigated and understood. In this PhD research, to the best of the author‟s knowledge, the author will investigate electrical and gas sensing performance of Pt/WO3 nanoplatelet/SiC Schottky diode based sensors towards hydrogen gas for the first time.


45

Similar to MoO3, WO3 is also a fascinating material with a wide range of nanostructured morphologies. In addition, WO3 can tolerate harsh and acidic environments, making it suitable for applications such as gas exhaust sensing and industrial monitoring. The gas sensing results of the WO3 based developed sensors will be discussed in detail later in this PhD thesis.

2.3.2. Graphene Nano-Sheets Based Gas Sensors Graphene is a two-dimensional material comprising carbon atoms arranged in six-membered rings [158, 208-210]. This material exhibits unique properties such as the quantum Hall effect [211-213], high carrier mobility [214-217], an ambipolar electric field effect along with ballistic electronic transport [158], tuneable band gap [218] and high elasticity [219]. Graphene can be classified as: single-layer graphene (SG), bilayer graphene (BG) and fewlayer graphene (FG, number of layers 10) [210]. Many techniques have been developed for the synthesis of graphene. For example, the micro-mechanical cleavage method was initially used for exfoliating highly oriented pyrolitic graphite using „Scotch-tape‟ [158]. Other popular methods include the epitaxial growth of graphene from a SiC wafer via thermal decomposition [220] and the synthesis of graphene via the chemical reduction of graphite oxide [221].

Graphene‟s gas sensing properties have been investigated by different researchers. Schedin et al. [222] showed that mechanically exfoliated graphene flakes can detect a single molecule of NO2 gas. The sensing performance of graphene nano-sheets deposited on LiTaO3 surface acoustic wave transducers towards hydrogen, NO2 and CO was investigated by Arsat et al. [223-225]. They showed that these gases are readily desorbed from the sensors even at room


46

temperature. Fowler et al. [226] reported graphene based chemical sensors for NO2, NH3 and 2,4-dinitrotouene. Furthermore, Joshi et al. [227] tested graphene based field effect transistors towards Cl2 and O2 gases. They showed that these gases are readily desorbed from graphenebased sensors. Joshi et al. [228] also reported the gas sensing properties of graphene films and ribbons towards O2, and 100 ppm of CO and NO2. They observed that the gas sensor mechanism is mainly dependent on the charge carrier transfer on conducting graphene surfaces caused by the adsorption of gases. NO2 sensing properties of a nanostructured graphite layer using a potentiometric detection technique was investigated by Qazi et al. [229].

There are many reports on theoretical analyses and experimental studies of graphene and metal contacts [230-235]. However, the performance of such contacts in the presence of different gas species is yet to be fully understood. In this PhD research, by studying the electrical and gas sensing properties of a metal/graphene device, the author will explain and hypothesize a mechanism for the transport of electrons at the interface between the metal contact and graphene, which will be discussed in chapter 3. The author developed Pt (film)/graphene-like (nano-sheets)/SiC (substrate) hydrogen gas sensors for the first time and the outcomes of her work were published in the Journal of Physical Chemistry C [236]. SiC is a favorable substrate for the formation of graphene based devices due to the fact that graphene can be synthesized from SiC directly by graphitization at high temperatures in a scalable process [237]. SiC can also produce large breakdown voltages in comparison to other semiconductors such as Si, which is beneficial for the fabrication of gas sensors. Pt is commonly used as an electrode material since it forms an electric contact with a high work function so as to produce non-linear behavior. In addition, Pt can act as an excellent hydrogen catalyst. In this PhD research, these devices were applied as sensors towards different


47

concentrations of hydrogen and NO2 gases in synthetic air with their electrical characteristics studied as a function of device temperature and target gas concentration. The effect of the Pt/graphene and graphene/SiC interfaces was also studied and will be presented in the next chapters.

2.4. Summary In this PhD research program, the author aims to develop highly sensitive gas sensors based on nanostructured Schottky diodes. In this chapter, a review of the available literature on Schottky diode based gas sensors using different substrates (Si, GaAs, InP, InGaP, GaN, AlGaN, SiC) was presented. Subsequently, the outcomes of previous reports on the development of nanostructured metal-oxides (TiO2, SnO2, ZnO, RuO2, MoO3 and WO3) and graphene nano-sheets based gas sensors were discussed to justify the author‟s rationale for proposing the structures to be developed in the course of her thesis.

The operating principles and gas sensing mechanisms of the conventional and nanostructured Schottky diodes will be presented in next chapter. Their operation in forward and reverse bias modes will be compared and a theoretical explanation regarding the differences of their behaviour will be presented.


48

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Chapter 3 Theoretical Study of Nanostructured Thin Film Schottky Diode Based Gas Sensors 3.1. Introduction The previous chapter discussed the most of up to date literature and reviews on nanostructured materials used in this thesis. In this chapter, the author will present a theoretical study of Schottky diode based sensors and the gas sensing mechanism that explains how these sensors operate.

The fundamental theories that explain the operation of conventional and nanostructured Schottky diodes will be presented. Their current-voltage (I-V) characteristics will be discussed and compared in detail. The effect of parameters such as electric field enhancement factor, carrier density, barrier height and temperature on the I-V characteristics of the nanostructured Schottky diodes will be demonstrated. Furthermore, the structure of the Schottky diode based sensors and gas sensing properties of these sensors will be presented and their gas sensing mechanisms will also be explained. Most of the sensors that will be discussed herein are based on metal/metal-oxide semiconducting devices. Additionally, the theory section will

Chapter 3: Theoretical Study of Nanostructured Thin Film Schottky Diode Based Gas Sensors

64

cover and discuss Pt/graphene/SiC based sensors. The chapter will then conclude with a summary of the discussions.

3.2. Fundamentals of Schottky Diodes A typical Schottky diode can comprise of a metal-semiconductor-metal configuration. To have a Schottky diode, one of the metal-semiconductor contacts should show a distinct rectifying and breakdown feature in its forward and reverse I-V characteristics, respectively (i.e. allows current to flow only in one bias direction). The other metal-semiconductor contact should be non-rectifying and is called the ohmic contact as it is linear in its both forward and reverse I-V characteristics. In this section, the theory of rectifying and breakdown metalsemiconductor contacts (Schottky contacts) based on conventional (polished surface) and nanostructured (n-type) metal-oxide semiconductors will be presented.

3.2.1. Conventional Schottky Diodes

3.2.1.1. Formation of Barrier When a metal is in contact with a semiconductor, a potential barrier is formed at the metalsemiconductor interface [1]. This potential barrier is responsible for controlling the conduction of current as well as its capacitive behaviour [1]. In this section, the formation of this barrier height, which is described by energy-band diagrams, will be presented.

3.2.1.1.1. Ideal Condition The barrier height of a metal-semiconductor contact is determined both by the metal work function and the interface states [2]. A general expression of the barrier height can be


65

obtained based on the following assumptions [2]: (1) the contact between the metal and the semiconductor has an interfacial layer of atomic thickness order; this layer is transparent to electrons but can withstand potential across it, and (2) the interface states per unit area per energy at the interface are the property of the semiconductor surface and is independent of the metal. First, the ideal case, in which there are no surface states and other anomalies, is considered. Figure 3.1 illustrates the energy-band diagrams of a metal and n-type semiconductor when they are (a) in separated systems, (b) connected as one system and (c) in intimate contact [1].

The work function of the metal (the magnitude of energy between the vacuum level and Fermi level), m, is larger than that of the semiconductor, ( 



n) (Figure 3.1a), where  is the

electron affinity (the magnitude of energy between the vacuum level and bottom conduction band EC), and n is the energy difference between EC and the Fermi level. When the metal is in contact with the semiconductor, the Fermi levels, EF, on both sides spontaneously adjust to reach the thermal equilibrium and therefore initially electrons flow from the semiconductor to the metal. This electron flow eventually slows down and results in the formation of a negative charge on the metal surface and depletion region, W, in the semiconductor near its surface (Figure 3.1b). When the metal is placed in intimate contact with the semiconductor, the gap distance, δ becomes zero; the electric field corresponds to a grade of the electron potential in the depletion region, resulting in the band-bending characteristic (Figure 3.1c). For an ideal contact between a metal and n-type semiconductor, the barrier height, Bn0, is given by [1]:

 Bn0  m  

(3.1)


66

and the depletion width, WD is given by [1]:

2 s  kT   bi  V   qN D  q 

WD 

(3.2)

where q is the electron charge, s is the permittivity of the semiconductor, ND is number of donors per cm3, k is the Boltzmann‟s constant, T is the absolute temperature and bi is the built in potential.

Vacuum

Gap 

m

n

EC EF

m Bno

 EC EF

qbi

Bno W

EC EF

EV Metal

Semiconductor

(a)

δ (b)

EV

EV (c)

Figure 3.1. Energy-band diagrams of a metal and n-type semiconductor (conventional): (a) neutral and isolated, (b) connected as one system and (c) in intimate contact.

3.2.1.1.2. Interface States Equation (3.1) shows that the barrier height of metal-semiconductor contacts is linearly dependant on the work function of the metal. The case is quite different as a large density of surface states is present in the semiconductor. These states can play an important role in the determination of the barrier height [2]. The surface states have energies in the semiconductor band gap, which are pinning the metal Fermi level. They are continuously distributed in the band gap and are characterised by a neutral level 0.


67

When the metal makes intimate contact with the semiconductor and equilibrium is reached, the semiconductor Fermi level changes by an amount equal to the contact potential, which is the result of charge exchange with the metal. If the surface state density is very large, the charge exchange between the metal and surface states take place predominantly, which results in a relatively unchanged depletion region. In this case, the barrier height becomes independent of the metal work function and is determined entirely by the doping and surface properties of the semiconductor [3]:

Bn 0  Eg  0

(3.3)

where Eg is the energy of the semiconductor band gap. For a general expression of the barrier height, the presence of a thin interfacial layer of thickness δ, which lies between the metal and the semiconductor, must be considered [2] and the barrier height can be given by [1, 4]:

 Bn 0  S m     1  S E g  0 

S

i

 i  q Ds

(3.4) (3.5)

2

where i is the permittivity of the interfacial layer and Ds is the density of interface states per unit area. It can be seen that as Ds approaches zero, the barrier height approaches the same value as in Equation (3.1).

In addition to the surface charges and the thin interfacial layer, the author will discuss the contribution of the image-force lowering (Schottky effect) on the barrier height in the next section. It will be seen later in this chapter that the image-force lowering has a significant effect on the shape of the I-V characteristics in nanostructured Schottky contacts.


68

3.2.1.1.3. Image-Force Lowering The image-force lowering, also known as the Schottky-barrier lowering or Schottky effect, is the image-force-induced lowering of the barrier energy for charge carrier emission, in the presence of an electric field [1]. In reference to the derivations by Sze et al. [1], a metalvacuum system is considered. When an electron is at a distance x from the metal, a positive charge is induced on the metal surface. Therefore by using the image theory [1], the attractive image force of the electron, with the charge of q, toward the metal can be expressed as:

F x  

 q2 16 0 x 2

(3.6)

where,  0 is the permittivity of free space and x is the distance from the metal into vacuum. Therefore, the potential energy Ep(x) for an electron transferred from infinity to the point x is given by [1]:

 q2 E p x    F x  dx   16 0 x x

(3.7)

When an external field ξ is applied, the total potential energy of the electron will be the difference between the potential energy (as calculated in Equation (3.7)) and the external field energy. It is given by [1]:

PE x  

 q2  q x 16 0 x

(3.8)

To represent a metal-semiconductor system, the potential energy of Equation (3.8) is used and a maximum value by d

dx

PE ( x)  0 is obtained to calculate both the magnitude of the image-


69

force lowering (change in barrier height)  and location of the lowering xm given in Equations (3.9) and (3.10), respectively [1]:

 

xm 

q m 4 s

q

(3.9)

(3.10)

16 0  m

where m is the maximum field at the metal-semiconductor interface. In a practical Schottkybarrier diode, the maximum value of the electric field at the surface, based on the depletion approximation, is as follows [1]:

m 

2qN D  s

(3.11)

s

where s is the surface potential at the metal-semiconductor interface on n-type semiconductor. Therefore substituting Equation (3.11) into Equation (3.9), the dependence of the barrier height change with respect to surface potential is obtained as [1]:

 q3 N D  S    3 2  8  S

  

1

4

(3.12)

Figure 3.2 shows the energy-band diagram incorporating the Schottky effect for a metal on conventional n-type semiconductor under different biasing conditions [1]. It can be seen that under the forward bias condition (V > 0), the field and the image force are smaller and the barrier height is smaller than the barrier height at zero bias (V = 0). For the reverse bias (V < 0), the barrier height is relatively large. The barrier height is significantly dependent on the bias condition and its magnitude [1].




F

70

R

Bn0

WD

Bn

EC (V > 0) EF (V > 0) WD

EC (V = 0) EF (V = 0) WD

Metal

Semiconductor (conventional)

EC (V < 0) EF (V < 0)

Figure 3.2. Energy-band diagram of a metal and n-type semiconductor (conventional) contact under forward, zero and reverse bias. Bn0 is the intrinsic barrier height.

3.2.1.2. Current-Voltage Characteristics For a Schottky contact, the forward current density-voltage (J-V) characteristics with VF > 3kT q can be given by [1]:

 q   q(  VF )  J F  A**T 2 exp   B 0  exp   kT  kT   

(3.13)

where, JF is the forward current density, A** is the effective Richardson constant, B0 is the barrier height,  is the image-force lowering and k is the Boltzmann‟s constant. Since both A** and  are weak functions of the applied voltage, the forward J-V characteristic (for VF > 3kT q ) can be simplified to [1]:

 qV  J F  J o exp  F   kT 

(3.14)


 q  J o  A**T 2 exp   B 0   kT 

71

(3.15)

where J0 is the saturation current density, which is determined using the extrapolation method of the current density at zero voltage [1]. Therefore, the forward barrier height is obtained from Equation (3.15) as [1]:

 B ( FWD ) 

kT  A**T 2   ln  q  J 0 

(3.16)

In reverse bias, for VR > 3kT q , the voltage dependence is dominantly affected by the imageforce lowering as the J-V characteristics is given by[1]:

    q B 0  q m   4  s    ** 2 J R  J 0  A T  exp    kT    

(3.17)

where m is the maximum electric field at the interface of metal-semiconductor [1]:

m 

2qN D  kT   VR  bi   s  q 

(3.18)

3.2.2. Nanostructured Thin Film Schottky Diodes Recent reports have shown that Schottky diodes based on metal-semiconductor nanostructures exhibit different I-V characteristics with respect to the conventional Schottky diodes, especially in the reverse bias voltage region [5-15]. In contrast with the conventional Schottky diodes, the nanostructured Schottky diodes exhibit a much lower breakdown voltage in their I-V characteristics [5-10, 16-19], which is attributed to the enhanced localized electric fields


72

induced in the proximity to the sharp edge and corner morphologies of the nanostructures [2026]. These enhanced localized electric fields lower the barrier energy to allow greater charge carrier flow [20-25]. Figure 3.3 illustrates typical I-V characteristics of conventional and nanostructured Schottky diodes.

I Nanostructured Schottky diode Conventional Schottky diode V

Figure 3.3. Typical I-V characteristics of conventional and nanostructured Schottky diodes.

For the nanostructured Schottky diodes, Equation (3.14) can be used to describe the forward current density. However, for the reverse current density, m the maximum electric field at the metal-semiconductor interface is replaced by γ the enhanced localized electric field [17]:

   q  B 0  q  4 s   J R  J 0  A**T 2  exp    kT   

       

(3.19)

For the nanostructured Schottky diodes, the enhanced localized electric field is a function of the reverse bias voltage VR [17]:

   a

2q  N D  kT   VR  bi   s  q 

(3.20)


73

where a is the electric field enhancement factor, which can be determined by curve fitting or simplified theories such as sphere on the post [18, 27]:

a 

 m

(3.21)

The effect of electric field enhancement factor, carrier density, barrier height and temperature on the I-V characteristics of nanostructured Schottky diodes will be presented in next subsections in reference to the investigations by Yu et al. [19].

3.2.2.1 The Effect of Electric Field Enhancement Factor According to Equation (3.14), the magnitude of the forward current density is proportional to  qVF  kT

exp 

  , 

which does not directly involve the electric field parameter [19]. Equations (3.19)

and (3.20) show that the reverse current density (at room temperature) is approximately 

proportional to exp  B a 

1

2 VR

1  4

 

, where B is a constant consisting of the variable terms

excluding VR and a [19] which appear in Equations (3.19) and (3.20). For simplification, both kT

q

and bi at room temperature in the presence of a large VR are neglected. The

enhancement factor increases the exponent and enhances the magnitude of current in the reverse bias condition [19].

3.2.2.2. The Effect of Carrier Density In forward bias, the current density is only a function of barrier height (B0) according to Equation (3.14) and the effects of electric fields from nanomaterials do not directly affect the I-V characteristics [19]. In reverse bias operation, the current density is a function of the




carrier density ND and is proportional to exp  C a 

1

2 ND

1  4

, 

74

where C is a constant representing

the variables in Equations (3.19) and (3.20) excluding ND and a [19].

3.2.2.3. The Effect of Barrier Height According to Equation (3.14), the forward current density is proportional to exp   qB0  , 

kT 

which is a function of the barrier height, B0 [19]. The reverse current density is proportional   to the term exp   qB 0  q 4 s  [19]. Considering the effect of exp   qB0  , the Fermi  

kT

 



kT 

level is shifted downwards as the reverse voltage magnitude increases. This will result in the barrier height increase and consequently decreases the flow of current density through the interface. However by further increasing the magnitude of the reverse voltage, the

exp



 q q 4  s   kT  

   

term becomes dominant [19]. This term is generated by the enhanced

localized electric fields, which opposes the barrier height increase. Consequently, it assists the thermionic emission of the free carriers to flow over the barrier in the reverse bias. Figure 3.4 shows the energy-band diagram of a nanostructured Schottky contact under different biasing conditions, with consideration of the enhanced localized electric fields [19]. The enhanced localized electric field effect has been considered for the calculation of the reverse barrier height which is given by:

B ( REV ) 

 2q3 N  kT  A**T 2  kT    4 a 2 D  VR   bi   ln  q  J0  8  s  q 

(3.22)


75

EC (V > 0) EF (V > 0)

Thermionic emission of carriers

EC (V = 0) EF (V = 0)

Metal

Semiconductor (nanostructures)

EC (V < 0) EF (V < 0)

Figure 3.4. Energy-band diagram of a metal and n-type nanostructured semiconductor contact under forward, zero and reverse bias with the effect of the enhanced localized electric fields.

3.2.2.4. The Effect of Temperature

In the forward bias condition, the current density is proportional to

 qV T 2 exp F  kT

  and 

is based on

Equation (3.14) [19]. While according to the Equations (3.19) and (3.20), the reverse bias current density is a function of the temperature as given by [19]:

1   3N    4 q kT 2 D   a  VR   b     q   8 2 s3  * * 2   J R  A T exp  kT    

         

(3.23)

At large values of VR and at low temperatures, the effect of  kT term is neglected. q Therefore, the reverse current density becomes dependent on the temperature by the 1  2   qN DV R 4 T 2 exp a kT  

    

[19].


76

3.3. Conventional and Nanostructured Schottky Diode Based Gas Sensors A typical Schottky diode based gas sensor consists of a thin catalytic metal layer (generally group VIII transition metal such as Pt or Pd) which is deposited on top of the semiconducting metal-oxide layer (as the gas sensing layer), substrate and ohmic contact.

Many authors have shown that when conventioanl metal-oxide thin films (polished surface) is applied as the gas sensing layer (Figure 3.5), the sensors exhibit a lateral voltage shift in their forward I-V characteristics upon exposure to the target gas [28-37]. This can be attributed to the change in the barrier height, which carriers with sufficient energies can flow over the lowered barrier easily via the thermionic emission mechanism.

Gas sensing mechanism is based on the catalytic dissociation of gas molecules. Generally the dissociated gas atoms diffuse through the transition metal and form a dipole layer at the metal-semiconductor interface. This dipole layer alters the effective Schottky barrier height energy at the metal-semiconductor interface [38-42], which causes a shift in the forward I-V characteristics of the sensor as shown in Figure 3.5. The change in the forward barrier height can be calculated using the extrapolation method and Equation (3.16) [1]. Upon exposure to reducing gases, such as hydrogen or propene, the sensors will exhibit a change in its I-V characteristics curves by a lateral voltage shift towards lower voltages making the curves more conductive. For oxidizing gases such as O2 and NO2, the curves will shift towards higher voltages making the sensors less conductive. The gas response is generally measured as a change in the voltage output upon exposure to the gas, when the sensor is forward biased at a constant current or a change in current output when the sensor is biased at a constant voltage.


I

77

Without gas (a)

(b)

ΔV

ΔV

V Figure 3.5. Typical structure of a conventional thin film Schottky diode based gas sensor and the voltage shift in forward I-V characteristics of the sensor towards (a) reducing and (b) oxidizing gases.

When nanostructured thin films are deposited in the Schottky diode based sensors structure, the electrical (I-V) and gas sensing characteristics of the sensors are significantly altered. The change in Schottky barrier height is an important factor for gas sensing [38-42], however the nanostructured materials surface also contributes to the performance of the sensors [43].

The gas sensing mechanism that describes how nanostructured metal-oxide materials operate can be explained by the reactions that occurs at the material‟s surface when exposed to the target gas molecules [44]. It involves adsorption of oxygen on the metal-oxide surface followed by a charge transfer during the reaction of the adsorbed oxygen with the gas molecules [44]. Upon exposure to reducing or oxidizing agents, carriers or electrons transfer into (or binds with) the metal-oxide, respectively [45] and therefore, results in a measurable change in the electrical properties of the metal-oxide material.

It is well known that the metal-oxide surface adsorbs oxygen molecules from air and forms O2, O and O2 ions by extracting electrons from the conduction band depending on the temperature [44, 46]. It was found that oxygen in molecular (O2) and atomic (O) forms


78

ionsorb over the metal-oxide surface in the operating temperature ranging between 100 and 500°C [47]; because O2 has a lower activation energy, it is dominates up to about 200°C and at higher temperatures beyond 200°C, the O form dominates.

In n-type semiconducting oxides, given sufficient adsorption of oxygen, the positively charged oxide surface and negatively charged adsorbed oxygen ions form an effective depletion layer at the surface. This layer causes a decrease in the carrier concentration and consequently an increase in the nanostructures‟ resistance [43, 48]. In addition, a high surface to volume ratio in nanostructured morphology provides a large number of surface atoms for interaction, which can lead to the insufficiency of surface atomic coordination and high surface energy [43, 48]. Therefore, when the surface is highly active, it promotes further adsorption of oxygen from the atmosphere.

As electron depletion occurs at the surface by a chemisorption process, a space charge layer is formed. The thickness of the space charge layer, D (also expressed by the Debye length) is defined using Poisson‟s equation [49]:

12

 2.K . 0 .Vs  Q D  s    e.N D  e.N D 

(3.24)

where ND is the number of ionized donor states per unit volume, Qs is the surface charge density, e is the carrier charge, K is the static dielectric constant of the oxide, εo is the permittivity of the vacuum and Vs is the surface potential barrier height.

The negatively charged oxygen adsobates play an important role in sensing gases such as hydrogen and NO2. When the nanostructure is exposed to a reducing gas such as hydrogen,


79

the gas reacts with the adsorbed O, as in Equation (3.25) and (3.26), and the electrons trapped by the oxygen adsorbates return to the conduction band. This results an increase in the carrier concentrations in the nanostructures [48, 50].

H2, gas H2, ads + Oads

H2, ads

(3.25)

H2O + e

(3.26)

The direct adsorption mechanism has also proposed for oxidizing gases such as NO 2 [51], which results in decreasing the nanostructures‟ carrier concentrations.

NO2, gas e + NO2, ads

NO2, ads

(3.27)

NO2, ads

(3.28)

The nanostructured Schottky diode based sensors have shown a larger lateral voltage shift in reverse bias mode operation than in forward bias [16-19, 52-56]. As aforementioned, the forward bias density changes with the barrier height (Equation (3.14)), which is independent to the carrier density (ND) and electric field parameters. The electric field plays a significant role in the electrical characteristics of the nanostructured Schottky diodes under reverse biased condition. The barrier height decreases significantly under reverse bias condition due to the enhanced localized electric fields emanating from the sharp edges and corners of the nanostructured morphologies. When the nanostructured based sensors are exposed to a reducing gas, the accumulation of hydrogen atoms at the metal-semiconductor interface causes lowering of the barrier height as well as increasing the carrier density (ND). As per Equation (3.20), the effect of increase in ND at the metal-semiconductor interface is amplified by the enhancement factor and causes a respective increase in the electric field and a decrease in the barrier height (in Equation (3.19)) and therefore produces a large lateral voltage shift in the reverse I-V characteristics. The change in the reverse barrier height can be calculated


80

using Equation (3.22). When the sensors are exposed to an oxidizing gas, the dissociated O atoms on the catalyst metal diffuse through the metal and results in the depletion of the carriers and decreases the parameter ND in Equation (3.20). The relative decrease in ND is amplified by the enhancement factor, which results in increase in the barrier height in Equation (3.19) and therefore, decrease of current density.

In this PhD research, the author developed Schottky diodes similar to that are shown in Figure 3.5. The nanostructured Schottky diode based sensors were fabricated using SiC and epitaxial-GaN/sapphire (Al2O3) substrates and nanostructured metal-oxides and graphene-like nano-sheets as the gas sensing layers. The transition metal Pt was implemented both as the Schottky contact and as the catalyst. Figure 3.6 shows the structure of nanostructured Schottky diodes fabricated on SiC and epitaxial-GaN/Al2O3 substrates. The fabrication techniques for development of the nanostructured Schottky diode based gas sensors will be presented in detail in chapter 4.

Nanostructured materials

Nanostructured materials

Figure 3.6. Schematic of nanostructured Schottky diodes fabricated on SiC and epitaxial-GaN/Al2O3 substrates.


81

3.4. Pt/Graphene/SiC Based Gas Sensors In this research, the theoretical analysis and gas sensing mechanism of the Pt/graphene/SiC devices have been investigated. Giovannetti et al. [57] reported that the transfer of electrons between the Pt and graphene results in the formation of an interfacial dipole layer accompanying a potential step (∆V) in the energy band; this is unlike the metal-semiconductor interface that produces band bending, which is clearly observed with a Schottky barrier.

To study the effect of the Pt/graphene and graphene/SiC interfaces, the author initially considers these two interfaces independent of each other as a first order assumption. Figure 3.7 shows a energy-band diagram of Pt on graphene [57]. As a Pt layer is deposited, it physisorbs onto the graphene surface, which leads to a weak interaction energy (van der Waals) at the Pt/graphene interface and hence preserves the electronic structure of graphene. Since the work function of Pt (WM) is larger than work function of graphene (WG), electrons transfer from graphene to Pt in order to achieve equilibrium (Fermi levels). Electron transfer between the Pt and graphene results in the formation of an interface dipole layer and with it, an associated potential step (∆V).

∆V WM

d

WG ∆EF

_ _

Pt

_ _

W EF

+ + +

Graphene

+

Figure 3.7. Theoretical energy-band diagram of Pt/graphene contact. WM is the Pt work function; WG is the graphene work function; W is the Pt coated graphene work function; d is the equilibrium separation distance; ∆EF is the Fermi-level shift and ∆V is the potential change generated by the Pt-graphene interaction.


82

Tongay et al. [58] reported the formation of a Schottky contact when using graphite on an ntype SiC. They measured the J-V characteristics of the graphite/SiC device, which showed a rectifying behaviour at room temperature using the assumption that graphene is a semi-metal. In the case of this PhD research, same assumption is used to explain the barrier that exists between graphene and SiC. From these arguments, it is believed that the electron transport from SiC to graphene is dominated by thermionic emission.

When a graphene layer is deposited between Pt and SiC, the I-V characteristics of the Pt/graphene/SiC device can be explained by the electron transport mechanism through these two interfaces (Pt/graphene and graphene/SiC) [59]. Initially, the electron transfer will encounter a transition from the n-type SiC to the semi-metallic (graphene) and then to the metallic Pt. The conduction behaviour will be similar to that between a metal to a metal junction. However, the non-linear I-V curves can be expected as the graphene behaves semimetal like.

Gas sensing mechanism of the Pt/graphene based sensors can be explained by the charge carrier transfer on conducting graphene surfaces, which is caused by the adsorption of gas species [60-62] as well as the change in the potential step at the Pt/graphene interface. The majority carriers in graphene are holes [61, 62]. Reducing gas such as hydrogen releases electrons [63, 64]; therefore causes depletion of the holes from the conduction band, which as a result, increases the resistance of graphene material [61]. Upon exposure to oxidizing gas such as NO2, the holes transfers from the NO2 to graphene [64] and therefore enhances the hole conduction. This results in a significant decrease in the graphene resistance [60, 61].

Three possible configurations of graphene adsorption sites including (A) upon one carbon, (B) upon the centre of a carbon-carbon bond and (C) upon the centre of a hexagon of carbon


83

atoms have been proposed by Leenaerts et al. [64] and Arellano et al. [63] using density functional theory (DFT).

It was assumed that hydrogen could either adsorb parallel or perpendicularly to the plane of graphene layer [63]. Binding energy of less than 0.1 eV and adsorbate surface distances between 5.07 Å and 5.50 Å were calculated. Table 3.1 shows the calculated binding energies and distances for adsorbed hydrogen by graphene surface [63]. From this investigation, it was concluded that the most stable configuration of hydrogen is physisorbed above the centre of a hexagon and the parallel position is slightly preferable than the perpendicular one.

Table 3.1. Binding energy and distance of physisorbed hydrogen on graphene layer. Position

A

B

C

C

Binding Energy (eV)

0.070

0.072

0.083

0.086

Distance ( Å)

5.50

5.49

5.25

5.07

Orientation

perpendicular

perpendicular

perpendicular

parallel

For the adsorption of NO2 onto the graphene layer, Leenaerts et al. [64] examined three different orientations of NO2 molecules including: the N-O bonds pointing up (u), down (d) and parallel to the graphene surface (n). They found that NO 2 adsorbs between 3.61 Å and 3.93 Å above the surface of the graphene layer with binding energies ranging between 0.05 eV and 0.07 eV. The details of the calculated binding energies and distances for adsorbed NO2 by graphene surface are presented in Table 3.2 [64].

From the aforementioned calculations, binding energy of less than 0.1 eV was obtained, hence, it can be concluded that these hydrogen and NO2 gas molecules are being physisorbed by the graphene surface [63].


84

Table 3.2. Binding energy and distance of physisorbed NO2 on graphene layer. Position

B

A

C

B

A

C

Binding Energy (eV)

0.067

0.065

0.063

0.055

0.055

0.067

Distance ( Å)

3.61

3.61

3.64

3.83

3.93

3.83

Orientation

d

d

d

u

u

n

Upon exposure to a reducing gas, the dissociation of hydrogen molecules occurs on the Pt surface. Then hydrogen atoms diffuse through the Pt catalyst onto graphene. Therefore, the free carrier concentrations in graphene increase [61]. Two assumptions are as follows [59]: (1) the work function of graphene (WG) increases; and/or (2) the separation distance between the Pt and graphene (d) increases, which causes an increase in the Fermi-level shift (∆EF) based on the results reported by Giovannetti et al. [57]. Both increases in ∆EF and WG decrease the potential step generated by the interface dipole layer according to Equation (3.29):

V  WPt  WG  EF

(3.29)

Therefore, a decrease in the potential step at the Pt/graphene interface results in an increase in the current density in the I-V characteristics.

3.5. Summary In this chapter, the electrical characteristics of metal in contact with semiconducting materials were described. The energy-band diagrams leading to the formation of barrier height were presented. Some effects such as interfacial states and image-force lowering, (which can alter the value of the barrier height) were explained. The I-V characteristics of the Schottky diodes based on conventional and nanostructured metal-oxides were presented. It was shown that the


85

breakdown voltages for nanostructured Schottky diodes are much lower than the breakdown voltages from the conventional diodes. This is caused by the enhanced localized electric fields emanating from the sharp edges and corners of the nanostructured morphologies.

The operating principals of conventional and nanostructured thin film Schottky diode based gas sensors were presented. They were used for describing the operation of nanostructured Schottky diode based gas sensors used in this research. Finally the theoretical evaluation of Pt/graphene/SiC based sensors as well as their gas sensing mechanism was presented. The details of the fabrication of nanostructured Schottky diode based gas sensors will be presented in next chapter. Thereafter, the material characterisation of the nanostructured layers will be explained in detail in chapter 5. Subsequently, the electrical and gas sensing properties of the developed nanostructured Schottky diode based sensors will be illustrated. Chapter 6 will present the experimental results, which are in an excellent agreement with the theories discussed in this chapter.


86

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Chapter 4 Nanostructured Thin Film Schottky Diode Based Gas Sensors Fabrication 4.1. Introduction In the previous chapter, the theory and fundamentals of Schottky diodes were presented. The electrical and gas sensing properties of the conventional and nanostructured Schottky diode based sensors were also discussed. Furthermore, their gas sensing mechanisms was presented. The theoretical study and gas sensing mechanism of the Pt/graphene/SiC based sensors were also presented. In this chapter, the process and different procedures used to fabricate the nanostructured thin film Schottky diodes are explained in detail. The fabrication processes include: the substrate preparation, ohmic contact formation, nanostructured materials deposition and the Schottky contact formation.

In this research program, Schottky diodes were fabricated using different substrates: SiC and epitaxial-GaN layers with the following structures:



Pt/nanostructured metal-oxide/n-6HSiC with Ti/Pt ohmic contacts: the nanostructured metal oxide includes TiO2 nano-dimensional grains, SnO2 nanowires, ZnO

Chapter 4: Nanostructured Thin Film Schottky Diode Based Gas Sensors Fabrication

91

nanostructured arrays, ZnO nanowires-nanoplatelets, RuO2 nanostructures, MoO3 nanostructures (nanoplatelets, nanoplatelets-nanowires and nano-flowers) and WO3 nanoplatelets. 

Pt/graphene-like nano-sheet/n-6HSiC with Ti/Pt ohmic contacts.



Pt/ZnO nanostructured array/n-epitaxial-GaN with Al/Cr/Au ohmic contacts. Epitaxial-GaN layers were grown on sapphire (Al2 O3) substrates.

Most of the devices were fabricated by the author at the Microelectronics and Materials Technology Centre (MMTC) at RMIT University using clean room (Class-1000) and vacuum laboratory facilities. Sensor technology laboratory colleagues contributed to the fabrication processes. The samples: Pt/SnO2 nanowire/SiC, Pt/ZnO nanowire-nanoplatelet/SiC and Pt/RuO2 nanostructure/SiC were fabricated in collaboration with Dr. Comini from University of Brescia, Italy. The Pt/graphene-like nano-sheet/SiC sample was fabricated in collaboration with Professor Kaner‟s group from UCLA, USA.

4.2. Wafer Cleaning Process The n-type 6H-SiC wafers (2 inch) were purchased from two different companies: Cree Inc. (USA) and Tankeblue Semiconductor Co. Ltd. (China). The wafers‟ specifications are presented in Appendix A.1. Cleaning of the samples is a vital step in the fabrication process. The wafers were cleaned to remove particles and contaminants left on the wafers‟ surface prior to the metal layers deposition at the backside (unpolished side) of the wafers for ohmic contact formation. The wafers were initially washed in acetone for 5 min to remove any organic impurities from the surface, then rinsed in isopropanol (IPA) and deionised (DI) water. The surface native oxide on the wafers was removed by etching in hydrofluoric acid


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(HF) (40%) + H2O, 1:2, for duration of 15-20 s. Thereafter, the wafers were rinsed in DI water and blown dry in nitrogen (N2) at room temperature.

The n-type 6H-SiC wafers (2 inch) from Sterling Semiconductor (USA) were used for the fabrication of the Pt/SnO2 nanowire/SiC and Pt/ZnO nanowire-nanoplatelet/SiC sensors at University of Brescia, Italy. Their specifications can be found in Appendix A.1. In this case, first the wafers were diced into individual 3×3 mm2 squares and then the substrates were cleaned using same cleaning process as aforementioned.

Epitaxial-GaN layers with Al/Cr/Au ohmic contacts were provided by the University of Western Australia. The (n-type) epitaxial-GaN layers were grown on a semi-insulating GaN template utilizing an underlying layer Fe-doped GaN buffer deposited via a metal organic chemical vapour deposition process (MOCVD) on Al2O3 substrates. The wafer specifications are presented in Appendix A.2. The wafers were first diced into 5×5 mm2 squares using a diamond scribe pen and then the substrates were cleaned before the deposition of the metal layers for forming the ohmic contact. The substrates were solvent cleaned as aforementioned and then they were dipped in hydrochloric acid (HCl) for 20 s, rinsed and blown dry in N2.

4.3. Substrate Preparation: Ohmic Contact Formation and Dicing Following the cleaning of the SiC wafers, the formation of ohmic contact was carried out. Review on the common types of ohmic contacts on n-type SiC can be found in [1-3]. A titanium (Ti) layer was chosen for the ohmic contact due to its low specific contact resistivity (approximately 10-4 Ωcm2) when deposited on SiC.


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A double metal layer of Ti/Pt with 40/100 nm thicknesses were deposited at the backside (unpolished) of the wafers using BalzersTM electron beam evaporation system (Figure 4.1) to form the ohmic contact after annealing at an elevated temperature. Pt layer was used to prevent the oxidisation of Ti during the annealing process.

Electron beam evaporation 1

2

Figure 4.1. Metal layers deposition at the backside of the SiC wafer using electron beam evaporation system for ohmic contact formation.

After the deposition of Ti/Pt metal layers at the backside of the SiC wafers, the wafers were first spin-coated with a thick photo-resist (AZ-1512) to be protected during the dicing process. Then, they were diced into 3×3 mm2 and 5×5 mm2 substrates using a DISCO DAD 321 automatic dicing saw. As the dicing process is delicate, the wafer was mounted on the cutting stage with careful attention so as to be cut along a crystallographic axis. Without such attention, it would result in the wafer damage and fraction. After dicing, the photo-resist was removed by immersing the diced wafers in an acetone bath for approximately 5-6 min. Then the prepared substrates were rinsed in IPA and DI water and blown dry in N 2 at room temperature. Subsequently, the SiC substrates with Ti/Pt contacts were annealed at 500°C for 30 min in N2 atmosphere to form the ohmic contact. A second cleaning process was essential to ensure that the gas sensitive layer is deposited on a clean and contamination free surface. A layer of photo-resist was spin coated over the ohmic contact to protect the layer during


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cleaning. The oxide layer grown on the polished side of SiC substrates during formation of the ohmic contact at high temperature was removed once again by HF (40%) + H2O, 1:2 etching for 15-20 s. Subsequently, the substrates were washed in acetone for 5 min to remove the photo-resist layer, followed by rinsing in IPA and DI water. Finally, they were blown to dry using N2 at room temperature.

In the case of Pt/SnO2 nanowire/SiC and Pt/ZnO nanowire-nanoplatelet/SiC samples developed at University of Brescia, SiC wafers were first diced into 3×3 mm2 substrates using a DISCO DAD 321 automatic dicing saw. Then, the metal layers deposition for ohmic contact formation was performed applying a shadow mask with 1 mm diameter hole. A circular pad of Ti/Pt metal layers was deposited by DC sputtering onto the backside (unpolished side) of the clean 3×3 mm2 substrates (Figure 4.2). The sputtering parameters were: 50 W DC power; 100% Ar process gas; working pressure of 5×10-3 mbar. The thickness of the Ti/Pt layers was 100 nm each.

2 DC Sputtering

(a)

1

(b)

1 mm

Figure 4.2. Metal layers deposition process for ohmic contact formation. (a) A shadow mask was placed in front of the backside of SiC substrate during deposition of Ti and Pt. The mask was made of a thin sheet of Al with a hole of approximately 1 mm in diameter. (b) Fabricated device after the removal of the shadow mask.


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In this case, the formation of the ohmic contact was carried out after deposition of the gas sensitive layer. Hence, the abovementioned cleaning process was performed on the SiC substrates followed by the deposition of the SnO2 nanowires and ZnO nanostructures as it will be described in the section 4.5. Finally, the samples were annealed in air at 450°C for 4 hrs and then at 600°C for another 2 hrs to form the ohmic contact.

For the Pt/ZnO nanostructured array/epitaxial-GaN samples, the epitaxial-GaN/Al2 O3 wafers were first diced into 5×5 mm2 squares and then metal layers comprising of Al/Cr/Au were deposited onto the top of surface of the clean 5×5 mm2 epitaxial-GaN/Al2O3 substrates using thermal evaporation technique. The thicknesses of Al/Cr/Au were 20/50/400 nm. The metal layers deposition was performed by applying a shadow mask with 1×1 mm2 hole at the corner (Figure 4.3). The formation of the ohmic contact was achieved by annealing the samples at 800°C for 30 s in a N2 atmosphere.

Thermal Evaporation

3

2

(a)

(b)

1

Figure 4.3. Metal layers deposition process for ohmic contact formation. (a) A shadow mask was used in contact with the top surface of epitaxial-GaN layers during deposition of Al/Cr/Au. The contacts were deposited at the corner of the surface using a mask with a hole of approximately 1×1 mm2 and (b) fabricated device after the removal of the shadow mask.


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4.4. Nanostructured Materials Synthesis and Deposition In the earlier chapters, the motivation to implement nanostructured thin films for gas sensing applications was presented. Based on this motivation, the author has investigated different materials: TiO2, SnO2, ZnO, RuO2, MoO3, WO3 and graphene with nanostructured morphologies in this research. The review of available literature on these nanostructured materials was presented in chapter 2. In this section, the synthesis and deposition of these nanostructured thin films onto the surface of the SiC and epitaxial-GaN/Al2 O3 substrates will be defined in detail. The preparation of these substrates and ohmic contact formation were explained in the sections 4.2 and 4.3.

Nanostructured materials are those with at least one dimension in the nano scale range (100 nm or less). They can be classified into different categories depending on the number of nano sized dimensions [4, 5]:



Zero-dimensional nanostructures: nanoparticles;



One-dimensional nanostructures: nanowires, and nanorods;



Two-dimensional nanostructures: thin films.

Nanostructured materials are expected to demonstrate unique mechanical, optical, electronic and magnetic properties substantially different from those observed for the bulk counterparts due to the high surface to volume ratio, change of surface energy and comparable dimensions with Debye length [4-6]. In order to investigate novel physical and chemical properties and phenomena and potential applications of these nanomaterials, the synthesis and characterisation with desired size, morphology, crystal structure and orientation as well as chemical composition are essential aspects in nanotechnology.


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Many approaches have been developed and applied for the synthesis and deposition of the nanomaterials. The two most common types are known as „top-down‟ and „bottom-up‟ approaches and both play very important roles in nanotechnology and have advantages and disadvantages [4, 5].

The top-down approach is considered when the bulk dimensions of a material are reduced until nanometer size features are produced [4]. Milling and photolithography are most common top-down methods. The bottom-up approach is that by which a material is built-up from the bottom atom-by-atom, molecule-by-molecule, or cluster-by-cluster [5]. Crystal growth is a bottom-up approach, where growth species such as atoms, ions or molecules, after impinging onto the growth surface and assemble into crystal structure layer by layer. The bottom-up approach has received great emphasis in nanotechnology literature. This method has shown the potential to obtain nanostructures with less defects and more homogeneous chemical composition.

In this PhD research program, different techniques such as anodization, thermal evaporation, radio frequency (RF) sputtering, hydrothermal growth method, and acid-etching have been applied for the synthesis and process of the nanostructured materials. The synthesis and deposition of these materials will be described in detail from subsections 4.4.1 to 4.4.7. The material characterisations and gas sensing performance of the nanostructured materials will be later presented in chapters 5 and 6, respectively.

4.4.1. Synthesis of TiO2 Nano-Dimensional Grains: Anodization Method Titanium dioxide (TiO2) is one of the most studied metal oxides, which has been widely used in applications such as gas sensors [7-9], dye-sensitized solar cells [10, 11], photocatalysis


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[12], batteries [13] and electrochromic cells [14]. Recently, a great deal of attention has been drawn to investigate nanostructured forms of TiO2 due to their highly active surfaces with a large surface to volume ratio and unique properties. TiO2 nanostructures can be synthesized by different methods such as chemical bath deposition [15], spray pyrolysis deposition [16], electrophoretic deposition method [17] and anodization [9, 18-20]. Anodization is one of the most promising methods for developing nanostructures. In this method, a metal such as Ti in the form of a foil or film is anodized in suitable electrolytes to form nanopores/tubes.

Many research groups have reported successful progress on the fabrication of nanopore/tubes via the anodization of Ti films [9, 18-20] since the first report by Zwilling et al. [21, 22] on self-organized nanotubular surfaces by anodic oxidation of Ti foil. There are some factors, which affect the anodization process including the deposition temperature, Ti films thickness, type of deposition and the stress mismatch of the substrate and Ti films [19]. The type of substrate plays an important role in the morphological formation and lattice orientation of Ti films [19]. In anodization process, different types of fluoride-ion-containing electrolytes such as neutral [23], acidic [24], aquesous or nonaqueous electrolytes [25] can be used. In this PhD research, the author has investigated the formation of nanostrucutred TiO 2 via anodization of deposited Ti films using filtered cathodic vacuum arc (FCVA) deposition system on the SiC substrates [8].

Ultra-smooth surfaces of Ti films with 250-300 nm thickness were first deposited onto the polished side of clean 5×5 mm2 SiC (Cree Inc.) substrates using FCVA deposition system. A 70 mm diameter, 99.99% purity Ti cathode was inserted into the FCVA system. The cathode was struck by a grounded mechanical striker to initiate the deposition plasma and an arc current of 120 A (average arc power 3 kW) was used to produce stable plasma conditions. A


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dual-bend magnetic filter prevented deposition of macro-particles onto the sample, thereby minimizing surface roughness. Samples were mounted onto a metallic holder, which slid onto an electrically isolated arm inside the chamber. A negative bias voltage of -100 V was applied to the holder via a high-voltage feed-through. This caused acceleration of the incident ions towards the substrates and this energetic deposition process assisted in the production of dense flat films. The schematic of FCVA system is shown in Figure 4.4. The roughness measurement (RMS) of these films was measured over a 1×1 µm2 area using tapping mode atomic force microscopy (AFM) and found to be less than 0.5% of the deposited film thickness.

Substrate Bias

Substrate Holder Retractable Shutter and Faraday Cup

Langmuir Probe “Double-Bend” Macroparticle Filter

Cathode Source Data Acquisition Figure 4.4. Schematic of the filtered cathodic vacuum arc (FCVA) deposition system.

Anodization of the Ti films was performed using a neutral electrolyte medium of 0.5% (wt) NH4F in ethylene glycol solution at 10 V for 1 hr. A platinum foil was used as a counter electrode for the anodization at room temperature (Figure 4.5). Afterward, the samples were annealed in 90% O2 in Ar at 600ºC for 4 hrs to obtain TiO2. Surface morphology characterisation of the anodized TiO2 film revealed that TiO2 nano-dimensional grains were


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produced through the anodization process. The material characterisation of the anodized TiO 2 will be described in detail in the next chapter. _

Pt Foil Cathode

+

Ti/SiC

Electrolyte Solution

Figure 4.5. Schematic representation of the anodization set-up.

4.4.2. Deposition of SnO2 Nanowires: Thermal Evaporation Method To date, many techniques have been developed for the fabrication and synthesis of onedimensional (1D) nanostructured materials employing top-down and bottom-up approaches. For example, lithography, focused ion beam, electron beam, nano-imprinting and scanning probe microscopy techniques are considered as a top-down approach and vapour phase transport, electrochemical deposition, solution-based techniques or template growth are bottom-up type approaches [26]. Both approaches have their respective advantages and disadvantages. In the top-down technique, highly ordered nanowires can be obtained [27-30], but this technology is generally unfavourable for large-scale applications due to elevated production costs and preparation time. Furthermore, the developed 1D nanostructured materials via these methods are not generally single-crystalline [26]. The bottom-up approaches allow producing high purity of the nanocrystalline materials with small diameters,


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and low fabrication cost. However, it could be difficult to obtain the 1D nanostructures well arranged and patterned [31].

1D nanostructured materials can be synthesized in different morphologies such as tubes, cylindrical wires, rods, nails, cables, belts, and sheets. Recently, many 1D nanostructures have been fabricated using bottom-up techniques. While some of theses structures can not be synthesized easily or economically using top-down technologies. The most important requirements in developing 1D nanostructures are morphology and dimensions control, uniformity and crystalline properties [26].

Vapour phase synthesis is extensively used to form 1D nanostructures such as nanorods [32], nanowires [33], and nanobelts [34, 35]. Typically, the vapour phase growth is performed in a horizontal tube furnace to obtain the proper temperature gradient with a carrier gas. The source material evaporates and transports towards the growth site by a gas carrier. The nucleation can start from the particles or catalyst following the vapour-liquid-solid (VLS) and vapour-solid (VS) deposition techniques.

In this PhD research, the evaporation method was used to synthesis 1D oxide nanostructures for gas sensing applications as this method is one of the promising, cheapest and quick approaches for single crystal production. The author will first describe VLS and VS processes for the 1D nanostructues growth and then detailed synthesis processes of the tin oxide (SnO 2) nanowires used to develop novel gas sensors will be given.


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Vapour-Liquid-Solid (VLS) Growth The VLS mechanism is one of the most popular methods for growing 1D nanostructures since it was discovered by Wang and Ellis in 1964 [36]. They applied the VLS method for crystal growth of Si whiskers. This process is an economical, scalable and controllable growth of different materials like semiconducting metal-oxides. Understanding the growth mechanism is critical to have a better control in the nanowires shape, and diameter. Typically, a VLS process starts with the dissolution of gaseous reactants into nanosized liquid droplets of a catalyst metal such as gold, platinum, or iron. This is then followed by nucleation and growth of single crystalline rods and wires. Figure 4.6 shows the principal steps of VLS growth technique.

Vapour Vapour Vapour Metal Catalyst

(a)

Nanowire/ Nanorod

(b)

(c)

(d)

Figure 4.6. The schematic illustration of principle steps for the VLS growth technique: from initial nucleation to continual growth.

The nanowires/rods section can be determined by the dimensions of the catalyst clusters either by direct matching of the size or by mechanism involving the catalyst curvature [26]. The growth process will end if the catalyst is fully consumed or evaporated during the growth, or when the source material no longer available or if the temperature is reduced below the


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required growth process temperature. In this method, the nanowires/rods can grow from the top or the bottom of the catalyst cluster as shown in Figure 4.7.

(a) Root Growth

(b) Float Growth

Figure 4.7. Nanowires/rod growth process using a catalyst metal. (a) Root growth, the catalyst metal stays at the bottom of the nanowires/rods. (b) Float growth, the catalyst metal remains at the top of the nanowires/rods.

Vapour-Solid (VS) Growth In the VS growth mechanism, the nanostructres are formed from the direct condensation of the vapour phase without using a metal catalyst [26]. In this method, the source materials are vapourized under high temperature condition and then condensed directly on the substrate, which is placed in the low temperature region. When the condensation process occurs, the initially condensed molecules form seed crystals as nucleation sites. It has been reported that the minimization of surface free energy governs the VS process [26]. Therefore, these nucleation sites provide directional growth to minimize the surface energy.

In the VS process, control of supersaturation is a key factor to control the morphology of the nanostructues [37]. Low supersaturation is required for growth of 1D nanostructures, while a medium supersaturation results in bulk crystal growth. At high supersaturation, powders can be formed by homogenous nucleation. The dimension of 1D nanostructures can be controlled by different factors such as temperature, pressure and substrate.


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Growth of SnO2 Nanowires In the past years, many techniques have been developed and used for the synthesis of 1D SnO2 nanostructures. These methods include thermal evaporation [37, 38], thermal decomposition [35], rapid oxidation [39], pulsed laser deposition [40] and redox reactions [41]. The vapour phase processes are preferred due to their simplicity and low costs [42]. In this deposition technique, the condensed or powder source material is vaporized at elevated temperatures, then transported and condensed onto the substrate placing in a colder region. In this PhD research, SnO2 nanowires were synthesized using thermal evaporation technique by Dr. Comini from University of Brescia (Italy), the collaborator of this work [37, 43]. The experimental set-up consists of a vacuum-sealed tube furnace with an inlet for the carrier gas and an output connected to a vacuum pump and control system as shown in Figure 4.8. The chamber has been designed in which the controlled pressure of the atmosphere and the temperature gradient within the furnace allows condensation and nucleation of the nanostructures downstream the gas flow. Such a peculiar thermodynamic condition promotes formation of nanosized 1D structures.

Furnace Heater Alumina Tube Ar gas

Gas Outlet Connected to Vacuum Pump

Gas Inlet

Source Material (SnO2 Powder)

Growth Substrate

Figure 4.8. Schematic diagram of experimental set-up for growth of SnO2 nanowires via thermal evaporation method.


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Platinum was used as a growth catalyst as the SnO2 nucleation occurs from the platinumbased clusters. First, small platinum clusters were dispersed on the 3×3 mm 2 SiC (Sterling Semiconductor) substrates by sputtering. Afterwards, the commercial 99.9% pure SnO2 powder was placed in an alumina tube inserted in a horizontal tube furnace. As crystal growth of SnO2 powders by thermal evaporation usually occurs at temperatures higher than 1300°C [37], the source was heated up to 1370°C. The transport of source to target was performed using a carrier Ar gas (75 SCCM) at pressure of 1×104 Pa. Furnace heating from room temperature (RT) to 1370°C took approximately 1.5 hrs. During furnace heating and cooling, a reverse Ar gas flow (from the substrates to the powder) was applied, to minimize uncontrolled mass deposition. Once desired temperature was reached, the deposition process lasted 30 min. Nanowires grew in a colder zone of the furnace at a temperature between 400 and 500°C. Nanostructural characterisation of the SnO2 nanowires based film developed by thermal evaporation is given in chapter 5.

4.4.3. Synthesis/Deposition of ZnO Nanostructures Recently, a great deal of attention has been devoted to the development zinc oxide (ZnO) nanostructures due to that it can be used in the diverse range of applications, which include, but not limited to; gas sensor technologies [44-46], nenostructured templates [47] and solar cell development [48, 49]. ZnO nanostructures can be synthesized in different morphologies such as nanodots, nanorods, nanowires, nanobelts, nanotubes, nanobridges and nanonails, nanowalls, nanohelixes, mesoporous single-crystal nanowires, and polyhedral cages [50, 51]. These ZnO nanostructures can be deposited onto substrates via a number of different processes including molecular beam epitaxy [52], thermal evaporation (vapour-liquid-solid) [53], chemical vapour deposition [54], hydrothermal decomposition of chemical solutions


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[48], successive ion layer adsorption reaction (SILAR) deposition [55], and radio frequency magnetron sputtering [56]. Among these numerous deposition techniques, the solution methods are one of the most versatile, cost effective, and low temperature synthesis [57].

In this PhD research, two different methods: thermal evaporation and hydrothermal growth were used for the synthesis of ZnO nanostructures. In subsections 4.4.3.1 and 4.4.3.2 the fabrication of nanostructured ZnO using hydrothermal growth and thermal evaporation methods will be explained in detail, respectively.

4.4.3.1. Synthesis of ZnO Nanostructured Arrays: Hydrothermal Growth Method In this work, ZnO nanostructured arrays were grown on 5×5 mm2 epitaxial-GaN layers using hydrothermal growth method [58]. The epitaxial-GaN layers were used as the substrate, while GaN has high lattice compatibility to ZnO and is a low cost material, which is becoming popular in industrial electronic devices rapidly. From the numerous deposition techniques available, the author has investigated the aqueous growth of ZnO nanostructures from the hydrothermal decomposition of an equimolar Zn+2/hexamethyleneteramine (HMT) solution, as it is one of the most versatile and cost effective methods of synthesis nanostructured ZnO arrays [58]. This method is inexpensive, simple, and environmentally friendly. Additionally, it is compatible with existing semiconductor processing technologies and can be easily scaled to suit commercial applications. Recently, Jang et al. [59, 60] have grown ZnO nanostructures on epitaxial-GaN layers via a similar hydrothermal method. They could produce a variety of structures including rods, sea-urchin, and flower like structures with dimensions ranging from µm to nm.


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Prior to the ZnO nanostructured arrays growth, the epitaxial-GaN layers were RF sputtered with a 1.2 μm ZnO nucleation layer. This nucleation layer provides an abundance of nucleation sites for the ZnO nanorods, and improves uniformity and directionality in the growth of nanostructured arrays. Furthermore, it provides the foundations of a thin ZnO protective film preventing an electrical short-cut from the Pt and epitaxial-GaN that may occur when depositing the Pt Schottky contact. During the RF sputtering process, the ohmic contacts (Al/Cr/Au) on the surface were covered by a mask. RF sputtering conditions of the nucleation layer were; 75 mm substrate to target distance; 100 W power, 60% Ar-40% O2 process gas; with substrates temperature of 260°C. The deposition was performed from a 99.9% pure ZnO target for duration of 60 min. The RF magnetron sputtering system and its schematic diagram are shown in Figure 4.9.

Nanostructured arrays of ZnO were grown in a sealed reaction vessel via the hydrothermal decomposition of 10 mM HMT and zinc nitrate hexahydrate (Zn(NO3)2·6H2O) solutions in a modified process [61], which was first described by Vayssieres [48]. In this process, the cleaned RF sputtered samples were placed into a sample holder inside a reaction vessel filled with the growth solution. The vessels were sealed at room temperature, and then placed inside a standard laboratory grade oven for 16 hrs at 80°C. Afterwards, the samples were removed and washed with DI water to remove any residual zinc salts and dried in a stream of N2. Figure 4.10 shows the schematic diagram of the experimental set-up for growth of ZnO nanostructured arrays. After the deposition, the ZnO nanostructed arrays were characterised by different techniques such as SEM, TEM, and XRD, which will be presented in next chapter.


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(a)

Matching Circuit Cooling Water Inlet/Outlet Sputtering Chamber

Thermocouple Heater Power Supply

(b)

Matching Circiut

RF Power (13.56 MHz)

Cooling water inlet/outlet

RF Sputter Connector

Magnet Array

Sputtering Target

Sample Heater

Heater Power Supply

Thermocouple

Stage at Ground potential

Sputtering Gas

Vacuum Pump

Figure 4.9. (a) Planar RF magnetron sputterer and (b) schematic diagram of the system.


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RF Sputtering

(a)

(b)

Autoclavable lid Reaction vessel placed in oven at 80°C for 16 hrs

Teflon sample holder Support rod Teflon chuck

Washing and drying 10 mM HMT and zinc nitrate hexahydrate dissolved in 200 mL DI water

Figure 4.10. ZnO nanostructured arrays fabrication via hydrothermal growth method. (a) Depostion of ZnO nucleation layer by RF sputtering. A mask was used to cover the contacts. (b) Sample holder and reaction vessel in hydrothermal method for growth of ZnO nanostructured arrays.

4.4.3.2. Deposition of ZnO Nanowires-Nanoplatelets: Thermal Evaporation Method In this PhD research, synthesis of ZnO nanostructures was also investigated using thermal evaporation as this method is one of the simplest and low cost methods for developing nanostructures [42]. The nanostructured ZnO was deposited onto the polished side of the 3×3 mm2 SiC (Sterling Semiconductor) substrates at Brescia University, Italy [37]. The same


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experimental set-up as shown in Figure 4.8 was used for the ZnO nanostructures growth. In this case, gold metal was deposited as the catalyst for crystal growth. Hence, the SiC substrates were coated with metallic gold nano-particles by RF sputtering. ZnO powder was placed at the hot spot in an alumina tube located in a horizontal tube furnace at temperature of 1370°C where the gold coated substrates were placed in a colder zone of the furnace with temperature between 400 and 500°C. The source temperature of 1370°C was reached after about 1.5 hrs. The deposition process was performed for 30 min using a carrier gas of Ar with 75 SCCM at pressure of 104 Pa. During furnace heating and cooling a reverse Ar gas flow (from the substrates to the source) was applied to avoid uncontrolled mass deposition. Morphological characterisations of the developed ZnO nanostructures carried out by SEM revealed a combination of the nanoplatelets and nanowires. The TEM investigation of the nanostructures indicated that the section of the nanowires is likely hexagonal. The detailed characterisation of this synthesized nanostructured ZnO will be presented in the next chapter.

4.4.4. Deposition of RuO2 Nanostructures: RF Sputtering Method Ruthenium oxide (RuO2) is a transition metal-oxide with unique combination of characteristics including high thermal and chemical stability, low resistivity, catalytic activities, field emitting behaviour and remarkable electrochemical redox properties [62, 63]. Due to such properties, RuO2 has been widely used in different applications such as: electrochemical devices [64], electrochemical capacitors for energy storage [65], as fieldemission cathodes for vacuum microelectronic devices and field emission displays [66] and as thick film resistor [67]. Many methods have been developed and applied for RuO 2 nanostructures deposition. These methods include; hydrothermal method [68], metal-organic chemical vapour deposition (MOCVD) [69], reactive magnetron sputtering [70],


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electrochemical deposition [71], thermal decomposition of aqueous salt solutions [72], modified chemical bath deposition (M-CBD) [73] and supercritical fluid deposition (SCFD) [74].

Korotcov et al. [70] successfully deposited 1D RuO2 nanocrystal using RF magnetron sputtering (RFMS). They demonstrated that RFMS is a simple method, which has several advantages including better control of the growth conditions and a single deposition step to obtain the nanostructures. In this PhD thesis, the RuO 2 nanostructures were deposited for sensing application using the RFMS technique reported by Korotcov et al. [70]. However, different RF sputtering parameters other than that reported by Korotcov et al. [70] were selected for the deposition of RuO2 nanostructures in order to improve the porosity (surface to volume ratio) of the nanostructures for gas sensing applications.

The nanostructures of RuO2 were deposited onto the SiC substrates (3×3 mm2, Tankeblue Semiconductor Co. Ltd.) using a RF magnetron sputterer with a 2-inch ruthenium target. The substrates were heated up at 300°C. The deposition of RuO2 nanostructures was performed at a working pressure of 9×10-3 mbar in Ar-O2 (73-27%) atmosphere for 45 min. The RF power was 50 W during the deposition process.

The structural and material characterisations of the RF sputtered RuO2 layer was investigated using SEM, TEM and XRD techniques. The Surface morphology observation revealed nanocubular structures. In chapter 5, the detail of the RuO2 nanostructures characterisations will be presented.


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4.4.5. Deposition of MoO3 Nanostructures: Thermal Evaporation Method Molybdenum trioxide (MoO3) is a n-type semiconducting material with a wide band gap between 2.75 and 2.95 eV [75]. It has been widely investigated in various applications including sensors [76, 77], catalysts [78, 79], and electrochromic and photochromic devices [80, 81]. Research has shown that a nanostructured form of MoO3 has the potential to enhance the performance of gas sensors due to its high surface to volume ratio and the alteration of the surface energy [4, 37, 76]. There are different growth techniques for the development and deposition of MoO3 nanostructured thin films such as thermal evaporation [76, 77], hydrothermal synthesis [82, 83], and pulsed laser vaporization-controlled condensation (LVCC) techniques [84].

In this PhD research, deposition of MoO3 nanostructures was performed in a horizontal quartz tube furnace by evaporating MoO3 powder (China Rare Metal Material Co.) as shown in Figure 4.11. The MoO3 powder was weighed at 10 mg and placed on an alumina boat inside a quartz tube (18.5 mm diameter and 680 mm length) at the hot spot of the furnace. SiC substrates purchased from Cree Inc. with size of 3×3 mm2 were used for the growth of MoO3 nanostructures. Prior to the deposition, the substrates were cleaned by the method as mentioned in section 4.2 and then they were located at different distance of 10, 15 and 20 cm from the source downstream in the quartz tube.

The author investigated different deposition parameters for the growth of nanostructured MoO3. Two different carrier gases: 10% O2 in 90% Ar gas and pure 100% Ar gas were used. The carrier gas was flown through the quartz tube at a constant rate during the deposition. Oxygen gas during the deposition can improve the formation of fully oxidized MoO 3, while it is believed that using pure 100% Ar gas increases the conductivity of the MoO 3


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nanostructures and significantly affects the gas performance of the sensor. With the carrier gas of pure 100% Ar, different parameters such as temperature, deposition duration, distance between the source and substrates and gas flow rate were studied. To see the effect of temperature, different evaporation temperatures of 700, 770 and 850ºC were used. The evaporation temperature was increased by heating at a rate of 2ºC per min until the desired temperature was reached and maintained for the deposition duration. As the deposition completed, the temperature was allowed to cool at rate of 2ºC per min. The deposition parameters used for the growth of MoO3 nanostructures via thermal evaporation technique are listed in Table 4.1.

Furnace

Quartz Tube Carrier Gas

Source (MoO3)

Substrates

Heater

Gas Inlet

Gas Outlet

10 cm

15 cm 20 cm

Figure 4.11. Schematic diagram of the experimental set-up for growth of nanostructured MoO3 via thermal evaporation method.

Surface investigation of the deposited MoO3 revealed different morphologies: nanoplatelets, nano-flowers,

nanowires

and

combination of these

nanostructures. The detailed

characterisation of these nanostructures will be presented in chapter 5. Afterwards, gas sensing performance of the developed sensors was conducted and will be given in chapter 6.


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Table 4.1. MoO3 nanostructures deposition parameters using thermal evaporation method.

Sample Distance (between source & substrate) (cm)

Temperature Deposition Gas Flow (°C) Duration Rate (min) (L/min)

Gas Carrier

a

15

770

30

0.7

10% O2 - 90% Ar

b c d e f g h i

10 15 20

770

30

0.7

100% Ar

15

700 850

30

0.7

100% Ar

15

770

0.7

100% Ar

15

770

0.9

100% Ar

15 60 30

4.4.6. Synthesis of WO3 Nanoplatelets: Acid-Etching Method Recently, a great deal of attention has been drawn to investigate tungsten trioxide (WO3) due to its interesting properties such as electrical, optical, defect and structural properties [85]. WO3 has been widely used in many applications including gas sensors [86-88], electrochromic devices [89, 90], and batteries [91]. Research has shown that nanostructured form of the WO3 films can increase efficiencies in some applications such as gas sensors [92, 93], smart windows [14, 94] and displays [85] due to their enhanced surface to volume ratio. To date, a number of techniques for the synthesis of nanostructured WO 3 films have been developed and employed such as chemical vapour deposition (CVD) [95, 96], electrodeposition [97], sol-gel [98, 99], thermal evaporation [100, 101], electron beam evaporation [102], sputtering [86, 101] and liquid synthesis (anodization [103-105] and hightemperature acid-etching [106]) techniques. Among these methods, the high-temperature acidetching is a fast, inexpensive synthesis process, which was first reported by Widenkvist et al. [106]. This process can be applied to produce nanostructured WO 3 on large substrate areas.


115

They developed a simple process in which tungsten (W) substrate is immersed in nitric acid solution at elevated temperature, resulting in the formation of nanostructured film of WO3.xH2O (hydrated tungsten oxide). Subsequently, the WO3.xH2O film is annealed at high temperature to obtain anhydrous WO3 nanostructures. They used tungsten foil (99.95%) for the synthesis of the WO3 nanostructures. The developed thin films were composed of platelike WO3 crystals with the edges directed out from the substrate surface. They investigated the effects of the deposition parameters such as acid concentration, temperature and immersion time on the shape and size of the plates. They found that the most important factor influencing the microstructure and shape of the tungstite crystallite is the deposition temperature.

In this PhD research, the nanostructured WO3 thin films were synthesized via the acid-etching method. The tungsten films were prepared by RF sputtering system. A W target of 99.95% purity was placed at 65 mm over the stage. SiC substrates (5×5 mm2, Tankeblue Semiconductor Co. Ltd.) were used for the growth of WO3 nanostructures. The base pressure of the sputtering chamber was 1×10-5 Torr. Pure Ar gas, as the process gas, was introduced into the chamber prior to sputtering and process pressure was 2.9×10-2 torr. In this process, a constant 60 W RF power was used and the substrates temperature was 300°C. The substrates were RF sputtered for duration of 45 min resulting in ~1 µm thick tungsten thin films. The tungsten coated SiC substrates were then placed into a reflux apparatus containing 200 mL of 1.5 M HNO3 solution. A constant temperature of 50°C was kept during the synthesis. Different deposition durations of 1 hr, 2 hrs and 3 hrs were investigated. After the acid treatment, the samples were washed with DI water and blown dried in N 2. The as-deposited WO3.xH2O samples were then annealed at 500°C for 4 hrs in 90% O2/Ar atmosphere.


116

The annealed synthesized WO3 thin films characterised by SEM revealed that the films consist of nanocrystallite with a regular plate-like shape. Chapter 5 will present characterisations of the developed WO3 nanostructures in detail.

4.4.7. Synthesis of Graphene-Like Nano-Sheets Graphene, a one-atom thick carbon layer, has shown to be a prospective material due to unique electrical and mechanical properties. Graphene is the basic building block of all graphitic materials with two-dimensional layers of sp2-bonded carbon forming sheets comprised of interconnected benzene rings [107, 108]. Most the graphene sheets are obtained by peeling off of highly oriented pyrolytic graphite; e.g. using „scotch-tape‟ technique [109]. This method obviously cannot be employed for large-scale applications, even though it is simple to perform. Other techniques that have been used to produce graphene sheets are through the reduction of SiC [110] and conversion from nanodiamond [111]. These methods are not suitable for large-scale applications due to high production costs. There are alternative methods to synthesis graphene including (a) pyrolysis of camphor under reducing conditions [111], (b) exfoliation of graphite oxide [111, 112] and (c) thermal exfoliation of reduced graphite oxide (GO) into single functionalized graphene sheets [113].

In this PhD research, the graphene-like nano-sheets were deposited onto the polished side of clean 5×5 mm2 SiC (Cree Inc.) substrates. They were synthesized via the reduction of spray coated GO using hydrazine [114] by Professor Kaner‟s group from UCLA, USA, the collaborator of this work. The GO was synthesized by a modified Hummers method [115, 116] and then dispersed in water. The existence of oxygen-containing group (including carboxylic acid and hydroxyl moieties) in GO makes it readily exfoliate upon sonication in water. In order to achieve a highly uniform deposition of graphene, the SiC substrates were


117

heated prior to the deposition. This step helped in freezing the GO upon contacting with the SiC substrates as the solvent evaporated. The deposited GO/SiC was placed in a flow cell and heated at 80°C under exposure to helium gas. Hydrazine vapour was supplied via a bubbler to the system to reduce GO to graphene.

The spray-coating method is quick, taking only a few hours to deposit sheets [114]. Unlike other syntheses, high temperatures are not required. By using this method, graphene sheets with any desired coverage density and great uniformity can be obtained. This process can also be scaled to deposit sheets onto a number of substrates of any size and even onto prepatterned electrodes. Therefore, this method has the potential for the large-scale deposition of graphene for use in electronic devices.

The material properties of the developed graphene-like nano-sheets/SiC samples were characterised and the results will be presented in chapter 5. Subsequently, the Pt contact was deposited onto the synthesized graphene-like nano-sheets to form the Schottcky contact (section 4.5) and the gas performance of the sensors was investigated. The gas sensing properties of the sensors will be presented in chapter 6.

4.5. Schottky Contact Formation The final stage of the nanostructured based Schottky diode fabrication was the formation of the Schottky contact. Pt layer was deposited on top of the nanostructured thin films using GATAN PECS™ (Precision Etching Coating System). A shadow mask with 1 mm diameter hole was utilized to obtain a circular pad of Pt. The thickness of this Pt layer was 25-30 nm.


118

In the case of Pt/SnO2 nanowire/SiC and Pt/ZnO nanowire-nanoplatelet/SiC samples, which were developed at Brescia University, Italy, a circular pad of Pt metal layer was formed by applying a shadow mask with 1 mm diameter hole and deposited using DC sputterer. The sputtering conditions were: 50 W DC power; 100% Ar process gas; operating pressure of 5×10-3 mbar. The thickness of the Pt layers was 100 nm. Figure 4.12 shows the schematic of the Schottky contact formation.

4.6. Summary In this chapter, the fabrication processes of the nanostrucutred thin film Schottky diode based gas sensors used in this PhD reserach were presented. Different steps were involved in these processes including the substrates preparation (cleaning and dicing), synthesis of nanostractured materials and formation of the ohmic and Schottky contacts. To ensure reproducible results are achieved, strong emphasis was placed on the cleanliness throughout the entire fabrication process.

The formation of the ohmic and Schottky contacts and the synthesis steps of the nanostructured materials were explained in detail. TiO 2 nano-dimensioanl grains, ZnO nanostructured arrays, MoO3 nanostructures (nanoplatelets, nanoplatelets-nanowires and nano-flowers), and WO3 nanoplatelets were successfully synthesized and fabricated using different deposition techniques at RMIT University. Synthesis of other nanostructures materials such as SnO2 nanowires, ZnO nanowires-nanoplatelets, RuO2 nanostructures and graphene-like nano-sheets were performed in collaboration with overseas researchers. These nanostructured materials were deposited onto different substrates (SiC and epitaxial-GaN layer) as sensing layers to develop the novel gas sensors. A summary of the developed nanostructured Schottky diodes is shown in Table 4.2.


119

Sputtering

(a)

1 mm

SiC

Ti Pt

Sputtering

(b) 1 mm

Figure 4.12. Process steps for Schottky contact formation: A shadow mask was used for the deposition of the nanomaterials onto (a) SiC and (b) epitaxial-GaN/Al2O3 substrates. The mask was made of a thin sheet of Al with a hole of approximately 1 mm in diameter.

After the fabrication of the devices, the nanostructured material films were characterised using different techniques such as SEM, TEM, XPS, AFM, Raman and XRD. The materials characterisations results will be presented in chapter 5 in detail. The sensors were then placed in a gas testing system in order to investigate their characteristics such as electrical properties and gas sensing performance. Chapter 6 will present the experimental results in detail.


120

Table 4.2. The developed nanostructured Schottky diodes.

a

Nanostructures

Deposition Technique

Substrate

Ohmic Contacts

Schottky Contact

TiO2 nano-dimensional grains

Anodizationa

5×5 mm2 SiC (Cree Inc.)

Ti/Pt (40/100 nm)a

Pt (100 nm)a

SnO2 nanowires

Thermal Evaporationb

3×3 mm2 SiC (Sterling Semiconductor)

Ti/Pt (100/100 nm)b

Pt (100 nm)b

ZnO nanostructured arrays

Hydrothermala

5×5 mm2 epitaxial-GaN/Al2O3c

Al/Cr/Au (20/50/400 nm)c

Pt (25-30 nm)a

ZnO nanowires-nanoplatelets

Thermal Evaporationb

3×3 mm2 SiC (Sterling Semiconductor)

Ti/Pt (100/100 nm)b

Pt (100 nm)b

RuO2 nanostructures

RF Sputteringb

3×3 mm2 SiC (Tankeblue Semiconductor)

Ti/Pt (40/100 nm)a

Pt (25-30 nm)a

MoO3 nanoplatelets

Thermal Evaporationa


Ti/Pt (40/100 nm)a

Pt (25-30 nm)a

MoO3 nanoplatelets-nanowires



Ti/Pt (40/100 nm)a

Pt (25-30 nm)a

MoO3 nano-flowers



Ti/Pt (40/100 nm)a

Pt (25-30 nm)a

WO3 nanoplatelets

Acid-Etchinga

5×5 mm2 SiC (Tankeblue Semiconductor)

Ti/Pt (40/100 nm)a

Pt (25-30 nm)a

Graphene-like nano-sheets

Chemical Reduction of GOd


Ti/Pt (40/100 nm)a

Pt (25-30 nm)a

RMIT University (MMTC, School of Electrical and Computer Engineering) University of Brescia, Italy c University of Western Australia d University of California, Los Angeles, USA b


121

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Chapter 5 Nanostructured Materials Characterisation 5.1. Introduction Following the fabrication of the nanostructured thin film Schottky diode based gas sensors, which were presented in previous chapter, structural and material properties of the sensing layers were characterised and analysed. Since their surface morphologies and properties strongly influence the gas sensing performance, it is essential to use characterisation tools to study and understand their physical and chemical properties.

In this chapter, the author focussed on the characterisation of the nanostructured materials synthesized for gas sensing applications in this research. A brief description of techniques, which were employed to characterise and analyse the developed nanomaterials is given in section 5.2. Following this, the structural and material characterisation results will be presented in detail in section 5.3. This section is divided into seven subsections; each addressing a different synthesized nanostructured material. The chapter concludes with a brief summary and discussion of the results obtained.

Chapter 5: Nanostructured Material Characterisation

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5.2. Characterisation Techniques There are many techniques that can be employed for nanostructured materials characterisations. Some of the techniques, which are widely used include: scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectroscopy. Nanostructured materials can be characterised by any combination of the above techniques depending on the specific applications. For example, XRD has been extensively used to determine crystallinity, crystal structures and lattice constants [1, 2]; electron microscopy (SEM and TEM) has been commonly used to obtain information regarding nanomaterial‟s topography, morphology and crystallographic structure [1, 3, 4]. This section outlines the characterisation techniques, which were utilized to analyse and characterise the nanostructured materials used in this research.

5.2.1. Scanning Electron Microscopy (SEM) The SEM is one of the techniques, which is extensively used for the characterisation of nanostructured materials. With this technique, it is possible to obtain an image by detecting the secondary electron of the organic and inorganic materials at nanoscale resolutions [1]. It provides morphological and structural information by scanning an electron probe across a surface and qualitatively monitoring the secondary electrons that are emitted [1]. The SEM can be used for monitoring the growth and formation of the material. It can map the morphology to illustrate the degree of the nanostructurization of the surface. In nanotechnology enabled sensing applications, it is essential to study surface of the gas sensing layers, since the surface morphology, topography of the nanomaterials and the surface to volume ratio of the gas sensing layers strongly influence the sensor performance [1].


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Throughout this PhD thesis, the secondary electron imaging mode was used for the SEM characterisations of the nanomaterials.

In this technique, a heated filament commonly a tungsten or lanthanum hexaboride emits an electron beam. The filament can be heated by either applying a voltage or via field emission. The electrons are accelerated towards the sample by applying an electric potential. The electron beam is focused first by condenser lens and then by objective lens. Subsequently, it scans over the sample by scanning coils. The primary electrons hit the sample and transfer part of their energy to electrons in the sample. This causes the emission of secondary electrons, which are subsequently collected by an Everhart-Thornley detector. Then they are converted to a voltage, amplified and construct a map image of the surface.

The surface morphology of all the nanostructured materials synthesized for this research was investigated by the author using FEI NovaNano SEM at Physics Department, RMIT Microscopy & Microanalaysis Facility (RMMF). The structural characterisation of the nanomaterials in terms of shape, length, width, and distribution on the film surface were studied in two conditions: as-deposited and after testing towards analyte gases. The results of SEM investigation of all the nanomaterials are given later in section 5.3 (subsections 5.3.15.3.7) in detail.

5.2.2. Transmission Electron Microscopy (TEM) TEM has been commonly used to obtain information about the materials‟ morphology, crystallography, particle size distribution and their elemental composition [1]. This technique can provide atomic-resolution lattice images, as well as chemical information at a spatial resolution down to 1 nm. TEM characterisation of nanomaterials can provide better


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understanding of their physical and chemical properties as well as the orientations of their crystallographic phases.

In the TEM process, a beam of focused high energy electrons is transmitted through a thin specimen. These electrons are emitted from an electron gun (cathode) and focused by condenser lens onto the target specimen. As the electron beam passes through the sample, certain parts of the beam are transmitted, either deflected or undeflected. The scattered electrons are focused by objective lens, then amplified by projector lens and finally produce the desired image on the screen [5]. A wide variety of materials including metals, minerals, ceramics, semiconductors, and polymers can be characterised by TEM. One of the conditions required is that the sample must be appropriately thin to allow the electrons transmit through them to acquire a readable image.

TEM has the capability to provide both images and diffraction patterns for the same region by adjusting the strength of the magnetic lenses [6]. By defocusing the condenser lens to produce parallel illumination at the specimen and using a selected area aperture to limit the diffracting volume, a selected area diffraction pattern (SADP) from an area as small as several hundreds to a few nanometers in diameter can be obtained. In nanotechnology, SADP offers a unique capability to determine the crystal structures of nanomaterials and different parts of a sample.

Recently, high resolution transmission electron microscopy (HRTEM) has become a powerful tool for nanostructured materials characterisation. It has the capability to characterise the nanostructures with resolution as low as one angstrom (0.1 nm). High resolution image characteristics can be interpretable directly in terms of projections of individual atomic positions by applying correct operating conditions and well-prepared samples.


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In this PhD research, the TEM images of the SnO2 nanowires, ZnO nanowires-nanoplatelets and RuO2 nanostructures were obtained using a FEI Tecnai F20 microscope operated at 200 kV in collaboration with Brescia University, Italy. The ZnO nanostructured arrays synthesized by hydrothermal growth method were also characterised via cross sectional TEM (XTEM) imaging with the help of Assistant Professor Kehagias at Aristotle University of Thessaloniki, Greece. A detailed analysis of TEM imaging outcomes for SnO2 nanowires, and ZnO nanostructures are given in subsection 5.3.2 and 5.3.3, respectively.

5.2.3. X-Ray Diffraction (XRD) The XRD is a crystallographic technique employed for identification and quantification of various crystalline phases in solid materials and powders [1, 7, 8]. The crystal structure and size of grains and nanoparticles can also be determined via the XRD [1]. In this technique, a beam of X-rays are directed at a sample and a proportion of them are diffracted to produce a pattern. By comparison of those diffracted pattern with the reference patterns, the crystal phases of the sample can be identified.

Figure 5.1 presents the diffraction of X-rays by parallel planes (illustrated as horizontal lines) in a crystal in the XRD technique [1]. A monochromatic X-rays beam with wavelength λ is directed onto the crystalline material with spacing d, at an angle θ and then diffracted when leaving the various planes. The diffracted wave pattern can be in or out of phase to produce either destructive or constructive interference. Each diffracted X-ray signal corresponds to a coherent reflection which is called Bragg reflection. The constructive interference patterns only occur when incident angles satisfy the Bragg‟s law:

n  2d sin

(5.1)


134

where n=1, 2, 3, … is the order of diffraction, λ is the X-rays wavelength, d is the spacing between atomics planes in the crystalline phase and θ is the incident angle. In a XRD pattern, the intensity of the diffracted X-rays is plotted versus the detector angle 2θ. The intensity and angle of a set of peaks is unique to the crystal structure of the examined material.

Reflected X-rays

Incident X-rays

θ

θ

d

Figure 5.1. The diffraction of X-rays by lattice planes in a crystal using XRD technique.

The crystalline phases of the synthesized ZnO nanostructured arrays, RuO 2 nanostructures, MoO3 nanostructures, and WO3 nanoplatelets employed for gas sensing were examined by a wide angle XRD (PW 1820, Philips) with a Cu-Kα source at RMIT University. The detailed results using XRD on these nanomaterials is presented in section 5.3.

5.2.4. Energy Dispersive X-Ray (EDX) The elemental composition of a nanomaterial can be analysed using EDX technique, which can be integrated with the SEM system. In this technique, the sample is bombarded by SEM‟s electron beam [2]. The electrons of the atoms in the sample‟s surface are ejected from their orbits and leave electron vacancies in the inner shell. These electron vacancies are filled by the electrons from outer shells by lowering their energy. In this case, the excess energies are released through X-rays emission.


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The X-ray energy identifies the characteristic of the element from, which is emitted. The amount of energy released by the transferred electrons depends on which shell it is transferred to and also which shell it is transferred from. During transferring process, the atoms of every element release a unique amount of X-rays.

The EDX spectrum shows how frequently an X-ray is received for each energy level. Each peak in the EDX spectrum corresponds with the particular energy level. These peaks are unique to individual atoms, therefore corresponds to a single element.

In this PhD research, the synthesized nanostructured materials: TiO2 nano-dimensional grains, SnO2 nanowires and ZnO nanowires-nanoplatelets were characterised using the EDX technique. These EDX characterisations were conducted by the author at RMMF, Physics department. The EDX characterisation results of these nanostructured materials will be presented later in this chapter.

5.2.5. X-Ray Photoelectron Spectroscopy (XPS) XPS technique is primarily used to quantify the chemical and electronic states of the elements existing within the first few atomic layers of a material‟s surface. By using this technique, the elements in the material‟s surface, their chemical bonds and hence the chemical composition and empirical formulae (stoichiometry) can also be identified [9-11]. In a XPS experiment, a beam of X-rays strike the sample surface and interact with the atomic electrons in the sample, mainly by photon absorption to produce the ejection of photoelectrons [1]. These X-rays are generally emitted from magnesium or aluminium source and therefore intensity of the ejected photoelectron from the sample is plotted as a spectrum of their binding energies. The obtained spectrum is compared with spectra from standard databases. The peak positions and shapes


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correspond to the material‟s electronic configuration, and therefore elements and compounds show their own unique characteristic peaks [1].

For this PhD research, the chemical bonding of the SnO2 nanowires were examined by XPS technique in collaboration with Professor Kaciulis‟s group at Institute of Nanostructured Materials (CRA), Monterotondo, Italy. The deposited graphene-like nano-sheets were also characterised by XPS technique. The author appreciates the assistance from Associate Professor Du Plessis at RMIT University to perform the analysis using XPS in Thermo KAlpha spectrometer using an A1-K source with a spot size of 400 m. Charging was minimized using a low energy electron and ion flood gun. Individual peaks were scanned at 50 eV pass energy. The XPS characterisation results of those nanomaterials will be presented in subsection 5.3.2 and 5.3.7, respectively.

5.2.6. Atomic Force Microscopy (AFM) The AFM is one of the scanning probe microscopy techniques, which developed by Binnig, Quate and Gerber in 1986 [12]. In the AFM, the image is produced by measuring attractive or repulsive forces between a scanning probe tip and the sample surface. Any kind of surface such as an insulator, conductor or organic can be imaged by the AFM [13]. The AFM can be used not only for topographical mapping of surfaces but it also can function as a current, chemical, physical and bio-sensors [14-17].

In the AFM system, there is a microscale cantilever, which has a sharp tip at one of its end. When this tip is brought close (a few nanometers) to the sample surface, forces between atoms in the tip and in the sample cause the cantilever to bend and deflect [1]. The deflection


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is quantified and correlated to the force. Therefore, the surface morphology can be mapped to 3-D images.

The AFM characterisation of the deposited graphene-like layers used in this research investigation was conducted in collaboration with Professor Kaner‟s group at UCLA, USA. The AFM results in detail will be presented in subsection 5.3.7.

5.2.7. Raman Spectroscopy Raman spectroscopy is a powerful analytical tool to determine the composition of the materials qualitatively and quantitatively [18, 19]. In this technique, the intensity and wavelength of light scattered inelastically from molecules or crystals are monitored [1]. The sample is irradiated with light generally in the visible or infrared ranges. When the light strikes molecules and interacts with the bonding electrons in the molecules, inelastic or Raman scattering occurs. The wavelength of the scattered light is shifted with respect to the incident light (Figure 5.2) [1]. The spectrum of the scattered light is then analysed to determine the shift in its wavelength.

hv°

h(v° ± ∆v)

Figure 5.2. Representation of Raman scattering from particles.

In a Raman spectrum, the wavenumbers of Raman shifts are plotted versus their respective intensities, which are generated from the interactions between photons and molecular


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vibrations (phonons in a crystal) [20]. When irradiation from a laser source interacts with phonons in the sample, an energy exchange between them can be occurred. These phonon modes are intrinsic properties, which are related to the chemical bonding [21, 22]. Hence, the Raman spectrum may provide information by which molecules can be identified.

In this PhD research, the synthesized graphene-like nano-sheets were characterised by Raman spectroscopy with help of Dr. Spizziri in Micro-analytical Research Centre, University of Melbourne. The results will be presented later in subsection 5.3.7.

5.3. Nanostructured Materials Characterisation Results In this PhD research, different nanostructured materials including TiO 2 nano-dimensioanl grains, SnO2 nanowires, ZnO nanostructured arrays, ZnO nanowires-nanoplatelets, RuO2 nanostructures, MoO3 nanostructures (nanoplatelets, nanoplatelets-nanowires and nanoflowers), WO3 nanoplatelets and graphene-like nano-sheets have been successfully synthesized for gas sensing applications. The material properties of these nanomaterials are strongly dependent on the deposition technique and parameters, as well as the substrate on which they are grown [2]. Therefore, every deposited film using different deposition conditions have been completely analysed and characterised.

This section will present the characterisation results of these nanostructured materials in detail from subsections 5.3.1 to 5.3.7. By depositing the nanostructured materials as the gas sensing layers, novel Schottky diode based sensors have been developed and their performance towards different gases will be presented in chapter 6.


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5.3.1. TiO2 Nano-Dimensional Grains In this research, TiO2 nano-dimensional grains were synthesized via anodization of Ti films deposited onto the SiC substrates by FCVA deposition system. The material and structural characterisation of the nanostructures were investigated using SEM and EDX techniques [23].

The Ti films (with a thickness ranging from 250 to 300 nm) prepared by FCVA deposition system were dense, smooth and highly adherent to the substrate. The RMS roughness of the films was measured over a 1×1 µm2 area using a tapping mode AFM and found to be less than 0.5% of the deposited film thickness. The AFM image of the deposited Ti films is shown in Figure 5.3.

Figure 5.3. AFM 3-D image of the FCVA deposited Ti films.

Figure 5.4 shows the surface morphology of the TiO2 nano-dimensional grains at different magnifications with the inset: 45° rotation. The SEM images reveal coverage of TiO2 nanodimensional grains over the substrate. As can be seen from Figure 5.4, most of the nanodimensional grains consist of edges and corners.


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Figure 5.4. SEM images of TiO2 nano-dimensional grains as deposited on SiC substrate. Inset: 45° rotation.

The EDX analysis of the TiO2 nano-dimensional grains was performed for identifying the existence elements. The EDX spectrum (Figure 5.5) indicates the existence of Ti, O elements, whereas the other peaks (Si and C) identify the substrate‟s (SiC) elemental contents.

2000

Si Ka

Counts

1500

1000

500

O Ka C Ka Ti Ka

0 0

1

2

3 4 5 Energy (keV)

6

7

8

Figure 5.5. EDX spectrum of the TiO2 nano-dimensional grains deposited on SiC substrate.

As discussed in chapter 3, nanostructured based Schottky diodes show high sensitivity towards analyte gases under reverse bias operation due to the enhanced localized electric fields, which emanate from the sharp edges, tips and corners of metal coated nanostructures [24-26]. As nanostructured TiO2 comprising of edges and corners were observed from the


141

SEM images, it is anticipated that the porosity and the grain geometry of the anodized thin films will affect the gas sensing characteristics.

The surface morphology of the TiO2 nano-dimensional grains were studied after testing towards hydrogen and propene at elevated temperatures up to 620°C. Figure 5.6 revealed that the TiO2 nanostructures morphology remained intact. Hence, TiO2 nanostructures fabricated by anodization method are suitable for gas sensing application at elevated temperatures up to 620°C. The electrical and gas sensing performance of the Pt/TiO 2 nano-dimensional grain/SiC Schottky diode based sensors will be presented in chapter 6.

Figure 5.6. SEM images of the TiO2 nano-dimensional grains after testing.

5.3.2. SnO2 Nanowires Morphological investigations of the SnO2 nanowires, which were synthesized by thermal evaporation technique were carried out by both SEM and TEM techniques [27]. SEM images (Figure 5.7) reveal that the evaporation-condensation (EC) process promotes formation of a large quantity of wire-like nanostructures over the SiC substrate. It is observed that the asgrown films consisted of (a) top layer of long and thin wires with typical lengths in the range


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of tens to several hundreds of micrometers and diameter ranging between 10 and 50 nm and (b) bottom layer of thick and short wires with diameter ranging between 60 and 150 nm. The EDX analysis (Figure 5.8) was also performed to indicate the surface elements.

Figure 5.7. SEM images of SnO2 nanowires as grown on SiC substrate. 7000 6000 Si Ka

Sn La

Counts

5000 4000 3000 O Ka 2000 1000

Pt Ma

C Ka

0 0

1

2

3 4 5 Energy (keV)

6

7

8

9

Figure 5.8. EDX spectrum of the SnO2 nanowires deposited on SiC substrate.


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TEM investigation of the SnO2 nanowires was conducted using a FEI Tecnai F20 microscope operated at 200 kV for conventional, scanning-transmission and high-resolution imaging. Figure 5.9 highlights the structural features of the nanowires. The sharp parallel lateral sides and the transverse section of the nanowires do not feature variations in detail. Electron diffraction (ED) and high-resolution imaging (see insets of Figure 5.9) reveal the singlecrystal arrangement for the nanowires. The image highlights the regular arrangement and the smooth lateral termination. In addition, Bragg reflections and the whole symmetry of the ED pattern agree with a cassiterite tetragonal SnO2 phase (P42/mnm-SG 136). The direction of the electron beam is parallel to the (010) zone-axis of the reciprocal lattice and the nanowire grows along to the (100) direction [28, 29].

The composition of nanowires and chemical states of constituent elements were analysed by using XPS (Figure 5.10). This analysis is the standard XPS quantification through main photoelectron peaks, the determination of modified Auger parameter (918.9 eV for tin and 1041.2 eV for oxygen) and the study of valence band (VB) spectra for the accurate determination of metals oxidation states in the nanowires. Table 5.1 presents the main XPS results for the four samples of nanowires, including the atomic concentrations and binding energy (BE) values of the main peaks.

The values of modified Auger parameter and the shape of VB spectrum [30, 31] indicate the presence of mixed oxidation states (SnO and SnO2). This finding is confirmed also by intermediate value of stoichiometric rate Sn:O [32].


144

Figure 5.9. Low-magnification TEM image of a long SnO 2 nanowire (main picture), high-resolution TEM image of the nanowire (upper inset), and indexed digital diffractogram of the high-resolution image (lower inset). Sn 3d

I (arb. un.)

I (arb. un.)

VB

0

2

4

O 1s

6 8 BE (eV)

10

12

Sn 3p O KLL Sn MNN C 1s 250

350

450

550

650

750

850

950

1050

Binding Energy (eV)

Figure 5.10. XPS spectrum of SnO2 nanowires. Detailed VB spectrum is shown in the inset: grey peaks – SnO, white peaks – SnO2. Table 5.1. Surface chemical composition of SnO 2 nanowires: atomic concentration (%) of the elements and their binding energy (BE).

C

Sn

Oox.

OOH

Sn/Oox.

Atomic (%)

17.3 - 32.0

24.7 - 27.8

34.9 - 46.1

8.1– 10.7

0.6 - 0.71

BE (eV)

284.8

486.4

530.3

531.8


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From a gas sensing point of view, structurally uniform and single crystalline SnO 2 nanowires are suitable for gas sensing applications as these nanowires possess high surface to volume ratio and a high probability of maintaining its structure and phase during and after operation at elevated temperatures. Figure 5.11 shows SEM images of the SnO2 nanowires after exposure to hydrogen and propene at elevated temperatures up to 620°C. As it can be seen, the nanowires remained intact. The electrical and gas sensing performance of the Pt/SnO2 nanowire/SiC Schottky diode based sensors are given in next chapter.

Figure 5.11. SEM images of SnO2 nanowires after testing.

5.3.3. ZnO Nanostructures This section shows the study of ZnO nanostructures grown on epitaxial-GaN layers and SiC substrates using hydrothermal growth and thermal evaporation techniques, respectively. The surface morphology investigations revealed that the as-grown ZnO films on epitaxial-GaN layers by hydrothermal method consisted of nanorods covered in smaller secondary nanowires; while the thermally grown ZnO films on SiC substrate comprised of nanoplatelets and nanowires. These ZnO nanostructures were characterised by SEM, TEM, EDX and XRD and the results are presented in detail as follows:


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5.3.3.1. ZnO Nanostructured Arrays The morphology of the ZnO nanostructured arrays were examined by SEM and crosssectional TEM (XTEM) techniques [33]. As seen in Figure 5.12a, b and inset, ZnO nanostructures have grown in a unique hierarchical morphology comprising of ZnO nanorods with widths of 150-450 nm and heights exceeding 1 µm. These ZnO nanorods are bridged with adjacent neighbouring structures. Smaller secondary ZnO nanostructures typically 30 nm in width and 150 nm high also grew atop of these bridges like vertical antennas. Furthermore, it is observed fine ZnO nanowires grown as an extension from the tip of the broadest nanorods.

(a)

(b)

Figure 5.12. SEM images (a) bridged ZnO nanorods, with fine nanowires emanating from the bridges (visible as intense white dots) and (b) ZnO nanostructured arrays 25° rotation; (inset: higher magnification).

Figure 5.13 (left) shows the cross sectional morphology of the epitaxial-GaN layer grown on Al2O3, together with the ZnO nucleation layer (nl) and nanostructures. Bridging between nanorods is observed only at the far right end of the image due to the relative geometry of XTEM specimens and the 2-D projection of the images in TEM.


147

The reduction for the ZnO sputtered nucleation layer from 1.2 µm to 0.3 µm confirms that the nucleation layer was partially consumed in the growth process of the nanorods. The density of the nanorods suggests that the majority of the nucleation crystallites are dissolved into the high pH solution and becoming a source of zinc ions for fortification larger nanorods (more thermodynamically favourable). The finer secondary nanostructures are thought to be generated in as similar manner, emerging from less crystalline bridges or from the tips of broader nanorods, growing as hexagonal nanowire. Figure 5.13 (right) illustrates a wurtzite ZnO nanowire emerged from a bridged area between two larger nanorods [33].

Figure 5.13. XTEM image; left: the overall architecture of the sample along the [1120] GaN/ZnO direction. The ZnO nucleation layer (ZnO-nl) has reduced from 1.2 μm to 0.3 μm; right: depicting a ZnO nanowire emanating from the bridged crystalline area between two large ZnO nanorods.

The lattice mismatch between ZnO and GaN is known to be extremely small, being 1.8% [34]. Previous study [33] have identified that ZnO nanostructured arrays deposited on epitaxial-GaN grew in coordination with the underlying GaN. The XRD pattern of the ZnO nanostructured array at different stages of fabrication is presented in Figure 5.14. It can be clearly seen that ZnO deposited via RF sputtering and aqueous hydrothermal methods have adopted epitaxial-GaN reflections, being intensified during the different stages of ZnO


148

deposition. Major reflections exist at 34.4° and 72.6° 2θ, and corresponds to (002) and (004) planes, respectively.

In this PhD research, the surface morphology observations have verified that the synthesized ZnO nanostructures consist of sharp tips in their structures. Hence, it is projected that the developed sensor based on these ZnO nanostructured arrays will exhibit promising results towards analyte gases under reverse bias operation.

18 6

Intensity (counts 10 )

16

/- GaN ZnO Sapphire

14 12

8





10

/-

(c)

6





4 (b) 2

(a)

-





-

0 25

30

35

40

45

50 55 60 2 (degrees)

65

70

75

80

Figure 5.14. XRD pattern of (a) epitaxial-GaN on sapphire, (b) ZnO nucleation layer on epitaxial-GaN and (c) ZnO nanostructures on epitaxial-GaN.

Electrical and gas sensing performance of the ZnO nanostructured Schottky diode were investigated towards NO2 gas at elevated temperatures up to 350°C. SEM observations (Figure 5.15) of the ZnO morphology after testing revealed that the larger bridged nanorods remained mostly unchanged; however the smaller ZnO nanowires had effectively degraded, fracturing from the underlying interconnected ZnO nanorod array. It was found that approximately 90% of the nanowires over the Schottky contact (1 mm in diameter) had


149

degraded in this manner. Interestingly, investigations of the uncoated Pt (Schottky contact) regions of the ZnO nanostructured arrays showed little evidence of nanowire fracture. In the following chapter, electrical properties and NO2 gas sensing performance of the Pt/ZnO nanostructured array/epitaxial-GaN Schottky diode based sensors will be presented in detail.

Figure 5.15. Left: SEM image of Pt/ZnO nanostructures after testing highlighting the different stages of nanowire degradation: ranging from ideal nanowires (intact), and nanowires which have “fallen” or are in the process of “falling”; right: Pt/ZnO nanostructures as deposited and after testing.

5.3.3.2. ZnO Nanowires-Nanoplatelets Surface morphological characterisations and crystallographic structure of the thermally evaporated ZnO nanostructures were carried out by SEM and TEM [26]. A combination between nanoplatelets and nanowires can be seen in SEM image (Figure 5.16) and the inset shows the TEM of the nanostructures. Cross-sectional SEM image of the sample reveals that the thickness of nanostructured ZnO layer is approximately 100 µm (Figure 5.17).


150

Figure 5.16. SEM image of ZnO nanowires-nanoplatelets as grown on SiC substrate. Inset: a panoramic TEM image of the nanowires and platelets.

Cursor Height=104.3 µm

Figure 5.17. Cross sectional SEM image of the ZnO nanowires-nanoplatelets layer on SiC substrate.

The TEM image in Figure 5.18 illustrates the uniformity of the micrometric platelets as well as their single crystalline assembly. For the purpose of determinating the crystalline phase and lattice parameters for the nanoplatelets and the nanowires, electron diffraction analysis was carried out with a convergent electron beam. The diffraction pattern arises from a small specimen area, allowing one to discriminate between the two different ZnO structures. Figure 5.18b shows the electron diffraction pattern obtained by focusing the convergent beam of electrons over a small area at the centre of the nanoplatelet, which is shown in


151

Figure 5.18a. Analysis of the full diffraction pattern allows the precise determination of the lattice symmetry and spatial group for the crystal. The experimental pattern agrees with the one

expected

for

the

hexagonal

ZnO

structure

(spatial

group

186)

[Ref

JCPDF pattern 891397]. Figure 5.19a shows a ZnO nanowire with an average diameter of about 90 nm. Figure 5.19b shows the full electron diffraction pattern from a single nanowire obtained with a focused (convergent) electron beam. The central part corresponds to the conventional SAD pattern, and the outer ring provides additional information about the lattice symmetry. The experimental pattern agrees with the expected one of the hexagonal phase of ZnO.

The surface elemental composition of the ZnO nanostructures was also characterised by EDX analysis. The EDX spectrum shown in Figure 5.20 indicates the existence of the elements Zn, and O on the surface of the device. The gold metal that was deposited as a catalyst for the growth of ZnO crystal does not appear on the surface. As the interaction between the analyte gas and sensing layer occurs on the surface, therefore gold does not play a significant role in the sensors‟ performance.

Figure 5.18. (a) Image of ZnO nanoplatelets and (b) full electron diffraction pattern from focused beam (at the marker +).


152

Figure 5.19. (a) Image of a ZnO nanowire and (b) the corresponding electron diffraction pattern from focused beam (at the marker +).

Zn L

350 300

Zn K

Counts

250 200 150 100

OK

50 0 0

5

10 Energy (keV)

15

20

Figure 5.20. EDX spectrum of the ZnO nanowires-nanoplatelets deposited on SiC substrate.


153

The sharp edges and corners of the ZnO nanostructures are believed to significantly affect the sensor‟s gas performance in reverse bias operation. The gas sensors based on these ZnO nanostructured films were tested towards hydrogen and propene at elevated temperatures up to 620°C. The SEM images after testing (Figure 5.21) revealed that the morphology of the ZnO nanostructures distorted throughout the substrate after testing. In some regions, broken platelets were observed. The author will present the electrical and gas sensing performance of the Pt/ZnO nanowire-nanoplatelet/SiC Schottky diode based sensors in next chapter.

Figure 5.21. SEM images of the ZnO nanowires-nanoplatelets after testing (Two different magnifications).

5.3.4. RuO2 Nanostructures RF sputtered RuO2 nanostructures were characterised using SEM, TEM and XRD techniques [35]. The SEM images in Figure 5.22 indicate that the as-grown RuO2 layer on the SiC substrate consist of nano-cubular structures with dimensions ranging between 10 nm and 50 nm.


154

For TEM characterisation, a Ru-O layer was directly deposited at the same operating conditions over the carbon film supported by the TEM grid. Such a specimen is expected to be representative of the Ru-O layer under the assumption that the substrate has a negligible effect over the morphology and texturing of Ru-O crystals. The bending of the carbon film in some areas of the specimen allowed for a side view of the layer: as visible in Figure 5.23, the layer turns out to be homogeneously formed by nicely sharp crystals, with lateral size averaging 10 nm. The overall thickness of the deposited film measured less than 50 nm.

XRD analysis (Figure 5.24) of the RF sputtered nanostructured RuO2 layer revealed the crystallographic growth of tetragonal ruthenium (IV) oxide (ICDD No. [40,1290] - RuO2) with preferred orientation in the (101) lattice plane (~35 2, 100%). The diffraction pattern also contains dominant features due to ruthenium (0) (ICDD No. [06-0663], Ru), in particular the peaks at 38.4 and 442 which correspond to the (100) and (101) planes, respectively. Peaks due to the 6H-SiC substrate, and due to partial oxidation of the substrate: tridymite ([85-0419] – SiO2, 20.3 (110/200), 21.5 (002), 23 (111), 29.8 2(112)) are also evident.

Due to the cubular like with sharp edges and corners in the deposited RuO2 layer, it can be expected that the nanostructured RuO2 based Schottky diode shows higher sensitivity towards analyte gas under reverse bias operation than in the forward bias operation. However, a large voltage shift is not expected for this sensor as the nanostructured layer is not fully oxidized, composing of mixed RuO2 and Ru phases. In addition, the film is not very porous; therefore it will not provide a large surface area for hydrogen gas adsorption.


155

Figure 5.22. SEM images of the as-deposited RuO2 nano-cubular structures on SiC substrate.

(101) Ru

Figure 5.23. TEM view of the Ru-O nanocrystals.

(101) RuO2 (100) Ru

140

Intensity (counts)

120 100 80 60 40 20 0 10

20

30

40 50 2 (degrees)

60

70

Figure 5.24. XRD pattern of the RuO2 nano-cubular structures.


156

The sensor was tested towards hydrogen at temperatures up to 240°C. The surface morphology observation of the RuO2 nanostructures after gas testing revealed no changes in the nanostructures morphology (Figure 5.25). The electrical and gas sensing performance of the Pt/RuO2 nanostructure/SiC Schottky diode based sensors will be presented in chapter 6.

Figure 5.25. SEM image of the RuO2 nano-cubular structures after testing.

5.3.5. MoO3 Nanostructures As mentioned previously in chapter 4, synthesis of MoO 3 nanostructures using thermal evaporation method with different deposition parameters were investigated in this research. Using the SEM technique, different morphologies were observed, which are presented in Table 5.2. The surface morphologies of the synthesized MoO3 are shown in Figure 5.26. The author firstly applied a carrier gas of 10% O2 in 90% Ar during the deposition to produce fully oxidized MoO3 nanostructures and as a result, orthorhombic nanoplatelets (sample “a”) were observed (Figure 5.26a). In a different deposition, pure 100% Ar gas was applied to increase the conductivity of the MoO3 nanostructures, which resulted in the growth of a combination of nanoplatelets and nanowires (sample “c”) as shown in Figure 5.26c. The author continued to use the pure 100% Ar carrier gas and the effect of other deposition parameters such as:


157

temperature, deposition duration, distance between the source and substrates and the gas flow rate were studied.

Table 5.2. Surface morphology of the MoO3 nanostructures as a function of deposition parameters

Sample

using thermal evaporation method.

Distance (source &

T (°C)

Deposition Duration (min)

Gas Flow Rate (L/min)

Gas Carrier

Surface Morphology

substrate) (cm)

a

15

770

30

0.7

10% O2 - 90% Ar

Nanoplatelets

b c d

10 15 20

770

30

0.7

100% Ar

Elongated nanobelts Nanoplatelets-nanowires No crystalline structures formed

e f

15

700 850

30

0.7

100% Ar

No crystalline structures formed Nano-wires/towers/platelets

g h

15

770

15 60

0.7

100% Ar

Nano platelets-nanowires Nano-flowers

i

15

770

30

0.9

100% Ar

Nano-flowers

As it can be seen in Figure 5.26b, a distance of 10 cm between the source and substrate caused the growth of elongated nanobelts (sample “b”). Figure 5.26d revealed that no crystalline structures were formed when the source was 20 cm far from the substrate. Hence, it was determined that a distance of 15 cm between the source and the substrate (at a substrate temperature of ~450°C) was optimal for the growth of MoO3 nanostructures using thermal evaporation method. The author investigated the effect of the deposition temperature on the nanostructures morphology by choosing deposition temperatures of 700 and 850°C. It was observed that most of the MoO3 powder remained unevaporated in the source area following the deposition process at 700°C, which meant that this temperature was insufficient to evaporate the MoO3 powder. SEM observation was also confirmed that no crystalline structures formed in this process (Figure 5.26e). The SEM image in Figure 5.26f revealed growth of a combination of nanowires, nanoplatelets and tall nano-towers (sample “f”) from the deposition process conducted at 850°C.


158

Figure 5.26. Surface morphology of the deposited MoO 3 based on the deposition conditions summarised in Table 5.2.

By reducing the deposition duration from 30 min to 15 min, growth of nanowires and nanoplatelets (sample “g”) were observed in the SEM image shown in Figure 5.26g. However, an increase in the deposition duration from 30 min to 60 min, nano-flowers with sharp tips were grown in sample “h” (Figure 5.26h). SEM image in Figure 5.26i showed randomly orientated thin nanoplatelets in a densely packed formation of nano-flowers (sample “i”) when the deposition gas flow rate was increased from 0.7 to 0.9 L/min.


159

In this PhD research, samples “a” (nanoplatelets), “c” (nanoplatelets-nanowires) and “i” (nano-flowers) were investigated for gas sensing applications. As these samples consist of sharp edges, tips and corners in their structures, the author can anticipate that they will show significantly great voltage shift upon exposure to hydrogen gas under reverse bias operation.

The crystallinity of these nanostructures were analysed by XRD. The surface morphologies of these samples were also investigated following the testing processes. The detail of the material characterisations of these nanostructures are discussed as follows:

MoO3 Nanoplatelets As shown in Figure 5.26a, the SEM image of the MoO3 layer (sample “a”) reveals orthorhombic MoO3 nanoplatelets with side dimensions ranging from 100 nm to several micrometers and thicknesses between 50 nm and 500 nm [24]. The inset of Figure 5.28a reveals that the corners of the platelets are atomically sharp and rigid.

The XRD spectrum (Figure 5.27) of the MoO3 nanoplatelets indicates the presence of (020), (110), (040), (021), and (060) orientations at angles of 12.73°, 22.33°, 25.71°, 27.34°, and 38.87°, respectively. The films were found to be nonstoichiometric and exhibited both (0 k 0) and (0 k 1) orientations representing an α-β mixed phase of MoO3 due to deposition at temperatures above 300°C [36]. There is also relative presence of additional (110) and (021) peaks that classify the MoO3 films of monoclinic β-MoO3 phase [37]. It should be highlighted that XRD was obtained with the presence of the Pt contact (Schottky contact).


160

(040)

1200

(1 1 1)



(060) Intensity (counts)

1000



MoO3 Pt

800

(021)

600



400

(020) 

(002) (200)

(110) 

(200)

200 0 5

10

15

20

25 30 35 2 (degrees)

40

45

50

55

Figure 5.27. XRD spectrum of MoO3 nanoplatelets; (α-β denotes crystallographic planes grow in their respective α-β phases).

Hydrogen gas sensing properties of the Pt/MoO3 nanoplatelet/SiC based Schottky diodes were investigated at different temperatures up to 360°C. It was found that the dimensions of the platelets degraded after testing above 300°C [25]. SEM images in Figure 5.28b show the platelets edges are no longer sharp and rigid after high temperature treatment. Chapter 6 will present the electrical and gas sensing performance of the sensor in detail.

(a)

(b)

As-deposited

After testing

Figure 5.28. SEM images of MoO3 nanoplatelets; (a) as-deposited and (b) after testing.


161

MoO3 Nanoplatelets-Nanowires and Nano-Flowers SEM images in Figure 5.29a indicates that sample “c” (Figure 5.26c) consist of two main types of randomly oriented nanostructures: (a) nanoplatelets in a tower like formation and (b) thin nanowires. The dimensions of the nanoplatelets within the towers range between 300 nm to 2 µm in width and the towers range from between 1 m to 5 m in height. Thin nanowires were found throughout the nanostructured film with a diameters ranging from 20 nm to 30 nm and lengths up to 1 µm. Randomly orientated thin nanoplatelets in a densely packed formation were found in sample “i” (Figure 5.26i) [38]. Figure 5.29b reveals nanoplatelets with dimensions ranging from 250 nm to 1 µm, which is similar to hexagon shaped MoO3 nanorods reported by Dhage et al. [39].

(a)

(b)

Figure 5.29. SEM images of as-deposited MoO3 (a) nanoplatelets-nanowires and (b) nano-flowers.

Figure 5.30 shows the measured XRD spectra of the MoO3 nanoplatelets-nanowires and nano-flowers samples (“c” and “i”). Both samples consisted of crystallographic planes common at (020), (110), (130) and (141). MoO3 nanoplatelets-nanowires sample comprises additional crystallographic planes (040), and (021) while MoO3 nano-flowers sample is found to have additional (040), (150), (221) and (002), (221) planes, respectively. These crystal


162

phases indicated the classification of a mixed α-β MoO3 due to deposition at temperatures above 300°C [37]. The crystallographic peaks when compared to the XRD library standards suggests the existence of the MoO3 with lattice parameters a, b and c as 3.962, 13.85 and 3.697, respectively [Card file #050508]. These results are similar to the nanowire MoO3 structures reported by thermal evaporation in a bell shape jar vacuum system [40, 41].

The MoO3 nanoplatelets-nanowires and nano-flowers samples were tested towards hydrogen at different temperatures up to 250°C; as it was previously discussed that the elevated temperatures above 300°C can affect the MoO3 nanostructures morphology and therefore the developed sensor‟s gas performance. SEM observations after gas testing clearly showed that at operating temperatures below 300°C, the nanostructure morphology was retained intact (Figure 5.31). The electrical and gas sensing performance of these samples are presented in chapter 6.

2800

(221)

(002)

(141)

(130)

(110)

1600

(020)

2000

SiC

(b)

1200 SiC (141)

400

(110) (040)  (021)  (130)

800

(020) 

Intensity (counts)

2400

(a)

0 10

15

20

25

30 35 40 2 (degrees)

45

50

55

60

Figure 5.30. XRD spectra of MoO3 (a) nanoplatelets-nanowires and (b) nano-flowers; (α-β denotes crystallographic planes grow in their respective α-β phases).


(a)

163

(b)

Figure 5.31. SEM images of MoO3 (a) nanoplatelets-nanowires and (b) nano-flowers after testing.

5.3.6. WO3 Nanoplatelets Structural and material characterisation of the sputtered tungsten (W) and the annealed nanostructured WO3 were investigated by SEM and XRD [42]. As described previously in chapter 4, these WO3 nanostructures were synthesized via an acid-etching method for durations of 1 hr (sample A), 2 hrs (sample B) and 3 hrs (sample C) and then they were annealed at 500°C for 4 hrs in 90% O2 in Ar atmosphere. Figure 5.32a reveals a uniform dispersion of the tungsten grains with dimension of approximately 20 to 70 nm over the SiC substrate. The RF sputtered tungsten film thickness is ~1 µm (Figure 5.32a inset). From Figure 5.32b, c and d, it can be seen that all annealed samples consist of nanoplatelets with thicknesses between 20 and 60 nm and lengths between 100 and 700 nm. The SEM observation confirmed that the platelets do not have sharp corners due to high temperature annealing (500°C) as reported by Kalantar-zadeh et al. [43]. The insets reveal an approximate thickness of the annealed acid-etched films to 3.3 µm consisting of two layers: (a) nanoplatelets on top and (b) non-porous layer at the bottom. As the duration of the acidetching increases, the height of the top nanoplatelets increases and bottom non-porous layer decreases (insets of Figure 5.32b, c and d).


164

XRD patterns for untreated tungsten and the annealed acid-etched samples (A, B and C) are shown in Figure 5.33. Following the acid-etching and high temperature treatment, a significant reduction in the intensity of the metallic W peaks (ICDD No. [04-0806] – 40.2, 73.2, 86.9 2 – (110), (211), and (220) planes respectively) were observed. It was found that the longer exposure of the sample to acid, the greater conversion of W to WO 3 was occurred. The greatest conversion of W to WO3 was observed in samples B and C. The SiC substrate peaks (ICDD No. [22-1273] Moissanite-6H, syn SiC – 34.2, 35.8, 41.6, 73.7, 75.6 2) were also observed in the diffractograms of all samples due to penetration of the X-ray beam through the sputtered thin film and onto the SiC substrates.

Figure 5.32. Surface morphology of (a) RF sputtered tungsten on SiC substrate and annealed nanostructured WO3 samples acid-etched for (b) 1 hr (sample A), (c) 2 hrs (sample B) and (d) 3 hrs (sample C). Insets: cross sectional SEM (45° tilted view).


165

The author highlights that the composition of the oxidised layer varied with acid exposure. In the sample A, the W appears to have undergone partial oxidation as evidenced by the presence of W3O (ICDD No. [41-1230]), together with partially hydrated- (ICDD No. [361142] – WO3 . 0.75 H2O) and protonated-tungsten oxides ((ICDD No. [42-1261] - H0.23WO3). Whereas, samples B and C showed significantly higher levels of oxidation. They also contain protonated tungsten oxide, together with WO3 (ICDD No. [24-0747]). The XRD analysis exhibited as the duration of acid-etching increases, the increase in the oxidation level of the film result in thicker upper nanoplatelet layer as shown in SEM data.

From the SEM and XRD observation, it is expected that sample C will exhibit higher sensitivity towards hydrogen gas than samples B and C due to a larger proportion of oxidized nanocrystallites on the films; providing a higher available surface area for gas adsorption.

16

*/ */ *

3


14 12

W

*

W

(a)

W

*W

10 8



(b)

6 4

 (c)

*  





 

 

*  

2 0

(d) 20

30

40

50 60 2 (degrees)

70

80

90

 = WO3  = SiC W = tungsten  = H0.23WO3  = W3O Figure 5.33. XRD patterns for (a) RF sputtered tungsten and annealed nanostructured WO3 samples, acid-etched for (b) 1 hr (sample A), (c) 2 hrs (sample B) and (d) 3 hrs (sample C).


166

As mentioned previously, the presence of sharp edges and corners in nanostructures can affect the sensor‟s sensitivity towards analyte gas in reverse bias operation. Although the WO 3 nanoplatelets do not possess sharp corners and edges, it is expected that the nanoplatelets will play a significant role in the sensor‟s electrical and gas sensing characteristics. Electrical and gas sensing measurements of the all samples were investigated towards hydrogen at different operating temperatures up to 300°C. SEM observations of the all samples after testing revealed that the nanoplatelets were degraded (Figure 5.34). It was found that in samples A and B, the nanoplatelets over the Schottky contact (1 mm in diameter) had degraded, while the Pt uncoated nanoplatelets remained mostly unchanged. For sample C, the nanoplatlets over the Schottky contact and also in the region out of Schottky contact had effectively degraded. As showed earlier in the insets of Figure 5.32b, c and d, the height of the top nanoplatelets increased and bottom non-porous layer decreased by increasing the duration of the acid-etching. Therefore, it can be concluded that the nanoplatelets in sample C are likely less stable at elevated temperatures than in samples A and B. In the following chapter, electrical and gas sensing performance of the Pt/WO3 nanoplatelet/SiC Schottky diode based sensors as well as temperature and analyte gas effect on the nanoplatelets morphologies will be presented in detail.

(a)

(b)

(c)

Figure 5.34. SEM images of WO3 nanoplatelets samples acid-etched for (a) 1 hr (sample A), (b) 2 hrs (sample B) and (c) 3 hrs (sample C) after testing.


167

5.3.7. Graphene-Like Nano-Sheets Different characterisation techniques were employed for material analysis of the synthesized graphene-like nano-sheets via the chemically reduction of graphite oxide (GO). The surface morphology of the deposited graphene-like nano-sheets on the SiC substrate were studied using SEM and AFM methods, while their chemical properties were investigated by Raman spectroscopy and XPS [44].

The SEM image of the graphene-like layers as shown in Figure 5.35a indicates that most of the SiC substrates are covered by graphene-like nano-sheets, which appear pale grey in the image. The AFM measurement was also conducted separately on the graphene-like layer deposited onto a Si substrate. The synthesized graphene-like material consists of multiple layers rather than a single atomic layer which is common for material fabricated in this way [45]. This is most evident in the AFM measurement (Figure 5.35b), which reveals that the material can have a thickness of up to 10 nm [46].

(b)

Figure 5.35. (a) SEM image of the graphene-like nano-sheets deposited on SiC substrate and (b) AFM image of graphene-like nano-sheets deposited on Si substrate.


168

Raman spectroscopy, which is a well established technique for studying carbon allotropes including graphene [47], was used to characterise the devices studied in this work. Figure 5.36 shows the full range of the low frequency spectrum (100-1000 cm-1), which exhibits bands consistent with first order scattering from the 6H polytype of SiC. Peaks are attributed to E symmetry phonon modes at 145, 149, 240, 265, 767, 789, 798 cm-1 and a symmetry modes at 504 and 514 cm-1 [48, 49]. Figure 5.37 shows bands, which are also consistent with second order scattering from the 6H polytype of SiC with peaks at 1710, 1619, 1543, 1531 and 1515cm-1 [50].

767, 789, 798 cm

-1

140000

100000

965 cm -1

-1

-1

890 cm

20000

504 & 514 cm

40000

240 & 265 cm

60000

-1

-1

80000 145 & 146 cm

Intensity (arb.units)

120000

0 100 200 300 400 500 600 700 800 900 1000 -1 Wavenumber (cm )

Figure 5.36. A representative low frequency Raman spectrum of the SiC device with deposited graphene-like material. The labelled peaks are characteristic of first order scattering for the 6H polytype of SiC.

In addition to phonon modes arising from the SiC substrate, Figure 5.37 also reveals the presence of sp2-bonded carbon phases with characteristic D (disorder) and G (graphitic, sp2) bands at ~1350 and ~1590 cm-1. Overtone (2D and 2G) and combination bands (D+G) associated with the graphitic phases are also evident in this spectrum and are consistent previous measurements of graphene films produced in a similar fashion [45].


169

Intensity (arb.units)

14000 12000

G

D

SiC+Graphene

10000 8000 6000 4000

2D

D+G 2G

2000 0 1000

1500

2000

2500

3000

3500

-1

Wavenumber (cm )

Figure 5.37. A Raman spectrum of the deposited grapheme-like material on a SiC device measured over the first and second-order scattering range for graphitic carbon. Scattering from the 6H polytype of SiC as well as characteristic D, G, 2D, D+G and 2G bands associated with the graphene-like material are indicated.

Shown in Figure 5.38a is a close up view of the first order scattering range for the carbon phases from regions of the device which: (i) have deposited graphene layers and (ii) are graphene free. The areas with graphene have an intense D band feature at ~1348cm -1 (2Γ = 88cm-1) and a G band feature comprised of at least 2 underlying components which are identified as: (i) an sp2 graphitic peak at 1589 cm-1 (2Γ = 61cm-1) and (ii) a SiC second order mode at 1619 cm-1 (2Γ = 28cm-1). Figure 5.38b shows the second order bands associated with both defected graphite and graphene including: (i) the 2D (i.e. G‟) band at ~2685 cm-1 (2Γ = 100 cm-1) which is an overtone of the D band, (ii) the D + G combination mode at ~2933 cm-1 (2Γ = 90 cm-1) and (iii) the second order 2G band at ~3192 cm-1 (2Γ = 50 cm-1). Simulated fits to each of these bands are also shown in this Figure. The D, 2D and D + G bands are all induced by local defects and disorder in sp2 phases, and have been previously observed along graphene and graphite edges [51].


170

The relative intensity of the D band (compared to that of the G band) is known to be related to the in-plane graphitic crystallite size, La [47, 52]. We have calculated the value of La from these measurements using the expression La = 4.4(IG/ID) which yields a value of 3.9 nm for these devices [47, 53]. This indicates that the graphene-like nano-sheets contain nanocrystalline graphitic phases with small sp2 domains resulting from the incomplete reduction of GO with hydrazine [54]. This result is consistent with the results from similarly prepared materials [45].

The 2D band of the Raman spectrum is also known to evolve with the number of graphene layers for single-, bi- and few-layers allowing unambiguous identification of the number of layers from 1 to 5 [47, 55]. Changes to: (i) the shape and frequency of this band and (ii) its relative intensity (to that of the first order allowed G-band) in response to the number of layers (n) can be used to make this assignment. From the relative 2D-band intensity, the deposited material looks like n ≥ 19 with the G-band intensity dominating the spectrum. This assessment is consistent with the AFM measurement shown in Figure 5.38b which reports the films as being up to 10 nm thick; equivalent to n ~ 30 graphene layers [55]. An alternative approach is to consider the 2D band position which is known to shift to higher frequencies and broaden with increasing values of n. The fitted 2D-band parameters may be considered consistent with fewer graphene layers (i.e. n ≤ 8).

Yet another reported method for determining n involves the analysis of the fitted G-band position [55]. Surprisingly for the sample prepared in this work, n was found to be ~ 1 when assessed using this criterion. This result is inconsistent with the previous assessments of n (using the 2D band parameters) possibly indicating that this spectral range is more complex because of the underlying SiC phonon bands which may require more fitting terms than used


171

in this simple model. In any case, some degree of inconsistency is still evident in these determinations of n which may be associated with the inhomogeneous deposition and reduction of GO resulting in material comprised of both graphitic and graphene phases

1619 cm-1

(a) 25000

1517, 1531 & 1543 cm-1

20000

SiC

15000

D

10000

G 1349 cm-1

Intensity (arb. units)

30000

1710 cm-1

containing many defects.

5000

SiC+Graphene 1589 & 1619 cm-1

0 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

Wavenumber (cm-1) 350

Intensity (arb. units)

(b)

D+G

300

2933 cm-1

2D

250

2685 cm-1

200

2G

150

3191 cm-1 100 50 0 2200

2400

2600

2800

3000

3200

3400

Wavenumber (cm-1) Figure 5.38. (a) Raman spectra depicting second-order scattering from the 6H polytype of SiC as well as characteristic first-order D and G (graphitic) bands associated with the applied graphene-like layer and (b) a second-order Raman spectrum arising from the scattering of sp2-bonded carbon phases with fitted profiles for the 2D, D+G and 2G bands depicted. Note that SiC does not make any spectral contribution to (b).

The XPS analysis would ideally show two types of C bond structures for graphene; C-C and C=C. Referring to the data of Figure 5.39, the XPS spectrum indicates the existence of C-C at


172

285 eV. The C1s spectrum also signifies C-OH, O=C-OH and Si-C bonds at 286.6, 289.1 and 283 eV, respectively. The absence of C=C suggests the imperfect structure of the deposited graphene. Arsat et al. [45, 56] suggested this may be due to the incomplete reduction with hydrazine, as well as the presence of adventitious carbon on the sample, which is also confirmed by the Raman measurements. The Si-C bond identifies the SiC substrate, which was used as the foundation in this device.

35 C-C 3


30 25 Si-C

20 C-OH

15 O=C-OH

10 5 292

290

288 286 284 282 Binding Energy (eV)

280

Figure 5.39. XPS analysis of graphene-like nano-sheets.

In this PhD research, a Pt/graphene-like nano-sheet/SiC based sensor was tested towards hydrogen at low operating temperatures up to 100°C. The thermogravimetric analysis (TGA) indicated that the graphene material begins to lose mass and structures as the temperature increases beyond this point. The graphene mass loss was almost 20% at 100°C and major weight loss occurred following heating at approximately 200°C and higher [45]. SEM image in Figure 5.40 shows that the graphene-like layer retained its structure after testing at low temperature up to 100°C. From a gas sensing perspective, it is expected that the large surface area of the deposited graphene layers should increase the interaction area available for the gas


173

molecules, thus enhancing the sensitivity. The electrical and gas sensing performance of the sensors based on the graphene-like nano-sheets will be presented in next chapter.

Figure 5.40.SEM image of the graphene-like nano-sheets after testing.

5.4. Summary In this chapter, material characterisation techniques including SEM, TEM, XRD, EDX, XPS, AFM and Raman spectroscopy, which were used for analysis and characterisation of the synthesized nanomaterials for this research were briefly described. Then, the material characterisation results of the TiO2 nano-dimensioanl grains, SnO2 nanowires, ZnO nanostructured

arrays,

ZnO

nanowires-nanoplatelets,

RuO2

nanostructures,

MoO3

nanostructures (nanoplatelets, nanoplatelets-nanowires and nano-flowers), WO3 nanoplatelets, and graphene-like nano-sheets were presented.

SEM characterisation of the as-deposited nanostructured materials was conducted to understand the morphology of the nanomaterials in terms of morphology, dimension and orientation, distribution on the film and porosity; as these parameters can significantly affect the performance of the sensor. Afterward, other characterisation techniques such as TEM,


174

XRD, EDX and Raman analysis were employed to obtain complete structural properties of the nanostructured materials in order to understand the sensors performance. Once the material characterisations were completed, the electrical and gas sensing properties of the developed sensors were investigated, which will be presented in the next chapter. The nanostructured materials were SEM characterised once again after gas testing.

The key outcomes from the above-presented nanomaterials characterisation results can be summarised as follow:

TiO2 Nano-Dimensional Grains 

AFM analysis of the Ti films deposited by FCVA system revealed dense, smooth films highly adherent to the substrate. SEM observation of the annealed anodized films showed that TiO2 nano-dimensional grains were grown over the SiC substrate.



Surface morphology investigation of the TiO2 nano-dimensional grains after testing towards hydrogen and propene at elevated temperatures up to 620°C showed that the nanostructures retained their initial morphology.

SnO2 Nanowires 

The SnO2 nanowires synthesized by thermal evaporation method were characterised by SEM, TEM and EDX techniques. SEM images revealed that the films consisted of top layer of long and thin wires (diameter: 10-50 nm) and bottom layer of thick and short wires (diameter: 60-150 nm). Furthermore, the presence of Pt catalyst was observed present at the tip of the nanowires and was verified by the EDX analysis.



TEM analysis highlighted a single-crystal arrangement for the nanowires.




175

SEM observation of the nanowires after testing revealed that elevated temperatures and analyte gas did not affect the nanostructures morphology.

ZnO Nanostructured Arrays 

Hydrothermally grown ZnO nanostructured arrays consisted of bridged ZnO nanorods (width: 150-450 nm and height: >1 µm) with fine nanowires (width: 30 nm and height: 150 nm) emanating from the bridges. The sharp tips in the ZnO nanostructres morphology are believed to affect significantly the performance of the sensor under reverse bias operation.



XTEM characterisation revealed a hexagonal crystal structure for the finer secondary nanowires.



SEM observation of the ZnO morphology after exposing to NO 2 gas at elevated temperatures revealed that the smaller nanowires had effectively degraded, while the larger bridged nanorods remained mostly unchanged.

ZnO Nanowires-Nanoplatelets 

A combination between nanoplatelets and nanowires were observed in the ZnO nanostructures grown by thermal evaporation. SEM and TEM characterisations revealed sharp edges and corners in the ZnO nanostructures. Hence, it is expected that the sensor will show high sensitivity towards target gases under reverse bias operation.



Selected-area diffraction pattern revealed hexagonal stable and stoichiometric phase for the ZnO structure.



It was found that the ZnO morphology was degraded after testing towards hydrogen and propene at elevated temperatures up to 620°C.


176

RuO2 Nanostructures 

Surface morphology investigation of RF sputtered RuO2 nanostructures revealed nano-cubular structures with dimensions ranging between 10 and 50 nm. It is anticipated the sensor will have higher sensitivity towards hydrogen in reverse bias operation than in the forward bias operation due to cubular like shape with sharp edges and corners in the RuO2 nanostructures.



Using SEM, it was observed that the RuO2 nanostructures morphology remained unchanged after testing towards hydrogen at low temperatures up to 260°C.

MoO3 Nanostructres 

Using different thermal evaporation deposition parameters for the synthesis of MoO 3 nanostructures resulted in different morphologies. Samples with morphologies of nanoplatelets, nanoplatelets-nanowires and nano-flowers were characterised for gas sensing applications in this research. SEM observation revealed that all these nanostructures consisted of sharp edges, tips and corners in their structures. Hence, it is expected highly sensitive sensors when they are exposed to hydrogen gas under reverse bias operation.



Using SEM, MoO3 nanostructures degradation was observed after high temperature treatment above 300°C.

WO3 Nanoplatelets 

Tungsten grains with dimensions of approximately 20-70 nm were RF sputtered onto the SiC substrate. Acid-etching of the tungsten grains (different durations of 1 hr, 2 hrs and 3 hrs) and annealing at elevated temperature (500°C) resulted in growth of


177

nanoplatelets with thicknesses of 20-60 nm and lengths of 100-700 nm. A crosssectional SEM observation of the annealed acid-etched films revealed that the WO3 nanostructred layer consisted of two layers: (a) nanoplatelets at top and (b) non-porous at the bottom. It was found that as the duration of the acid-etching increased, the height of the top nanoplatelets increased and bottom non-porous layer decreased. 

XRD analysis revealed that the rate of oxidation of tungsten increased as the duration of acid-etching increased.



SEM images of the WO3 nanoplatelets after gas testing showed that the exposure to hydrogen at temperatures above 200°C resulted in degradation of the WO 3 nanostructures.

Graphene-like Nano-Sheets 

Structural morphology investigation of the deposited graphene-like layer via SEM and AFM revealed that most of the SiC substrates were covered by graphene-like nanosheets (in pale grey region).



Raman characterisation revealed that the deposited graphene material contained defects and was heterogeneous in nature comprising both graphitic and graphene phases. The graphene material ranged in thickness from ~8 to ~30 graphene layers.



The XPS spectrum indicated the absence of C=C bond. This can be concluded due to incomplete reduction of GO to graphene.



Surface morphology of the deposited graphene-like nano-sheets maintained their structure after testing towards hydrogen at low temperatures up to 100°C.

The synthesized and characterised nanomaterials presented in chapters 4 and 5 were applied as sensing layers in developed gas sensors based on Schottky diodes. Chapter 6 will present


178

the electrical and gas sensing performance of the sensors based on these nanomaterials towards different gas species in detail.


179

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Chapter 6 Nanostructured Thin Film Schottky Diode Based Gas Sensors Characterisation 6.1. Introduction In chapter 4, the deposition of nanomaterials and fabrication of the nanostructured Schottky diode based gas sensors were presented. The nanostructured materials were successfully deposited onto the different substrates (SiC and epitaxial-GaN layer). These include: TiO2 nano-dimensional grains, SnO2 nanowires, ZnO nanostructures (nanostructured arrays and nanowires-nanoplatelets),

RuO2

nanostructures,

MoO3

nanostructures

(nanoplatelets,

nanoplatelets-nanowires and nano-flowers), WO3 nanoplatelets and graphene-like nanosheets. Following the deposition of the nanomaterials, the structural and material properties were characterised and analysed to acquire a better understanding of their physical and chemical properties. The characterisation results were presented in chapter 5.

In order to perform electrical and gas sensing characterisations, a circular pad of Pt metal was deposited onto the nanostructured materials to form the Schottky contact and also function as a catalyst for the dissociation of gas molecules. The nanostructured thin film Schottky diode

Chapter 6: Nanostructured Thin Film Schottky Diode Based Gas Sensors Characterisation

184

based sensors were placed in a test chamber using a computerized multi-channel gas calibration system and were exposed towards different gases such as hydrogen (H2), propene (C3H6) and nitrogen dioxide (NO2).

This chapter is divided into four sections. The first section (6.1) outlines the introduction. The experimental set-up and the gas testing system are presented in section 6.2. The experimental procedures for testing are described in detail in this section. The third section (6.3) presents the electrical and gas sensing results of the fabricated sensors and is divided into seven subsections. Each subsection details the results for one of the developed sensors. The currentvoltage (I-V) characteristics of each sensor and the barrier height change upon exposure to target gas are presented in detail. The electrical properties of each sensor are discussed with respect to the temperature and free carrier concentration (ND). Subsequently, the performance of the sensors towards target gases is characterised by the sensitivity, response/recovery time (t90%) and baseline stability. A summary of the results is presented in section 6.4.

6.2. Experimental Set-Up and Gas Testing System Throughout this research, a fully automated multi-channel gas testing system was calibrated and used to test the developed nanostructured Schottky diode based sensors. This system was based on a volumetric mixing of gases using four mass flow controllers (MFCs). Certified gas cylinders of high purity (99.999%) dry synthetic air and low concentrations of analyte gases such as: 1% hydrogen, 1% propene and 9.9 ppm NO2 balanced in synthetic air were used. The concentration of the analyte gases is diluted with synthetic air by adjusting the flow rates of each MFCs while maintaining a total constant flow rate of 200 SCCM (mL/min). Figure 6.1 shows a schematic representation of the complete mutli-channel gas testing system. It


185

incorporates a test chamber, a MFC processing unit, four MFCs, a DC power supply, a digital multimeter, a source meter, a K-type thermocouple and data acquisition computer.

The test chamber system was comprised of a Teflon block with 30 mm thickness and a quartz lid. The quartz lid was clamped to the Teflon base using a Viton O-ring to maintain an air tight sealed chamber. The total volume of the chamber was approximately 175 mm3. The sensor was mounted on a planar alumina micro-heater, which controlled the operational temperature. A thermocouple attached to a multimeter was used to obtain a real-time reading of the temperature. The back electrode of the sensor was contacted to the biasing circuit using a thin copper foil, while the Pt surface was connected via a mechanical needle contact as shown in Figure 6.2.

Data acquisition/System control

Mass flow controller (MFC)

Power supply

MFCs Multimeter

Source meter

Gas exhaust

Thermocouple

Test chamber

Gas cylinders

Figure 6.1. Block of gas testing system set-up for the nanostructured Schottky diode based sensors.


Prob

186

e Connections to biasing circuit

M

eta l

foi l

ter

SiC

Hea

Figure 6.2. Illustration of a nanostructured Schottky diode based sensor with electrical connections.

As previously mentioned, for all the experiments, a constant total gas flow rate of 200 SCCM was applied. Therefore, the time required to fill the chamber with volume of 175 mm3 was determined to be approximately 50 s. Prior to the commencement of the measurements, several stabilizing processes were performed. In these processes, the sensor was subjected to different temperatures and exposed to the target gas pulses in sequence. This involved sequential increase in temperature then allowing it to cool down, while at the same time introducing different gas concentrations into the chamber. This cycling process was conducted so that a stable baseline voltage could be obtained as well as the repeatability and coherence in the measured data could be ensured. The duration of the stabilizing process required approximately an average, two days per sensor. Afterwards, the I-V characteristics and dynamic response of the sensors were measured with exposure to the target gas. The I-V measurements were performed using a Keithly 2602 source meter. The dynamic response of the sensor to the target gas was measured as a change in the voltage magnitude, while the sensor was biased at a constant current and the gas concentrations were adjusted in sequence. An Agilent 34410A digital multimeter was used to record the response in terms of the change in voltage.


187

6.3. Electrical and Gas Sensing Results In this section, the electrical properties and gas sensing performance of the developed nanostructured Schottky diode based sensors will be presented in detail.

6.3.1. TiO2 Nano-Dimensional Grains Based Sensor The electrical measurements of the fabricated Pt/TiO 2 nano-dimensional grain/SiC Schottky diode based sensor were measured with respect to the temperature and free carrier concentration by exposure to hydrogen and propene.

The I-V characteristics of this sensor towards 1% hydrogen at different temperatures (25, 90, 200, 310, 420, 530 and 620ºC) are shown in Figure 6.3. As can be seen from the I-V curves at 25°C, the forward I-V characteristics are similar to the conventional forward bias Schottky diode characteristics. In contrast with conventional Schottky diodes, the breakdown of nanostructured Schottky diodes occurs at a higher voltage as discussed previously in chapter 3. In chapter 5, the characterisation of the anodized TiO2 surface morphology revealed nanodimensional grains comprising of structures with edges and corners. Although these edges and corners of the grains are not sharp and rigid, they still play a role in producing enhanced localized electric fields. Hence, the low breakdown voltage in the sensor I-V characteristics can be attributed to the presence of these edges and corners in the nanostructured TiO2 morphologies.

From the I-V characteristics (Figure 6.3), larger lateral voltage shift in reverse bias mode operation than that in the forward bias operation was observed as expected for a nanostructured Schottky diode. At 25°C, upon the introduction of 1% hydrogen into the


188

chamber, a voltage shift of approximately 200 mV at reverse bias current of 10 µA was measured. However, as hydrogen was purged from the chamber, the sensor response did not recover and return to the baseline.

Figure 6.4 shows the lateral voltage shifts of the sensor upon exposure to 1% hydrogen and 1% propene with respect to temperatures ranging from 25 to 620°C. The sensor was biased at a constant reverse bias current of 10 µA. For hydrogen, the largest lateral voltage shift was observed at 420°C. Voltage shifts of 139, 147 and 98 mV were recorded for 1% hydrogen at 310, 420 and 530°C, respectively. The preferred operating temperature of 420°C was considered for hydrogen sensing. It was found that at lower temperatures (less than 400°C); the sensor response towards propene was not significant enough for a reliable measurement, as higher temperatures were required for dehydrogenation on the catalytic metal surface [1]. The sensor showed the largest voltage shift towards propene at 620°C; hence, the sensor was tested towards propene at the optimum operating temperature of 620°C.

25°C Air 25°C 1% H2 90°C Air 90°C 1% H2

10

200°C Air 200°C 1% H2

8

310°C Air 310°C 1% H2

6

420°C Air 420°C 1% H2

4

530°C Air 530°C 1% H2

-4

-3

2

620°C Air 620°C 1% H2

-2

0 -1

-2 -4 -6 -8 -10

0

1

Current (A)

Voltage (V)

Figure 6.3. I-V characteristics of Pt/TiO2 nano-dimensional grain/SiC Schottky diode based sensor measured at temperatures ranging from 25 to 620°C towards 1% hydrogen.


189

0.16

Voltage shift (V)

1% H2 1% Propene

0.12

0.08

0.04

0.00 0

100

200 300 400 Temperature (C)

500

600

Figure 6.4. Plot of voltage shift of Pt/TiO2 nano-dimensional grain/SiC Schottky diode based sensor as a function of temperature towards 1% hydrogen and 1% propene at a constant reverse bias current of 10 µA.

The I-V characteristics of the sensor towards 1% hydrogen and 1% propene at 420 and 620°C, respectively is shown in Figure 6.5. Using Equation (3.22), the change in reverse barrier height of the sensor was calculated to be 3.94 and 10.53 meV upon exposure to 1% hydrogen at 420C and 1% propene at 620C, respectively.

1.0

Air 1% H2

(b)

0.8

0.8

0.6

0.6

0.4

0.4

0.2

Voltage (V)

1.0

Air 1% Propene

0.2

Voltage (V)

0.0

0.0 -3

-2

-1

-0.2 -0.4 -0.6 -0.8 -1.0

0

1 Current (mA)

-4

2

-5

-4

-3

-2

-1

-0.2 -0.4 -0.6 -0.8 -1.0

0

1

2

Current (mA)

(a)

Figure 6.5. I-V characteristics of Pt/TiO2 nano-dimensional grain/SiC Schottky diode based sensor towards (a) 1% hydrogen at 420°C and (b) 1% propene at 620°C.

Figure 6.6 shows the dynamic responses of the sensor towards 1% hydrogen and 1% propene at 420 and 620°C, respectively. The sensor exhibited voltage shifts of 0.147 and 0.157 V upon


190

exposure to 1% hydrogen at 420°C and 1% propene at 620°C, respectively. The sensor was biased at a constant reverse current of 10 µA. The sensor showed a slightly faster response and recovery to hydrogen than to propene. The response and recovery time for 1% hydrogen at 420°C were 75 and 162 s, respectively while for 1% propene at 620°C, the time were 87 and 183 s, respectively.

(a)

-2.15

1% hydrogen

-2.75

(b)

1% propene

-2.20 Voltage (V)

Voltage (V)

-2.80

-2.85

-2.25

-2.30

-2.90 10 µA constant reverse bias current at 420°C

10 µA constant reverse bias current at 620°C

-2.35

-2.95 0

3

6

9 12 Time (min)

15

18

0

3

6

9 12 Time (min)

15

18

Figure 6.6. Dynamic responses of Pt/TiO2 nano-dimensional grain/SiC Schottky diode based sensor towards (a) 1% hydrogen at 420°C and (b) 1% propene at 620°C with a constant reverse bias current of 10 µA.

6.3.2. SnO2 Nanowires Based Sensor Figure 6.7 shows the I-V characteristics of the developed Pt/SnO2 nanowire/SiC Schottky diode based sensor towards 1% hydrogen at temperatures of 25, 90, 200, 420 and 530°C. As can be seen, the I-V curve in synthetic air at 25°C exhibited a low breakdown voltage. Furthermore, a greater lateral voltage shift was observed in reverse bias than the forward bias towards hydrogen. The SEM analyses presented in chapter 5 revealed that the thermally evaporated SnO2 films comprised of long and thin wires in the upper half of the film and thick and short wires in the lower half of the film. Figure 5.7 showed the short nanowires with rounded tips at the lower half. These nanowires should play role in producing localized


191

electric fields, which cause a lowering reverse barrier height resulting in increase in reverse current density. The reverse barrier height changes of 57.89 and 24.12 meV were calculated using Equation (3.22) for the sensor responding towards 1% hydrogen at 420 and 530°C, respectively.

25°C Air 25°C 1% H2

1.0

90°C Air 90°C 1% H2

0.6

420°C Air 420°C 1% H2

0.4

530°C Air 530°C 1% H2

0.2

-4

-3

-2

0.0 -1 0 -0.2 -0.4 -0.6 -0.8 -1.0

1

2

3

4 5 Voltage (V)

Current (A)

-5

0.8

200°C Air 200°C 1% H2

Figure 6.7. I-V characteristics of Pt/SnO2 nanowire/SiC Schottky diode based sensor measured at temperatures ranging from 25 to 530°C towards 1% hydrogen.

Figure 6.8 shows the plot of the voltage shift magnitude of the sensor towards 0.125%, 0.25% and 0.5% hydrogen as a function of the operating temperature. The sensor was biased at a constant reverse current of 1 µA. The voltage shift increased as the temperature increased and peaks at a maximum at approximately 530°C; then becames smaller for any further increase in the temperature. It was observed that at low concentrations of 0.125% and 0.25%, the difference in voltage shift between 420 and 530°C was not significantly large. Therefore, operating temperatures of 420 and 530°C were chosen.

The dynamic response of the sensor upon exposure to different concentrations of hydrogen at 420 and 530°C with respect to time are shown in Figure 6.9. The sensor was biased at a constant reverse current of 1 µA. At 420°C, voltage shifts of 44, 70, 96 and 132 mV were


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measured for 0.125%, 0.25%, 0.5% and 1% hydrogen, respectively; while the measured voltage shifts at 530°C were 57, 95, 149 and 310 mV for the aforementioned hydrogen concentrations, respectively. At 420°C, poor baseline stability was observed. The response time of 189 and 180 s were measured towards 1% hydrogen at 420 and 530ºC, respectively. In Figure 6.10, a non-linear relation between the voltage shift and hydrogen concentration can be observed at both operating temperatures of 420 and 530°C.

Voltage shift (V)

0.20

0.125% 0.25% 0.5%

0.16

0.12

0.08

0.04 200

300

400 500 Temperature (C)

600

700

Figure 6.8. Plots of voltage shift of Pt/SnO2 nanowire/SiC Schottky diode based sensor measured as a function of temperature towards 0.125%, 0.25% and 0.5% hydrogen at a constant reverse bias current of 1 µA. -1.11

-0.85

(a)

-1.17

-0.95

0.5%

-1.20

Voltage (V)

Voltage (V)

1%

-0.90

-1.14

0.25%

-1.23

0.125%

-1.26 -1.29

(b)

1%

0.5%

-1.00 0.25%

-1.05

0.125%

-1.10 -1.15


0

10

20

30 40 Time (min)

50


0

60

10

20

30 40 Time (min)

50

60

Figure 6.9. Dynamic responses of Pt/SnO2 nanowire/SiC Schottky diode based sensor towards different hydrogen concentrations at (a) 420°C and (b) 530°C at a constant reverse bias current of 1 µA.


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0.32

Voltage shift (V)

0.28

420°C 530°C

0.24 0.20 0.16 0.12 0.08 0.04 0.0

0.2 0.4 0.6 0.8 Hydrogen gas concentration (%)

1.0

Figure 6.10. Plots of voltage shift as a function of hydrogen concentration for Pt/SnO2 nanowire/SiC Schottky diode based sensor at 420°C and 530°C with a constant reverse bias current of 1 µA.

6.3.3. ZnO Nanostructured Based Sensor This section will present the electrical and gas sensing performance of sensors based on ZnO nanostructured Schottky diodes. As mentioned previously, two gas sensing layers with different morphologies were used. They were nanostructured arrays and nanowiresnanoplatelets. As previously shown in chapters 4 and 5, ZnO nanostructured arrays were grown on the epitaxial-GaN layers using a hydrothermal growth method and ZnO nanowiresnanoplatelets were deposited on the SiC substrates via a thermal evaporation technique. The fabricated Pt/ZnO nanostructured array/epitaxial-GaN Schottky diode was tested towards NO2 and the Pt/ZnO nanowire-nanoplatelet/SiC Schottky diode was tested towards hydrogen and propene. The results are shown in detail in the following subsections.

6.3.3.1. ZnO Nanostructured Arrays Based Sensor Figure 6.11 shows the I-V characteristics of the Pt/ZnO nanostructured array/epitaxial-GaN and Pt/epitaxial-GaN Schottky diodes at room temperature. A non-linear I-V characteristic


194

was observed for the Pt/epitaxial-GaN diode. However, by depositing nanostructured ZnO between the Pt and epitaxial-GaN layer, the reverse I-V curves were observed to have almost ohmic behaviour. This indicates that the ZnO nanostructures may have caused a significant reduction in the Schottky barrier height at the interface of Pt and ZnO nanostructured layer in reverse bias, which resulted in a significant increase in the current density. The morphological characterisations of the ZnO nanostructures presented in chapter 5 showed the presence of sharp tips at the ZnO nanowires, which grew as an extension from the tip of the broad nanorods. Therefore, a significant increase in the current density can be attributed to the enhanced localized electric field emanating from the Pt coated sharp tips in the ZnO nanostructures.

1000 Pt/ZnO nanostructured array/epitaxial-GaN Pt/epitaxial-GaN

750 500 250

Voltage (V) -20

0 -16

-12

-8

-4

0

4

-500 -750 -1000

Current (A)

-250

Figure 6.11. I-V characteristics of Pt/ZnO nanostructured array/epitaxial-GaN and Pt/epitaxial-GaN Schottky diodes at 25ºC.

Figure 6.12 shows the I-V characteristics of the Pt/ZnO nanostructured array/epitaxial-GaN Schottky diode based sensor towards 9.9 ppm NO2 at different temperatures of 25, 100, 270 and 350ºC. As can be seen, exposure to NO2 (an oxidising gas) causes a lateral voltage shift in the I-V curves towards larger voltages due to depletion in the carrier concentration at the


195

surface of the Pt/ZnO nanostructures interface. The larger lateral voltage shift in the reverse I-V curves than in the forward can be explained in terms of barrier height change, which results in a smaller reverse barrier than the forward barrier due to the enhanced localized electric field effect as described in chapter 3.

300

25°C Air 25°C 9.9 ppm NO2 100°C Air 100°C 9.9 ppm NO2

200

270°C Air 270°C 9.9 ppm NO2

100

350°C Air 350°C 9.9 ppm NO2

-6

-4

0 -2

0 -100 -200 -300

2 4 Voltage (V) Current (µA)

-8

Figure 6.12. I-V characteristics of Pt/ZnO nanostructured array/epitaxial-GaN Schottky diode based sensor measured at temperatures ranging from 25 to 350°C towards 9.9 ppm NO2.

Figure 6.13 shows the response magnitude (voltage shift) of the sensor measured towards 0.6 and 9.9 ppm NO2 at a constant reverse bias current of 300 µA as a function of temperature. The response gradually increases with increasing temperature, reaches a maximum at 270°C and decreases for any further increase in temperature. Figure 6.14 shows the dynamic response of the sensor towards different NO2 concentrations at 270°C. Voltage shifts of 0.647, 3, 3.5, 4.56 and 5.79 V were obtained upon exposure to 0.6, 1.2, 2.5, 5 and 9.9 ppm NO2, respectively. The response time of 489 s was recorded for 9.9 ppm NO2. It is important to highlight that the sensor response did not recover to the baseline after purging with synthetic air for a long period of 30 min, which could suggest that the desorption of the gas is difficult


196

and requires a very long time for the sensor to return to its initial state. The results show long response and recovery for this sensor. Figure 6.15 shows the plot of voltage shift with respect

Voltage Shift (V)

to NO2 concentration.

0.6 ppm NO2

6.6 6.0 5.4 4.8 4.2 3.6 3.0

9.9 ppm NO2

0.6 0.4 0.2 0.0 100

150 200 250 Temperature (°C)

300

350

Figure 6.13. Plots of voltage shift of Pt/ZnO nanostructured array/epitaxial-GaN Schottky diode based sensor as a function of temperature towards 0.6 ppm and 9.9 ppm NO2 at a constant reverse bias current of 300 µA.

-8 0.6 ppm

Voltage (V)

-10 1.2 ppm

-12

2.5 ppm 5 ppm

-14

9.9 ppm

-16 300µA constant reverse bias current at 270°C

-18 0

50

100 150 Time (min)

200

250

Figure 6.14. Dynamic response of Pt/ZnO nanostructured array/epitaxial-GaN Schottky diode based sensor towards different NO2 concentrations at 270°C with a constant reverse bias current of 300 µA.


197

6

Voltage shift (V)

5 4 3 2 1 0 0

2 4 6 8 NO2 gas concentration (ppm)

10

Figure 6.15. Plot of voltage shift as a function of NO2 concentration for Pt/ZnO nanostructured array/epitaxial-GaN Schottky diode based sensor at 270°C with a constant reverse bias current of 300 µA.

The initial sensor‟s response to NO2 was incredibly large; with a voltage shift of 5.79 V towards 9.9 ppm NO2 at 270°C. However, subsequent to multiple exposures, the shifts were reduced to approximately 3 V and then followed by measurements recording shifts of less than 1 V. The SEM (Figure 5.15) of the ZnO morphology after testing revealed the degradation of the ZnO nanowires. This suggests that the enhanced localized electric field are no longer present as the degraded ZnO nanowire structures, thus proving detrimental to this sensor‟s performance.

6.3.3.2. ZnO Nanowires-Nanoplatelets Based Sensor Figure 6.16 shows the measured I-V characteristics of the fabricated Pt/ZnO nanowirenanoplatelet/SiC Schottky diode based sensor towards 1% hydrogen at temperatures of 25, 200, 310, 420 and 620°C. The sensor exhibited a low breakdown voltage and larger lateral voltage shift towards hydrogen in reverse bias operation than in forward bias due to the sharp edges and corners of the ZnO nanostructures. According to the I-V characteristics, the operating temperature of 620°C was chosen as it exhibited the largest response towards


198

hydrogen. At this temperature, carriers have more energy to conduct over the Schottky barrier resulting in a lateral voltage shift in the I-V characteristics [2-4]. Using Equation (3.22), a barrier height change of 10.81 meV was calculated for 1% hydrogen at reverse bias operation at 620°C. The I-V characteristics of the sensor towards 0.06%, 0.125%, 0.25%, 0.5% and 1% hydrogen were measured at 620°C as can be seen in Figure 6.17.

25°C Air 25°C 1% H2

1.0

200°C Air 200°C 1% H2

0.8

310°C Air 310°C 1% H2

0.6

420°C Air 420°C 1% H2

0.4 0.2

-2.5

-2.0

-1.5

-1.0

0.0 -0.5 0.0 -0.2 -0.4 -0.6 -0.8 -1.0

0.5

1.0 1.5 Voltage (V)

Current (A)

620°C Air 620°C 1% H2

Figure 6.16. I-V characteristics of Pt/ZnO nanowire-nanoplatelet/SiC Schottky diode based sensor measured at temperatures ranging from 25 to 620°C towards 1% hydrogen. Voltage (V) -3.0

-2.5

Air 0.06% H2

-2.0

-1.5

-1.0

-0.5

0 0.0 -5

0.25% H2 0.5% H2 1% H2

-10

Current (A)

0.125% H2

-15

-20

Figure 6.17. Reverse I-V characteristics of Pt/ZnO nanowire-nanoplatelet/SiC Schottky diode based sensor towards different hydrogen concentrations at 620°C.


199

The dynamic response of the sensor towards different concentrations of hydrogen at 620°C is shown in Figure 6.18. The sensor was biased at reverse constant current of 1 µA. Voltage shifts of 0.114, 0.174, 0.213, 0.251 and 0.325 V were recorded for 0.06%, 0.125%, 0.25%, 0.5% and 1% of hydrogen, respectively which verifies the measured voltage shifts from the I-V characteristics. At 620°C, the response and recovery time of the sensor were measured to be 108 and 210 s for 1% hydrogen, respectively. The relationship between voltage shift and the hydrogen concentration for the sensor is shown in Figure 6.19. The plot indicates that the voltage shift increases logarithmically with the increase of hydrogen concentration.

-0.7

1% 0.5% 0.25%

-0.8 Voltage (V)

0.125% 0.06%

-0.9

-1.0


-1.1 0

10

20

30

40 50 Time (min)

60

70

80

Figure 6.18. Dynamic response of Pt/ZnO nanowire-nanoplatelet/SiC Schottky diode based sensor towards different hydrogen concentrations at 620°C with a constant reverse bias current of 1 µA.

The sensor was also tested towards propene at different temperatures ranging from 25 to 620°C. However, the sensor did not show any significant response to propene at temperatures less than 400°C. This is previously explained as high temperatures are required for the dehydrogenation process of propene on the catalytic metal surface. The measured dynamic response (at a constant reverse bias current of 1 µA) shows the largest voltage shift at 620°C.


200

Voltage shifts of 0.087, 0.197 and 0.304 V were recorded upon exposure to 1% propene at 420, 530 and 620°C, respectively.

Voltage shift (V)

0.30

0.25

0.20

0.15

0.10 0.0

0.2

0.4

0.6

0.8

1.0

Hydrogen gas concentration (%)

Figure 6.19. Plot of voltage shift as a function of hydrogen concentration for Pt/ZnO nanowirenanoplatelet/SiC Schottky diode based sensor at 620°C with a constant reverse bias current of 1 µA.

At an operating temperature of 620°C, the dynamic responses of the sensor towards 0.125%, 0.5% and 1% propene were measured as shown in Figure 6.20. The sensor did not show a stable baseline and the response was noisy at low concentrations of 0.125% and 0.5% propene. Voltage shifts of 108, 177 and 304 mV were recorded for 0.125%, 0.5% and 1% propene at constant reverse bias current of 1 µA. The sensor‟s response towards propene was smaller than that towards hydrogen for the same condition. This is most likely due to the rate of dehydrogenation of the propene on the surface, resulting in fewer hydrogen atoms available to diffuse to the interface and cause the lowering of the barrier height [5]. In contrast to the hydrogen response, the relation between the voltage shift and propene concentration was almost linear as observed in Figure 6.21. The sensor exhibited a quicker response towards propene than towards hydrogen whilst the recovery time for propene was larger than for hydrogen. The response and recovery time of the sensor for 1% propene were measured to be 60 and 378 s, respectively at 620°C.


1%

-0.65

-1.25

201

0.5%

-0.70

-1.30

Voltage (V)

Voltage (V)

-0.75 -1.35

0.125%

-1.40

-0.80 -0.85 -0.90

-1.45

-0.95 -1.50

-1.00


-1.55


-1.05 0

5

10

15 20 25 Time (min)

30

35

40

45

0

5

10

15 20 Time (min)

25

30

Figure 6.20. Dynamic responses of Pt/ZnO nanowire-nanoplatelet/SiC Schottky diode based sensor towards different propene concentrations at 620°C with a constant reverse bias current of 1 µA.

Voltage shift (V)

0.30

0.25

0.20

0.15

0.10 0.0

0.2 0.4 0.6 0.8 Propene gas concentration (%)

1.0

Figure 6.21. Plot of voltage shift as a function of propene concentration for Pt/ZnO nanowirenanoplatelet/SiC Schottky diode based sensor at 620°C with a constant reverse bias current of 1 µA.

The sensor‟s performance did not exhibit any measureable electrical changes after continuously testing towards hydrogen and propene (at elevated temperatures) over three week period. However, SEM observation of the nanostructured ZnO layer revealed changes in the morphology after the three week testing period. As seen in Figure 5.21, the nanostructured morphology degraded and many nanoplatelets were cracked and broken on. From the observations, a new morphology was formed, which consisted of sharp corners. The device‟s gas sensing performance can be linked to the localized electric field induced from these sharp


202

edges and corners of the new morphology. The author believes that the ZnO nanostructures morphology deteriorates after long time measurements resulting in the degradation of the sensor‟s sensitivity. Thus, it is suggested that this sensor should be operated at low temperatures below 620°C.

6.3.4. RuO2 Nanostructured Based Sensor The I-V characteristics of the developed Pt/RuO2 nanostructure/SiC Schottky diode based sensor towards 1% hydrogen at different operating temperatures of 25, 110, 160, 200 and 240ºC is shown in Figure 6.22. Due to the presence of nanocubular like structures with sharp edges and corners in the deposited RuO2 layer as shown in chapter 5, this sensor exhibited a low breakdown voltage and larger lateral voltage shift in reverse bias operation than in forward bias. The largest lateral voltage shift was observed at 240°C. Above the 240°C temperature, the I-V curves became almost ohmic.

Voltage (V)

25°C Air 25°C 1% H2

1.0

110°C Air 110°C 1% H2

0.8

160°C Air 160°C 1% H2

0.6

200°C Air 200°C 1% H2

0.4

240°C Air 240°C 1% H2

0.2 0.0

-3

-2

-1

-0.2 -0.4 -0.6 -0.8 -1.0

0

Current (mA)

-4

Figure 6.22. I-V characteristics of Pt/RuO2 nanostructure/SiC Schottky diode based sensor measured at temperatures ranging from 25 to 240°C towards 1% hydrogen.


203

The reverse I-V characteristics of the sensor towards 0.06%, 0.125%, 0.25%, 0.5% and 1% hydrogen at 240ºC is shown in Figure 6.23. Using Equation (3.22), the change in reverse barrier height of the sensor was calculated to be 14.58 meV upon exposure to 1% hydrogen at 240C.

Voltage (V) -1.2

-1.0 Air 0.06% H2

-0.8

-0.6

-0.4

-0.2

0.0 0.0 -0.2

0.25% H2

-0.4

0.5% H2 1% H2

-0.6

Current (mA)

0.125% H2

-0.8

-1.0 Figure 6.23. Reverse I-V characteristics of Pt/RuO2 nanostrucutre/SiC Schottky diode based sensor towards different hydrogen concentrations at 240°C.

Dynamic response of the sensor as shown in Figure 6.24 was measured towards different concentrations (0.06%, 0.125%, 0.25%, 0.5% and 1%) of hydrogen at 240C. The sensor was biased at a constant reverse bias current of 1 mA. The sensor exhibited voltage shifts of 57, 94, 145, 208 and 304 mV upon exposure to the aforementioned concentrations of hydrogen, respectively. The exposure and purge time were 300 and 1200 s, respectively. As can be seen, the sensor response did not fully saturate within the exposure time of 300 s and also did not recover to the baseline after purging with air. Figure 6.25 shows that the voltage shift of this sensor increases logarithmically with increase of hydrogen concentration.


-0.7

1%

-0.8

Voltage (V)

204

0.5%

0.25%

-0.9 0.125% 0.06%

-1.0

-1.1

1 mA constant reverse bias current at 240°C

0

40

80 120 Time (min)

160

200

Figure 6.24. Dynamic response of Pt/RuO2 nanostructure/SiC Schottky diode based sensor towards different hydrogen concentrations at 240°C with a constant reverse bias current of 1 mA.

Voltage shift (V)

0.30 0.25 0.20 0.15 0.10 0.05 0.0


1.0

Figure 6.25. Plot of voltage shift as a function of hydrogen concentration for Pt/RuO2 nanostructure/SiC Schottky diode based sensor at 240°C with a constant reverse bias current of 1 mA.

6.3.5. MoO3 Nanostructured Based Sensor As mentioned previously, nanostructured MoO3 with different morphologies: nanoplatelets, nanoplatelets-nanowires and nano-flowers were used as the gas sensing layers in the Schottky diode structure and tested towards hydrogen. The MoO3 nanostructured films were deposited on SiC substrates via a thermal evaporation method using different deposition parameters as


205

presented in chapter 4. The electrical and gas sensing performance of the developed Pt/MoO3 nanostrucutre/SiC Schottky diodes will be presented in the following subsections.

6.3.5.1. MoO3 Nanoplatelets Based Sensor The I-V characteristics of the Pt/MoO3 nanoplatelet/SiC Schottky diode based sensor towards 1% hydrogen at different temperatures ranging from 25 to 360ºC are shown in Figure 6.26. As previously presented (chapter 5), the MoO3 nanoplatelets consisted of sharp edges and corners, which resulted in a low breakdown voltage and larger reverse bias lateral voltage shift towards hydrogen than for the forward bias. The most significant reverse bias lateral voltage shift towards hydrogen was observed at 180ºC. Figure 6.27 shows the reverse I-V characteristics of the sensor towards 0.06%, 0.125%, 0.25%, 0.5% and 1% hydrogen at 180ºC. At 180ºC, the calculated change in reverse bias barrier height was 5.17 meV upon exposure to 1% hydrogen using Equation (3.22).

25°C Air 25°C 1% H2

-10

-8

-6

80

180°C Air 180°C 1% H2

60

250°C Air 250°C 1% H2

40

360°C Air 360°C 1% H2

20

-4

0 -2

-20 -40 -60 -80 -100

0

Current (A)

Voltage (V)

100

100°C Air 100°C 1% H2

Figure 6.26. I-V characteristics of Pt/MoO3 nanoplatelet/SiC Schottky diode based sensor measured at temperatures ranging from 25 to 360°C towards 1% hydrogen.


Voltage (V) -3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0 0.0 -20

Air 0.06% H2 0.125% H2

-40

0.25% H2 0.5% H2 1% H2

-60

Current (A)

-3.5

206

-80

-100

Figure 6.27. Reverse I-V characteristics of Pt/MoO3 nanoplatelet/SiC Schottky diode based sensor towards different hydrogen concentrations at 180°C.

At 180ºC, the dynamic response of the sensor towards different hydrogen concentrations was measured and shown in Figure 6.28. The sensor was biased at 100 µA reverse constant current. The voltage shifts of 0.275, 0.484, 0.702, 0.914 and 1.343 V were obtained for 0.06%, 0.125%, 0.25%, 0.5% and 1% hydrogen, respectively, which were in good agreement with the voltage shifts at the same bias current in the I-V curves shown in Figure 6.27. The response and recovery time of 40 and 270 s, respectively, were measured at 1% hydrogen. Figure 6.29 shows the voltage shift of the measured response for the sensor with respect to hydrogen concentration. This plot indicates that the voltage shift increases logarithmically with the increase of hydrogen concentration.

1%

-2.4

Voltage (V)

0.5%

-2.8 0.25% 0.125%

-3.2 0.06%


-4.0 0

20

40

60 80 Time (min)

100

120

Figure 6.28. Dynamic response of Pt/MoO3 nanoplatelet/SiC Schottky diode based sensor towards different hydrogen concentrations at 180°C with a constant reverse bias current of 100 µA.


207

1.4

Voltage shift (V)

1.2 1.0 0.8 0.6 0.4 0.2 0.0


1.0

Figure 6.29. Plot of voltage shift as a function of hydrogen concentration for Pt/MoO3 nanoplatelet/SiC Schottky diode based sensor at 180°C with a constant reverse bias current of 100 µA.

Upon multiple exposures to hydrogen at temperatures above 300ºC, the sensor performance deteriorated and no longer exhibited the same response. As presented previously in chapter 5, the SEM (after a series of high temperature testing) revealed the decrease in the dimensions of the platelets, which caused the structures to degrade. These MoO 3 nanostructures dominantly consisted of horizontally oriented plates, which exhibited that electric fields could no longer contribute to the normal component, which explains the loss of the enhanced localized electric fields and degradation in the sensor‟s performance [6]. Although there was presence of vertical standing nanoplatelets deep within the nanostructured morphology as shown in Figure 5.27, it is assumed that they could not exhibit the same enhancement of electric field due to their roundness in morphology as they lack their original sharpness as illustrated in Figure 6.30.


Ambient

m electric field

Ambient

m electric field lines

(a)

208

γ

lines

Localized electric field lines

Pt metal

γ

Localized electric field lines

(b)

Figure 6.30. Illustration of localized electric field lines emanated from MoO 3 nanoplatelets (a) asdeposited and (b) after testing.

6.3.5.2. MoO3 Nanoplatelets-Nanowires Based Sensor The developed Pt/MoO3 nanoplatelet-nanowire/SiC Schottky diode based sensor was tested towards hydrogen at operating temperatures in a range from 25 to 250ºC. A limitation of temperature to 250C was applied as previous results from the Pt/MoO3 nanoplatelet/SiC sensor indicated that the temperatures above 300ºC resulted in the degradation of the nanostructures morphology and the sensor‟s performance.

Figure 6.31 shows the I-V characteristics of the sensor towards 1% hydrogen at temperatures of 25, 100, 170 and 250ºC. This sensor also exhibited low breakdown voltage as well as large lateral voltage shifts towards hydrogen in reverse bias operation due to the presence of sharp edges and corners in the MoO3 nanoplatelets-nanowires morphology as discussed in chapter 5. The sensor showed the most significant reverse bias lateral voltage shift at 170ºC. The reverse I-V characteristics of the sensor towards different hydrogen concentrations (0.06%, 0.125%, 0.25%, 0.5% and 1%) are shown in Figure 6.32. The change in the reverse barrier height for 1% hydrogen at 170°C was calculated to be 11.79 meV using Equation (3.22).


100

25°C Air 25°C 1% H2

80

100°C Air 100°C 1% H2

Voltage (V) -9

-8

-7

-6

-5

60

170°C Air 170°C 1% H2

40

250°C Air 250°C 1% H2

20

-4

-3

209

-2

0 -1 0 -20

Current (A)

-40

1

-60 -80 -100

Figure 6.31. I-V characteristics of Pt/MoO3 nanoplatelet-nanowire/SiC Schottky diode based sensor measured at temperatures ranging from 25 to 250°C towards 1% hydrogen. Voltage (V)

Air 0.06% H2 0.125% H2 0.25% H2

-3

-2

0 -1

0

-20 -40

0.5% H2 1% H2

-60

Current (µA)

-4

-80 -100

Figure 6.32. Reverse I-V characteristics of Pt/MoO3 nanoplatelet-nanowire/SiC Schottky diode based sensor towards different hydrogen concentrations at 170°C.

The measured dynamic response at 170ºC (Figure 6.33) indicated voltage shifts of 0.129, 0.24, 0.456, 0.756 and 1.123 V towards 0.06%, 0.125%, 0.25%, 0.5% and 1% hydrogen, respectively at a constant reverse bias current of 100 µA. The response and recovery time of this sensor was larger than that of the Pt/MoO3 nanoplatelet/SiC sensor. The response and recovery time were 60 and 849 s, respectively, for 1% hydrogen at 170ºC. Figure 6.34 shows


210

the voltage shifts of the sensor as a function of hydrogen concentration. The plot suggests that at low concentrations of less than 0.25%, the voltage shift increases linearly with increase in the hydrogen concentration, while for higher concentrations (of 0.5% and 1%) the gradient of the plot decreases.

1%

-3.9

Voltage (V)

0.5%

-4.2 0.25%

-4.5 0.125%

-4.8

0.06%


-5.4 0

40

80 120 Time (min)

160

200

Figure 6.33. Dynamic response of the Pt/MoO3 nanoplatelet-nanowire/SiC Schottky diode based sensor towards different hydrogen concentrations at 170°C with a constant reverse bias current of 100 µA.

1.2

Voltage shift (V)

1.0 0.8 0.6 0.4 0.2 0.0 0.0


1.0

Figure 6.34. Plot of voltage shift as a function of hydrogen concentration for Pt/MoO3 nanoplateletnanowire/SiC Schottky diode based sensor at 170°C with a constant reverse bias current of 100 µA.


211

6.3.5.3. MoO3 Nano-Flowers Based Sensor Figure 6.35 shows the I-V characteristics of the developed Pt/MoO3 nano-flower/SiC Schottky diode based sensor towards 1% hydrogen at different temperatures of 25, 100, 170 and 250ºC. Significant lateral voltage shifts were observed at 170 and 250°C as the sensor was operated at reverse bias condition. Therefore, the author investigated the gas sensing performance of the sensor at two different operating temperatures of 170 and 250°C. The measured I-V characteristics in reverse bias towards different hydrogen concentrations at these two operating temperatures are shown in Figure 6.36. The change in the barrier height at reverse bias operation for 1% hydrogen were 35.67 and 6.09 meV at 170 and 250°C, respectively (using Equation (3.22)).

10

25°C Air 25°C 1% H2

8

100°C Air 100°C 1% H2

-14

-12

-10

-8

4

250°C Air 250°C 1% H2

2

-6

-4

0 -2

-2 -4 -6 -8 -10

0

2

Current (A)

Voltage (V)

6

170°C Air 170°C 1% H2

Figure 6.35. I-V characteristics of Pt/MoO3 nano-flower/SiC Schottky diode based sensor measured at temperatures ranging from 25 to 250°C towards 1% hydrogen.

As can be seen from the I-V characteristics, the sensor showed a large lateral voltage shift in the order of many volts at a very small bias current of 10 µA. This observation becomes more


212

apparent as the operating temperature increased further. Therefore, the sensor was biased at a constant current of 10 µA.

Voltage (V) -10 Air 0.06% H2

-8

-6

(b)

0 -4

-2

0

-7

-2

-5

-4

-3

0 -2

-1

0 -2

0.125% H2

-6

Current (A)

-4

0.5% H2 1% H2

-6 Air 0.06% H2

0.125% H2 0.25% H2

Voltage (V)

0.25% H2 0.5% H2

-4

1% H2

-6

-8

-8

-10

-10

Current (A)

(a)

Figure 6.36. Reverse I-V characteristics of Pt/MoO3 nano-flower/SiC Schottky diode based sensor towards different hydrogen concentrations at (a) 170°C and (b) 250°C.

Figure 6.37 shows the dynamic response of the sensor at different temperatures of 170 and 250°C at a constant reverse bias current of 10 µA. The sensor exhibited a voltage shift of 3.16 V towards 1% hydrogen at 170°C, which was far larger than that exhibited by the MoO3 nanoplatelets sensor (which a voltage shift of 1.343 V was recorded at 180°C using a larger reverse current bias of 100 µA). The response was also larger than that exhibited by the MoO 3 nanoplatelets-nanowires sensor, which 1.123 V voltage shift was measured with a 100 µA reverse bias current at 170°C. However, the baseline at 170°C was not entirely stable and fluctuated unpredictably during the measurements (Figure 6.37a). This is likely due to the use of such a small bias current. It was also observed that at 170C, the sensor is yet to be fully saturated. Therefore the sensor was operated at 250°C. At this temperature, voltage shifts of 0.406, 0.86, 1.51, 4.176 and 5.7 V were obtained towards 0.06%, 0.125%, 0.25%, 0.5% and 1% hydrogen, respectively (Figure 6.37b). The response and recovery time of 171 and 1323 s were recorded when the sensor was exposed to 1% hydrogen at 250°C. It can be concluded


213

that this sensor exhibited a much slower response in comparison to the two other MoO3 nanostructured based sensors discussed previously. The plot in Figure 6.38 shows the relation between the voltage shift and the hydrogen concentration.

-4.5

(a)

0

1%

-5.0

(b)

1%

-1 0.5%

-2

-6.0

0.5%

Voltage (V)

Voltage (V)

-5.5

-6.5 0.25%

-7.0 0.125%

-7.5

-3 -4 0.25%

-5

0.06%

-8.0 -8.5 -9.0

0.125% 0.06%

-6 -7


0

50

100 150 Time (min)

200

250


-8 0

50

100 150 Time (min)

200

250

Figure 6.37. Dynamic responses of Pt/MoO3 nano-flower/SiC Schottky diode based sensor towards different hydrogen concentrations at (a) 170°C and (b) 250°C with a constant reverse bias current of 10 µA. 6

Voltage shift (V)

5 4 3 2 1 0 0.0


1.0

Figure 6.38. Plot of voltage shift as a function of hydrogen concentration for Pt/MoO3 nanoflower/SiC Schottky diode based sensor at 250°C with a constant reverse bias current of 10 µA.

Table 6.1 presents a summary of the experimental results for the three developed MoO3 nanostructured based sensors towards 1% hydrogen. The MoO3 sensor with nano-flowers


214

morphology exhibited the largest voltage shift upon exposure to hydrogen. The MoO3 nanoflowers film comprises of small nanoplatelets in a clustered form of flowers, which is believed to concentrate a strong localized electric fields in proximity to the clustered areas. However, the MoO3 nano-flowers based sensor showed the largest response and recovery towards hydrogen compared to MoO3 nanoplatelets and nanoplatelet-nanowires based sensors, which is attributed to the small adsorption surface areas of the nanostructures.

Table 6.1. Summary of the experimental gas sensing results for the developed MoO 3 nanostructured Schottky diode based sensors.

Sensor

Voltage Shift (V)

T (°C)

Constant Bias Current ( µA)

Response Time (s)

Recovery Time (s)

Pt/MoO3 nanoplatelet/SiC

1.343

180

-100

40

270

Pt/MoO3 nanoplatelet-nanowire/SiC

1.123

170

-100

60

849

Pt/MoO3 nano-flower/SiC

3.16 5.70

170 250

-10 -10

171

1323

6.3.6. WO3 Nanoplatelets Based Sensor The electrical and gas sensing performance of Pt/WO3 nanoplatelet/SiC Schottky diode based sensors were investigated towards hydrogen at different temperatures ranging from 25 to 300°C. It was observed that the magnitude of the response toward hydrogen reduced as the sensors were subsequently operated at temperatures above 200°C. The SEM characterisations (chapter 5) of the WO3 layer after testing revealed the degradation of the platelets. The author limited the sensing performance of this type of sensor (towards hydrogen) within the operating temperatures between 25 and 200°C.

Figure 6.39 shows the I-V characteristics of the sensors based on the annealed WO 3 nanostructures (acid-etched over a duration of 1 hr (sample A), 2 hrs (sample B) and 3 hrs


215

(sample C)) towards 1% hydrogen at 25, 70, 120 and 200°C. All the sensors exhibited significantly small breakdown voltages in their I-V characteristics at 25°C, while greater lateral voltage shifts with respect to hydrogen were observed in forward bias operation than in the reverse bias. This observation may be attributed to the incomplete conversion of tungsten to WO3 in the underlying thin film (non-porous layer) and/or the presence of protonated metal-oxide (H0.23WO3) (as shown in XRD analysis in chapter 5), which was not observed in any other samples tested.

500

500

400

25°C Air 25°C 1% H2

70°C Air 70°C 1% H2

300

70°C Air 70°C 1% H2

300

120°C Air 120°C 1% H2

200

120°C Air 120°C 1% H2

200

200°C Air 200°C 1% H2

100

200°C Air 200°C 1% H2

100

0 -1

-100 -200 -300 -400 -500

0

0

1

2 3 Voltage (V)

-2

-1

-100

0

-200

Current (A)

-2

400

-300

WO3 nanoplatelets-1 hr (sample A)

-400 -500

Current (A)

25°C Air 25°C 1% H2

1

2 3 Voltage (V)

WO3 nanoplatelets-2 hrs (sample B)

500

25°C Air 25°C 1% H2

400

70°C Air 70°C 1% H2

300

120°C Air 120°C 1% H2

200

200°C Air 200°C 1% H2

100 0

-2

-1

-100 -200 -300 -400 -500

0

1 Current (A)

-3

2

3 4 Voltage (V)

WO3 nanoplatelets-3 hrs (sample C)

Figure 6.39. I-V characteristics of Pt/WO3 nanoplatelet/SiC Schottky diode based sensors (acid-etched for 1 hr (sample A), 2 hrs (sample B) and 3 hrs (sample C)) measured at temperatures ranging from 25 to 200°C towards 1% hydrogen.


216

Figure 6.40 shows the forward I-V characteristics of the sensors towards 1% hydrogen at 200°C. Upon exposure to 1% hydrogen at 200°C, the forward barrier height change (∆B) was calculated as 32.64, 73.43 and 97.91 meV (using Equation (3.16)), for the samples A, B and C, respectively. As seen in Figure 6.40, the greatest lateral voltage shift was observed for the sample C. This is due to the available surface area of the WO3 nanostructures in the sample C, which is greater than in the other two sensors, resulting in largest sensitivity towards hydrogen. In addition, the thicker upper nanoplatelet layer [7, 8] and higher percentage of W converted to WO3 in sample C; as confirmed by SEM and XRD analyses (shown in chapter 5) may be responsible for the recorded larger voltage shift.

500

Current (µA)

400

300 Air

200

1% H2

Air 1% H2

100

Air

0 0.0

1% H2

0.5

1.0

1.5

2.0 2.5 Voltage (V)

(WO3 nanoplatelets-1 hr, sample A) (WO3 nanoplatelets-1 hr, sample A) (WO3 nanoplatelets-2 hrs, sample B) (WO3 nanoplatelets-2 hrs, sample B) (WO3 nanoplatelets-3 hrs, sample C) (WO3 nanoplatelets-3 hrs, sample C) 3.0

3.5

4.0

Figure 6.40. Forward I-V characteristics of the Pt/WO3 nanoplatelet/SiC Schottky diode based sensors (acid-etched for 1 hr (sample A), 2 hrs (sample B) and 3 hrs (sample C)) towards 1% hydrogen at 200°C.

The dynamic responses of these three sensors towards 1% hydrogen at 200°C are shown in Figure 6.41. The sensors were biased at constant forward current of 500 µA. The voltage shifts of 0.45, 0.93 and 2.37 V were recorded for samples A, B and C, respectively. The


217

results verified that sample C exhibited the greatest voltage shift upon exposure to hydrogen as previously shown in the I-V characteristics.

The response time of 1239, 760 and 660 s were measured for samples A, B and C, respectively, as they were exposed to 1% hydrogen for 30 min at 200°C. It was observed that the response of sample C was faster than the other two sensors as it provides larger surface

Voltage (V)

0.93 V 0.45 V

area for hydrogen adsorption than the other sensors.

1% H2 (WO3 nanoplatelets-1 hr, sample A)

2.37 V

1% H2 (WO3 nanoplatelets-2 hrs, sample B) 1% H2 (WO3 nanoplatelets-3 hrs, sample C)

0

20

40

60 Time (min)

80

100

Figure 6.41. Dynamic responses of Pt/WO3 nanoplatelet/SiC Schottky diode based sensors (acidetched for 1 hr (sample A), 2 hrs (sample B) and 3 hrs (sample C)) towards 1% hydrogen at 200°C with a constant forward bias current of 500 µA.

The author chose to study the sample C as it exhibited the largest sensitivity as well as the fastest response as compared with the other sensors. The I-V characteristics of the sample C towards 0.06%, 0.125%, 0.25%, 0.5% and 1% hydrogen were performed at 200°C as shown in Figure 6.42. It can be observed that as the concentration of hydrogen increases, the lateral voltage shift in the I-V curves increases, which can be due to greater free carrier concentration in WO3 as well as a lowering of barrier height as the concentration of hydrogen increases.


218

Figure 6.43 shows the dynamic response of the sample C towards different hydrogen concentrations at 200°C with a constant forward bias current of 500 µA. The voltage shifts of 0.14, 0.31, 0.61, 1.283 and 2.37 V were measured for 0.06%, 0.125%, 0.25%, 0.5% and 1% hydrogen, respectively. Figure 6.44 shows a plot of voltage shift as a function of hydrogen concentration for the sample C. It can be seen that the voltage shift increases almost linearly with the increase of hydrogen concentration.

500

Current (A)

400

300 Air 0.06% H2

200

0.125% H2 0.25% H2

100

0.5% H2 1% H2

0 0.0

0.5

1.0

1.5 2.0 Voltage (V)

2.5

3.0

3.5

Figure 6.42. Forward I-V characteristics of Pt/WO3 nanoplatelets-3 hrs/SiC Schottky diode based sensor (sample C) towards different hydrogen concentrations at 200°C. 3.5 0.06%

0.125% 0.25%

0.5%

1%

Voltage (V)

3.0 2.5 2.0 1.5 1.0 0.5

500 µA constant forward bias current at 200°C

0

50

100

150 200 Time (min)

250

300

Figure 6.43. Dynamic response of Pt/WO3 nanoplatelets-3 hrs/SiC Schottky diode based sensor (sample C) towards different hydrogen concentrations at 200°C with a constant forward bias current of 500 µA.


219

2.5

Voltage shift (V)

2.0 1.5 1.0 0.5 0.0 0.0


1.0

Figure 6.44. Plot of voltage shift as a function of hydrogen concentration for Pt/WO3 nanoplatelets3 hrs/SiC Schottky diode based sensor (sample C) at 200°C with a constant forward bias current of 500 µA.

6.3.7. Graphene-like Nano-Sheets Based Sensor The electrical characteristics of the Pt/graphene-like nano-sheet/SiC and Pt/SiC devices were measured in a synthetic air environment at room temperature to study the properties of the graphene-like layer in this type of device (Figure 6.45). As can be seen in the I-V characteristics, without the presence of the graphene-like layer, the Pt/SiC Schottky diode exhibits a distinctive rectifying (forward bias) and large breakdown (reverse bias) characteristics. With the presence of the graphene-like layer, the I-V curve significantly altered to be more ohmic-like and differs most notably from the rectifying and breakdown behaviour, which is in agreement with the theory discussed in chapter 3.

The I-V characteristics of the Pt/grapheme-like nano-sheet/SiC device were measured upon exposure to 1% hydrogen at temperatures between 25 and 100°C (Figure 6.46). The experiments were limited to low temperature of 100°C as the thermogravimetric analysis (TGA) indicated that the graphene material began to lose mass and structure as the temperature increases beyond this point. The graphene mass loss was almost 20% at 100°C


220

and major weight loss occurred following heating at approximately 200°C and higher [9]. The largest lateral voltage shift at a constant bias current of 1 mA was observed at 100°C.

1.00

Pt/graphene-like nano-sheet/SiC Pt/SiC

0.75 0.50 0.25

Voltage (V)

-0.50 -0.75 -1.00

1

2

Current (mA)

0.00 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 -0.25

Figure 6.45. I-V characteristics of Pt/graphene-like nano-sheet/SiC and Pt/SiC devices in synthetic air at room temperature.

25C Air 25C 1% H2 36C Air 36C 1% H2 70C Air 70C 1% H2 100C Air 100C 1% H2

-2.5

-2.0

-1.5

-1.0

0.8 0.6 0.4 0.2

0.0 -0.5 0.0 -0.2 -0.4 -0.6 -0.8 -1.0

0.5

1.0

Current (mA)

Voltage (V)

1.0

Figure 6.46. I-V characteristics of Pt/graphene-like nano-sheet/SiC based sensor measured at temperatures ranging from 25 to 100°C towards 1% hydrogen.


221

Upon exposure to 1% hydrogen at 100°C and a constant current of 1 mA, lateral voltage shifts of 0.07 and 0.1 V were recorded for the forward and reverse biased sensor, respectively. The electrical and gas sensing performance of the sensor was investigated under reverse bias operation as a result of the larger voltage shift towards hydrogen. As previously discussed in chapter 3, a gradual increase in hydrogen concentration causes a gradual decrease in the potential step. This behaviour can be seen in the I-V characteristics of the sensor towards 0.06%, 0.125%, 0.25%, 0.5% and 1% hydrogen at 100°C under reverse bias operation (Figure 6.47).

Votage (V) -1.5

-1.2 Air 0.06%H2

-0.9

-0.6

-0.3

0.0 0.0 -0.2

0.25%H2

-0.4

0.5%H2 1%H2

-0.6

Current (mA)

0.125%H2

-0.8

-1.0

Figure 6.47. Reverse I-V characteristics of Pt/graphene-like nano-sheet/SiC based sensor towards different hydrogen concentrations at 100°C.

The dynamic response of the sensor towards different concentrations of hydrogen was measured at 100°C with two different constant reverse bias currents of 100 µA and 1 mA. Figure 6.48 shows the voltage shift of the sensor towards hydrogen as a function of time. Voltage shifts of approximately 12, 29 and 63 mV were recorded for 0.25%, 0.5% and 1% hydrogen, respectively at a constant reverse bias current of 100 µA. The sensor exhibited response time of 150 s towards 1% hydrogen, while recovery time was 120 s. Under a reverse bias current of 1 mA, voltage shifts of approximately 20, 47 and 100 mV were measured for


222

0.25%, 0.5% and 1% hydrogen, respectively. At this condition, the sensor exhibited a faster response (144 s), whilst a similar recovery time of 120 s was observed upon exposure to 1% hydrogen. The magnitude of the response for the sensor towards different hydrogen concentrations at 100°C under constant reverse bias currents of 100 µA and 1 mA are shown Figure 6.49. The voltage shifts increase linearly with respect to the increase in hydrogen concentrations.

1%

-0.30 0.5%

Voltage (V)

-0.33 0.25%

0.125%

-0.36

100 A constant reverse bias current at 100°C

-0.39

1%

-1.29 0.5%

-1.32 -1.35

0.25%

0.125%

-1.38


-1.41

0

15

30 45 Time (min)

60

75

Figure 6.48. Dynamic response of the Pt/graphene-like nano-sheet/SiC based sensor towards different hydrogen concentrations at 100°C with different constant reverse bias currents of 100 µA and 1 mA.

Voltage shift (V)

0.10

100 µA 1 mA

0.08 0.06 0.04 0.02 0.00 0.0

0.2

0.4 0.6 0.8 Hydrogen gas concentration (%)

1.0

Figure 6.49. Plot of voltage shift as a function of hydrogen concentration for Pt/graphene-like nanosheet/SiC based sensor at 100°C with constant reverse bias currents of 100 µA and 1 mA.


223

The electrical characteristics of the Pt/graphene-like nano-sheet/SiC device were also investigated towards NO2 at temperatures ranging from 25 to 100°C. Figure 6.50 shows the I-V characteristics of the device towards 9.9 ppm NO2 at 25, 36, 70 and 100°C. The sensor was most sensitive towards the NO2 at 70°C. Under constant bias current of 1 mA, voltage shifts of 80 and 60 mV were recorded in the reverse and forward bias operation, respectively.

The dynamic response of the sensor was measured towards different concentrations of NO2 at an optimum temperature of 70°C, at a constant reverse bias current of 1 mA. The voltage change upon exposure to 0.6, 1.2, 2.5, 5 and 9.9 ppm NO2 as a function of time is shown in Figure 6.51. It was observed that the sensor exhibited very small voltage shifts toward these concentrations of NO2 at 70°C and the sensor response did not return to the baseline after purging the NO2.

36C air 36C 9.9 ppm NO2 70C air 70C 9.9 ppm NO2 100C air 100C 9.9 ppm NO2

-2.5

-2.0

-1.5

-1.0

1.0 0.8 0.6 0.4 0.2 0.0 -0.5 0.0 -0.2 -0.4 -0.6 -0.8 -1.0

0.5

1.0 1.5 Voltage (V)

Current (mA)

25C air 25C 9.9 ppm NO2

Figure 6.50. I-V characteristics of Pt/graphene-like nano-sheet/SiC based sensor measured at temperatures ranging from 25 to 100°C towards 9.9 ppm NO2.


-1.784

224

0.6 ppm 1.2 ppm

-1.785 Voltage (V)

2.5 ppm

-1.786

5 ppm

-1.787

9.9 ppm

-1.788 -1.789


-1.790 0

20

40

60 80 Time (min)

100

120

140

Figure 6.51. Dynamic response of the Pt/graphene-like nano-sheet/SiC based sensor towards different NO2 concentrations at 70°C with a constant reverse bias current of 1 mA.

6.4. Summary This chapter presented the experimental set-up utilized and outcomes of the measurements obtained for the gas sensors testing by the author at RMIT University, Australia. Two main types of sensors were investigated: (1) Pt/nanostructured metal-oxide based Schottky diodes and (2) Pt/graphene-like nano-sheet/SiC devices. The electrical and gas sensing properties of these devices were characterised and studied in detail. Table 6.2 summarises the gas sensing measurements for the sensors that have been investigated in this research.

Pt/Nanostructured Metal-Oxide Based Schottky Diodes

The fabricated Pt/nanostructured metal-oxide Schottky diodes were tested towards reducing/oxidizing gases. The key outcomes from the electrical and gas sensing characteristics results can be summarised as follows:



The measured room temperature I-V characteristics of the Pt/nanostructured metaloxide based Schottky diodes exhibited a much lower reverse breakdown voltage as


225

compared to conventional Schottky diodes, which is in agreement with the theory discussed in chapter 3. The results are attributed to enhanced localized electric fields emanating from the edges and corners of the high aspect ratio nanostructures. These enhanced localized electric fields respectively lower the Schottky barrier height energy at the interface between Pt and the metal-oxide to allow greater charge carrier flow.

Table 6.2. Summary of the experimental gas sensing results for the developed nanostructured based sensors. Sensor

T (°C)

Analyte Gas (in air)

Voltage Shift

Response Time (s)

Ref.

Pt/TiO2 nano-dimensional grain/SiC

420 620

1% H2 1% propene

0.147 V at -10 µA 0.157 V at -10 µA

75 87

[10]

Pt/SnO2 nanowire/SiC

420 530

1% H2 1% H2

0.132 V at -1 µA 0.310 V at -1 µA

189 180

[11]

Pt/ZnO nanostructured array/GaN

270

9.9 ppm NO2

5.790 V at -300 µA

489

_

Pt/ZnO nanowire-nanoplatelet/SiC

620 620

1% H2 1% propene

0.325 V at -1 µA 0.304 V at -1 µA

108 60

[12]

Pt/RuO2 nanostructure/SiC

240

1% H2

0.304 V at -1 mA

279

[13]

Pt/MoO3 nanoplatelet/SiC

180

1% H2

1.343 V at -100 µA

40

[14]

Pt/MoO3 nanoplatelet-nanowire/SiC

170

1% H2

1.123 V at -100 µA

60

_

Pt/MoO3 nano-flower/SiC

250

1% H2

5.700 V at -10 µA

171

[15]

Pt/WO3 nanoplatelets-3 hrs/SiC

200

1% H2

2.370 V at +500 µA

660

[16]

Pt/graphene-like nano-sheet/SiC

100 100 70

1% H2 1% H2 9.9 ppm NO2

0.063 V at -100 µA 0.100 V at -1 mA 0.080 V at -1 mA

150 144 -

[17] [18]



For all the reverse biased Pt/nanostructured metal-oxide based Schottky diodes excluding the Pt/WO3 nanoplatelet/SiC, larger lateral voltage shifts were observed than for the forward biased diodes. Upon exposure the sensor to the target gas, the effect of change in free carrier density (ND) at the Pt/metal-oxide interface is amplified by the enhancement factor (Equation (3.20)) causing a respective increase in the


226

electric field. Therefore, these enhanced localized electric fields decrease the barrier height (Equation (3.19)), resulting in a large lateral voltage shift in the reverse I-V characteristics. 

For the Pt/WO3 nanoplatelet/SiC Schottky diodes, the greater lateral voltage shifts were observed in forward bias operation than in the reverse bias due to the incomplete conversion of tungsten to WO3 in the underlying thin film (non-porous layer) and/or the presence of protonated metal-oxide (H0.23WO3) (as shown in XRD analysis in chapter 5), which was not observed in any other samples tested.



From these investigations, the MoO3 nanostructured based Schottky diodes were found to have the largest sensitivity comparing to other nanostructured metal-oxide based Schottky diodes. These sensors also showed stable baseline and fast response.



MoO3 nanoplatelets, MoO3 nanoplatelet-nanowires and MoO3 nano-flowers based Schottky diode sensors were tested towards hydrogen at different temperatures of 25 to 250°C. The Pt/MoO3 nano-flower/SiC sensor exhibited the highest sensitivity. A significant large voltage shift of 5.7 V was recorded for the Pt/MoO3 nano-flower/SiC sensor upon exposure to 1% hydrogen at 250°C. However, faster response was observed for the Pt/MoO3 nanoplatelet/SiC and Pt/MoO3 nanoplatelet-nanowire/SiC sensors than the Pt/MoO3 nano-flower/SiC sensor. It is concluded that the MoO3 nanostructured based sensors are promising candidates for gas sensing applications at low temperatures below 300°C.



Pt/TiO2 nano-dimensional grain/SiC sensor showed voltage shifts of 0.174 and 0.157 V for 1% hydrogen at 420ºC and 1% propene at 620ºC, respectively. The small voltage shifts can be attributed to that the edges and corners of the grains are not sharp and rigid. The morphology of the TiO2 nanostructures remained intact after several


227

exposures towards hydrogen and propene at elevated temperatures. Hence, this sensor is suitable for gas sensing applications at elevated temperatures up to 620ºC. 

Voltage shifts of 0.132 and 0.31 V were recorded at 420 and 530ºC, respectively upon exposure of the Pt/SnO2 nanowire/SiC sensor to 1% hydrogen. The presence of short wires with tips at the bottom layer of the deposited SnO2 nanowires could contribute to the response at the reverse bias condition. This sensor is also suitable for high temperature environments as the nanowires morphology and the sensor‟s performance did not degrade upon several exposures to hydrogen at elevated temperatures.



Comparison between the I-V characteristics of the Pt/epitaxial-GaN and Pt/ZnO nanostructured array/epitaxial-GaN Schottky diodes exhibited ohmic-like (linear) behaviour for the Pt/ZnO nanostructured array/epitaxial-GaN diode. A large voltage shift of 5.79 V was measured for 9.9 ppm NO2 at 270ºC. However, poor baseline stability and slow response was observed. The sensor‟s performance degraded upon multiple exposures at elevated temperatures, which the degradation of nanowires was confirmed by the SEM investigations after testing.



Hydrogen and propene sensing performance of the Pt/ZnO nanowire-nanoplatelet/SiC Schottky diode were investigated at the elevated temperature of 620ºC under reverse bias operation. The response towards propene was slightly smaller than towards hydrogen. The SEM observation of the nanostructures after high temperature testing revealed the changes in the nanostructures morphology and broken platelets in some regions. The author believes that the sensor should operate at low temperatures below 620ºC.



The Pt/RuO2 nanostructure/SiC sensor was tested towards hydrogen at operating temperatures between 25 and 240C. Above the 240°C temperature, the I-V curves


228

became almost ohmic. At 240C, The sensor exhibited a low magnitude response (voltage shift of 304 mV) towards 1% hydrogen at 1 mA constant reverse bias current. This behaviour can be attributed to the low surface to volume ratio of the deposited nanostructured layer as well as the presence of mixed RuO2 and Ru phases. 

The sensors based on the annealed WO3 nanostructures (acid-etched for durations of 1 hr (sample A), 2 hrs (sample B) and 3 hrs (sample C)) were tested towards hydrogen at different temperatures ranging from 25 to 300°C. It was observed that the response to hydrogen was deteriorated after the sensors were operated at temperatures above 200°C due to degradation of the nanoplatelets. Hence, operating temperatures below 200°C were chosen for these sensors. At 200°C, the largest voltage shift obtained for sample C. Voltage shifts of 0.45, 0.93 and 2.37 V were recorded towards 1% hydrogen for samples A, B and C, respectively. The WO3 nanoplatelets based sensors showed very long response and recovery.

Pt/Graphene-like Nano-Sheet/SiC Device

The electrical characteristics of the Pt/graphene-like nano-sheet/SiC and Pt/SiC devices were investigated. Ohmic-like behaviour was observed for the Pt/graphene-like nano-sheet/SiC as compared to Pt/SiC device. The Pt/graphene-like nano-sheet/SiC device was tested towards hydrogen and NO2. At 100ºC, a voltage shift of 100 mV was recorded towards 1% hydrogen at a constant reverse current bias of 1 mA; while the response of the sensor towards NO2 was not significant. Therefore, these devices could sense selectivity hydrogen at the presence of NO2.

It is the author‟s opinion that the MoO3 nanostructured based Schottky diodes are the best candidates for hydrogen sensing applications compared to the other nanostructured metal-


229

oxide based Schottky diodes investigated throughout this research program. Although, the Pt/ZnO nanostructured arrays/epitaxial-GaN and Pt/WO3 nanoplatelet/SiC sensors showed high sensitivity, their response was very slow (489 and 660 s, respectively). In addition, poor baseline stability was observed for the Pt/ZnO nanostructured arrays/epitaxial-GaN sensor.


230

References

[1]

B. P. Luther, S. D. Wolter, and S. E. Mohney, "High temperature Pt Schottky diode gas sensors on n-type GaN," Sensors and Actuators B, vol. 56, pp. 164-168, 1999.

[2]


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T.-H. Tsai, H.-I. Chen, K.-W. Lin, Y.-W. Kuo, C.-F. Chang, C.-W. Hung, L.-Y. Chen, T.-P. Chen, Y.-C. Liu, and W.-C. Liu, "SiO2 passivation effect on the hydrogen adsorption performance of a Pd/AlGaN-based Schottky diode," Sensors and Actuators B: Chemical, vol. 136, pp. 338-343, 2009.

[4]

K. Matsuo, N. Negoro, J. Kotani, T. Hashizume, and H. Hasegawa, "Pt Schottky diode gas sensors formed on GaN and AlGaN/GaN heterostructure," Applied Surface Science, vol. 244, pp. 273-276, 2005.

[5]


[6]

J. Yu, M. Shafiei, W. Wlodarski, Y. X. Li, and K. Kalantar-Zadeh, "Enhancement of electric field properties of Pt/nanoplatelet MoO3/SiC Schottky diode," Journal of Physics D-Applied Physics, vol. 43, p. 025103, 2010.

[7]

A. Z. Sadek, H. Zheng, M. Breedon, V. Bansal, S. K. Bhargava, K. Latham, J. Zhu, L. Yu, Z. Hu, P. G. Spizzirri, W. Wlodarski, and K. Kalantar-zadeh, "High-temperature anodized WO3 nanoplatelet films for photosensitive devices," Langmuir, vol. 25, pp. 9545-9551, 2009.

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231

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M. Shafiei, J. Yu, M. Breedon, R. B. Kaner, K. Galatsis, K. Kalantar-zadeh, and W. Wlodarski, "Pt/MoO3 nano-flower/SiC Schottky diode based hydrogen gas sensor," in Proceedings of IEEE Sensors Conference, Waikoloa, HI, USA, 2010, pp. 354-357.

[16]

M. Shafiei, A. Z. Sadek, J. Yu, K. Latham, M. Breedon, K. Kalantar-zadeh, and W. Wlodarski, "A hydrogen gas sensor based on Pt/nanostructured WO3/SiC Schottky," Sensor Letters, vol. 9 (1) pp. 11-15, 2011.

[17]

M. Shafiei, R. Arsat, J. Yu, K. Kalantar-zadeh, S. Dubin, R. B. Kaner, and W. Wlodarski, "Pt/graphene nano-sheet based hydrogen gas sensor," in Proceedings of IEEE Sensors Conference, Christchurch, New Zealand, 2009, pp. 295-298.

[18]


Chapter 7 Conclusions and Future Work The research reported in this PhD thesis, was a comprehensive investigation of novel nanostructured thin film Schottky diode based gas sensors. The author developed Pt/nanostructured semiconducting metal-oxide/(on SiC and epitaxial-GaN substrates) and Pt/graphene-like nano-sheet/SiC devices and investigated their sensing performance towards different gases (hydrogen, propene and NO2) at elevated temperatures. The author sought to investigate the gas sensing potential of such devices as well as to explain and analyse the theory and gas sensing mechanisms of the developed sensors. Based on the results presented in this thesis, the electrical properties, gas sensing performance and gas sensing mechanism of the nanostructured thin film Schottky diode sensors were presented and analysed in detail. The research has been highly successful and the results have been published in refereed high impact factor international journals and presented at international conferences. These include 12 publications in: Journal of Physical Chemistry C, Sensor Letters, Sensors & Actuators B: Chemical, Journal of Physics D: Applied Physics, Thin Solid Films, Applied Physics Letters, Surface Interface Analysis, Sensors & Transducers Journal and International Journal on Smart Sensing & Intelligent Systems. The author‟s work has also resulted in 13 publications in

Chapter 7: Conclusions and Future Work

233

prestigious international conference proceedings. A complete list of publications by the author can be found in Appendix B.

The results presented in this thesis can be summarised as follows:



Synthesis and development of novel morphologies of nanostructured semiconducting metal-oxides including: TiO2 nano-dimensional grains, SnO2 nanowires, ZnO nanostructured arrays, ZnO nanowires-nanoplatelets, RuO2 nanostructures, MoO3 nanoplatelets, MoO3 nanoplatelets-nanowires, MoO3 nano-flowers, WO3 nanoplatelets and graphene-like nano-sheets on SiC and epitaxial-GaN substrates.



Development and fabrication of Schottky diodes incorporating the above gas sensing nanostructured thin films. To the best of the author‟s knowledge, most of the devices and structures were reported for the first time.



Structural and material characterisations of the nanostructures employing scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and atomic force microscopy (AFM) techniques.



Morphological analyses of nanostructured materials after the testing.



Electrical measurements (current-voltage, I-V) of each sensor and analysis of their gas sensing performance.



Theoretical evaluation of the nanostructured Schottky diode based sensors as well as detailed explanation of their gas sensing mechanism.

In the following sections, the author will present a summary of the major findings in this research and her recommendations for future work.


234

7.1. Conclusions This PhD research encompassed the theoretical analysis and the results of the electrical properties and gas sensing performance of the sensors (Pt/nanostructured semiconducting metal-oxides and Pt/graphene-like nano-sheet devices). The experimental results were in excellent agreement with the theoretical analysis. A summary of the major outcomes in this research can be summarized as follows:

Pt/Nanostructured Metal-Oxide Based Schottky Diodes 

To the best of the author‟s knowledge, for the first time the Pt/TiO2 nano-dimensional grains/SiC, Pt/SnO2 nanowires/SiC, Pt/ZnO nanostructured arrays/epitaxial-GaN, Pt/ZnO

nanowires-nanoplatelets/SiC,

Pt/RuO2

nanostructures/SiC,

Pt/MoO3

nanoplatelets/SiC, Pt/MoO3 nanoplatelets-nanowires/SiC, Pt/MoO3 nano-flowers/SiC and Pt/WO3 nanoplatelets/SiC sensors were successfully fabricated and investigated for gas sensing applications. 

The morphological investigation (via SEM) of the as-deposited nanomaterials revealed sharp edges and corners in many of the nanostructures morphology, which contributes immensely in increasing the sensitivity of the developed sensors.



The electrical characteristics (current-voltage, I-V) of sensors were measured in the presence and absence of target gas (hydrogen, propene and NO2 in synthetic air) at different selected temperatures ranging between 25 and 620°C. In an environment of room temperature and ambient air, the measured I-V characteristics of the Pt/nanostructured metal-oxide based Schottky diodes exhibited a much lower breakdown voltage compared to conventional metal-oxide Schottky diodes. For the first time, the author and her colleagues showed that this is attributed to the enhanced


235

localized electric fields induced in the proximity to the sharp edge and corner morphologies of the nanostructures. These enhanced localized electric fields respectively lower the Schottky barrier height energy at the interface between the Pt and the metal-oxide to allow greater charge carrier flow. 

The sensitivity (a change in voltage at a constant reverse bias current) of these sensors was studied as a function of different concentrations of hydrogen and propene (up to 1% in synthetic air) and NO2 (up to 9.9 ppm in synthetic air) gas. From the measured I-V characteristics of these sensors towards a target gas, a larger lateral voltage shift in reverse bias mode operation was observed over that in forward bias. This result is attributed to the sharp edges and corners of the nanostructures. The only exception to this behaviour was found in WO3 nanoplatelets based sensors, which the author attributed to the incomplete conversion of tungsten to WO3 in the underlying thin film (non-porous layer) and/or the presence of protonated metal-oxide (H0.23WO3), which was not observed in any other samples tested. Some of the key properties of the studied sensors are shown in Table 6.2.



The sensors based on nanostructured MoO3 were found to have the highest sensitivity with excellent baseline stability and a fast response as compared to the other sensors investigated in this research. A voltage shift of 5.7 V at 250°C was recorded for the Pt/MoO3 nano-flower/SiC sensor upon exposure to 1% hydrogen at 250°C under a constant reverse bias current mode of operation at 10 µA. To the best of the author‟s knowledge, such a large voltage shift has not been reported previously in literature.


236

Pt/Graphene-like Nano-Sheet/SiC Device 

For the Pt/graphene-like nano-sheet/SiC devices, a theoretical description was developed and gas sensing performance of Pt/graphene-like nano-sheet/SiC device was investigated for the first time.



Raman characterisation revealed that the deposited graphene material contained defects and was heterogeneous in nature comprising of both graphitic and graphene phases. The graphene material ranged in thickness from ~8 to ~30 graphene layers. The XPS spectrum indicated the absence of the C=C bonds. The author ascribed this to an incomplete reduction of GO to graphene has occurred during the deposition process.



Measured I-V characteristics of the Pt/graphene-like nano-sheet/SiC device in an air environment and room temperature exhibited a much more ohmic-like behaviour as compared to Pt/SiC based devices. The results can be attributed to the semi-metal like behaviour of graphene layer as it operates between Pt and SiC substrate.



The developed graphene-like nano-sheet based sensor was tested towards hydrogen and NO2. At 100ºC, a voltage shift of 100 mV was recorded towards 1% hydrogen at a constant reverse current bias of 1 mA; while the response of the sensor towards NO2 was not significant. Therefore, these devices could sense selectivity hydrogen at the presence of NO2.

In summary, the author has successfully accomplished the research objectives set out in chapter 1. The research on nanostructured thin film Schottky diode based sensors has involved many experimental and theoretical aspects. This research resulted in a number of significant contributions to the field of nanostructured semiconducting metal-oxide and graphene based sensors. Several key outcomes have been identified and the results and


237

findings have been respectively published in highly reputed referred journals and proceedings of international conferences. Furthermore, the author was invited to referee for the Journal of Sensors and Sensors and Actuators B during the course of this PhD program.

7.2. Future Work This PhD research has presented some major advancement in the field of nanostructured thin film Schottky diodes for gas sensing applications. Throughout the course of this research, several areas of interest, which has tremendous research potential, have been identified. Therefore, the author highlights the future recommendations to researchers whom seek to follow with these investigations:



The author believes this work can be extended into investigating the origins of the electric field enhancement at the metal-nanostructured contacts. Within this research, the results showed an enhancement in the gas sensitivity of the sensors based on nanostructured Schottky diodes due to strong local electric fields induced at the metal coated sharp edges and corners in the nanostructures‟ morphologies. Therefore, understanding the factors responsible for the enhancement of the electric fields can provide the information needed for the development of devices for maximum performance.



Surface modifications of the gas sensing layers (nanostructured materials) using catalytic metal (such as Pt or Pd) can improve gas sensitivity of the nanostructured Schottky diode sensors as the catalytic metals enhance the target gas dissociation onto the gas sensitive layer.




238

Application of different device geometries such as different size of Schottky contacts can alter the sensor sensitivity and shall be investigated.



Optimisation of these nanostructured Schottky diode structures, especially aimed at long term stability, was not possible in the time frame of this thesis. Therefore, such research will assist in establishing the durability of these sensors.



The sensors based on RuO2 nanostructures exhibited a smaller sensitivity towards hydrogen relative to the other sensors based on metal-oxide materials. This result is attributed to the low surface to volume ratio of the RF sputtered nanostructured layers as well as the presence of mixed RuO2 and Ru phases. The author believes that the sensing performance of these devices can be improved by further modifications in the gas sensing layer structure in order to make it more porous with sharp edges and corners as well as making the nanostructured layers fully oxidized.



The Pt/graphene-like nano-sheet/SiC based sensor exhibited a small sensitivity towards hydrogen (100 mV at 1 mA constant bias current). The author believes that applying a single layer graphene in this structure could improve the gas sensing performance.

There are still many considerations, which can also enhance the performance of the sensors studied in this research. Some of which are viable to address the requirements for industrial production and use. More research and development is also required to address the stability and repeatability of the synthesis of nanostructures using the methods as addressed in this work. The author strongly believes that if the nanostructured based sensors, which are presented in this thesis, are further optimised, their performance will be highly suitable for commercialisation purposes.

Appendix A Silicon Carbide (SiC) and Gallium Nitride (GaN) Wafers Specifications A.1. SiC N-type 6H-SiC conducting wafers used in this PhD research were purchased from different companies: Cree Inc. (USA), Tankeblue Semiconductor Co. Ltd. (China) and Sterling Semiconductor (USA). Their properties are summarized in Table A.1.

Table A.1. Properties of SiC wafers purchased from different companies. Properties

Cree Inc.

Tankeblue Semiconductor

Sterling Semiconductor

Diameter

50.8 ± 0.38 mm

50.8 ± 0.38 mm

50.8 ± 0.4 mm

Thickness

254 µm ± 10%

254 µm ± 10%

254 µm ± 10%

Resistivity

0.02 - 0.2 Ω.cm

0.03 - 0.12 Ω.cm

 0.1 Ω.cm

Dopant

Nitrogen

Nitrogen

Nitrogen

Carrier Concentration

1.2  10 /cm

1  10 /cm

1.56  1018 /cm3

Prototype

6H

6H

6H

Crystal Structure

Hexagonal

Hexagonal

Hexagonal

Micropipe Density

20-50 per cm2

< 100 per cm2

< 100 per cm2

Usable area

80-85%

≥ 90%

≥ 85%

Orientation

On axis (0001) Si face polished



18

3

18

3

Appendix A: Silicon Carbide (SiC) and Gallium Nitride (GaN) Wafers Specifications

240

A.2. GaN The GaN epitaxial layers used in this PhD research were provided by University of Western Australia. The n-type GaN layers were grown on a semi-insulating GaN template (an underlying layer Fe-doped GaN buffer deposited via a metal-organic chemical vapour deposition process) on sapphire (Al2O3) substrates. The layered structure is shown in Figure A.1.

GaN:Si shoot for ND = 4-5E16 cm-3

1.5 µm

GaN undoped; ND = 1-5E16 cm-3

520 nm

cm-3

670 nm

GaN:Fe; approx. NFe = 6E18

Sapphire

0.5 mm

5 mm

ND is the donor doping concentration NFe is the Fe impurity concentration Figure A.1. Schematic representation of epitaxial-GaN layer as grown on sapphire (Al2O3).

Appendix B Author’s Publication List B.1. Refereed Journal Articles

[1] J. Yu, M. Shafiei, E. Comini, M. Ferroni, G. Sberveglieri, K. Latham, K. Kalantarzadeh and W. Wlodarski, “Pt/nanostructured RuO2/SiC Schottky diode based hydrogen gas sensor,” Sensor Letters (2010). (Under review) [2] R. Arsat, X. He, P. Spizzirri, M. Shafiei, K. Kalantar-zadeh and W. Wlodarski, “Hydrogen gas sensor based on highly ordered polyaniline/multiwall carbon nanotubes composite,” Sensor Letters (2010). (Under review) [3] M. Shafiei, A.Z. Sadek, J. Yu, K. Latham, M. Breedon, K. Kalantar-zadeh, and W. Wlodarski, “A hydrogen gas sensor based on Pt/nanostructured WO3/SiC Schottky diode,” Sensor Letters, vol. 9 (1) (2011) pp. 11-15. [4] J. Yu, M. Shafiei, C. M. Oh, T. B. Jung, K. Kalantar-zadeh, J. H. Kang, and W. Wlodarski, “Pt/nanograined ZnO/SiC Schottky diode based hydrogen and hydrocarbon sensor,” Sensor Letters (2010). (In press) [5] M. Shafiei, R. Arsat, J. Yu, P. G. Spizzirri, S. Dubin, R. B. Kaner, J. du Plessis, K. Kalantar-zadeh and W. Wlodarski, “Platinum/graphene nano-sheet/SiC contacts and their application for hydrogen gas sensing,” Journal of Physical Chemistry C, vol. 114 (32) (2010) pp. 13796-13801.


242

[6] M. Shafiei, J. Yu, R. Arsat, K. Kalantar-zadeh, E. Comini, M. Ferroni, G. Sberveglieri and W. Wlodarski, ”Reversed bias Pt/nanostructured ZnO Schottky diode with enhanced electric field for hydrogen sensing,” Sensors & Actuators B: Chemical, vol. 146 (2010) pp. 507-512. [7] J. Yu, M. Shafiei, W. Wlodarski, Y. X. Li, K. Kalantar-zadeh, “Enhancement of electric field properties of Pt/nanoplatelet MoO3/SiC Schottky diode,” Journal of Physics D: Applied Physics, vol. 43 (2010) p. 025103 (8pp). [8] M. Breedon, Th. Kehagias, M. Shafiei, K. Kalantar-zadeh, W. Wlodarski, “ZnO nanostructures grown on GaN epitaxial layer,” Thin Solid Films, vol. 518 (4) (2009) pp. 1053-1056. [9] M.B. Rahmani, S. H. Keshmiri, M. Shafiei, K. Latham, W. Wlodarski, J. du Plessis and K. Kalantar-zadeh, “Transition from n- to p-type of spray pyrolysis deposited Cu doped ZnO thin films for NO2 sensing,” Sensor Letters, vol. 7 (2009) pp. 621-628. [10] R. Arsat, M. Breedon, M. Shafiei, P. G. Spizziri, S. Gilje, R. B. Kaner, K. Kalantarzadeh and W. Wlodarski, “Graphene-like nano-sheets for surface acoustic wave gas sensor applications,” Chemical Physics Letters, vol. 467 (2009) pp. 344–347. [11] J. Yu, S. J. Ippolito, M. Shafiei, D. Dhawan, W. Wlodarski, and K. Kalantar-zadeh, “Reverse biased Pt/nanostructured MoO3/SiC Schottky diode based hydrogen gas sensors,” Applied Physics Letters, vol. 94 (2009) p. 013504 (3pp). [12] M. Shafiei, K. Kalantar-zadeh, M. Kocan, G. Parish, J. Antoszewski, L. Faraone, and W. Wlodarski, “'Pt/GaN Schottky diode for propene (C3H6) gas sensing,” Sensors & Transducers Journal (ISSN 1726-5479), vol. 98 (11) (2008) pp. 38-44. [13] M. Shafiei, K. Kalantar-zadeh, W. Wlodarski, E. Comini, M. Ferroni, G. Sberveglieri, S. Kaciulis and, L. Pandolfi, “Hydrogen gas sensing performance of Pt/SnO 2 nanowires/SiC MOS devices,” International Journal on Smart Sensing & Intelligent Systems, vol. 1 (3) (2008) pp. 771-783. [14] S. Kaciulis, L. Pandolfi, S. Bianchi, E. Comini, G. Faglia, M. Ferroni, G. Sberveglieri, M. Shafiei, S. Kandasamy, and W. Wlodarski, “Nanowires of metal oxides for gas sensing applications,” Surface Interface Analysis, vol. 40 (2008) pp. 575-578.B.2.


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B.2. Reviewed Conference Proceedings

[1] M. Shafiei, J. Yu, M. Breedon, A. Moafi, R. Kaner, K. Galatsis, K. Kalantar-zadeh and W. Wlodarski, “Pt/MoO3 nano-flower/SiC Schottky diode based hydrogen gas sensor,” in Proceedings of IEEE Sensors Conference, Waikoloa, HI, USA (2010) pp. 354357. [2] A. Moafi, M. Shafiei, A. Z. Sadek, D. W.M. Lau, K. Kalantar-zadeh, W. Wlodarski and D. G. McCulloch, “A hydrogen sensor based on oriented graphitic carbon,” in Proceedings of IEEE Sensors Conference, Waikoloa, HI, USA (2010) pp. 378-381. [3] J. Yu, G. Chen, C. X. Li, M. Shafiei, K. Kalantar-zadeh, P.T. Lai, W. Wlodarski, “Pt/RF sputtered tantalum oxide Schottky diode based hydrogen sensors,” Procedia Engineering, vol. 5 (2010) pp. 147-151. [4] M. Shafiei, A. Z Sadek, J. Yu, K. Kalantar-zadeh, W. Wlodarski, “Pt/TiO2 nanotubes/SiC for hydrogen gas sensing applications,” in Proceedings of IEEE, International Conference on Nanoscience and Nanotechnology (ICONN) Conference, Sydney, Australia (2010). (Under review) [5] J. Yu, M. Shafiei, M. Breedon, K. Kalantar-zadeh, and W. Wlodarski, “Comparison studies of forward and reverse biased Pt/nanotextured ZnO/SiC Schottky diode based hydrogen sensor,” Procedia Chemistry, vol. 1 (2009) pp. 979-982. [6] M. Shafiei, R. Arsat, J. Yu, K. Kalantar-zadeh, S. Dubin, R.B. Kaner, W. Wlodarski, “Hydrogen gas sensing from Pt/graphene nano-sheets Schottky diode,” in Proceedings of IEEE Sensors Conference, Christchurch, New Zealand (2009) pp. 295-298. [7] M. Shafiei, J. Yu, R. Arsat, K. Kalantar-zadeh, E. Comini, M. Ferroni, G. Sberveglieri and W. Wlodarski, “Reverse biased Schottky contact hydrogen sensors based on Pt/nanostructured ZnO/SiC,” in Proceedings of American Institute of Physics (ISOEN Conference, Brescia, Italy), vol. 1137 (2009) pp. 353-356. [8] J. C. W. Yu, M. Shafiei, C. Ling, W. Wlodarski, K. Kalantar-Zadeh, “Pt/ZnO/SiC thin film for hydrogen gas sensing,” Smart Structures, Devices and Systems IV; Proceedings of SPIE-The International Society for Optical Engineering, Melbourne, Australia, vol. 7268 (2008) pp. 72680L-10. [9] M. Shafiei, A. Z. Sadek, J. Yu, R. Arsat, X. F. Yu, J. G. Partridge, K. Kalantar-zadeh and W. Wlodarski, “Pt/anodised TiO2/SiC based MOS device for hydrocarbon gas


244

sensing,” Smart Structures, Devices and Systems IV; Proceedings of SPIE-The International Society for Optical Engineering, Melbourne, Australia, vol. 7268 (2008) pp. 72680K-8. [10] R. Arsat, M. Breedon, M. Shafiei, K. Kalantar-zadeh, S. Gilje, R.B. Kaner, F.J. Arregui, W. Wlodarski, “Graphene-like nano-sheets/36° LiTaO3 surface acoustic wave hydrogen gas sensor,” in Proceedings of IEEE Sensors Conference, Leece, Italy (2008) pp. 188-191. [11] R. Arsat, M. Breedon, M. Shafiei, K. Kalantar-zadeh, S. Gilje, R.B. Kaner and W. Wlodarski, “Graphene-like nano-sheets based LiTaO3 surface acoustic wave NO2 gas sensor,” in Proceedings of 22nd International Eurosensors Conference, Dresden, Germany (2008) pp. 1132-1135. [12] M. Shafiei, W. Wlodarski, K. Kalantar Zadeh, L. Pandolfi, E. Comini, S. Bianchi, S. Kaciulis and G. Sberveglieri, “Pt/SnO2 nanowires/SiC based hydrogen gas sensor,” in Proceedings of IEEE Sensors Conference, Leece, Italy (2007) pp. 166-169. [13] S. Bianchi, E. Comini, M. Ferroni, G. Sberveglieri, L. Pandolfi, S. Kaciulis, W. Wlodarski, M. Shafiei and S. Kandasamy, “Preparation and characterization of tin oxide nanowires on SiC,” in Proceedings of Transducers Conference- The 14th International Conference on Solid-State Sensors, Actuators and Microsystems, Lyon, France (2007) pp. 183-186.