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Republic of Iraq Ministry of Higher Education & Scientific Research University of Baghdad College of Education for Pure Science Ibn Al-Haitham

Study of Photodetector Properties ZnTe:Al/Si prepared by Thermal Evaporation A thesis Submitted to the college of education for pure science ( Ibn AlHaitham) / Baghdad University in partial fulfillment of the requirements for the Degree of Doctor of Philosophy in Physics By

Hanan Kadhem Hassun (B.Sc. 1997) (M.Sc. 2006) Supervised by:

Prof. Dr. Samir A. Maki 2017 A.D.

1438 A.H.

Supervisor Certification I certify that this thesis, entitled " Study of Photodetector Properties ZnTe:Al/Si prepared by Thermal Evaporation " was prepared by Hanan Kadhem Hassun under my supervision at the Physics Department in the College of education for pure science (Ibn Al-Haitham), University of Baghdad as partial requirement for the degree of Doctor of Philosophy in Physicsal of Science.

Signature: Name: Dr. Samir A. Maki Title: Professor Date:

/ / 2017

Recommendation the head of the Department of Physics: In view of the available recommendations, I forward this thesis for debate by the examination committee.

Signature: Name: Dr. Kareem Ali Jasim Title: professor Address: Chairman of the Department of Physics, College of Education for pure science ( Ibn Al-Haitham), University of Baghdad. Date:

/ / 2017

Certification We certify that we have read this thesis, entitled " Study of Photodetector Properties ZnTe:Al/Si prepared by Thermal Evaporation " and as examination committee, we examined the student " Hanan Kadhem Hassun " on its contents, and that in our opinion it is adequate for the partial fulfillment of requirements for the Degree of Doctor of philosophy of science in Physics. Signature Name: Dr. Ziad T. Al-Dahan Title: Professor (Chairman) Date: / / 2017 Signature Name: Dr. Raad M.S. Al-Haddad Title: Professor (Member) Date: / / 2017

Signature Name: Dr. Ramiz A.M. Al-Anssri Title: Assistant professor (Member) Date: / / 2017

Signature Name: Dr. Ibrahim R.Agool Title: Professor (Member) Date: / / 2017

Signature Name: Dr. Iman Hameed Khudayer Title: Assistant professor (Member) Date: / / 2017

Signature Name: Dr. Samir A. Maki Title: Professor (Supervisor) Date: / / 2017

Signature: Name: Dr. Khalid Fahd Ali Title: Professor Address: The Dean of College of Education for Pure Science Ibn Al-Haitham /University of Baghdad. Date: / / 2017

.

Thanks to God " Allah " who helped me to accomplish this work which I hope will serve our community.

I am grateful to the college of education for pure science (Ibn AlHaitham) university of Baghdad and special thanks go to the physics department for their help in completing my studies. I would like to express my deep gratitude and appreciation to my supervisor Prof. Dr. Samir A. Maki for suggesting the topic of the thesis, continuous advice and their guidance throughout this work. I am grateful to the staff of thin film laboratory, especially Dr. Alia A.A. Shehab. I would like to thank Dr. Ayad A. Salih , Dr. Ali H.A. Alrazak and Mohamed H. Mustafa for their advice in this study. In the way to the completion of this thesis, my teachers, colleagues, and all friends contributed in different ways. At this moment I am very thankful to all of them. Finally, my special thanks, great appreciation and sincere gratitude goes to my family for all their patience, great efforts and facilities offered to make this thesis possible.

Abstract In this thesis (ZnTe) alloy prepared in an evacuated quartz tube, XRD analysis show that alloy (powder) was polycrystalline and have cubic structure. (ZnTe) film of different thickness (400, 450 , 500)±20nm were prepared by thermal evaporation technique with a deposition rate about (1.2∓0.1) nm/sec. The films have been doped with Aluminum (Al) with different doping ratios (0.05,0.1,0.15,0.2,0.25)%. X-ray diffraction results show that the structure of all films are polycrystalline and have cubic structure with a preferred orientation along (111) plane for all ZnTe films. It was found the FWHM of the prominent peaks decreases with increasing thickness. Also, the prominent peaks shift to large 2θ value and intensity of all peaks rapidly decreases with the Al percentage increases. In addition to, the results of atomic force microscope (AFM) show all films have a homogeneous distribution of grains, and the roughness of the surface was decreased with increase of thickness and the surface roughness increased with Al percentage ratio increased. The optical measurement showed that the nature of the optical transition was been direct allowed and the transmittance value decreases with increase of Al doping concentration which subsequently increase absorption, the optical energy gap for all films decreases with increase film thickness and the optical band gap could be controlled by Aldoping. In addition, the absorption coefficients and the optical constant were calculated as a function of photon energy. The study of electrical properties for thin films including two mechanisms of activation energy and the electrical conductivity increased when the thickness and Al-dopant increased. The results of Hall effect show that all films were (p-type), and the concentration of the charge carriers and the carriers mobility increases with increase thickness and Al-dopant.

The (C-V) measurement have shown that the heterojunction were of abrupt type. The built in potential and the depletion width increases with increasing of thickness and Al doping ratio. In addition, the I-V characteristics of ZnTe /Si heterojunctions show the forward dark current varies with applied voltage and the dark current increases while the saturation current and the ideality factor decrease with increasing of thickness. Also, under reverse bias condition the value of dark current for Al-doped ZnTe/Si heterojunction is slightly less than dark current for pure heterojunctions. The photoelectric properties indicated an increase illumination current of heterojunctions with increasing both of incident illumination intensity and Al-dopant. The values of specific detectivity and quantum efficiency shift to red shift with increase of thickness, and the highest value of the spectral response reached for{ZnTe: Al (0. 2%)/Si} at {666, 673 and 746}nm for thickness {400, 450 and 500}nm, respectively. Also, it has been observed from the spectral responsively results that the photodetectors are sensitive for visible and near IR regions, and the best spectral response occurs when thickness equal to 500 nm and Al doping ratio 0.2%, which has a peak response about (0.475A/W) at (746nm) wavelength. It can be concluded that this value of thickness and doping is the optimum condition for prepared ZnTe photovoltaic detector.

Table of contents Contents

Pag. No

Table of Contents

I

List of Tables

IV

List of Figures

V

Symbols & Abbreviations

VIII

Chapter One 1-1 1-2 1-3 1-3-1 1-3-2 1-3-3 1- 4 1-5

General Concept of ZnTe

Chapter Two 2-1 2-2 2-3 2-4 2-5 2-5-1 2-5-2 2-5-3 2-5-4 2-5-5 2-5-6 2-6 2-6-1 2-6-2 2-6-3 2-6-4 2-6-5 2-7 2-7-1 2-7-2 2-8 2-9 2-9-1 2-9-2

1 1 2 3 3 5 6 12

Introduction General Concept of ZnTe Basic Properties Zinc and Tellurium Properties Zinc Tellurium The Properties of Aluminum Review of Literatures The Aim of Work

Theoretical Part

Introduction Semiconductors Doping Methods Doping by Thermal Diffusion Method Diffusion Process Structural Properties X-Ray Diffraction Lattice Constants Average Crystallite Size Dislocation Density Number of Crystals Atomic Force Microscopy Optical Properties Transmittance Absorbance Absorption Coefficient Electronic transitions and the Band Gap Optical Constants Electrical Properties of Semiconductors D.C Conductivity Hall Effect Heterojunctions Abrupt Heterojunctions Abrupt Anisotype Heterojunction Abrupt Isotype Heterojunction

I

13 13 14 14 16 16 18 18 18 18 19 19 20 20 20 21 23 24 24 25 26 28 28 31

2-10 2-10-1 2-10-2 2-11 2-12 2-12-1 2-12-2 2-13 2-13-1 2-13-2 2-13-3 2-13-4 2-13-5

Electrical Properties of Heterojunctions Current-Voltage (I-V) Characteristics Capacitance-Voltage Characteristics Optoelectronic properties of heterojunction Photodetectors Thermal Detectors Photon Detectors Photodetector Parameters Responsivity Quantum Efficiency Noise Equivalent Power Detectivity Response Time

Chapter Three 3-1 3-2 3-3 3-4 3-4-1 3-4-2 3-5 3-6 3-6-1 3-6-2 3-6-3 3-7 3-8 3-8-1 3-8-2 3-9 3-9-1 3-9-2 3-10 3-11

Experimental Part

Introduction Preparation of ZnTe Alloy Vacuum Thermal Evaporation System Samples Preparation Substrates Preparation and Cleaning Evaporation of Thin Films Thickness Measurement of Thin films Structural Measurements X-Ray Diffraction Atomic Force Microscope Measurements Energy Dispersive X-ray Spectrometer Optical Measurements Electrical Measurements Electrical Conductivity Measurement Hall Effect Measurement The Electrical Measurement of ZnTe /SiHeterojunction Current-Voltage Measurements in the Dark state Capacitance-Voltage Measurements in the Dark state Current-Voltage Measurement under Illumination Photodetector Measurements

Chapter Four 4-1 4-2 4-2-1 4-2-2 4-2-3 4-2-4 4-3 4-3-1 4-3-2

32 32 34 35 36 37 38 40 40 40 41 41 42 43 43 45 47 47 48 49 50 50 51 51 51 52 52 52 53 53 54 54 55

Results And Discussion

Introduction Structural Properties X-Ray diffraction Results of ZnTe Alloy X-Ray diffraction Results of ZnTe Thin Films Atomic Force Microscope Results Elemental Composition results Optical Properties Transmission Spectrum Absorbance Spectrum and absorption coefficient

II

56 56 56 58 64 70 70 70 73

78 81 85 85 86

4-3-3 4-3-4 4-4 4-4-1 4-4-2 4-4-3

The Optical Energy Gap Optical Constant The Electrical Properties The Effects of Thickness on D.C Conductivity The Effects of Doping on D.C Conductivity Hall Effect for ZnTe Films in Different Thickness

4-4-4 4-5 4-5-1

4-7

Hall Effect for ZnTe Films in Different Doping Ratio Electrical Properties of ZnTe /Si Heterojunctions Effect of Thickness on C-V Characteristic of ZnTe /Si Heterojunctions Effect of Doping on C-V Characteristic of ZnTe /Si Heterojunctions Effect of Thickness on I-V Characteristic of ZnTe /Si Heterojunctions under Dark Effect of Doping on I-V Characteristic of ZnTe /Si Heterojunctions under Dark Optoelectronic Properties of ZnTe/ Si Heterojunction Effect of Thickness on Illuminated I-V Characteristics for ZnTe/Si Heterojunction Effect of Doping on Illuminated I-V Characteristics for ZnTe/Si Heterojunction ZnTe /Si Detectors Measurements

4-7-1

Spectral Measurements at Different Thickness

114

4-7-2 4-8 4-9 4-10

Spectral Measurements at Different Al doping ratio Response Time and Carrier Life Time Conclusions Suggestions for Future Work References Appendix

118 124 125 127 128 137

4-5-2 4-5-3 4-5-4 4-6 4-6-1 4-6-2

III

90 91 93 93 95 98 100 104 104 106 114

LIST OF TABLES Table no.

Title

Page no.

Table. (1-1)

Some properties of II–VI compounds……………

2

Table. (1-2)

Some properties of Zinc and Tellurium………….

3

Table. (1-3)

Shows Physical and Chemical properties of ZnTe ...

5

Table. (4-1)

The X-ray diffraction parameters of ZnTe alloy…...

57

Table. (4-3)

The elemental composition concentration of alloy (EDS) ………………………………… Optical band gap and optical constant at λ ≈ 600 nm of ZnTe films …………………………..

Table. (4-4)

The activation energy for thin ZnTe films for different thickness…………………………….

Table. (4-2)

Table. (4-5) Table. (4-6) Table. (4-7)

The activation energy for ZnTe films for different dopant ratio….. Values of Co, W, NA, and Vbi, for ZnTe/Si hetrojunection …………….. the saturation current and ideality factor for ZnTe/ Si heterojunctions at different thickness. ………..

70 84 86 89 94 99

Table. (4-8)

The saturation current and ideality factor for ZnTe/ Si heterojunctions with different doping ratio at different thickness……………………..

101

Table. (4-9)

Gain factor for ZnTe/Si heterojunctions with different Al doping ratio and for different thickness…………………………

113

Table. (4-10)

the values of Rλ, η % and D* for ZnTe/Si detectors

118

IV

LIST OF FIGURES Figure no.

Titles

Page no.

Fig. (1-1)

The lattice structure of Zinc telluride crystal

4

Fig. (2-1)

Thermal diffusion techniques for doping

14

Fig. (2-2)

diagram of Bragg reflection from crystalline lattice planes.

17

Fig. (2-3)

three regions A, B and C of the absorption edge

22

Fig. (2-4)

The Optical Transitions, direct and indirect band gap semiconductor

23

Fig. (2-5)

Geometry for measuring the Hall Effect.

25

Fig. (2-6)

Energy-band diagram for two semiconductors

27

Fig. (2-7)

Energy band diagrams. (a) For two isolated semiconductor (b) For n-p heterojunction at equilibrium

29

Fig. (2-8)

Energy-band diagrams for ideal (a) n-n and (b) p-p isotype heterojunctions at thermal equilibrium Forward bias characteristics of a p-Ge/n-GaAs heterojunction at different temperatures The capacitance –voltage characteristic for n-p heterojunction

31

37

Fig. (2-12)

Relative spectral response for a photo detector and a thermal detector Processes of photoconductive for semiconductor

Fig. (2-13)

relation of the rise time with light output power

42

Fig. (3-1)

Schematic for experimental work.

44

Fig. (3-2)

Illustration of the thermal evaporation system

46

Fig. (3-3)

The most commonly used materials for evaporating sources

47

Fig. (3-4)

Circuit diagram for I-V measurement in the dark state.

53

Fig. (3-5)

Circuit diagram for I-V measurement under illumination.

54

Fig. (3-6)

Diagram for measuring spectral responsivity.

55

Fig. (4-1)

XRD patterns for ZnTe alloy

57

Fig. (4-2)

XRD patterns for thin ZnTe films at different thicknesses.

58

Fig. (4-3)

X-ray diffraction pattern of Al doped ZnTe thin films for 400nm thickness and in different ratio. X-ray diffraction pattern of Al doped ZnTe thin films for 450nm thickness and in different ratio X-ray diffraction pattern of Al doped ZnTe thin films for 500nm

60

Fig. (2-9) Fig. (2-10) Fig. (2-11)

Fig. (4-4) Fig. (4-5)

V

33 34

39

60 61

Fig. (4-6) Fig. (4-7) Fig. (4-8) Fig. (4-9) Fig. (4-10) Fig. (4-11) Fig. (4-12)

thickness and in different ratio Effect of doping concentration on crystallite size with different thickness. Effect of doping concentration on lattice constant, planes spacing, and crystallite size. Effect of doping concentration on dislocation density and the number of crystals. Surface Morphology images of ZnTe thin films for (400, 450 and 500) nm. Surface Morphology images of ZnTe thin films for (400 nm) with different Al dopant Percentage ratio Surface Morphology images of ZnTe thin films for (450 nm) with different Al dopant Percentage ratio. Surface Morphology images of ZnTe thin films for (500 nm) with different Al dopant Percentage ratio

61 62 63 65 66 67 68

Surface roughness and grain size change as a function of dopant percentage ratio for Al with different thickness . Optical transmittance spectra as a function of wavelength for ZnTe thin films with different thickness Transmittance spectra of Pure and Al doped ZnTe thin films for different thickness Absorption spectra of ZnTe thin films with different thicknesses

69

74

Fig. (4-21)

The absorption coefficient of ZnTe thin films with different thicknesses. Absorption spectra of Pure and Al doped ZnTe thin films for different thickness The absorption coefficient of Pure and Al doped ZnTe thin films for different thickness Variation of (αhv)2 with photon energy of ZnTe thin films with different thicknesses (ahυ) 2 versus hυ of Pure and Al doped at different thickness.

Fig. (4-22)

Energy gap versus of Pure and Al doped at different thickness

81

Fig. (4-23)

The refractive index change with Pure and Al doped at different thickness The extinction coefficient change with Pure and Al doped at different thickness. The real (ε1) part of the dielectric constant change with Pure and Al doped at different thickness. The imaginary (ε2) part of the dielectric constant change with Pure and Al doped at different thickness. Ln σ versus 1000/T for ZnTe films at different thickness.

82

Ln σ versus 1000/T for ZnTe films at different thickness and different ratio of Al Variation of carrier concentration and mobility as a function of thickness for ZnTe films Variation of carrier concentration and mobility as a function of

88

Fig (4-13) Fig. (4-14) Fig. (4-15) Fig. (4-16) Fig. (4-17) Fig. (4-18) Fig. (4-19) Fig. (4-20)

Fig. (4-24) Fig. (4-25). Fig. (4-26) Fig. (4-27) Fig. (4-28) Fig. (4-29) Fig. (4-30)

VI

71 72 73

75 77 79 80

82 83 83 85

90 92

Fig. (4-31)

Fig. (4-32)

Fig. (4-33) Fig. (4-34) Fig. (4-35) Fig. (4-36)

Fig. (4-37)

Fig. (4-38) Fig. (4-39) Fig. (4-40) Fig. (4-41) Fig. (4-42) Fig. (4-43) Fig. (4-44) Fig. (4-45) Fig. (4-46) Fig. (4-47) Fig. (4-48)

doping ratio for ZnTe films with thickness(400,450,500)nm [a- Variation of capacitance as a function of reverse bias voltage bthe variation of 1/C2 as a function of reverse bias voltage] for ZnTe/Si heterojunction at different thickness Variation of capacitance as a function of reverse bias voltage for ZnTe/Si heterojunction with different doping ratio at different thickness Variation of 1/C2 as a function of reverse bias voltage for ZnTe/Si heterojunction with different doping ratio at different thickness I-V Characteristic of ZnTe /Si heterojunctions under dark with different thickness ln (J) versus V for forward bias of dark of ZnTe/ Si heterojunction at different thickness I-V characteristics under dark for ZnTe/Si heterojunction at forward and reverse bias voltage with different Al doping ratio at different thickness A semi logarithmic of the forward dark current versus V for fabricated ZnTe/ Si heterojunction with different doping ratio at different thickness. I-V characteristics for ZnTe/Si heterojunction with different thickness at different incident power density I-V characteristics for ZnTe/Si heterojunction at thickness 400nm with different doping ratio and different incident power density I-V characteristics for ZnTe/Si heterojunction at thickness 450nm with different doping ratio and different incident power density I-V characteristics for ZnTe/Si heterojunction at thickness 500nm with different doping ratio and different incident power density The spectral responsivity for ZnTe/Si detectore with different thickness The quantum efficiency for ZnTe/Si detectore with different thickness The specific detectivity for ZnTe/Si detectore with different thickness The spectral responsivity for ZnTe/Si detectors with different Al doping ratio at different thickness The quantum efficiency for ZnTe/Si detectors with different Al doping ratio at different thickness. The specific detectivity for ZnTe/Si detectors with different Al doping ratio at different thickness The response time and carrier lifetime for different thickness

VII

94

96

97 99 99 102

103

105 108 110 112 115 116 117 120 121 123 124

Symbols & Abbreviations Symbol a A S Bz C C' Co C-V d hkl

D D* E Ea Eg F h hkl hv Id If Iph In IR Is I-V kB k L n n' p q R' R RH t T' T

Description

Units

Lattice constant Absorbance Film cross section area Magnetic field in the z-direction Junction capacitance Dopent concentration Capacitance at zero bias voltage Capacitance-Voltage Inter planer spacing diffusion coefficient specific detectivity Photon energy Activation energy Energy gap Flux of dopant atoms Plank constant Miller indices Incident photon Dark current Forward current Illumination current noise current Infrared Dark saturation current Current-Voltage Boltzmann constant Extinction coefficient Distance between electrodes Electron concentration Refractive index Hole concentration Electronic charge Resistance Spectral Responsivity Hall coefficient Film thickness Transmittance Absolute temperature

nm cm2 Tesla F/m2

VIII

m-3

F/m2 nm m2/s

cm.Hz-1/2 .W-1 eV eV eV m-3 J.s eV amp amp amp amp amp J.K-1 cm m-3 m-3 C Ohm Amp/W Cm3.C-1 nm Kelvin

V Vbi VH Vr W α χ β λ εr εi

ρ σ

μH

μn μp τresponse τLife ASTM AFM XRD

Applied voltage Built-in potential Hall voltage Reverse bias voltage Total width of depletion region Absorption coefficient Electron affinity Ideality factor Photon wavelength Real part of dielectric constant Imaginary part of dielectric constant Resistivity Electrical conductivity Hall mobility Electron mobility Hole mobility Response time Carrier life time American Society of Testing Materials Atomic Force Microscopy X-Ray Diffraction

IX

volt volt volt volt nm cm-1 eV nm Ωm Ω-1m-1 cm2.V-1s-1 cm2.V-1s-1 cm2.V-1s-1 nm nm -

Chapter one

General Concept of ZnTe

1-1 Introduction: This chapter includes the general introduction in the semiconductor device field , the applications, some of semiconducting properties of II–VI compounds, basic properties Zinc Telluride compound and literatures review for study and fabrication the thin films of Zinc Telluride.

1-2 General Concept of ZnTe: A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. Thin films are applied to modify or enhance the surface of a material or to build functional devices such as light emitting diodes. A small selection of applications where thin films are used are; eyeglasses, microelectronics, solar cells, mirrors, flat screens and windows. The properties of a film and its area of application is, mainly determined by the choice of material [1]. The semiconductor-device field is a relatively new area of study and it has enormous impact on our society and the global economy. Due to the semiconductor devices serve as the foundation of the largest industry in the world. The conductivity of a semiconductor is generally sensitive to temperature, illumination, magnetic field, and minute amounts of impurity atoms, this sensitivity in conductivity makes the semiconductor one of the most important materials for electronic applications [2]. Semiconductors are a class of materials whose electrical properties are intermediate between good metallic conductors and good insulators. They are of enormous practical interest, forming the basis for a wide variety of devices used in electronic circuitry, including diodes, transistors, photocells, particle detectors, and integrated circuits [3]. Semiconductor materials are mostly divided into two large classes: elemental semiconductors (group IV of the periodic table): silicon, germanium,

1

Chapter one General Concept of ZnTe diamond,… etc, and compound semiconductors: IV-IV (SiC), III-V (GaAs, InP, InSb, GaN) and II-VI (CdTe, ZnSe, ZnS, ZnTe, etc.)[4], where the widebandgap II–VI compounds semiconductors are expected to be one of the most vital materials for high-performance optoelectronics devices such as lightemitting diodes (LEDs) and laser diodes (LDs) operating in the blue or ultraviolet spectral range [5]. Zn and Cd are the most important members of this family and are semiconductor with wide-band gaps varying from 3.6 eV(ZnS) to 1.61 eV(CdTe), which are all direct in nature[6], some of semiconducting properties of II–VI compounds are given in table (1-1) [7]. Energy band gap is one of the important properties of the semiconductors which distinguish them from the metals and insulators. In fact, this property determines the wavelength of light which can be absorbed or emitted by the semiconductor. Table (1-1): some properties of II–VI compounds [7]

Compound

Melting Point (ºC)

band gap(e V)

Effective mass m

Mobility (cm2V-1S-1)

ZnS

1830

3.66

0.28

150

ZnSe

1520

2.67

0.17

200

ZnTe

1295

2.26

0.11

100

CdS

1475

2.42

0.20

350

CdSe

1239

1.73

0.13

650

CdTe

1092

1.61

0.14

1200

1-3 Basic Properties: Binary and ternary II–VI compounds presented in wide range of optical and electrical properties, they became an important class of materials competing candidates for silicon and other semiconductors [8], group II-VI compound semiconductors including cadmium selenide (CdSe), zinc selenide (ZnSe), zinc 2

Chapter one General Concept of ZnTe telluride (ZnTe), and cadmium telluride (CdTe) have received significant attention due to their low-cost but high absorption coefficients in their applications to photovoltaic and photoelectrochemical cells [9]. ZnTe with a direct band gap of 2.26 eV at room temperature is excellent material for optoelectronic devices in the visible region of electromagnetic spectra where this wide and direct band property leads to the applications of II–VI group semiconductor in a variety of solid state devices [8].

1-3-1 Zinc and Tellurium Propertie: In this work the Zinc Tellurium (ZnTe ) was selected to fabrication photodetector. The physical and chemical properties of utilized elements is clearly shown in Table (1-2) . Table (1-2) some properties of Zinc and Tellurium Material

zinc

Tellurium

Zn

Te

Density (g/cm )

7.14

6.24

Melting point (K)

692.68

722.66

Boiling point (K)

1180

1261

Atomic number

30

52

Atomic Weight (amu)

65.38

127.60

Crystal structure

hexagonal

hexagonal

Lattice constant A˚

2.660

4.450

Ref

[10],[11]

[12],[13]

Chemistry Symbol 3

1-3-2 Zinc telluride: Zinc telluride is a binary chemical compound with the formula ZnTe. This solid is a semiconductor material with band gap of 2.26eV and it is usually a P-

3

Chapter one General Concept of ZnTe type semiconductor. ZnTe has the appearance of grey or brownish-red powder, or ruby-red crystals, typically had acubic (sphalerite, or "zincblende") crystal structure, but it can be also prepared as hexagonal crystals. Its lattice constant is 0.6101 nm [14]. It can be grown in thin-film polycrystalline form on substrates such as glass [15, 16]. Fig.(1-1) show the lattice structure of Zinc telluride crystal.

Figure (1-1): The lattice structure of Zinc telluride crystal [14]

Table (1-3) shows Physical and Chemical properties of ZnTe. Zinc telluride can be easily doped, and for this reason it is one of the more common semiconducting materials used in optoelectronics.. It is important for development of various semiconductor devices [17], founds applications in green radiation semiconductor lasers for projection systems. Also, the ZnTe crystals are nontoxic and moisture resistant. These qualities make ZnTe, an excellent material for multipurpose radiation detectors. They are used for radiation monitoring, medical and technical tomography and X-ray medical devices [18].

4

Chapter one General Concept of ZnTe Table (1-3): shows Physical and Chemical properties of ZnTe [14, 15, 19] Basic Properties Chemical formula

ZnTe

Lattice constants

a = 6.101 Å

Molecular Weight Crystal structure Dielectric Constant

193.01 Cubic 10.4

Electron Mobility

340 cm2/Vs

Hole Mobility

100 cm2/Vs

Electron affinity χ

3.53 eV

Effective density of state in conduction band Nc

0.22×1019 cm-3

Effective density of state in conduction band Nv

0.078×1019 cm-3

Density

6.34 g/cm3

Melting Point

1295°C

Appearance

red crystals

Refractive Index

3.56

Direct band gap

2.26 eV

1-3-3

The Properties of Aluminum:

Aluminum is the third most abundant element in the Earth's crust (after oxygen and silicon) and its most abundant metal. Aluminum makes up about 8% of the crust by mass. Instead, it is found combined in over 270 different minerals [20]. Aluminum appearance ranging from silvery to dull gray, depending on the surface roughness and have remarkable for the metal's low density and its ability to resist corrosion, also aluminum has about one-third the density and stiffness of steel [21,22], some properties of aluminum are shown in Appendix (1).

5

Chapter one

General Concept of ZnTe

1- 4 Review of Literatures: Many research have been devoted to study and fabrication the thin films or detectors as a function of compound that used to recognize the variation on structure, optical and electrical properties. John V et al. [23 ] in (2002) have prepared the ZnTe:Cu films by an electrochemical method yielded stoichiometric and uniform films. A copper added to the allowed controllable ptype doping of the ZnTe films. The X-ray diffraction studies have not revealed any change in the film structure but the grain size of the Cu-doped film increases (56 nm). The optical studies of the Cu doped films illustrated a decrease in the transmission, an increase in the refractive index and also a slight shift in the optical band gap. The SEM examination presents the globular and flake-like morphology for the as-deposited and Cu-doped ZnTe films. Liang Li et al. [24] in (2005) have been synthesized ZnTe Nanowire by the electrochemical deposition from aqueous solutions into porous anodic alumina membranes, X-ray diffraction analyses showed highly preferential orientation and the scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy indicate that high-filling, ordered, and single-crystalline nanowire arrays have been obtained. The optical absorption spectra of the nanowire arrays show that the optical absorption band edge of the ZnTe nanowire array exhibits a blue shift compared with that of bulk ZnTe. In (2006) Haibin Huo [25] have studied electrical properties of Cu doped p-ZnTe nanowires by used vapour phase transport method , electrical transport measurements showed that the as-grown ZnTe NWs are p-type and very high resistivity. After 30 min immersion in Cu(NO3)2 solution, their conductivity can be increased by about three orders of magnitude. The hole concentrations of the p-type ZnTe nanowires could be controlled in a range from 7.0 × 10 17 to 3.5 × 1018 cm−3 by changing the immersion duration.

6

Chapter one General Concept of ZnTe Vacuum evaporation technique is used in (2007) by Khalid Z. Yahiya et al. [26] to study optical constants of ZnTe thin films in the visible and nearInfrared regions, the optical parameters, such as extinction coefficient, value and type of energy gap, type of the dominant absorption processes, were determined. Their results stated that both the real and complex refractive indices vary fast with photon wavelength until they reach an approximately constant value at NIR wavelengths. Potlog T et al. In (2009) [27] has studied a structure and an optical properties of ZnTe thin films by used close spaced sublimation technique. Structural investigations performed by X-ray diffraction technique showed that studied samples are polycrystalline and have a cubic (zinc blende) structure. XRD patterns have been used to determine the microstructural parameters (crystallite size, lattice parameter) of investigated films. Surface morphology studies SEM showed that the grains are uniformly distributed over the entire surface of the substrate. Optical properties of ZnTe films were studied extensively in the range of incident photon energy (0.5-4.0) eV. In the same year Jayadev Pattar et al. [28 ] have prepared the ZnTe thin films using thermal evaporation method and successfully doped with Indium using ion exchange process. XRD characteristics reveal that ZnTe thin films become crystalline after doping and annealing at 150°C. Optical band gap was calculated from the absorption data. Optical band gap of the doped films decreased with increased in a doping concentrations, which confirms the incorporation of indium atoms into the ZnTe thin films. There was a two order of magnitude enhancement in the conductivity of the doped films. In (2010) Zhong Li et al. [29] have prepared a single nanowire ZnTe photoconductors

by

metal-organic

chemical

vapor

deposition.

Room

temperature I-V characteristics with and without green light ( 530 nm), illumination, at different power density were measured. Single ZnTe NW

7

Chapter one General Concept of ZnTe photodetectors with a record visible wavelength at ( 3 Volt ) bias with a fast carrier lifetime of ( 0.26 –1.3) µs . Furthermore, in the same year Shanying Li, [30] have studied an enhanced p-type conductivity of ZnTe nanoribbons by nitrogen doping by using the thermal evaporation method.

X-ray diffraction peaks of N-doped ZnTe

nanoribbons had an obvious shift toward higher angle direction as compared with intrinsic ZnTe. X-ray photoelectron spectroscopy detection confirmed that the dopant content of nitrogen in ZnTe nanoribbons was close to 1%. Fieldeffect transistors based on both intrinsic and N-doped ZnTe nanoribbons were constructed. Electrical measurements demonstrated that N-doping led to a substantial enhancement in p-type conductivity of ZnTe nanoribbons with a high hole mobility of 1.2 cm−2 V−1 S−1 and a low resistivity of 0.14 Ω cm in contrast to the 6.2 × 10−3 cm−2 V−1 S−1 and 45.1 Ω cm for intrinsic nanoribbons . In (2010) M. S. Hossain et al. [31 ] have prepared (ZnTe:V) thin films of various thicknesses for a particular composition of 2.5wt% V onto glass substrates by evaporation technique in vacuum at a pressure of 8×10 -4 Pa. The deposition rate of the ZnTe:V films was maintained at 2.05 nms -1. The film surface was found dense, smooth and compact in nature by using scanning electron microscopy (SEM) technique. Energy dispersive analysis of x-ray (EDAX) method suggested that the elemental compositions of all samples are non-stoichiometric. The optical parameters of both the as-deposited and annealed ZnTe:V films such as, the Urbach tail energy, optical band gap, refractive index, extinction coefficient and real part of optical dielectric constant were evaluated for different film thicknesses. To explain the enhancement in the photoresponse on the basis of structural and compositional changes due to the substrate temperature and annealing, Gowrish K. R. [32] studied in the same year photoconductivity of vacuum deposited ZnTe as a function of substrate temperature. Detailed analyses were first carried out to understand the effect of substrate temperature and annealing 8

Chapter one General Concept of ZnTe on the structure, composition, optical and electrical properties of the films. The films deposited at elevated substrate temperatures showed faster and improved photoresponse. In (2011) Weichang Zhou et al. [33] have studied and discussed a structure and photoluminescence of pure and indium-doped ZnTe microstructures , the morphology of doped ZnTe microstructures showed multilayered periodical structure, which is due to the dopant ion resulting in lattice mismatch and is different from the morphology of pure ZnTe microstructures. The PL spectra of pure ZnTe microstructures showed band-edge or near band-edge emission under different excitation powers while the PL spectra of doped ZnTe microstructures with different dopant concentrations show infrared emission. The infrared emissions are related to the recombination between In donor levels and acceptor levels at varied doping concentrations, respectively. In the same year for application as high-performance green/ultraviolet photodetector, Y. L. M. Chen et al. [34] fabricated Photodetectors of individual ZnTe nanowires to study photoconductivity of the nanowires by a simple vapor transport and deposition method, the ZnTe nanowire photodetector shows a best sensitivity at wavelength of 500 nm. The ZnTe nanowire photodetector also exhibit a relatively fast response speed of 1.3 s. The unique properties are attributed to the high crystal quality of the fabricated ZnTe nanowires. In (2012) Di Wu and Y. Jiang [35] have studied a

Sb-doped ZnTe

nanoribbons (NRs) with enhanced p-type conductivity by a simple thermal evaporation method, the transport properties of the ZnTe nanostructures play a critical role in determining the device performances as well as the device structures and the appropriate p-type doping, nano-photodetectors (nanoPDs) based on the ZnTe:Sb NRs exhibit excellent device performances, such as high responsivity and photoconductive gain, fast response speed, large detectivity and so on.

9

Chapter one General Concept of ZnTe In same year effect of Mn-doping on optical properties of ZnTe thin films from Dinesh C.Sharma et al. [36] were prepared ZnTe and Zn1-xMnxTe for x=0.1 & 0.2 thin films by using the thermal evaporation method. The optical absorbance and refractive index decrease with the increasing concentration of Mn due to incorporation of Mn atoms into ZnTe films. At the same year R. Amutha [ 37] has studied copper doped ZnTe thin films, were deposited onto cleaned glass substrates by thermal evaporation. The decreasing of atomic percentage value of copper with increase of the ZnTe film thickness is confirmed by energy dispersive analysis of X-rays (EDAX) analysis. The X-ray diffraction analysis indicates that the films undergo a phase change from hexagonal to cubic structure. Copper doped ZnTe films have very low transmittance when compared to pure ZnTe films. A sharp increased in extinction coefficient is observed near the fundamental absorption edge. The optical transition of these films is found to be direct allowed. In (2013) Liu Z.Chen G. et al. [38] fabricated individual ZnTe nanowires based field-effect transistors. Single ZnTe nanowire based photodetectors on silicon substrate exhibited high sensitivity and excellent stability to visible incident light with responstivity and quantum efficiency as high as 1.87 × 105A/W and 4.36 × 107 %, respectively and are stable in a wide temperature range (25-250 °C). Comparative study of ZnTe thin films prepared in (2014) from M. U. Farooq et al. [39] by using close space sublimation (CSS) and electron beam evaporation (EBE) thin film fabrication techniques for optoelectronic applications. Structural, morphological, optical and electrical properties of ZnTe films were studied before and after doping. The ZnTe thin films fabricated by EBE technique showed improvement in microstructure, morphology, crystalline orientation and physical parameters such as porosity and density as compared to CSS fabricated thin films. The highest density with very small porosity is observed in EBE fabricated films. The improvement in electrical parameters 10

Chapter one General Concept of ZnTe such as resistivity, mobility, sheet concentration, type of semiconductor (p or n type) and bulk concentration were observed in EBE fabricated thin films. The highest decreased in electrical resistivity (8.34×102 Ω cm) was observed by incorporating Ag dopant via EBE technique. In the same year Shailaja Jeetendra et al. [40 ], prepared ZnTe thin films by using thermal evaporation method for different thickness. The structural, optical and morphological properties of thin films were studied in the visible region. Based on the results, the 300nm ZnTe thin film showed low transmittance in visible wavelength with maximum absorption coefficient, optimum surface roughness, appropriate band gap and work function. These results strongly support ZnTe thin film as a back contact for CdTe solar cells. In (2015) W.A.SYED et al. [41] have study effect of Cu-doping on ZnTe thin films were thin films deposited on the glass by the two source evaporation method. The annealing was carried out 300oC at a vacuum of 10-3 mbar for about an hour. The effect of varying thickness and doping of copper on the physical properties of ZnTe thin film has been investigated. The structural properties including crystallite size, micro strain and dislocation density were determined by X-ray diffraction (XRD) and surface morphology and grain size by scanning electron microscopy. The UV-VIS-NIR spectrophotometery was carried out to investigate optical transmittance, which was decreased from 98% to 60% with increasing thickness 129 to 1514 nm. In the same year for devices,

fabricating high performance nano-optoelectronic

Lin-BA Luo. [42] have synthesized ZnTe and

ZnTe:Ga

nanostructures via a simple thermal evaporation method , when the Ga content in the ZnTe NWs increases from 1.3 to 5.1 to 8.7% the hole mobility will increase from 0.0069 to 0.33 to 0.46 cm2 V−1 s−1, respectively. It was also found that the photodetector composed of a ZnTe:Ga NW/graphene schottky diode exhibited high sensitivity to visible light illumination with an on/off ratio as high as 102 at reverse bias. The responsivity and detectivity were estimated to be 4.17 11

Chapter one × 103 A W−1 and 3.19 × 1013 cm Hz

1/2

General Concept of ZnTe W−1, higher than other ZnTe

nanostructure based photodetectors.

1-5 The Aim of Work: 1- Fabrication of photovoltaic detector of (p-ZnTe:Al /n-Si) heterojunction and Providing

overview

of

basic

photovoltaic

detector

characterization

measurements. 2- Discussion of key characteristics and challenges for ZnTe devices and comparison the effect of different thicknesses and also the effect of doping on the structural, optical, and electrical properties in order to get thin device works with high specifications in the visible and near infrared region to use in scientific and practical applications.

12

Chapter two

Theoretical part

2-1 Introduction: This chapter includes the general descriptions of the theoretical part of the current thesis, physical concepts, scientific explanation, relationships, and the laws used to interpret the obtained results.

2-2 Semiconductors Doping Methods: Semiconductors in their natural state are poor conductors because a current requires the flow of electrons, and semiconductors have their valence bands filled, preventing the entry flow of new electrons. There are several developed techniques available in literature that allow semiconducting materials to behave like conducting materials, such as doping . This modification has two outcomes( n-type and p-type). These refer to the excess or shortage of electrons, respectively. An unbalanced number of electrons would cause a current to flow through the material [43 ]. Doping is a procedure used for controlling the carrier concentration and hence the conductivity of semiconductors. It can be achieved by introducing into the semiconductor impurity atoms possessing a different number of valence electrons from those of the component elements of the semiconductor [44 ]. There are five types of doping methods [45, 46 ] : 1- Doping by mixture method. 2- Doping by co-evaporation method. 3- Doping by thermal diffusion method. 4- Doping by ion implantation. 5- Doping by laser.

13

Chapter two

2-3

Theoretical part

Doping by Thermal Diffusion Method:

Thermal diffusion is a high temperature process, in this method the dopant atoms are placed on or near the surface of the wafer by deposition from the gas phase of the dopant, see Fig (2-1). The doping concentration decreases monotonically from the surface, and the profile of the dopant distribution is determined mainly by the temperature and the diffusion time[47,48].

Figure (2-1): Thermal diffusion techniques for doping [45].

2-4 Diffusion process: Diffusion was the mass flow process in which atoms change their positions relative to neighbors in a given phase under the influence of thermal and a gradient [49]. In other words, the transport of matter from one point to another by random molecular motions, diffusion play a key role in the kinetics of many microstructural changes that occur during the processing of metals, alloys, ceramics, semiconductors, glasses, and polymers. It is occurs in the direction of decreasing concentration of a substance and leads to a uniform distribution of matter over the entire volume it occupies [ 50,51]. The diffusion rate of impurities into semiconductor lattice depends on the following [52 ]. 14

Chapter two 1- Mechanism of diffusion.

Theoretical part

2- Temperature. 3- Physical properties of impurity. 4- The properties of the lattice environment. 5- The concentration gradient of impurities. 6- The geometry of the parent semiconductor. The basic mechanisms of diffusion can by classified into [53 ]: 1- Interstitial mechanism. 2- Atomic vacancy inter change mechanisms. 3- Atomic inter change mechanism. 4- Ring diffusion mechanism.

Atoms in the thermal equilibrium can be considered to vibrate about their lattice positions. By

increasing the temperature these may acquire enough

energy to overcome the potential barriers and thus jump to adjacent substitutional or interstitial sites. In the presence of a concentration gradient, more impurity atoms will jump in the direction of the gradient and then in the opposite direction and hence constitute a flux of diffusing atoms. Thus diffusion is governed by Fick’s first law [54]. F = −D

dc′

………..………..…... (2-1)

dx

Where (F) be the flux of diffusion dopant atoms, (D) is the diffusion coefficient, (C' ) is the dopant concentration, and (x) is the distance in one dimensional and (

𝒅𝒄′ 𝒅𝒙

) is the concentration gradient. The negative sign indicates that the flux is

from high to low doping concentration, and (D) depends on the type of individual atom and is strongly dependent on temperature being given by equation [55]. D = Do exp (−

εa κB T

) …….…..…………….. (2-2)

15

Chapter two

Theoretical part

Where ( Do ) denotes the diffusion constant, (𝜺𝒂 ) stands for the Arrhenius activation energy, ( KB) Boltzman constant and (T) absolute temperature. When the concentration of the dopant atoms is low, the diffusion coefficient can be considered to be independent of doping concentration, and we get Fick’s second law [54 ]. ∂c ∂t

Where the (

𝝏𝒄′ 𝝏𝒕

= D(

∂2 c′ ∂x2

)

……………..…………... (2-3) 𝝏𝟐 𝒄

) is the rate of accumulation of concentration and ( 𝟐 ) is the 𝝏𝒙

2nd derivative (or curvature) of the concentration.

2-5 Investigation of Structural Properties: The structures of the alloy and deposited films have been examined by used the following characterization:

2-5-1

X-Ray Diffraction:

X-ray diffraction (XRD) is the most common characterization technique that used to investigate the quality of lattice parameters, orientation, defects, stress and strain in semiconductor materials [56 ]. The crystal structure of materials examines through the style of the diffraction, which is be in a shape of sharp bright spots in a single crystalline materials, as a form of fine rings in polycrystalline material and in the form concentric broad holes in the case of random materials [ 57 ]. The geometrical conditions which must be satisfied for diffraction to occur in a crystal were first established by Bragg. He was considered a monochromatic (single wavelength) beam of X-rays with coherent radiation (X-rays of common wave front) to be incident on a crystal, as shown in Fig (2-2), when X-rays are scattered from a crystal lattice, peaks of scattered intensity are observed which correspond to the following conditions [58] : 16

Chapter two 1. The angle of incidence = angle of scattering.

Theoretical part

2. The path length difference is equal to an integer number of wavelengths. Bragg concluded that the path difference between the two X-rays diffracted from two consecutive lattice planes is 2dsinθ, and it leads to Bragg’s law, which states that the condition for diffraction of X-rays for a crystalline material is[59]: nλ= 2dhkl sinθ……………………………………….(2-4) Where θ is the angle of incidence and λ is the wavelength of the X-rays, n is an integer and it is the order of reflection, and d hkl is the distance between the lattice planes. From (X-Ray) we can identify some compositional parameters such as: the lattice constants, rate of Crystal size, the dislocations density and number of crystals and in comes illustration of how some of these compositional parameters account:

Fig. (2-2): Diagram of Bragg reflection from crystalline lattice planes [58].

17

Chapter two

Theoretical part

2-5-2 Lattice Constants: Bragg’s law can be used to obtain the lattice spacing of a particular cubic system through the following relation [58]: 1 d2

=

(h2 +k2 +l2 ) a2

……………….…….……………… (2-5)

Where (hkℓ) was the Miller indices and (a) lattice constants. The accuracy of the film structure can be obtained by comparing the resultant data from knowing (d) and from comparison with the ASTM (American Society of Testing Materials) card.

2-5-3 Average Crystallite Size: The average grain size (𝑪. 𝑺), which can be estimated by using the Scherer’s formula [60]: (C. S) =

0.94 (λX−Ray ) ẞ cosθ

…………………………………… (2-6)

Where (λ) is the x-ray wavelength (Å), (β) is FWHM (is equal to the width of the line profile (in radian) at the half of the maximum intensity) and (θ) is the Bragg diffraction angle of the XRD peak (degree).

2-5-4 Dislocation Density: The dislocation density (𝜹) can be calculated by using the following relation [61]. δ=

1 (C.S)2

………………………………..…. (2-7)

2-5-5 Number of Crystallites per unite area: The number of Crystallites (N0) per unit area can be evaluated by using the following relation [62]: No =

t (C.S)3

…………………..……….…..… (2-8)

Where (t) was thickness of the film units (nm). 18

Chapter two

Theoretical part

2-5-6 Atomic Force Microscopy: The atomic force microscope (AFM) is one kind of scanning probe microscopes (SPM). SPM is based on the strong distance-dependent interaction between a sharp probe or tip and a sample, therefore the atomic force microscope uses the force existing between the probe and the sample to build an image of an object. AFM micrographs can provide information about surface roughness of the deposit on a nanometric scale and can measure local properties, such as height, friction, magnetism, with a probe. The central part of an AFM is therefore the tip that literally feels the sample, a nanometer-sharp AFM tip made by microfabricating technology is grown at the free end of a flexible cantilever that is used as the transductor of the interaction between the tip and sample [63]. AFM applications in the field of solid state physics include: (a) The identification of atoms at a surface. (b) The evaluation of interactions between a specific atom and its neighboring atoms. (c) The study of changes in physical properties arising from changes in an atomic arrangement through atomic manipulation.

2-6 Optical Properties: Rapid advances in semiconductor manufacturing and associated technologies have increased the need for optical characterization techniques for materials analysis and applications. Optical measurements have many unique and attractive features for studying and characterizing semiconductor properties. All optical measurements of semiconductors rely on a fundamental understanding of their optical properties. The optical properties of a semiconductor can be defined as any property that involves the interaction between electromagnetic radiation or light and the semiconductor, including absorption, transmittance, reflection, refraction, and others [64] . 19

Chapter two

Theoretical part

2-6-1 Transmittance (T'): The transmission is defined as the ratio of transmitted light intensity (I) to incident intensity of light (I0) [65]: T′ =

I Io

………………….……..………………. (2-9)

2-6-2 Absorbance (A): The Absorbance (A) can be defined as the ratio between absorbed light intensity (IA) by material and the incident intensity of light (Io) [65]. 𝐴=

IA Io

……………………...……………………. (2-10)

The fundamental relations between photon transmittance (T) and absorbance (A) 1

A = Log ( ) = Log ( T′

Io I

) …………….….. (2-11)

2-6-3 Absorption Coefficient: The absorption coefficient determines how far into a material light of a particular wavelength can be penetrated before it is absorbed [66] or defined as a ratio decrement in flux of incident rays energy relative to the distance unit in the direction of incident wave diffusion. The absorption coefficient depends on the material and also on the wavelength of light which is being absorbed [67]. The optical absorption coefficient α is the most important optical constant for photodetectors. The absorption of photons in a photodetector produce carrier pairs and thus a photocurrent, depends on the absorption coefficient α for the light in the semiconductor used to fabricate the detector. The absorption coefficient determines the penetration depth 1/α

of the light in the

semiconductor material according to Lambert–Beer’s law [68]: I = Io e−αt

)Lambert Law) ……………….……. (2-12)

20

Chapter two Theoretical part Where I is transmitted light intensity, Io incident light intensity and t is the thickness. Calculating the absorption coefficient (α) which is depends on the film thickness and absorbance, given as the following equation α = (2.303)

A t

…………………….…….…. (2-13)

Where A is the absorbance, and t is the thickness. The energy of a photon can be transferred to an electron in the valence band of a semiconductor, which is brought to the conduction band, when the photon energy is larger than the bandgap energy Eg. The photon is absorbed during this process and an electron–hole pair is generated. Photons with an energy smaller than Eg, however, cannot be absorbed and the semiconductor is transparent for light with wavelengths longer than (λ cut off) [68] λcut off (nm) =

hc Eg (eV)

…….….……………… (2-14)

Where (c) is the velocity of light in vacuum and (h) is Planck’s constant.

2-6-4 Electronic Transitions and the Band Gap: Based on the intrinsic location of the top of the valence band (V.B) and the bottom of the conduction band (C.B) in the band structure, the electron–hole pair generation occurs directly or indirectly. The absorption of radiation that leads to electronic transitions between the valence and conduction bands is split into direct and indirect transitions. These transitions are described by the equation[69]. opt.

αhυ = Bo (hυ − Eg )r ………….………………. (2-15) Where (α) is the absorption coefficient,(hυ) is the incident photon energy, (r) is constant which takes the values (1/2, 3/2, 2, 3) depending on the the type of the optical transition whether it is direct or indirect and (B0) is constant involving the properties of bands [69].

21

Chapter two Theoretical part The absorption diagram for all semiconductor materials refers to rapidly increase for absorption when the energy of absorption light equal to fundamental absorption edge. Fig. (2-3) show the absorption edge has three distinct regions

Fig.( 2-3): Shows three regions A, B and C of the absorption edge [69].

(1) High Absorption Region (A) (2) The Exponential Region (B) (3) The Weak Absorption Region (C)

The band gap represents the minimum energy difference between the top of the valence band and the bottom of the conduction band. As shown in Fig (2-4), In a direct band gap semiconductor the top of the valence band and the bottom of the conduction band occur at the same value of momentum, in this case the value of (r) has been taken to be (1/2) and the direct transition is called allowed, but

if the direct transition occur at different

position, this transition is called the forbidden transition and (r) has value (3/2). In an indirect band gap semiconductor, the maximum energy of the valence band occurs at a different value of momentum to the minimum in the conduction band energy. In this case the transition require absorption or emission of phonon to verify the conservation of the crystal momentum.

22

Chapter two

Theoretical part

(b)

(a)

Fig. (2-4): The Optical Transitions (a) direct band gap semiconductor , (b) indirect band gap semiconductor [70].

A photon of energy equal Eg, where Eg is the band gap energy, can be produced an electron-hole pair in a direct band gap semiconductor quite easily, because the electron does not need to be given very much momentum. The same principle applies to recombination of electrons and holes to produce photons. The recombination process is much more efficient for a direct band gap semiconductor than for an indirect band gap semiconductor, where the process must be mediated by a phonon [70].

2-6-5 Optical Constants: The refractive index n' defined as the ratio of the speed of light in vacuum, and the phase velocity v of light in the medium [71]. n′ =

c v

…………………………………. (2-16)

The complex index of refraction that can be defined as. nc = n' -i K ………………………………………..... (2-17) Where (nc) the complex refractive index, (i) the square root of (-1), (n') the refractive index and (K) the extinction coefficient. The refractive index value can be calculated from the formula [72]. 1

4R

2 2

1+R

n′ = [(1−R) 2 − K ] + (

) … … … … ….…………… (2-18)

1−R

23

Chapter two Theoretical part Where (R) is the reflectance and (K) is the extinction coefficient which is determined from the formula [73] K = α/4 ………………………………....………… (2-19) The real and imaginary parts of the dielectric constant were calculated from the relations [74]:  r = n'2 – K 2 ……………………………..………… (2-20)  i = 2n' K …………………...……….………………… (2-21)

2-7 Electrical Properties of Semiconductors: Electrical properties can provide a great deal of information about a semiconductor. The electrical properties measurement of semiconductor thin films allow the determination of the impurity levels present in the materials and the parameters that are critical to their utilization in various electronic and optoelectronic applications [75]. Some electrical properties of semiconductor are:

2-7-1 D.C Conductivity: Electrical conductivity (σ) is defined as the proportional factor between the current density (J) and the electric field (E), and it is given by Ohm’s law [76,77]: J = σ E………………………..………………… (2-22) In semiconductors the relation between the current density and electric field is given by [68, 69]: J = q (n μe + p μh) E …………………….………..… (2-23) Where n and p are the electron and hole concentration and µn and µp are the mobility of electron and hole, respectively, then the relation between the conductivity and electron–hole concentration is [78]: σ = q (n μe + p μh) ……….………………………. (2-24)

24

Chapter two Theoretical part For most cases of semiconductor the following equation gives the change of the electrical conductivity with temperature [76]. σ = σo exp (

−Ea

KB T

)………………………….....…. (2-25)

Where (𝜎𝑜 ) is the minimum electrical conductivity at [0 K], (Ea) is the activation energy which corresponds to (Eg /2) for intrinsic conduction, (T) is the absolute temperature and (kB) is the Boltizman’s constant.

2-7-2 Hall Effect: The Hall effect is an important measurement in the analysis of semiconductor material (in which the charge carriers can be either positively or negatively charged). It also has many important practical applications in detecting and measuring magnetic fields [79]. In this measurement, the magnetic field is applied at right angles to the direction of current flowing in a conductor, and the electric field is created in a direction perpendicular to both, see Fig (2-5) [80]. The effect may be used to determine the sign of the moving charges that form the electric current.

Fig. (2-5): Geometry for measuring the Hall Effect. [80]

25

Chapter two Theoretical part The Hall coefficient (RH) is determined by measuring the Hall voltage (vH ) that generates the Hall field across the sample of thickness (t), by[73] V

R H = ( H) I x

t Bz

…….……..……..…..…..... (2-26)

Where (Bz) is the magnetic field intensity. The Hall electron concentration (nH) in n-type semiconductor and the hole concentration (pH) in p-type semiconductor are related to Hall coefficient and were obtained by the relations [73]: n = - 1/q RH ............…...…………….......... (2-27) p = 1/ q RH

……………………………..... (2-28)

The Hall mobility (µH) of the films was found according to the relation: μH = |R H | σ ………..………………..… (2-29) σ=

1 ρ

=

L R′

S

………….……………..... (2-30)

Where (q) is the electron charge, (ρ) is the resistivity, (R') is the resistance, (S) is the cross sectional area of the film, and (L) is the distance between the electrodes.

2-8 Heterojunctions: Heterojunctions play an important role in semiconductor devices such as solar cells, light-emitter diode, photo detector, solid state laser and integrated circuits [81]. A heterojunction is a junction formed between two dissimilar semiconductors, which are assumed to have different band gaps (Eg), permittivities (εs), work functions (  m ), and different electron affinities ( χ). From the Fig. (2-6 a and b) it is easy to see in the difference in energy of the conduction band edges ΔEC, and the difference in energy in the valence band edges ΔEV [82]. 26

Chapter two Theoretical part The presence of large barrier in the valence band ΔEV for pn junction impedes injection holes from p side to n side. As for conduction band, the high barrier ΔEC is a little compared with the barrier formed in valence band, so that the electrons are responsible for transfer of current in that kind of pn junction, see Fig (2-6)b. Also the two barrier do not appear in homojunction, see Fig (26)a [83]. Heterojunction can be classified into two kinds; abrupt and graded according to distances during which the transition from one material to the other is completed near the interface. Besides, there is another classification according to the type of the conductivity presents on either side of the junction. So, isotype heterojunction (p–P) or (n–N) is formed when the two semiconductors have the same type of conduction, while an isotype heterojunction (p–n) or (n–p) is formed when the conductivity type differs [84].

Fig. (2-6): Energy-band diagram for two semiconductors [82] (a) homojunction

(b) heterojunction.

27

Chapter two

Theoretical part

2-9 Abrupt Heterojunctions: A good approximation models for many heterojunctions is abrupt heterojunction. One of the earliest theoretical models for the behavior of the abrupt heterojunction based on the depletion approximation was developed by Anderson[85]. There are two types of abrupt heterojunction: anisotypes heterojunctions and isotypes heterojunctions.

2-9-1 Abrupt Anisotype Heterojunction: When two semiconductors are brought into contact to form a junction, the Fermi level must be continuous across the interface region as shown from the energy-band profile at equilibrium in Fig. (2-7). The Fermi level must coincide on both sides in equilibrium and the vacuum level is everywhere parallel to the band edges and is continuous [86]. The discontinuity in band edges (ΔE C) and (ΔEV) (difference energy gaps) have a signification effect on the properties of transition of the carriers through the junction, therefore, these two values are essential factors in heterojunctions devices [87]. From Fig (2-7) a (the subscripts 1and 2 refer to p-and n-type semiconductors). The discontinuity in the conduction band edges (ΔEC) is equal to the difference in electron affinities of the two semiconductors [83, 88]: ∆EC = χ1 − χ2 = ∆χ …………………………...…..…. (2-31) ∆EV = (χ2 + Eg2 ) − (χ1 + Eg1 ) = ∆Eg − ∆χ ……..… (2-32) ∆EC + ∆EV = ∆Eg

where

∆Eg = Eg2 − Eg1 …… (2-33)

The total built-in voltage (Vbi) due to difference in work function (∅𝟐 − ∅𝟏 ) is equal to the sum of built-in voltage on both sides (Vbi = Vbi1 + Vbi2 ). If the coordinate of the interface as shown in Fig. (2-7b) is denoted by 𝑿𝒐 , then the transition widths on either side of the interface for an abrupt p-n heterojunction,

28

Chapter two

Theoretical part

Fig. (2-7) Energy band diagrams. (a) For two isolated semiconductors (b) For n-p heterojunction at equilibrium [83]

obtained by generalizing the solution of the Poisson's equation for junctions in the presence of an applied voltage (Va) are given by [83-88]: Xn = (Xo − X1 ) = [

2NA2 ε1 ε2 (Vbi −Va ) qND1 (ε1 ND1 +ε2 NA2 )

29

1/2

]

……. (2-34)

Chapter two

Theoretical part

Xp = (X2 − Xo ) = [

2ND1 ε1 ε2 (Vbi −Va ) qNA2 (ε1 ND1 +ε2 NA2 )

1/2

]

……. (2-35)

Where (𝑋𝑜 − 𝑋1 ) and (𝑋2 − 𝑋𝑜 ) depletion widths in the doner and accepter sides respectively, q is the electronic charge, 𝜀1 and ND1 are dielectric constant and concentration of donors in p-type semiconductor, respectively, 𝜀2 and NA2 are dielectric constant and concentration of acceptors in n- type semiconductor. The total width (W) of the transition region is [88,89]: W =Xn+ Xp = (X0-X1) + (X2-Xo) ………………..….… (2-36) W=[

2ε1 ε2 (Vbi −Va )(NA2 +ND1 )2 q(ε1 ND1 +ε2 NA2 ) ND1 NA2

1/2

]

………….…..……... (2-37)

The relative built-in voltages in each of the semiconductors are [88]: Vbi1 Vbi2

=

ND1 ε2 X2n NA2 ε1 X2p

………………..……....……….…. (2-38)

In this model, Anderson assumed that because of discontinuities in the band edges at the interface, the diffusion current will consist almost entirely of electrons or holes [83]. For the n-p heterojunction, the predominant current carriers will be holes because increase of barrier for electrons by ∆𝐸𝐶

and

reduction of barrier for holes by ∆𝐸𝑣 [84]. There are many models of mechanisms for charge carrier transport in abrupt anisotype heterojunction such as [83,90]: 1- Diffusion model. 2- Emission model. 3- Emission –recombination model. 4- Tunneling model. 5- Tunneling –recombination model.

30

Chapter two

Theoretical part

2-9-2 Abrupt Isotype Heterojunction: An isotype heterojunctions are different from isotype heterojunction in that the adopts of two sides are of the same type [91].These types of heterojunctions which include n-n and p-p heterojunctions are majority carrier devices like Schottky diodes, so that the contribution of minority carriers to the electrical current in this type of heterojunction is negligible. In an n-n heterojunction, since the work function of the wide-band gap semiconductor is smaller, the energy bands will be bent oppositely to those for n-p heterojunctions as shown in Fig.(2-8 a) [90]. In this case the depletion region is developed only on one side of the interface (on the side of the wide-band gap semiconductor) and an accumulation region is formed on the other side of the interface.

q

q

Fig. (2-8): Energy-band diagrams for ideal (a) n-n and (b) p-p isotype heterojunctions at thermal equilibrium [90]

31

Chapter two Theoretical part There are many models of mechanisms for charge carrier transport in abrupt isotype heterojunction such as [83, 90]: 1- Emission model. 2- Diffusion model. 3- Tunneling model. 4- Double – schottky- diode model.

2-10 Electrical Properties of Heterojunctions: Current-voltage and the capacitance-voltage characteristics are the electrical measurements which give a complete knowledge of the heterojunction. Also, give information about the band structure of a heterojunction (the type of heterojunction and the built–in potential) [84]. These properties of a heterojunction depend strongly on the fabrication method and doping levels of the two semiconductors [90].

2-10-1 Current-Voltage (I-V) Characteristics: The current-voltage (I-V) characteristics in heterojunctions is influenced by various mechanisms depending on the band discontinuities at the interface and the density of interface states. I-V characteristics can be classified as forward or reverse bias depending on the polarity of the applied voltage. The plot of forward bias characteristics of heterojunction is shown in Fig. (2-9). A. Low – Voltage Region: At low-voltage region the total dark current of the heterojunctions can be represented by a sum of several components of currents. If generationrecombination and diffusion mechanisms are dominant, then the dark current (I) obeys the following formula [92]: If = Is exp (

qV

βkB T

) ………………………….... (2-39)

32

Chapter two Theoretical part Where Is is the saturation current, and β is the ideality factor having a value between 1.0 and 2.0 [93]

Fig. (2-9): Forward bias characteristics of a p-Ge/n-GaAs heterojunction at different temperatures: (1) 333K;(2) 298K;(3) 250K;(4) 200K; (5) 77K [90].

B. High – Voltage Region: At high– voltage region, tunneling current is dominated across the junction and is given by the expression [83]: If ∝ exp(AV) exp(BV) ….…….…………………… (2-40) Where A and B are constants essentially independent of voltage and temperature. This expression applies in forward bias and high voltages, where the tunneling –recombination mechanism dominated across the junction [86]. The reverse bias characteristics divided into two regions: 1-Low – Voltage region of these heterojunction show a linear variation between reverse current and applied reverse voltage, ( Ir ∝ V ). 2-High – Voltage region, the relation above becomes ( Ir ∝ Vm ), where (m >1). This behavior was explained on the basis of the tunneling model [90, 94].

33

Chapter two

2-10-2

Theoretical part

Capacitance-Voltage Characteristics:

C-V measurements can be used to study the most basic properties of semiconductors devices, such as potential barrier, analyze the depletion region potential and the charge distribution in a heterojunction. Anderson showed that the junction capacitance per unit area can be written as [84, 95]: C=

1/2 q ( N n N p εn εp ) [ 2 (ε N + ε N )] (Vbi n n p p

− V)−1/2 ...……... (2-41)

Where Nn and Np are the donor and acceptor concentrations, 𝜀𝑛 and 𝜀𝑝 are the dielectric constants of n- and p- type semiconductors respectively, Vbi is the built-in junction potential, and V is the applied voltage. From expression (2-41), a plot of C-2 against applied reverse voltage V is linear and its extrapolated intercept on the voltage axis gives the built-in junction potential Vbi as shown in Fig. (2-10), and the linear variation of the experimental curve C-2 versus V gives an indication of the presence of abrupt heterojunction [95].

Fig. (2-10): the capacitance –voltage characteristic for n-p heterojunction [86]

34

Chapter two Theoretical part The width of the depletion region can be calculated by [96, 97]: W = εs / Co ……………….………………….……. (2-42) Where Co is the capacitance at zero biasing voltage, and εs is the permittivity of semiconductor calculated from the equation:

εs = Where

εn . εp

……………………….…………… (2-43)

εn +εp

𝜀𝑛 and 𝜀𝑝 are the semiconductor permittivity for the two

semiconductor materials.

2-11 Optoelectronic properties of heterojunction: When a semiconductor is illuminated with light, the photons may be absorbed or propagate through the semiconductor, depending on the incident photon energy and on the band gap energy (Eg) of semiconductor. The expression for photocurrent (Iph) can be written as [97, 98]: Iph = q A Gph (W + Ln + Lp) …………….…… (2-44)

Where q is the electronic charge, A the heterojunction area, W the depletion layer width, Gph the generation rate of carriers and (Ln, Lp) the diffusion length of electrons and holes, respectively. The modified expression for current-voltage relation of an ideal anisotype heterojunction under illumination [83,88]. IL = Is [exp (

qV

kB T

) − 1] + Iph ……….….….. (2-45)

∴ Iph = IL − Id …………………..…...….. (2-46) Where 𝐼𝐿 is the total current under illumination, 𝐼𝑠 the saturation current and 𝐼𝑑 the dark current. When illuminated heterojunction, (Eg1> Eg2) the photons are incident on the front surface of the wide-band gap material, photons with energy less than energy gap (Eg1) will pass through the wide-band gap material, which acts as an optical window, and are absorbed in the narrow-band gap material 35

Chapter two Theoretical part near the interface, while photons with an energy greater than Eg1 will be absorbed in the wide-band gap material [90, 99]. If the absorption coefficient was high in the narrow gap semiconductor E g2, the carriers are generated in the depletion layer or in nearby area. Often lead this phenomenon to sufficient optical response for anisotype Heterojunction (p-n) [98, 100].

2-12 Photodetectors: A photodetector is an optoelectronic device that measures photon flux or optical power by converting the energy of the absorbed photons into a measurable form. There are generally three steps involves in the photodetection process [101]:  Absorption of optical energy and generation of carriers.  Transportation of the photo generated carriers across the absorption and/or transit region, with or without gain.  Carriers collection and generation of a photocurrent, which flows through external circuitry. When a semiconductor material is illuminated by photons of an energy greater than or equal to its bandgap, the absorbed photons promote electrons from the valence band into excited states in the conduction band, where they behave like free electrons able to travel long distances across the crystal structure under the influence of an intrinsic or externally-applied electric field. The separation of electron-hole pairs generated by the absorption of light gives rise to a photocurrent, which refers by definition to the fraction of the photo generated free charge-carriers collected at the edges of the material by the electrodes of the photodetecting structure, and whose intensity at a given wavelength is an increasing function of the incident light intensity[4].

36

Chapter two Theoretical part Two principal classes of photodetectors are in common use: thermal detectors and photoelectric detectors, from Fig.(2-11) it can be seen clearly the typical spectral dependence of the output of photon detectors as a function of wavelength [102].

2-12-1 Thermal detectors: Thermal detectors are sensing the changing in temperature produced by absorption of incident radiation. In thermal detectors, are rather inefficient and relatively slow as a result of the time required to change their temperature. The change in the temperature of the lattice by the absorption causes a change in the electrical properties. Fig (2-11) shows the mechanism responsible for the absorption of the radiation which is itself wavelength independent [103,104].

Fig. (2-11): Relative spectral response for a photo detector and a thermal detector [102]. The type of thermal detector are [87]. 1- Thermoelectric Junction Devices: (Thermocouple, Thermopile). 2- Bulk Devices: (Bolometer, Golay Cell, Pyroelectric).

37

Chapter two

Theoretical part

2-12-2 Photon detectors: The operation of photon detectors is based on the photoeffect, in which the absorption of photons by some materials results directly in an electronic transition to a higher energy level and the generation of mobile charge carriers. Under the effect of an electric field these carriers move and produce a measurable electric current [103]. Photon detectors have a small sizes, minimum noise, low biasing voltage, high sensitivity, high reliability, and fast response time. Most of photon detectors have a detectivity that is one or two orders of magnitude greater than thermal detector, and the response time of photon detectors is very short due to direct interaction between the incident photons and the electrons of the detector material. Photon detectors include photoconductive (PC) and photovoltaic (PV) detectors [105]. 1- Photoconductive Detectors: The photoconductive detector consists of a single crystal of semiconductor material with two ohmic contacts, and a voltage applied between them. When photons are absorbed by a semiconductor material, mobile charge carriers are generated, the electrical conductivity of the material increases in proportional to the photo flux. An electrical field applied to the material by an external voltage source causes the electrons and holes to be transported [106]. These results in a measurable electric current in the circuit. There are two basic types of photoconductive detector show in fig (2-12): A. The intrinsic photoconductive type. This process occurs if the incident photon energy larger than or equal the energy gap of the semiconductor (hv ≥ Eg), then an electron-hole pairs will be generated. The absorption coefficient (α) depends on the type and amount of E g and energy of radiation incident [83,107].

38

Chapter two

Theoretical part

Fig.( 2-12): Processes of photoconductive for semiconductor [108] (a) Intrinsic

(b) Extrinsic

B. Extrinsic photoconductive detectors. This process occurs if (hv < Eg), the incident photon energy larger than ionize energy of impurity atom, thus transition of electron from donor level to the conduction band for n-type or, transition of holes from the valence band to acceptor levels for p-type takes place but not both. In both cases the concentration of carrier increases so that the conductivity of semiconductor will be increased also. The absorption coefficient (α) proportional to the concentration of ionize impurities and absorption cross section area [83]. 2- Photovoltaic Detectors: Photovoltaic detectors based on its work on the generation of electrical driving force when absorbed light as a result of an internal electric field generated due to the transition carriers from higher concentration to low regions [87]. In photovoltaic devices, a built-in electric field is created by a diode structure in the semiconductor junction. This can be done by doping the semiconductor in layers (junction diode) or it can be done by forming a thin semi-transparent conductive layer on the front surface (Schottky-barrier diode). The absorption of a photon near this diode then gives rise to a voltage. 39

Chapter two Theoretical part Photovoltaic effect can be observed in several structures. { p-n junctions, Heterojunctions, Shottky barriers and Metal-insulator-semiconductor (MIS) photo-capacitors }[109].

2-13 Photodetector Parameters: There are many parameters affecting the performance of the detectors. These parameters are:

2-13-1 Responsivity 𝓡 : The basic function of a detector is to convert the radiant input to an output signal of some convenient type. The output is electrical either a current or a voltage. Responsivity is defined as the ratio between the output electrical signals (voltage or current) to the incident radiation power Ps, the responsivity for monochromatic light of wavelength incident normally is given by [109,110]. ℛ=

Iph Ps

or

Vph Ps

…………………..……. (2-49)

Where ( Iph) is photocurrent flowing between the electrodes and (Vph) is signal voltage.

2-13-2 Quantum Efficiency 𝛈 : The quantum efficiency of a photodetector is defined as the probability that a single photon incident on the device generates a photo carrier pair that contributes to the detector current. Quantum efficiency is the ratio of the flux of generated electron-hole pairs that contribute to the detector current to the flux of incident photons. It can be expressed [103]. η=

Ncarriers Nphotons

=

(Iph /q) (Ps /hv)

…………..……..….. (2-50)

40

Chapter two

Theoretical part

η = ℛ(λ)

hc qλ ℛ(λ)

η = 1.24

λ

…………………..………. (2-51) …………………….…… (2-52)

Where 𝓡(𝝀) is responsivity, (q) is the electron charge and (hc) is the photon energy.

2-13-3 Noise Equivalent Power NEP: Noise equivalent power (NEP) is defined as the root mean square (r.m.s) incident radiant power falling on the detector that is required to produce an (r.m.s) signal voltage or current equal to the (r.m.s) noise voltage or current at the detector output. It is expressed as [110]: (NEP) =

IN ℛ

………………..…..…... (2-53)

IN = √2 q Id ∆f …………………...……... (2-54) Where (𝑰𝑵 ) is noise current, Id is the dark current and ∆𝒇 is the bandwidth.

2-13-4 Detectivity D : The detectivity (D) is defined as the signal to noise ratio per unit incident radiation power and it is defined as: 𝐷=

1 𝑁𝐸𝑃

, (watt)-1....….……………..… (2-55)

The specific detectivity (D*) is the detector signal to noise ratio when 1 Watt of optical power is incident on the detector with optical area 1 cm2 and the noise is measured with a band width of 1 Hz . It is normally dependent on the size of the detector and the bandwidth of the measurement circuit. The peak value of D* at specific wavelength can be written as [111]. D∗ =

(S . ∆f)1/2 NEP

= (S . ∆f)1/2 . D …………….…….... (2-56)

41

Chapter two D∗ = ℛλ

Theoretical part (S . ∆f)1/2 IN

= ℛλ [

S 2 q Id

1/2

]

.……………... (2-57)

Where S is the area of the sample.

2-13-5 Response Time: Response time is an important limiting factor for the speed of operation of all semiconductor photodetectors. It can be defined as time required for detector to respond to an optical input. The response time is related to (resistance R and Capacitance C), which is related to the rise time (τ) that gives the relationship (τ = R.C) [113]. The rise time is the time required for the detectors to increase its output from 10% to 90% of final output level.

Fig.( 2-13) relation of the rise time with light output power [87]. The carrier lifetime (recombination lifetime) is defined as the average time it takes an excess minority carrier to recombine [83,114], and it can be calculated from photocurrent gain (g) which defined as the number of charge carriers flowing between the two contact electrodes of a detector per second for each photon absorbed per second [83,113].

42

Chapter three

3-1

Experimental Part

Introduction:

This chapter includes the preparation of (ZnTe) alloy, and involves the description of the technique which is used for preparation and testing the structure of the thin films and heterojunctions are described, also includes the instrument and devices that active in this work. The deposition of (ZnTe) were carried out using the thermal evaporation method on glass substrates and on (111) n-type Si wafers. Investigating the effect of thickness and Al doping with different ratio on the structural, optical properties, Hall effect, and junction characteristics in dark and under illumination are conducted. A schematic illustrating of the experimental work is shown in Fig. (3-1).

3-2 Preparation of ZnTe Alloy: The exact amount of high purity (99.999%) (Zn, Te) elements, in accordance with their atomic percentages, are weighed using a sensitive electronic balance with the least count of (10-4 gm). The material mixed well and then appropriate weight of Zn and Te were placed in the quartz tube (length ~ 25 cm and internal diameter ~ 8 mm) which was attached to the evacuated system, then sealed under (~10-3) Torr vacuum. The melting processes done in an electric furnace, of type (carbolite), the temperature of the furnace arises gradually at a rate of 10 ºC /min, to reach the thermal class limit to (1200K), and keep at this temperature for 5 hours. The temperature then lowered slowly in order to make quartz tube cool and after cooling the quartz tube took out the electric furnace and broke to extract the sample and then grinding sample to get powder .

43

Chapter three

Experimental Part Experiment work and procedure

Preparing ZnTe alloy

XRD

Thermal Evaporation System in Vacuum Masks preparation

Substrate cleaning (glass and Si wafer)

Preparing ZnTe thin film with different thickness (400, 450, 500 ) nm on Si wafer and glass substrate

Doping ZnTe thin film in Al with different ratio ( 0.05, 0. 1, 0. 15, 0. 2 , 0. 25 )% Measurement for ZnTe:Al films on glass substrate

Measurement for ZnTe:Al / Si heterojunction

Electrical Photovoltaic

I-V phot

Electrical

C-V

Iph , Isc , Voc

D.C

I-V dark

Optical

Structur e XRD

UV AFM

Hall effect

T, A α , Eg kº , nº Ɛ1 ,Ɛ2

Fabrication photodetector

Measurement Rλ, ɳ% , D

Fig. (3-1) Schematic for experimental work. 44

EDS

Chapter three

Experimental Part

3-3 Vacuum Thermal Evaporation System: The vacuum unit system, which is used to prepare thermally evaporated (ZnTe) films was Edward Coating unit model 306A, shown in Fig.(3-2) in the preparation of films and heterojunction under study, this system consists of four basic parts:-

a. Evaporation Chamber Fig.(3-2) shows several basic parts of vacuum unit system were include:  Cover (Bell jar) which is a metal cylinder anti rust.  Boat which manufacturing from material that have very high melting temperatures.  Substrate Holder that the best distance between target and substrate was 18 cm.

b. Vacuum Pumps  The mechanical pump (Rotary pump ). Rotary pump is designed to work in the vacuum pressures to about 10 -2Torr.  The diffusion pump This pump works to complete the working of the first mechanical pump, the diffusion pump is designed to operate with an outlet pressure of less than 10-2 Torr and can be achieved vacuums on the order of 10-6 Torr.

c. Vacuum gauges  Pirani gauge  Penning gauge

d. Cooling System The cooling system used in the thermal evaporation system played an important role for grades the air pressure dump degrees required in times of short time, depending on the type and efficiency of the fluid used in the cooling process though running the steaming system, is a that liquid nitrogen gas or water radiator (which was used in the cooling system) Fig.(3-2). 45

Chapter three Experimental Part The most commonly used materials for evaporating sources are metals with a high melting point and do not react with the evaporate, in this research a suitable shape design for molybdenum boats were used for films evaporation, and the films that doping with Al, the suitable shape design for Tungsten boats, also used spiral cord from Tungsten for deposition aluminum poles, see Fig. (3-3).

Fig. (3-2) Illustration of the thermal evaporation system.

46

Chapter three

Experimental Part

Fig. (3-3) a. molybdenum boat used for ZnTe films evaporation b. Tungsten boat for aluminum evaporation c. spiral cord from Tungsten for deposition aluminum poles

3-4 Samples Preparation: 3-4-1 Substrates Preparation and Cleaning: The effectiveness of cleaning the substrates has strong effect on the adhesion properties of the deposited films. Two types of substrates were used in this study, glass slides substrate was used in order to investigate the structural, optical, and electrical properties of ZnTe thin films and n-Si wafer with crystal orientation (111), electrical resistivity of 1.5-4 (Ω.cm), which used to form a ZnTe/Si heterojunction and study the photovoltaic and the electrical properties of heterojunction.  Glass Slides The substrate cleaning could be summarized as follows: A. Glass Slides are cleaned using detergent with water to remove any oil or dust that might be attached to the surface of substrate and then placed under tap water and rubbing gently for 15 minutes. B. Placed in a clean beaker containing distilled water and then rinsed in ultrasonic unit for 15 minutes. 47

Chapter three Experimental Part C. Replacing the distill water with pure alcohol solution in ultrasonic unit for 15 minutes. D. The slides eventually were dried by air blowing and wiped with soft paper.  Silicon Wafer Substrate: The n-type Si (111) wafer was used as a substrate, the samples cut from the wafer and cleaned using distilled water, then rinsed in ultrasonic unit for 15 minutes. In order to remove the native oxide layer on the samples, they were immersed and stirred in a chemical solution consists of 1 ml HF and 10 ml H 2O for 3 minutes. Then these specimens were rinsed by distill water several times and dried using soft paper.  Masks: Various shapes of thick aluminum (Al) foil masks used on glass substrates to make a suitable shape design for both film and electrodes according to the type of the electrical measurements, the cleaning involve washed the masks in distilled water then immersed in a pure alcohol for 10 min and later dried by heating air .

3-4-2 Evaporation of Thin Films: Thin

films

of

undoped

and

doped

ZnTe

with

various

weight

(0.05,0.1,0.15,0.2,0.25)% of Al grown with a thickness of (400, 450, and 500) nm of ZnTe on Si wafers and glass substrates at RT, by thermal evaporation method using the Coating Unit 306 Edward. A high purity (99.999%) of ZnTe powder was used as a source for undoped and doped ZnTe with Al thin film deposition using molybdenum boat. The thicknesses of films (400, 450, 500) nm were determined with ( Precisa -Swiss) microbalance by using weighing method and with deposition rate about (1.2∓0.1) nm/sec.

48

Chapter three Experimental Part The material was placed into molybdenum boat with a small dimple at the center to act as a point source. The boat was heated indirectly by passing current through the electrodes. The material evaporated in vacuum at room temperature onto two types of substrates, glass substrates with (2.5 x 2 x 1) cm3size were used to study the structural, electrical and optical properties of ZnTe films. Secondly, n -type Si wafer substrates with crystal orientation (111), indirect energy gap of 1.1eV, diameter 76.2mm, and thickness (508±15)µm which used to study the photovoltaic and the electrical properties of heterojunction. The distance between the substrate and the boat is (18) cm. After reaching high vacuum in the vacuum chamber, after end of vaporation process leave sample in the chamber to cool then extracted samples for different tests.

3-5 Thickness Measurement of Thin films: Thickness is the most significant thin films parameter, because it largely affects the properties of the films. There are several methods used to determine the film thickness. 1. In this method gives an approximate deposited film thickness to obtain the thickness of the used required films. The theoretical prescription is given by equation [82]: t = m/4π 𝜌𝑓 R'2

………………………………………. (3-1)

Where, t is the thickness of the film, m is the mass of the material, 𝝆𝒇 is the density of the material and R' is the distance between the substrate and the boat.

2. The second method was weighting method where use of electronic microbalance type, very sensitive where measure until (10-4 g), the thickness is determined using the formula [86] :

t=

∆m S . ρf

=

(m2 −m1 ) S . ρf

………………………..….. (3-2)

Where m1 was the mass of substrate before deposition, m2 was the mass of substrate after deposition and S was the thin film area. 49

Chapter three

Experimental Part

3-6 Structural Analysis : 3-6-1 X-Ray Diffraction: X-ray diffraction (XRD) was employed to detect the effect of various deposition conditions on the crystallization behavior of the investigated thin films. In this study investigated X-Ray Diffraction for alloy prepared and the deposition thin films, the scanning angle 2θ was varied in the range of 10°-80°. Thicknesses and effect of doping have been examined by X-ray diffractions using (SHIMADZU-Japan-XRD 6000) diffractometer system which records the intensity as a function of Bragg's angle, the characteristics of diffractometer system are:

X-Ray Tube Target: Cu kα. Wave Length: 1.5406 Ǻ Voltage: 40 kVolts. Current: 30 mA.

Measurements Condition Axis: Theta – (2θ). Scan Mode: Continuous Scan. Step: 0.05 (deg.). Speed: 5 (deg /min).

The general structure of ZnTe alloy and thin films, including inter planer distance d(hkl) for different planes, lattice constants, grain size, the dislocation density, and the number of crystals are determined and compared with value in the American Standard for Testing Materials (ASTM) cards for ZnTe.

50

Chapter three

Experimental Part

3-6-2 Atomic Force Microscope Measurements (AFM): It was used atomic force microscope to study the effect of thickness and doping ratios on mapping topography surfaces of the thin films. The study was recorded by using scanning probe microscope (type AA3000 , supplied by Angstrom Advanced Inc .USA), for two-and three-dimensional images describing the surface in terms of roughness and grain size, and these images of high precision values.

3-6-3 Energy Dispersive X-ray Spectrometer (EDS): Is an analytical technique used for the elemental analysis or chemical characterization of a sample. It relies on an interaction of some source of X-ray excitation and a sample. The main principle of spectroscopy Its characterization capabilities are due in large part to the fundamental principle that each element has a unique atomic structure allowing a unique set of peaks on its electromagnetic emission spectrum [115].

3-7

Optical Measurements:

Optical measurement constitutes the most important means of determining the band structure of semiconductors. Where the optical properties of ZnTe thin films deposited on glass slide which include the transmittance and absorbance spectra were studied over the wavelength range (400-1100) nm by using UVVisible 1800 SpectraPhotometer. Optical absorbance data were used to calculate the transmittance, absorption coefficient (α), band gap energy (E g) and the optical constants (extinction coefficient, refractive index, and real and imaginary parts

of

dielectric

constant)

for

undoped

and

Al

with

ratio

(0.05,0.1,0.15,0.2,0.25)%doped ZnTe thin films deposited with different thickness of (400, 450, and 500) nm on glass slides substrate.

51

Chapter three

Experimental Part

3-8 Electrical Measurements: For electrical measurements of ZnTe thin films, the deposition of aluminum electrodes was made by Al wire of high purity (99%) which were deposited with thickness of (200nm) from a tungsten spiral boat on glass and Si substrates by vacuum thermal evaporation method using Edward type E306A unit for this purpose. The distance between the substrate and boat was 18 cm. Electrical properties can provide a great deal of information about a semiconductor. Among these properties:

3-8-1 (D.C) Electrical conductivity measurement: The electrical conductivity for ZnTe thin films has been measured as a function of temperature over the range (293 – 423) ˚K by using the electrical circuit. The measurements have been done using sensitive digital electrometer type Keithley 616 and electrical oven. The resistivity (ρ) of the films is calculated by using equation (2-30) where A was equal [87]: S = b × t …………..……...…... (3-3) Where S is the cross sectional area of the film, t is the thickness of the film and 𝒃 is the width of the electrode. By using the formula (2-25), the temperature dependence of the dark conductivity where plotting lnσ versus 1000/T, the activation energy of the deposited thin films was obtained, (this was measured in the Ministry of Science and Technology).

3-8-2 Hall Effect Measurement: Hall effect measurement is determined by using HMS3000 Hall measurement setting. The measurements have been used in determining majority carrier concentrations, type of carrier and their mobility in thin film materials. The 52

Chapter three Experimental Part principle Hall effect refers to potential difference (Hall voltage) on opposite sides of a thin sheet through which an electric current is flowing, created by a magnetic field (B=0.550 Tesla) applied perpendicular to the Hall element. The Hall electron concentration (PH) are related to Hall coefficient and obtained by using Eq. (2-28), and the Hall mobility (μH) of the films was found according to the Eq. (2-29) (this was measured in the Ministry of Science and Technology).

3-9 The Electrical Measurement of ZnTe /Si Heterojunction: 3-9-1 Current-Voltage Measurements in the Dark state: The current-voltage measurements in the dark as a function of forward and reverse bias voltage were done for all prepared ZnTe /Si junction (undoped and doping with Al in different ratio) with different thickness, in the range (0.0-3.0 Volt) and (–3.0-0.0 Volt) respectively, using D.C power supply (F30-2, Farnell Instrument) and (keithley digital electrometer 616) , see Fig. (3-4). From the plot of the forward current (If) versus applied forward bias voltage (Vf), the ideality factor (β) and (Is) the saturation current was obtained according to the relation [87]. 𝜷=(

𝒒

𝒌𝑩

𝒅𝑽

) . [𝒅 𝐥𝐧(𝑰 ) ] ………………………. (3-4) 𝑻 𝒇

Where V is the bias voltage, If is the forward current and slope= [

𝒅𝑽 ] 𝒅 𝐥𝐧(𝑰𝒇 )

Fig. (3-4) Circuit diagram for I-V measurement in the dark state. 53

Chapter three

Experimental Part

3-9-2 Capacitance-Voltage Measurements in the Dark state: For all prepared ZnTe /Si junction (undoped and doping with Al in different ratio) with different thickness, the capacitance-voltage characteristics (C-V) have been measured as a function of the reverse bias voltage at the range (0-2.8) Volt with fixed frequency of 10 kHz using (LRC meter GWinstek 8105G). The measurements determining the type of heterojunction (abrupt or graded), built-in potential and the width of the depletion region. A plot of 1/C2 versus the reverse bias voltage gives the value of the built-in potential from the intercept of the straight line with the voltage axis and gives Co the capacitance at zero biasing voltage (intercept point V=0). The width of the depletion region (W) was estimated from Eq. (2.42).

3-10 Current-Voltage Measurement under Illumination: The illuminated I-V characteristics for all prepared ZnTe /Si junction were made at reverse bias voltages in the range (–4.0-0.0) when they were exposed to Halogen lamp light Philips (1000W) with different intensities (200-30) mW/cm2 using Keithley Digital Electrometer 616, and D.C. power supply as shown in Fig. (3-5)

Fig. (3-5) Circuit diagram for I-V measurement under illumination.

54

Chapter three

Experimental Part

3-11 Photodetector Measurements: To determine the photodetector measurements, the spectral responsively (R λ), quantum efficiency (η) and specific detectivity (D*) of the all samples, a suitable system used which is consisted of monochrometer in the range (200-900) nm, electrometer and power meter for measuring the radiation power for each wavelength and d.c power supply to supply bias voltage on each side of the detector (this was measured in the University of Technology) see Fig.(3-6).

Sample

Monochrometer

Halogen Lamp

Under Test

Sensitive Ammeter

Fig. (3-6) Diagram for measuring spectral responsivity. The results responsivity, the quantum efficiency (η) was obtained using Eq. (2-52) and specific detectivity (D*) was obtained using Eq.(2-57). Also to determine the response time and carrier lifetime we used (Digital Storage Oscilloscop–Twintex–TSO1202)in(200)MHz.

55

Chapter four

Results And Discussion

4-1 Introduction: This chapter include the results and discussion of the experimental measurements of the undoped and doped ZnTe films with various thickness (400,

450,

500)nm,

and

with

various

doping

percentage

ratio

(0.05,0.1,0.15,0.2,0.25)% of Al which were prepared by thermal evaporation method and doping by thermal diffusion method, the structural, optical, electrical properties of all samples films deposition on a glass and n-Si substrate have been studied, also determined the photoelectric measurements for ZnTe/Si heterojunction and finally discussed the photodetector measurement.

4-2 Structural Properties: 4-2-1 X-Ray Diffraction Results of ZnTe Alloy: The X-ray diffraction pattern (XRD) of the ZnTe alloy is shown in Fig. (4-1), the pattern included a sharp peaks at 2θ equal to [25.22o, 29.20o, 41.78o, 49.46o, 60.56o, 66.72o, and 76.34o] referred to (111), (200),(220),(311),(400),(331) and (422) direction respectively, where the values of d and 2θ alloy nearly similar to that in the ASTM card, with a number card (15-0746). Also, it can be observed that the preferential orientation is in the (111) direction, the ZnTe alloy have polycrystalline structure in nature, and have zinc blend structure. From Table (41), it can be seen the value of lattice constant which has been calculated by using equation (2-5) and the value of 2θ and dhkl for standard card and experiment work respectively.

56

Chapter four

Results And Discussion

600

( 111 )

(220)

Intensity (arb.unit )

500

400

300

( 311 )

200

( 331 )

(200 )

100

( 422 )

( 400 )

0 20

30

40

50

60

70

80

2Ɵ (Degree)

Fig. (4.1) XRD patterns for ZnTe alloy

Table. (4-1) The X-ray diffraction parameters of ZnTe alloy. 2θ (deg)

2θ (deg)

stand

Observed

25.258

25.2431

29.247

d (Å)

d (Å)

a (Å)

a (Å)

stand

observed

stand

observed

(111)

3.523

3.5252

6.1058

29.2200

(200)

3.051

3.0538

6.1076

41.804

41.8041

(220)

2.159

2.1590

49.496

49.4801

(311)

1.840

1.8406

6.1045

60.632

60.6124

(400)

1.526

1.5264

6.1056

66.745

66.7453

(331)

1.4003

1.4003

6.1037

76.398

76.3687

(422)

1.2456

1.2460

6.1041

(hkl)

57

6.1026

6.1065

Chapter four

Results And Discussion

4-2-2 X-Ray diffraction Results of ZnTe Thin Films: A. Effect of Thickness on ZnTe Thin Films Structure: Fig.(4-2) shows XRD patterns of deposited ZnTe thin films for different thickness. It is observed from XRD pattern of films that the all thickness films have polycrystalline and have cubic structure, the prominent peak was (111), this is agreement with other studies [26,32,116]. The lattice constant, crystallite size, dislocation density, and the number of crystals have been determined with the help of equations [(2-5)- (2-8)] respectively. The variation in lattice constant were measured as [6.1034, 6.1141and 6.1208] Å for the thin films of thicknesses [400, 450 and 500] nm, respectively. The FWHM of the prominent peak decreases with increasing thickness. This shows a drop in the defects due to shrinking of the grain boundaries, large crystallite size has been observed for film of thickness 500nm because of fast growth of crystallite. The increasing crystallite size and decrease dislocation density are due to the reduction in grain boundaries, this process results removal of the defects in the films, a similar behavior was observed in other studies for different thickness [117-119]. For the same reason the number of crystals decrease, where the crystallite size change in opposite manner to that of dislocation density and number of crystals.

Intensity (arb.unit)

(111)

(200)

10

20

30

(220)

40

(311)

50

60

400 nm 450 nm 500 nm

70

80

2Ɵ (Degree)

Fig. (4-2) XRD patterns for thin ZnTe films at different thicknesses. 58

Chapter four

Results And Discussion

B. Effect of Doping on ZnTe Thin Films Structure: The XRD patterns of the deposited ZnTe thin films on glass with different ratio of Al (0.05,0.1,0.15,0.2,0.25)%for thickness (400, 450, and 500) nm are illustrated in Fig. (4-3), (4-4), and (4-5), respectively. From the figures it can be observed that the Al doped ZnTe thin films have polycrystalline with cubic phases and highest sharp peak corresponding to (111) planes, similar results were obtained from other studies [27,120,121]. From the XRD patterns the structural changes in Al doped polycrystalline films it is obvious that the diffusion and location of Al atoms play an important role, where the prominent peak (111) shift to large 2θ value when the doping gradually increased, and this due to the relatively small Al atom doping entering to the lattice structure and moving to interstitial positions in the ZnTe, the resultant crystal unit cell become smaller, thus the 2θ is increased according to Bragg's Law, which that depending on the (Ionic Radius) for impurity atom, this results are a good agreement with other studies [120,121]. However the FWHM values of the highly doped Al increase, and intensity of all peaks rapidly decreases with the Al percentage increases, and this indicates that increase doping concentration attributed to the deformation in the ZnTe lattice induced by the ion size difference between Zn and Al and excess Al may also occupy interstitial positions in ZnTe lattice resulting in distorted crystal structure, similar behavior has reported by other studies [20, 31,122]. According to the Scherrer's equation the grain size change in opposite manner to the FWHM values, where the grain size decreases with the Al percentage increases while the dislocation density and number of crystals increases according to the variation in the crystallite size value. Fig.(4-6) show effect of doping concentration on crystallite size with different thickness. Also, Fig.(4-7) and (4-8) show effect of doping concentration on lattice constant, planes spacing, crystallite size and dislocation density, the number of crystals respectively. When doping concentration increase (more than 0.0.2%) the crystallite size starts increasing. This is because more and more Al atoms diffuses ZnTe and reduce the 59

Chapter four Results And Discussion dislocations of ZnTe film, this results are a good agreement with other studies [123].

Intensity (arb.unit)

(111)

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

400 nm 10

20

30

40

50

60

70

80

2Ɵ (Degree)

Fig. (4-3) X-ray diffraction pattern of Al doped ZnTe thin films for 400nm thickness for different ratio.

Intensity (arb.unit)

(111)

pure 0.05 % Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

450 nm 10

20

30

40

50

60

70

80

2Ɵ (Degree)

Fig. (4-4) X-ray diffraction pattern of Al doped ZnTe thin films for 450nm thickness for different ratio

60

Chapter four

Results And Discussion

Intensity (arb.unit)

(111) pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

10

20

30

40

50

60

70

80

500 nm

2Ɵ (Degree)

Fig. (4-5) X-ray diffraction pattern of Al doped ZnTe thin films for 500nm thickness for different ratio

34 32

Crystallite Size(nm)

30 28

400 nm

26

450 nm 500 nm

24 22 20 18 16 14 0

1

2

Doping Concentration x

3

10-1

Fig. (4-6) Effect of doping concentration on crystallite size with different thickness.

61

Chapter four

Results And Discussion 30

20

6

15 10

5.95

Lattice constant (a) planes spacing d hkl Crystallite Size (C.S )

5

5.9

0

0

1

2

3

Doping Concentration x10-1

35

6.1

30 25

6.05

20

6

15 10

Lattice constant (a) planes spacing d hkl Crystallite Size (C.S )

5.95 5.9 0

1

2

Doping Concentration x10-1

5 0

3

35

500nm 6.1

Lattice Constant a Planes spacing d hkl x1.7 (Aº)

Crystallite Size (nm)

(Ao )

Lattice Constant a Planes spacing d hkl x1.7 (Aº)

450nm

(Ao )

Crystallite Size (nm)

25

6.05

30 25

6.05 20

6

15 10

5.95

Lattice constant (a) planes spacing d hkl Crystallite Size (C.S )

5.9 0

1

Crystallite Size(nm)

Lattice Constant a (Ao ) Planes spacing d hkl x1.7 (Aº)

35

400nm

6.1

5 0

2

3

Doping Concentration x10-1

Fig. (4-7) Effect of doping concentration on lattice constant, planes spacing, and crystallite size.

62

Results And Discussion

Number of Crystale No x1016 (m-2)

15

4.5 4

400nm

3.5

10

3 2.5 2

5

1.5 1

Number of Crystals Dislocation Density

0.5

0

0

0

0.5

1

1.5

2

2.5

3

Dislocation Density x1015(m-2)

Chapter four

3.5

450nm

3

10

2.5 2 1.5

5

1

Number of Crystals Dislocation Density

0.5

0

0

0

0.5

1

1.5

2

2.5

3

Dislocation Density x1015(m-2)

Number of Crystale No x1016 (m-2)

Doping Concentration x10-1

3

500nm 10

2.5 2 1.5

5 1

Number of Crystals Dislocation Density

0.5

0

0

0

0.5

1

1.5

2

2.5

3

Dislocation Density x1015(m-2)

Number of Crystale No x1016 (m-2)

Doping Concentration x10-1

Doping Concentration x10-1

Fig. (4-8) Effect of doping concentration on dislocation density and the number of crystals.

63

Chapter four

Results And Discussion

4-2-3 Atomic Force Microscope Results (AFM): A. Effect of Thickness on the Surface Morphology of ZnTe Thin Films: The average roughness and grain size of ZnTe thin films for different thickness (400, 450, and 500) nm deposited on glass substrates have been measured by using AFM, from Fig.(4-9) it can be observed that the films are found to be uniform and densely packed without any cracks or pinholes and the average grain size was increased as the film thickness increase, and the roughness of the surface was decrease, this may be due to aggregation of grains into the larger clusters and also growth of some crystal planes. Our results were in a good agreement with other studies [32, 124].

B. Effect of doping on the Surface Morphology of ZnTe Thin Films: Figs.(4-10), (4-11) and (4-12) show the AFM images obtained for the pure and Al-doped ZnTe films deposited onto glass substrate with different ratio of Al, each film exhibits a homogeneous distribution of grains, there is a observable change in the morphology of the films when they were doped with aluminum. From Fig (4-13) one can see the surface roughness increase with increase Al percentage ratio, the roughness increase to (1.24, 1.05, 0.91) nm for figs.(4-10), (4-11) and (4-12), respectively for the films doped with Al, this increase may be due to the different kinetics of the dopant atoms and the host atoms on the film surface, the same described can see in other studies for different dopant material [25,123]. The surface roughness is less significant for the doping ratio of 0.25%. However, there will be an abrupt decrease in the surface roughness for the doping ratio more than 0.0.2%. Increasing the content of Al may increase the grain size and hence in decrease in the roughness of the films and hence significant modifications in the surface topographies have been observed [125, 126].

64

Chapter four

Results And Discussion

400nm

450nm

500nm

Fig. (4-9) Surface Morphology images of ZnTe thin films for (400, 450 and 500) nm.

65

Chapter four

Results And Discussion

pure

0.05%

0.1%

0.15%

0.25%

0.0.2%

Fig. (4-10) Surface Morphology images of ZnTe thin films for (400 nm) with different Al dopant Percentage ratio

66

Chapter four

Results And Discussion

0.05%

Pure

0.1%

0.15%

0.0.2%

0.25%

Fig. (4-11) Surface Morphology images of ZnTe thin films for (450 nm) with different Al dopant Percentage ratio.

67

Chapter four

Results And Discussion

0.05%

Pure

0.15%

0.1%

0.0.2%

0.25%

Fig. (4-12) Surface Morphology images of ZnTe thin films for (500 nm) with different Al dopant Percentage ratio.

68

Chapter four

Results And Discussion 80

1.4

70 65

1

60

0.8

55

0.6

50

0.4

Roughness (nm)

0.2

Grain Size (nm)

45

Grain Size (nm)

Roughness (nm)

75

400 nm

1.2

40 35

0

30

0

0.5

1

1.5

2

2.5

3

Doping Concentration x10-1

80

Roughness (nm)

75

450 nm

1.2

Grain Size (nm)

1.4

70

1

65

0.8

60

0.6

55

Roughness (nm)

0.4

50

Grain Size (nm )"

0.2

45

0

40

0

1

2

3

Doping Concentration x10-1

1

85

500 nm

0.9

Roughness (nm)

0.7

75

0.6

70

0.5 65

0.4 0.3 0.2 0.1

Roughness (nm)

60

Grain Size (nm )"

55

0

Grain Size(nm)

80

0.8

50

0

1

2

3

Doping Concentration x10-1

Fig (4-13). Surface roughness and grain size change as a function of dopant percentage ratio for Al with different thickness.

69

Chapter four

Results And Discussion

4-2-4 Elemental Composition results (EDS): The results of elemental compositions are shown in Table (4-2). It can be seen from table that for alloy of ZnTe that the amount or (concentration) of the elements (Zn, Te) examined by (EDS) technique are close to the theoretical concentration values. Table. (4-2) The elemental composition concentration of alloy (EDS)

4-3

Elements

Actual concentration %

Experimental concentration %

Zn

33.878

33.87

Te

66.122

66.13

Total%

100

100

Optical Properties:

The optical properties of the deposited ZnTe films on glass for thickness (400, 450 and 500) nm with different percentage ratio for Al have been determined which includes transmission, absorption, absorption coefficient , energy band gap of thin films and optical constants. 4-3-1 Transmission Spectrum: A. Effect of Thickness on Transmission Spectrum: Fig. (4-14) shows the optical transmittance spectra with wavelength from 400 nm to 1000 nm of the ZnTe thin films with different thicknesses. It is shown that the transmission of these films increases rapidly within the range 500-850nm reaching the maximum value. After this maximum, the transmission approximately remains constant at near-Infrared wavelengths. Also, from these curves one can be see the transmittance of the thin films varies between (60.7% - 88.4%), maximum of 88.4% being reached for ZnTe thin films with lower thicknesses (400nm). By increasing the thicknesses (from 400 to 500) nm, the 70

Chapter four Results And Discussion spectra show decreasing in the transmission. There is a sharp fall from 88.5% to 60.7% in the transmittance as the thicknesses rises from 400 to 500 nm, due to increase in the density of the film and due to highly transparent in visible and IR region, similar behavior has reported by other studies [127, 128]. The same described can be noticed in other studies for different dopant material where the transmittance spectra of the films decrease as film thicknesses increases, which is identifies a good crystallinity of obtained thicknesses [16]. The transmittance spectra of the films decrease as film thickness increases, which subsequently increase absorption. The onset of absorption edge for films became less sharp which is due to the fact that bigger crystalline sizes are deposited and because in the case of more atoms are present in the film so more states will be available for the photons to be absorbed, similar behavior has

transmittance %

reported by other studies [129, 130]. 100 90 80 70 60 50 40 30 20 10 0

400nm 450nm 500nm 400

500

600

700

800

900

1000

wave length (nm)

Fig. (4-14) Optical transmittance spectra as a function of wavelength for ZnTe thin films with different thickness.

B. Effect of doping on Transmission Spectrum: Optical transmittance spectra with wavelength from 400nm to 1000 nm was shown in Fig.(4-15) of the ZnTe thin films pure and doped with Al for thickness [400,450 and 500]nm. Itis seen the transmittance value decreases with the increase of Al doping concentration, there is sharp fall from (73.36% to 43.88% 71

Chapter four Results And Discussion ), (60.98% to 31.16% ) and (59.0.2% to 28.81% ) for (400, 450 and 500),

transmittance %

respectively. 100 90 80 70 60 50 40 30 20 10 0

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

400nm

400

500

600

700

800

900

1000

transmittance %

wave length ( nm)

100 90 80 70 60 50 40 30 20 10 0

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

450nm 400

500

600

700

800

900

1000

transmittance %

wave length (nm)

100 90 80 70 60 50 40 30 20 10 0

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

500nm 400

500

600

700

800

900

1000

wave length ( nm)

Fig. (4-15) Transmittance spectra of Pure and Al doped ZnTe thin films for different thickness 72

Chapter four Results And Discussion This decrease in the transmission can be attributed to the introduction of impurity level between valance band and conduction band [131, 132]. This behavior is well agreed with the report of other worker used different dopant material, and this decrease in the transmittance is due to the increased scattering of photons by crystal defects by doping [133-135].

4-3-2 Absorbance Spectrum and absorption coefficient : A. Effect of Thickness on Absorbance Spectrum and absorption coefficient: From Figs.(4-16) and (4-17) it can be seen the absorption spectra and absorption coefficient of ZnTe films deposited onto a glass substrate, the absorption edge shifts to higher wavelength for higher thicknesses and it changes with film thicknesses. This increase in absorption spectrum with increase thickness due to decrease in transmittance where bigger crystalline sizes case more atoms in the film so more states will be available for the photons to be absorbed, similar behavior has reported by other studies [129, 130], and the increase in absorption suggesting a decrease in the band gap due to increase thickness. 100

400nm

80

Absorbance %

450nm 500nm

60 40 20 0 400

500

600

700

800

900

1000

wave length (nm)

Fig. (4-16) Absorption spectra of ZnTe thin films with different thicknesses.

73

Chapter four Results And Discussion The ability of a material to absorb light is measured by its absorption coefficient and it is a very strong function of the photon energy and band gap energy [136]. The variation of the optical absorption coefficient with photon energy for various thicknesses is shown in Fig. (4-17). The variation of optical absorbance with wavelength reveals a high absorption of energy at shorter wavelength and vice versa. The spectra also confirms that with increasing film thicknesses the absorption effect increase exponentially due to the effect of index of refraction of films, similar behavior has reported by other studies [137]. 6.00E+04 400nm

a (cm -1)

450nm 500nm

4.00E+04

2.00E+04

0.00E+04 1

1.5

2

2.5

3

photon Energy(ev)

Fig. (4-17) The absorption coefficient of ZnTe thin films with different thicknesses. B. Effect of doing on Absorbance Spectrum and absorption coefficient: Absorbance spectra with wavelength is shown in Fig.(4-18) from 400nm to 1000 nm of the ZnTe thin films pure and doped with Al with different ratio for thickness [400,450 and 500]nm. It is seen the absorption value increase until it reaches great value (88.43, 85.8 and 83.6)% for thickness (400,450 and 500)nm respectively and this in percentage ratio 0.2% of Al, the high absorption occurs due to decrease in the transmittance, the wavelength of the incident radiation is less than the wavelength cut off (λ < λ cut off) and that are within the visible

74

Chapter four

Results And Discussion

100

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

400nm Absorbance %

80 60 40 20 0 400

500

600

700

800

900

1000

wave length ( nm)

100

450nm

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

Absorbance %

80 60 40 20 0 400

500

600

700

800

900

1000

wave length (nm)

100

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

500nm Absorbance %

80 60 40 20 0 400

500

600

700

800

900

1000

wave length (nm)

Fig. (4-18) Absorption spectra of Pure and Al doped ZnTe thin films for different thickness

75

Chapter four Results And Discussion region of the electromagnetic spectrum. Also, we note from the absorption spectra that percentage ratio caused by 0.2% for thickness (500nm) show better absorption of other samples between (56.64-88.43)% within the visible region (400-700) nm of the electromagnetic spectrum, mainly those that satisfy (λ < λ cut off),

and when we return to the transparent spectra can see the same ratio have

more demonstrate transparency from other samples within the same area in the spectrum of transparent [Fig.(4-15) thickness 500nm]. On this basis, we can take advantage of this film in manufacturing photodetector where this high absorption for (500nm film ratio 0.2%) is very much useful and the best in application for ZnTe detectors. Fundamental absorption edge aberration towards long wavelengths (low energy). This is due to the dopant ratio where the doping increase the density of the donor localized states near the conductivity package. So the possibility of absorption of electrons to photons with energies less than the value of energy gap of ZnTe films perfectly possible. This agrees with characteristic of measurement (XRD), where increase the dislocation density and number of crystals with increasing dopant ratio will increase the absorbency film of light, similar behavior has reported by other studies [131, 132] with the different dopant material where the absorption value increase with the increase of Al doping concentration due to decrease in the transmittance and this due to attributed to the introduction of impurity level. The absorption coefficients as a function of photon energy (hν) are shown in Fig.(4-19). The behavior of the absorption coefficients is very similar to the behavior of the absorption spectrum according to the relationship between them as in Eq. (2-13), we find lower absorption coefficient α with lower photon energies under (1.5) eV, then begins (α) with a gradual increase to all films with increasing energy of the photon energies, in the range of (1.5-1.8) eV, and then quickly increase sharply over energies that equal to or greater than the optical energy gap of all films in the range of energies (2 -2.4) eV, this is due to a

76

Chapter four

Results And Discussion 400nm

6.00E+04 5.00E+04

a (cm -1)

4.00E+04 pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

3.00E+04 2.00E+04 1.00E+04 0.00E+04 1

1.5

2

2.5

3

Photon Energy(eV)

6.00E+04

450nm

a (cm -1)

4.00E+04 pure 0.05% Al 0.1% Al

2.00E+04

0.15% Al 0.2% Al 0.25% Al

0.00E+04 1

1.5

2

2.5

3

Photon Energy(eV) 6.00E+04

500nm

a (cm -1)

4.00E+04

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

2.00E+04

0.00E+04 1

1.5

2

2.5

3

Photon Energy(eV)

Fig. (4-19) The absorption coefficient of Pure and Al doped ZnTe thin films for different thickness

77

Chapter four Results And Discussion process of absorption occurs within the range includes the optical absorption edge, the absorption coefficient α indicates a long band tail in the absorption curves, which may cause due to defects and impurities of the samples, this agree in other studies for different dopant material [30]. Compared to undoped ZnTe, the ZnTe:Al shows significantly very high absorption in the visible region. The absorption was observed to be very high at shorter wavelength 600nm for doped films. As the doping concentration increased, the absorption coefficient increases, this is probably due to the increase in fundamental absorption as photon striking increases with increase in carrier concentration. The photon energy is just above the band gap due to which the electrons are excited from valence band into conduction band. This generates electron-hole pairs and a new charge carrier distribution is created. This behavior is in a good agreement with other studies [45].

4-3-3 The Optical Energy Gap: A. The effect of thickness on Optical Energy Gap: The fundamental absorption, which corresponds to electron excitation from valance to conduction band, can be used to determine the nature and value of the optical band gap [83]. The optical band gap energy Eg was obtained from the intercept on the photon energy axis after extrapolating of the straight line section of the curve of (αhν)2 versus (hν) plot as shown in Fig.(4-20). Due to increase in film thicknesses the results are decrease of energy band gap from (2.24 to 1.98) eV, the individual levels of free atoms will broaden the energy bands and create overlapping levels. This occurs when atoms become closer to each other. Hence, with high film thicknesses there are several energy levels resulting in several overlapping energy bands in the band gap of these films. The overlapping energy bands therefore tend to reduce the energy band gap, resulting in lower band gaps for increment on films thicknesses, this is a good agreement with other studies [138]. 78

(ahv)2 (eV .cm-1)2

Chapter four

Results And Discussion 400nm 450nm 500nm

1.00E+02

Eg(400nm)=2.24 eV Eg(450nm)=2.1 eV Eg(500nm)=1.98 eV

0.00E+00 0.5

1

1.5

2

2.5

3

photon Energy(eV)

Fig. (4-20) Variation of (αhv)2 with photon energy of ZnTe thin films with different thicknesses. B. The effect of doping on Optical Energy Gap: When compared the band gap value of pure ZnTe films with Al doped films, see decreased in band gap value due to the segregation of impurities at the grain boundaries. The optical band gap value decreased for 400nm thickness from (2.24 to 1.86)eV, for 450nm thickness from (2.1 to 1.84)eV, and for 500nm thickness from (1.98 to 1.7) eV, see Fig.(4-21). Associated with the introduction of the Al impurity level between the valance and conduction bands. Similar observation have been made for In doped ZnTe films [139], and for Cu doped ZnTe films [31]. The Band gap values are found to decrease with increase of Al doping contents (up to 0.2%) and then start to increase again with higher Al doping content, where increase to (1.92, 1.9 and 1.74) eV, when percentage ratio for Al (0.25%) for thickness (400, 450 and 500) nm, respectively. We expect increase in band gap of ZnTe thin films when increase Al ratio above 0.2% due to increase in carrier concentrations, the impurities act as effective scattering centers and trap centers of grain bounders occurs from

crystal

structure deformation similar observation have been made from other studies [140].

79

Chapter four

Results And Discussion

2.00E+02

400nm (ahv)2 (eV .cm-1)2

1.50E+02 pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

1.00E+02

5.00E+01

0.00E+00 1

1.5

2

2.5

3

photon Energy(eV) 2.00E+02

450nm (ahv)2 (eV .cm-1)2

1.50E+02

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

1.00E+02

5.00E+01

0.00E+00 1

1.5

2

2.5

3

photon Energy(eV)

(αhv)2 (eV .cm-1)2

1.00E+02

500nm

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

5.00E+01

0.00E+00 0.5

1

1.5

2

2.5

3

photon Energy(eV)

Fig. (4-21) (ahυ) 2 versus hυ of Pure and Al doped at different thickness. Fig.(4-22) explain the band gap values decrease with increase of Al doping for different thickness.

80

Chapter four

Results And Discussion 400 nm 450 nm 500nm

Energy gap (eV)

2.7

2.2

1.7

1.2 0

0.5

1

1.5

2

2.5

Doping ratio x10-1 %

Fig. (4-22) Energy gap versus of Pure and Al doped at different thickness.

4-3-4 optical constant: Optical constants included refractive index (n), extinction coefficient (k), real part (Ɛ1) and imaginary parts (Ɛ2) of optical dielectric constant. It is obvious from Fig.(4-23) effect of thichness and doping, the refractive index values increase with the increase photon energy, than at higher photon energy the refractive index values decreases reaching the lowest value with increase thichness, and decreases with the increase of Al dopants. Fig.(4-24) shows the extinction coefficient increases with the increase thichness and with increase Al dopants, this behavior of refractive index and extinction coefficient may be due to the density of the localized states increases with increasing film thickness. This behavior of the extinction coefficient values similar for all the range of the wavelength spectrum to that of the absorption coefficients for the same reasons as mentioned before. Results are close to those obtained by other studies [30]. Fig.(4-25) and (4-26) show effect of thickness and effect of Al dopants on the values of the real (ε1) and imaginary (ε2) parts of the dielectric constant. From this Figs we can notice that the real part of the dielectric constant (ε 1) decrease with the increase of thickness and decrease with increase Al dopants in

81

Chapter four Results And Discussion all the range of the spectrum while the imaginary part of the dielectric constant (ε2) showed an reverse trend because the variation of (ε1) mainly depend on the value of the refractive index while the (ε2) value mainly depend on the extinction coefficient values which are related to the variation of absorption coefficient. Table.(4-3) shows the evaluated some of the optical constants values.

Refractive Index (n)

3

400 nm 450 nm 500nm

2.5

2

1.5

1 0

0.5

1

1.5

Doping ratio

x10-1

2

2.5

%

Fig. (4-23) The refractive index change with Pure and Al doped at different thickness at λ ≈ 600 nm.

Extinction coefficient (K)

0.2

0.15

0.1

400 nm 450 nm 500nm

0.05

0 0

0.5

1

1.5

2

2.5

Doping ratio x 10-1 %

Fig. (4-24) The extinction coefficient change with Pure and Al doped at different thickness at λ ≈ 600 nm.

82

Chapter four

Results And Discussion

Real Part of Dielectric Constant

8

400 nm

7

450 nm 6

500nm

5 4 3 2 1 0

0.5

1

1.5

2

2.5

Doping ratio x10-1 %

Fig. (4-25) the real (ε1) part of the dielectric constant change with Pure and Al

Imagenry Part of Dielectric Constant

doped at different thickness at λ ≈ 600 nm.

0.8 0.75 0.7 0.65 0.6 0.55 0.5

400 nm

0.45

450 nm

0.4

500nm

0.35 0.3 0

0.5

1

1.5

2

2.5

Doping ratio x10-1 %

Fig. (4-26) the imaginary (ε2) part of the dielectric constant change with Pure and Al doped at different thickness at λ ≈ 600 nm.

83

Chapter four Results And Discussion Table. (4-3) optical band gap and optical constant at λ ≈ 600 nm of ZnTe films

Thick nm

Al ratio%

Eg (eV)

n

k

ε1

ε2

0

2.24

2.63492

0.0909

6.93452

0.448

0.05

2.22

2.63208

0.10994

6.91578

0.5112

0.1

2.2

2.57

0.12464

6.321

0.598

0.15

2.08

2.27

0.15

5.1643

0.6245

0.2

1.86

1.91264

0.18

3.62067

0.6451

0.25

1.92

2.0875

0.17993

4.119

0.6351

0

2.1

2.21412

0.1145

4.884

0.47903

0.05

1.98

2.1922

0.1237

4.786

0.557

0.1

1.94

2.1267

0.14131

4.5032

0.635

0.15

1.92

1.9282

0.1632

3.694

0.667

0.2

1.84

1.61474

0.19021

2.57122

0.693

0.25

1.9

1.77141

0.18114

3.10507

0.47903

0

1.98

2.15361

0.13514

4.4312

0.5078

0.05

1.9

2.05546

0.139

4.1985

0.598

0.1

1.86

1.85

0.15495

3.754

0.67

0.15

1.8

1.66722

0.1745

2.76009

0.7234

0.2

1.7

1.4074

0.19675

1.9483

0.75133

0.25

1.74

1.5837

0.1656

2.5238

0.73561

400

450

500

84

Chapter four

Results And Discussion

4-4 The Electrical Properties: 4-4-1 The Effects of thickness on D.C Conductivity: Fig.(4-27) shows the variation of d.c conductivity for thin ZnTe films as a function of the reciprocal temperature at different thickness (400, 450 and 500) nm, respectively, there are two mechanisms of activation energy (Ea1 and Ea2)

ln [σd.c (Ω.cm)-1 ]

throughout the temperature range (239- 423) K, see Table. (4-4).

400 nm

6 5.8 5.6 5.4 5.2 5 4.8 4.6 4.4 4.2 4

450 nm 500 nm

2.2

2.7

3.2

1000/T (K)-1

Fig. (4-27) Ln σ versus 1000/T for ZnTe films at different thickness.

At higher temperature range (353 - 423)K the conduction mechanisms is due to carrier excited into the extended states beyond the mobility edge by thermal excition and at lower temperature range (293 – 353)K, the conduction mechanisms due to the carriers transport to localized states near the valence and conduction bands and hopping between restricted levels within the energy gap [83]. From d.c measurement we can see that the conductivity increases and the resistivity decreases with increasing the temperature, this is due to the property of semiconductors which have resistance of negative thermal coefficient. Results are close to those obtained by other studies [78]. Also, the conductivity increases with the film thickness increases while the activation energy(Ea) decreases, where this increases can be attributed to the increase in the mobility and the 85

Chapter four Results And Discussion carrier concentration [26].This behavior due to increase of grain size as shown in XRD results. Increasing thickness of the film causes fall in the resistivity due to the corresponding grain size, with higher grain size the defects decreasing causing decrease in the resistivity. Results are close to those obtained by other studies [141-142]. Table. (4-4) the activation energy for thin ZnTe films for different thickness Film thickness

Ea1

Tem. range

Ea2

Tem. range

nm

( e V)

(K)

( e V)

(K)

400

0.0362

(293 – 353)

0.1455

(353 -423)

450

0.0311

(293 – 353)

0.1166

(353 -423)

500

0.0271

(293 – 353)

0.1078

(353 -423)

4-4-2 The Effects of doping on D.C Conductivity: The plots of (Ln σ) versus 1000/T for ZnTe films at different percentage ratio of Al are shown in Figs. (4-28 a, b, c for thickness (400, 450,500)nm, respectively) . Also, there are two stages of d.c conductivity mechanism throughout the temperature range (293- 423 )K as shown in Table (4-5). The conductivity increases with Al doping increase which lead to decrease in the activation energy, this can be attributed to that impurity had led to the formation of donor levels in energy gap and near the conductivity edge, which leads to reduced energy gap and that leads to stimulate a greater number of electrons which have lower energy cross to conduction band, which increase the concentration of carriers and thus increase the electrical conductivity accord with equation (2-23). Reduction in the resistivity can be interpreted by the increase in the number of the charge carriers coming from the ions Al incorporated in the substitutional sites from cation of Zn [143], the electrical resistivity of ZnTe thin films after Al-doping was dropped with significant

86

Chapter four Results And Discussion increase in mobility, results are close to those obtained by other studies [142] with different dopant atoms. Also, the

electrical resistivity of

films doping more than (0.2%) Al,

increase due to high value energy gap of films, which is actually found in optical measurements of films doping with (0.25%) Al, where the electrons will need more energy for pass to conduction band, as well as a decrease in the concentration of charge carriers (who proved in Hall effect measurements), which lead to decreased electrical conductivity when percentage ratio of Al increase more than (0.2%), this result agrees with X- ray diffraction measurements (XRD) where the crystallinity of films deterioration when percentage ratio of Al increase more than (0.2%)[140]. The value of activation energy decreases with increasing the dopant ratio with aluminum, where the energy that electrons require to reach to conduction band decreases with increasing dopant ratio of Al until (0.2%), due to formation donor levels within the energy gap near conduction band and the Fermi level approaches more toward conduction band, this is lead to increase conductivity and decrease the values of activation energy depending on doping, results are close to those obtained by other studies [144, 145]. We can see from section (4-4-1 and 4-4-2) there are two stages of d.c conductivity mechanism throughout the temperature range (293- 423K), this supports the combination results for XRD, where all films have polycrystalline structure.

87

ln [σd.c (Ω.cm)-1 ]

Chapter four

Results And Discussion

6 5.8 5.6 5.4 5.2 5 4.8 4.6 4.4 4.2 4

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

a

2.2

2.7

3.2

1000/T (K)-1

ln [σd.c (Ω.cm)-1 ]

6.5

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

b

6 5.5 5 4.5 4

ln [σd.c (Ω.cm)-1 ]

2.2

2.7

3.2

1000/T (K)-1

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

c

6.3 6.1 5.9 5.7 5.5 5.3 5.1 4.9 4.7 4.5 2.2

2.7

3.2

1000/T (K)-1

Fig. (4-28) Ln σ versus 1000/T for ZnTe films at different thickness and different ratio of Al.

88

Chapter four Results And Discussion Table. (4-5) the activation energy for ZnTe films for different doping ratio. Thick nm

400

450

500

Ratio % doping

Ea1 ( e V) At Tem.range (K) (293 – 353)

Ea2( e V) At Tem.range (K) (363 – 423)

0

0.0362

0.1455

0.05

0.0334

0.1286

0.1

0.0261

0.11942

0.15

0.0244

0.10546

0.2

0.0196

0.09441

0.25

0.0197

0.10476

0

0.0271

0.11664

0.05

0.0220

0.11542

0.1

0.01867

0.11329

0.15

0.01610

0.1025

0.2

0.01164

0.08845

0.25

0.01339

0.0956

0

0.0183

0.10787

0.05

0.01314

0.10658

0.1

0.01239

0.10424

0.15

0.01095

0.09819

0.2

0.00861

0.0731

0.25

0.01050

0.0926

89

Chapter four

Results And Discussion

4-4-3 Hall Effect for ZnTe films in different thickness: The type of charge carriers, concentration and Hall mobility, have been estimated from Hall measurements. The positive sign of Hall coefficient indicates the conductivity nature of the film is p-type. Fig.(4-29) shows the variations of carriers concentration and Hall mobility with thickness, we can notice from figure that the carriers concentration increased with increasing thickness, where increase thickness of the film causes fall in the resistivity, the ultimate change in resistivity is due to the corresponding grain size [141-142]. With higher grain size, the defects were found decreasing causing decrease in the resistivity. Also, we see increase in the Hall mobility with increasing thickness, where there is a direct proportion between the carriers concentration and the conductivity then this proved in the d.c. conductivity, results are close to those obtained by other studies [26] with different deposition technique and

3.80E+17 800

3.30E+17 2.80E+17 2.30E+17

600

1.80E+17

n M

1.30E+17 8.00E+16

Carrier Mobility µ (cm2/V.S)

Carrier Concentration n (cm-3)

thickness.

400

350

400

450

500

550

Thicknees (nm)

Fig. (4-29) Variation of carrier concentration and mobility as a function of thickness for ZnTe films

90

Chapter four 4-4-4

Results And Discussion

Hall Effect for ZnTe films in different doping ratio:

Fig.(4-30) show the variations of carriers concentration and Hall mobility with different doping ratio for different thickness, both the carriers concentration and Hall mobility in general increases and the Hall coefficient decrease with the increasing Al ratio for all films under 0.2% ratio. The electrical resistivity of ZnTe films doping with Al is lower than that of the undoped ZnTe films due to the free electrons released by the interstial of Al+3 at the sites occupied by Zn+2, the decrease in resistivity for Al doped films may be due to a increase in carrier concenteation density. At higher thickness 500nm and with Al ratio 0.2%, ZnTe films the improvment crystallinity is obtained, which in turn may reduce grain boundary scattering with a consequent reduction in the resistivity of the films, results are agree with worker obtained by other studies [14,18,24] with different doping atoms. the electrical resistivity of films doping more than (0.2%) Al, increase due to high value energy gap of films, which is actually found in

optical

measurements of films doping with (0.25%) Al. This result is related to the structure which mean that with (0.25% )Al ratio the grain size decrease and this led to increases of the trapping centers which decrease both the number of charge carriers and their mobility essentially decrease because of the increases the band gap. In addition, a decrease in the concentration of charge carriers (who proved in Hall effect measurements), which lead to decreased electrical conductivity when percentage ratio of Al increase more than (0.2%) due to increases the band gap, this result agrees with X- ray diffraction measurements (XRD) where the crystallinity of films deterioration when percentage ratio of Al increase more than (0.2%), results are agree with worker obtained by other studies [140]. So, in general the doping with aluminum improves the electrical properties of the ZnTe films, these results showed that films of

thickness 500nm and

percentage ratio 0.2% Al has achieved the highest value of the carrier 91

Chapter four Results And Discussion concentration, conductivity mobility and lower values for activation energies,

800 700

n

600

M 500

0

0.5

1

1.5

2

2.5

Doping ratio %

Carrier Concentration n (cm-3)

1E+18

900

450 nm

9E+17 8E+17

800

7E+17 6E+17 5E+17

700

4E+17 3E+17

n

2E+17

600

M

1E+17 0

500

0

0.5

1

1.5

2

2.5

Doping ratio %

2E+18

Carrier Concentration n (cm-3)

Carrier Mobility µ (cm2/V.S)

900

400 nm

Carrier Mobility µ (cm2/V.S)

9E+17 8E+17 7E+17 6E+17 5E+17 4E+17 3E+17 2E+17 1E+17 0

900

500 nm 1.5E+18

800

1E+18

700

5E+17

600

n M

0

500

0

0.5

1

1.5

Doping ratio %

2

2.5

Carrier Mobility µ (cm2/V.S)

Carrier Concentration n (cm-3)

which make him suitable for use in optoelectronic devices application.

Fig. (4-30) Variation of carrier concentration and mobility as a function of doping ratio for ZnTe films with thickness(400,450,500)nm

92

Chapter four

Results And Discussion

4-5 Electrical Properties of ZnTe /Si Heterojunctions: The

measurement

(Capacitance–Voltage)

of

important

electrical

measurements used to determine some characteristics of heterojunction, from CV can be calculated the built in potential (Vbi), give an idea of the depletion width layar (W) and capacity at zero biasing voltage (Co). The variation of capacitance as a function of reverse bias voltage in the range of (0-2.8) Volt at frequency equal to 10 kHz has been studied; and all the measurements were performed in air under dark at room temperature. 4-5-1 Effect of thickness on C-V Characteristic of ZnTe /Si Heterojunctions: Fig.(4-31 a) shows change heterojunction capacity of unit area with reverse bias voltage at frequency (10 KHz) for different thickness. It is clear that capacitance decrease with increasing reverse bias voltage, such behavior is attributed to increase the depletion width, which leads to increase the value of built in potential as a result of improved crystal structure of the films. From Table.(4-6) we can observe that the capacitance at zero bias voltage (C o) decreases with the increasing of ZnTe thickness. We relate such data as behavior due to the increasing of the depletion region. Where the evolving defects and dislocations have effect on mobility of charge carrier at interface and also these defects evolutions allow energy levels to be within the energy gap, which act as active recombination centers. The inverse capacitance square is plotted against applied reverse bias voltage at different thickness as shown in Fig.(4-31 b), the plots revealed straight line relationship which means that the junction was of an abrupt type, our results good agreement with other studies [146].

93

Chapter four Results And Discussion Table. (4-6) Values of Co, W, NA, and Vbi, for ZnTe/Si hetrojunection Thickness nm

Co nf/cm2

W nm

NA (cm-3)

Vbi(volt )

400

98.80

49.34

5.59 E+15

1.7

450

88.76

54.92

6.31 E+15

1.8

500

82.09

59.38

7.13 E+15

1.85

100

a 400 nm

90

450 nm 500 nm

C (nF/cm2)

80 70 60 50 40

-3

-2.5

-2

-1.5

-1

-0.5

0

V (volt)

45

b

1/C2 *10-5 (nF/cm2)-2

40 35 30

400 nm

25

450 nm

20

500 nm

15 10 5 -3.5

-2.5

-1.5

0 -0.5

0.5

1.5

2.5

3.5

V (volt)

Fig.(4-31) [a- Variation of capacitance as a function of reverse bias voltage bthe variation of 1/C2 as a function of reverse bias voltage] for ZnTe/Si heterojunction at different thickness 94

Chapter four

Results And Discussion

4-5-2 Effect of doping on C-V Characteristic of ZnTe /Si Heterojunctions: Fig.(4-32) shows change heterojunction capacity of unit area with reverse bias voltage at frequency (10 KHz) with different doping ratio for different thickness. We can observe a decrease in the value of capacity with increased Al doping ratio, and this is can explain that aluminum atoms have better-quality of electrical characteristics where reduce resistivity and increase connectivity, leading to increased carrier concentration which increases the built in potential. Therefore, an increase in depletion region width (W) which results a decrease in heterojunction capacity [90 , 96]. The inverse capacitance square is plotted against applied reverse bias voltage with different Al doping ratio at different thickness shown in Figure (4-33), the plots revealed straight line relationship which means that the junction was of an abrupt type [90]. Also, Fig.(4-32) shows the capacity of films doping more than (0.2%) Al, increase due to high ratio of Al doping led to increases of the trapping centers which decrease the number of charge carriers, built in potential and depletion region width. These results indicate that the capacitance at zero bias voltage (C0) decrease with doping and this is attributed to the decrease in the surface states which may be leads to a increases in the depletion layer[83,98]. pure

400nm

0.05% Al

100

0.1% Al

0.2% Al

80

0.25% Al

70 60 50 40 -3

-2.5

-2

-1.5

V (volt)

95

-1

-0.5

0

C (nF/cm2)

90

0.15% Al

Chapter four

Results And Discussion

pure

450nm

0.05% Al

90

0.1% Al 80

0.15% Al 0.2% Al

C (nF/cm2)

70

0.25% Al

60 50 40 30 -3

-2.5

-2

-1.5

-1

-0.5

0

V (volt)

pure

500nm

90

0.05% Al 0.1% Al

70

0.2% Al 0.25% Al

60

C (nF/cm2)

80

0.15% Al

50 40 30 -3

-2

-1

0

V (volt)

Fig.(4-32) Variation of capacitance as a function of reverse bias voltage for ZnTe/Si heterojunction with different doping ratio at different thickness

96

Chapter four

Results And Discussion 35

1/C2 *10-5 (nF/cm2)-2

400nm

Vbi=1.7 V for pure Vbi=1.75 V for 0.05% Vbi=1.85 V for 0.1% Vbi=1.95 V for 0.15% Vbi=2 V for 0.2% Vbi=1.975 for 0.25%

30 25 20

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

15 10 5 0

-2.5

-1.5

-0.5

0.5

1.5

2.5

V (volt) 60

1/C2 *10-5 (nF/cm2)-2

450nm

Vbi=1.8 V for pure Vbi=1.85 V for 0.05% Vbi=1.9 V for 0.1% Vbi=2 V for 0.15% Vbi=2.12 V for 0.2% Vbi=2.05 V for 0.25%

50 40 30 pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

-2.5

20 10 0 -1.5

-0.5

0.5

1.5

2.5

V (volt)

1/C2 *10-5 (nF/cm2)-2

60

500nm

Vbi=1.85 V for pure Vbi=1.9 V for 0.05% Vbi=1.95 V for 0.1% Vbi=2.05 V for 0.15% Vbi=2.2 V for 0.2% Vbi=2.15 V for 0.25%

50 40 30

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

20 10 0

-2.5

-1.5

-0.5

0.5

1.5

2.5

V (volt)

Fig.(4-33) Variation of 1/C2 as a function of reverse bias voltage for ZnTe/Si heterojunction with different doping ratio at different thickness. 97

Chapter four

Results And Discussion

4-5-3 Effect of thickness on I-V Characteristic of ZnTe /Si Heterojunctions under Dark.: Current – voltage (I-V) characteristic is one of important parameters of junction measurement which explains the behavior of the resultant current with applied forward and reverse bias voltages. Fig.(4-34) shows the I-V characteristics of ZnTe /Si heterojunctions deposited with different thickness, the forward currents have exponential rise at low voltage where the forward dark current is generated due to the flow of majority carriers and the applied voltage inject majority carriers which leads to the decrease of the built – in potential, as well as the width of the depletion layer. As the majority and minority carrier concentration is higher than the intrinsic carrier concentration, which generate the recombination current at the low voltage region. This is due to the excitation of electrons from valence band (V.B.) to conduction band (C.B.) will recombine them with the holes which are found at the V.B., and this is observed by the little increase in recombination current at low voltage region [147], however at the high voltage region the tunneling current occurs. After that there is a fast exponential increase in the current with increasing of the voltage and this is called diffusion current, which is dominated [148]. the reverse currents are very weak and also contains two regions. At

low voltages, the current slightly

increases with increasing of the applied voltage, and the generation current dominates, while at high voltage region, the diffusion current dominates [148]. Also, the value of the current increases with increasing of thickness of ZnTe films which is attributed to evolving defects and dislocations that have effect on mobility of charge carrier at interface and also these defects evolutions allow energy levels to be within the energy gap. Table (4-7) and Fig.(4-35) show the saturation current and the ideality factor have been calculated, it is clear that the ideality factor decrease with increasing of thickness, where this behavior attributed to improvement of crystal structure[149].

98

Chapter four Results And Discussion Table. (4-7) the saturation current and ideality factor for ZnTe/ Si heterojunctions at different thickness. Thickness nm

Ideality factor (β)

Saturation current (Is) µA

400

2.02

1.28

450

1. 81

3.85

500

1.61

5.22

260 400 nm 450 nm 500 nm

210

I (µA)

160 110 60 10

-3

-2

-1

-40

0

1

2

3

V (volt)

Fig (4-34) I-V Characteristic of ZnTe /Si heterojunctions under dark with different thickness V (volt) -6 0

1

2

3

ln (J) (Amp/cm^2)

-8 -10 -12 -14

pure

0.05% Al

-16

0.1% Al

0.15% Al

-18

0.2% Al

0.25% Al

-20

Fig. (4-35) ln (J) versus V for forward bias of dark of ZnTe/ Si heterojunction at different thickness

99

Chapter four

Results And Discussion

4-5-4 effect of doping on I-V Characteristic under Dark.: Fig.(4-36) shows change dark current as a function of forward and reverse bias voltages of ZnTe /Si heterojunctions and the effect of different doping ratio on dark current in forward and reverse bias. In general, under the forward bias condition the I-V curves of all heterojunction show the exponential rise at low voltage where this is due to decrease in the width of depletion region at the junction as a result of an increase in the majority carriers injected by the applied voltage which lead to the decrease in the built-in potential. At low voltages (V< 0.4 Volts), the current exponential rise with increasing of the applied voltage and the recombination current dominates, while at high voltage region (V>0.4 Volts) , the diffusion current dominates [83,88]. Also, it can be seen from these figures that for all heterojunction, the forward current increases with doping of Al, where this is due to decrease the energy gap and activation energy. Moreover, increaseing of carrier concentration which leads to increase conductivity and increase current with increase doping ratio, this result agrees with optical measurements and Hall measurements results. While under reverse bias condition the value of dark current for Al-doped ZnTe/Si heterojunction is slightly less than dark current for pure heterojunctions, also dark current and saturation current decrease with doping [150]. Also, under reverse bias there are two regions, first at low voltage where the generation current dominates, while at high voltage region, the diffusion current dominates. Furthermore, when doping increases more than (0.2%) Al, the dark current increase with applied voltage due to high ratio of Al doing led to increases of the dislocation density and trapping centers which decrease the number of charge carriers, and due to increases the energy gap and activation energy. A semi logarithmic plot of the forward dark current versus applied voltage for undoped and doped ZnTe/Si heterojunction are shown in Fig.(4-37), by using Eq.(2-39) and (3-4) the values of saturation current and ideality factor are listed in Table.(4-8). The ideality factor greater than unity (𝛽 > 1) and the ideality factor decrease with increases Al doing ratio and reach to (1.411) for 500nm with 0.2% doping ratio, this 100

Chapter four Results And Discussion means that each electron excitation from valence band to conduction band will recombine with the holes which are found at the valence band [83]. The tunneling effect depending on both sides of the heterojunction and the presence of defect states attributed to make the value of ideality factor greater than unity [151. Table. (4-8) The saturation current and ideality factor for ZnTe/ Si heterojunctions with different doping ratio at different thickness. Thickness nm

Doping ratio%

Ideality factor (β)

Saturation current (Is) µA

0

2.021

1.28

0.05

2.01

1.91

0.1

1.971

2.33

0.15

1.853

3.15

0.2

1.817

6.35

0.25

1.862

4.26

0

1.803

3.85

0.05

1.74

6.35

0.1

1.642

7.7

0.15

1.53

9.48

0.2

1.494

14.3

0.25

1.517

13.2

0

1.61

5.22

0.05

1.572

6.35

0.1

1.543

9.48

0.15

1.491

13.2

0.2

1.411

21

0.25

1.453

19.7

400

450

500

101

Chapter four

Results And Discussion 500

I (µA)

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

400nm

400 300 200 100 0

-3

-2

-1

0

1

2

3

-100

V (volt) 500

I (µA)

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

450nm

400 300 200 100 0

-3

-2

-1

0

1

2

3

-100

V (volt)

I (µA)

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 2.5 % Al

590

500nm

490 390 290 190 90 -10

-3

-2

-1

-110

0

1

2

3

V (volt)

Fig. (4-36) I-V characteristics under dark for ZnTe/Si heterojunction at forward and reverse bias voltage with different Al doping ratio at different thickness

102

Chapter four

Results And Discussion V (volt)

ln (J) (Amp/cm^2)

-6 0 -8

1

2

3

-10 -12 -14

pure

0.05% Al

-16

0.1% Al

0.15% Al

-18

0.2% Al

0.25% Al

400nm

-20

V (volt)

ln (J) (Amp/cm^2)

-8 0

1

2

3

-10 -12 pure 0.1% Al 0.2% Al

-14 -16

0.05% Al 0.15% Al 0.25% Al

450nm

-18

V (volt)

ln (J) (Amp/cm^2)

-8 0

1

2

3

-10 -12 -14 -16

pure

0.05% Al

0.1% Al

0.15% Al

0.2% Al

0.25% Al

500nm

-18

Fig. (4-37) A semi logarithmic of the forward dark current versus V for fabricated ZnTe/ Si heterojunction with different doping ratio at different thickness.

103

Chapter four

Results And Discussion

4-6 Optoelectronic Properties of ZnTe/ Si heterojunction: 4-6-1 Effect of thickness on illuminated I-V Characteristics for ZnTe/Si Heterojunction: The measurements were carried out under different incident power density equal to (30, 69, 100, 157 and 200) mW/cm2, Fig.(4-38) shows the relation between the photocurrent (Iph) and the reverse bias voltage of the ZnTe/Si heterojunction at different thickness, from this figures we observe that the photocurrent increases with increasing of the reverse bias voltage, where the height of the depletion region increases with increasing of the applied reverse bias voltage, due to the separation of the electron-hole pairs and then increases the photocurrent [148]. It can be noticed for all samples that the illumination current increases with increases thickness, when the thickness of the film is thin, the crystal property of film will be very poor because of the defects and dislocations in the interface. Also, the resistivity of the film is relatively high, and the mobility of carriers is much lower because the carriers are easily seized by these defects, then the photocurrent is comparatively weak, while when the film thickness increases, the defects converge at the interface of Si and ZnTe and the crystal property of the film is significantly improved. The photocurrent also increases rapidly with the thickness increasing to 500 nm. These results are conform with Ref.[21,152] We also note from Fig.(4-38) that all fabrication detectors

have high

response with increased incident power density and that through increased illumination current values with increased incident power density. This is due to the increasing intensity of incident light means an increase in the number of incident photons which lead to an increase in the number of generated charge carriers and which will spread within the depletion region and within carriers diffusion region

and then increase the generated illumination current with

increasing incident power density [90].

104

Chapter four

Results And Discussion

-4

-3

-2

0 -1 -3

-1

-5 -7 -9

400nm

dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

ZnTe /Si

-11 -13

photo current Iph (mA)

Reverse Bias Voltage V (volt)

-15

-4

-3

-2

0 -1 -3

-1

-5 -7 -9

450nm

dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

ZnTe /Si

-11 -13 -15

photo current Iph (mA)

Reverse Bias Voltage V (volt)

-4

-3

-2

0 -1 -3

-1

-5 -7 -9 dark current 10 mW/cm2 30 mW/cm2 100 mW/cm2 170 mW/cm2 250 mW/cm2

500nm ZnTe/Si

-11 -13 -15

photo current Iph (mA)

Reverse Bias Voltage V (volt)

Fig. (4-38) I-V characteristics for ZnTe/Si heterojunction with different thickness at different incident power density.

105

Chapter four

Results And Discussion

4-6-2 Effect of doping on illuminated I-V Characteristics for ZnTe/Si Heterojunction: Figure (4-39),(4-40) and (4-41) show the relation between the photocurrent (Iph) and the reverse bias voltage of the ZnTe/Si heterojunction at different Al doping ratio with different thickness. The Figures represent the dark current curves and change the illumination current as a function of the reverse bias voltage for detectors under different incident power density. It can be noticed for all prepared detectors that increase illumination current value with increasing reverse bias voltage, and this is due to increase the depletion region with increase the reverse bias voltage, where the generated electron-hole pairs within the depletion width and within a diffusion length outside of the depletion width are quickly swept away due to the strong electric field producing the photocurrent in the reverse-bias direction which increases with the incident light due to the increase in the number of photo generated electrons and holes in the depletion region [83,98]. We find from the curves in figures the impact of doping aluminum is very clear on the characteristics of detectors, where illumination current increased with increasing Al doping ratio, the aluminum atoms had improve optical and electrical properties for pure ZnTe which acts as a window to the incident light, as well as the aluminum atoms increase the optical energy gap decrease and increase the absorption coefficient and reduced electrical resistivity and this lead to increase in the concentration carriers and there mobility and that make the illumination current increased with increasing Al doping ratio [83]. Also, detector of 500nm thickness in {ZnTe: Al (0.2%)/Si} have the highest increase in illumination currents as in figure (4-41), where {ZnTe: Al (0.2%)/Si}detector get the best optical and electrical properties from others also less electrical resistivity and greatest number of pairs (electron-hole) which contributes significantly to increase the illumination current.

106

Chapter four

Results And Discussion

-4

-3

-2

0 -1 -3

-1

-5 -7 -9 dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

(a) ZnTe /Si

-11 -13 -15

photo current Iph (mA)

Reverse Bias Voltage V (volt)

-4

-3

-2

-1

(b) ZnTe:Al (0.05 %) /Si

0 0 -2 -4 -6 -8 -10 -12 dark current -14 30 mW/cm2" -16 69 mW/cm2" -18 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

photo current Iph (mA)

Reverse Bias Voltage V (volt)

-4

-3

-2

(c ) ZnTe:Al (0.1 %) /Si

107

-1

0 0 -2 -4 -6 -8 -10 -12 dark current -14 30 mW/cm2" -16 69 mW/cm2" -18 100 mW/cm2 -20 157 mW/cm2" 200 mW/cm2"

photo current Iph (mA)

Reverse Bias Voltage V (volt)

Chapter four

Results And Discussion Reverse Bias Voltage V (volt) 0 -3

-2

-1

0 -5 -10 -15 dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

(d ) ZnTe:Al (0.15 %) /Si

-20 -25

photo current Iph (mA)

-4

Reverse Bias Voltage V (volt) 0 -4

-3

-2

-1

0

-10 -15 -20 dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

(e ) ZnTe:Al (0.2 %) /Si

-25 -30

photo current Iph (mA)

-5

Reverse Bias Voltage V (volt) 0 -4

-3

-2

-1

0 -10 -15 -20

(f ) ZnTe:Al (0.25 %) /Si

dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

-25 -30

photo current Iph (mA)

-5

Fig. (4-39) I-V characteristics for ZnTe/Si heterojunction at thickness 400nm with different doping ratio and different incident power density.

108

Chapter four

Results And Discussion Reverse Bias Voltage V (volt) -3

-2

0 -1

-1

-3 -5 -7 -9 dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

(a) ZnTe /Si

-11 -13

photo current Iph (mA)

-4

-15

-4

-3

-2

(b) ZnTe:Al (0.05 %) /Si

-1

dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

0 0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20

photo current Iph (mA)

Reverse Bias Voltage V (volt)

Reverse Bias Voltage V (volt) 0 -3

-2

-1

0 -5 -10 -15

(c ) ZnTe:Al (0.1 %) /Si

109

dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

-20 -25

photo current Iph (mA)

-4

Chapter four

Results And Discussion Reverse Bias Voltage V (volt) 0 -3

-2

-1

0 -5 -10 -15 -20 dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

(d ) ZnTe:Al (0.15 %) /Si

-25 -30

photo current Iph (mA)

-4

Reverse Bias Voltage V (volt) -3

-2

0 -2

-1

-7 -12 -17 -22 dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

(e ) ZnTe:Al (0.2 %) /Si

-27 -32

photo current Iph (mA)

-4

Reverse Bias Voltage V (volt) 0 -3

-2

-1

0 -5 -10 -15 -20

(f ) ZnTe:Al (0.25 %) /Si

dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

-25 -30

photo current Iph (mA)

-4

Fig. (4-40) I-V characteristics for ZnTe/Si heterojunction at thickness 450nm with different doping ratio and different incident power density.

110

Chapter four

Results And Discussion

-4

-3

-2

0 -1 -3

-1

-5 -7 -9 dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

(a) ZnTe (pure)/Si

-11 -13 -15

photo current Iph (mA)

Reverse Bias Voltage V (volt)

-4

-3

-2

(b) ZnTe:Al (0.05 %) /Si

-1

dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

0 0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20

photo current Iph (mA)

Reverse Bias Voltage V (volt)

Reverse Bias Voltage V (volt) 0 -3

-2

-1

0 -5 -10 -15

(c ) ZnTe:Al (0.1 %) /Si

111

dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

-20 -25

photo current Iph (mA)

-4

Chapter four

Results And Discussion Reverse Bias Voltage V (volt)

-4

-3

-2

0 -1

0

-10 -15 -20 dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

(d ) ZnTe:Al (0.15 %) /Si

-25

photo current Iph (mA)

-5

-30

Reverse Bias Voltage V (volt) 0 -3

-2

-1

0

-5 -10 -15 -20

-25 dark current 30 mW/cm2" -30 69 mW/cm2" 100 mW/cm2 -35 157 mW/cm2" 200 mW/cm2"

(e ) ZnTe:Al (0.2%) /Si

photo current Iph (mA)

-4

Reverse Bias Voltage V (volt) 0 -3

-2

-1

0 -5 -10 -15 -20

(f ) ZnTe:Al (0.25%) /Si

dark current 30 mW/cm2" 69 mW/cm2" 100 mW/cm2 157 mW/cm2" 200 mW/cm2"

-25 -30

photo current Iph (mA)

-4

Fig. (4-41) I-V characteristics for ZnTe/Si heterojunction at thickness 500nm with different doping ratio and different incident power density.

112

Chapter four Results And Discussion We also note a good response of all heterojunction to illumination with increased incident power density where the illumination current values increased and this attributed to the same reason mentioned earlier for effect of thickness. Moreover, the illumination current values for heterojunction doped with more than (0.2%) Al decrease for all thickness and for all incident power density, Fig { ZnTe:Al (0.25 %) /Si} for all thickness. This is due to high ratio of Al doing led to increases dark current and that reflected a decrease in illumination current, the high ratio of doing led to increases of the dislocation density and trapping centers which increases the energy gap and activation energy and also decrease the number of charge carriers, mobility, depletion region and the diffusion length [83,114]. Gain factor can be calculated from the ratio between the photocurrent to the dark current at the same bias voltage. The values of gain factor for undoped ZnTe/Si heterojunctions and doped with Al are listed in Table.(4-9), where by using the values of illumination current and dark current for (Ps = 100 mW/cm2)and V=3volt. The results showed that the gain factor increases with increasing doping ratio according to improve optical and electrical properties, also increase number of charge carriers and conductivity. The greater gain factor can show in thickness 500nm ZnTe:Al (0.2%) /Si which is a best heterojunction comparable to the other heterojunction. Table. (4-9) gain factor for ZnTe/Si heterojunctions with different Al doping ratio and for different thickness. Gain factor Thickness

Al doping ratio %

nm 0

0.05

0.1

0.15

0.2

0.25

400

80.5

101.4

113

121

131.4

127.7

450

97.5

117

135.3

141.7

175.8

159.5

500

124.3

138.5

167.8

186.3

209.6

196.7

113

Chapter four

4-7

Results And Discussion

ZnTe /Si Detectors Measurements:

4-7-1 Spectral Measurements at Different Thickness: A.

Spectral Responsivity: Spectral responsivity(Rλ) is the most important parameter by which the range

of heterojunction operation can be determined and determined suitable application of detectors, the Fig.(4-42) show the variation of spectral responsivity with the wavelength for ZnTe/Si heterojunction prepared at different thickness. spectral responsivity is investigated in the wavelength rang (400-950)nm with (3 volt) bais. As shown, there is three regions in the Fig; the first one represents low wavelength

region(𝜆 < 500 𝑛𝑚),

the

second

region

represents

rang

wavelength (500 < 𝜆 < 680)𝑛𝑚 , then the last was the wavelength longer than 680nm. It can be seen that the responsivity of the ZnTe/Si device is rather low when the wavelength is shorter than 500 nm, the responsivity reaches a maximum at ~500 nm, and for the wavelength longer than 680 nm. The response spectrum is directly related to the energy band structure of ZnTe, and the spectral character of the density of states, which is reflected by the photocurrent measurement. Thus for enhancement in second region, it can be concluded that the enhancement of the spectral responsivity is due to the electron-hole pairs excited by the incident light with energy larger than the band gap ,only photon with enough energy is able to induce a significant increase in conductance. Photon with a smaller energy does not have enough energy to excite electrons from the valence band to the conduction band and thus contributes little to the photocurrent. The slight increase of responsivity in the long wavelength side is possibly due to transition of carriers from defect states in the band gap to the conduction band [21,154]. The smaller of responsivity on the shorter wavelength side, which was also observed in the other studies [155], is attributed to the enhanced absorption of high-energy photons at or near the surface region of the semiconductor. The electron-hole pairs generated near the

114

Chapter four Results And Discussion suface region typically have a lifetime shorter than those in the bulk; thus they contribute less to the photo conductance, the smaller behaver was also observed in the other studies [156].

0.3

400 nm 450 nm

Rλ , (A/W)

500 nm 0.2

0.1

0 300

500

700

900

λ, (nm)

Fig. (4-42)The spectral responsivity for ZnTe/Si detectore with different thickness

Different thickness is used in our work, the maximum photo responsivity peaked at about (553, 620 and 650) nm for thickness (400, 450 and 500)nm, respectively. We can notice from these figures that the spectral responsivity value is increased with increase thickness, and the peaks of Rλ are shifted to longer wavelength as thickness increases due to the decrease of optical energy gap, also the increased of the responsivity can be attributed to the decreasing in the resistance at large value of ZnTe thickness (500nm) and tend to increased photocurrent, and because of decreasing the density of states which act as recombination centers concentrated on two sides of interface, the smaller behaver was also observed in the other studies [157]. From this figure it can be seen that all the samples have a good response in the visible region, and there is region around (600-650)nm for thickness 500nm which is in according with the band gap (1.98 eV) of ZnTe film, it clearly shows that it has higher response intensity for longer wavelength. 115

Chapter four

Results And Discussion

B. Quantum Efficiency (η): Quantum efficiency is a quantity defined for a photosensitive device, relates to the percentage of photons hitting a photo reactive surface that will produce the electron-hole pairs [21], where it is represents the ratio between the numbers of generated electrons in the heterojunction to the number of incident photons on the effective area of the heterojunction. Quantum efficiency is related with the spectral responsivity according to Eq.(2-52), Fig.(4-43) show the variation of quantum efficiency with the wavelength for ZnTe/Si heterojunction at different thickness (biased at 3V), this Fig shows that the maximum value of quantum efficiency (55.387%) at about 650nm for thickness 500nm, and the variation of quantum efficiency is due to the same reason mentioned earlier for responsivity. Table.(4-10) shows the maximum value the spectral responsivity, quantum efficiency and specific detectivity according to the corresponding wavelengths.

60

400 nm 450 nm

50

500 nm

ɳ%

40 30 20 10 0 200

400

600

800

1000

λ, (nm)

Fig. (4-43)The quantum efficiency for ZnTe/Si detectore with different thickness.

116

Chapter four

Results And Discussion *

C. Specific Detectivity (D ): According to the Eq.(2-57) specific detectivity values were calculated, Fig.(4-44) and Table.(4-10) show the change of the specific detectivity with the wavelength for ZnTe/Si heterojunction at different thickness (biased at 3V). It is clear from the figure that the specific detectivity behavior is similar to the behavior for spectral responsivity and quantum efficiency. It is obvious that the specific detectivity increases with increasing thickness, where the maximum value of specific detectivity (1.93804x1011 cm.Hz1/2.W-1) at about 650nm for thickness 500nm. The variation of quantum efficiency is due decrease the defect that leads to decrease of the recombination centers which effect the value of specific detectivity, and decrease the noise current according to decrease the dark current, also the same reason mentioned earlier for responsivity according to the Eq.(2-57).

D*, (cm.Hz1/2 .W-1)

2E+11 400 nm 1.5E+11

450 nm 500 nm

1E+11

5E+10

0 200 300 400 500 600 700 800 900 1000

λ, (nm)

Fig. (4-44)The specific detectivity for ZnTe/Si detectore with different thickness.

117

Chapter four Results And Discussion * Table. (4-10) the values of Rλ, η % and D for ZnTe/Si detectors Thick (nm)

λ (nm)

Rλ (Amp/W)

η%

D*x 1011(cm.Hz 1/2.W-1)

400

553

0.187

42.061

1.26156

450

620

0.253

50.641

1.70117

500

650

0.288

55.387

1.93804

4-7-2 Spectral Measurements at Different Al doping ratio: A. Spectral Responsivity: Fig.(4-45) show the variation of spectral responsivity with the wavelength for ZnTe/Si heterojunction prepared with different Al doping ratio at different thickness, the spectral responsivity for all heterojunctions have similar curves behavior for the the spectral responsivity in

pure ZnTe/Si heterojunction.

However, the spectral response values have increased with increase the Al ratio, and the highest value of the spectral response reached for{ZnTe: Al (0.2%)/Si} at {666, 673 and 746}nm for thickness {400,450 and 500}nm, respectively. The effect of Al on the spectral response attributed to improved optical, electrical and optoelectronic properties, where decrease in electrical resistivity lead to increased electrical connectivity and increased carriers concentration which lead to increase the illumination current then the spectral response increase. According to the peak position, it can be seen that all the samples have a good response in the visible and near infrared regions. In the visible region, it clearly shows that it has higher response intensity for wavelength rang (550-700), so the advantage of ZnTe which has a strong visible response emerges. Also, near infrared regions photon through ZnTe is absorbed in Si substrate to generate electrons and holes. Thus, the advantage of Si which has a strong response in long wavelength has been appeared. Moreover, for samples, the response

118

Chapter four Results And Discussion intensity is enhanced with Al content in ZnTe film, the smaller behaver was also observed in the other studies [158]. Also, from figures can see the spectral responsivity value is increased with increase doping ratio, and the peaks of R λ are shifted to longer wavelength until 0.2%,and more than 0.2% spectral responsivity value decreases due to the increase the defect structure with increase doping ratio.

400nm

0.4

Rλ , (A/W)

0.3

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

0.2

0.1

0 200

400

600

800

1000

λ, (nm)

450nm

Rλ , (A/W)

0.4

0.3 pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

0.2

0.1

0 200

400

600

λ, (nm)

119

800

1000

Chapter four

Results And Discussion

0.5

500nm

Rλ , (A/W)

0.4 pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25%Al

0.3

0.2

0.1

0 200

400

600

800

1000

λ, (nm)

Fig. (4-45)The spectral responsivity for ZnTe/Si detectors with different Al doping ratio at different thickness.

B. Quantum Efficiency 𝛈: According to Eq.(2-52), quantum efficiency is related with

the spectral

responsivity, therefore the behavior curves of quantum efficiency for all heterojunctions have similar behavior for the the spectral responsivity. Fig.(446) shows the variation of quantum efficiency with the wavelength for ZnTe/Si heterojunction at different Al doping ratio (biased at 3V), this figure show effect of Al on quantum efficiency value, where the maximum value of quantum efficiency (78.85%) at about 746nm for thickness 500nm, and the variation are attributed to the increasing in the absorption of incident radiation in thin film which is generating electron-hole pairs, and that lead to increase illumination current and decreased resistivity. Also, decreased trapping center where leading to decreased recombination process.

120

Chapter four

Results And Discussion 80

400nm

70 60

ɳ%

50

pure 0.05% Al 0.1% Al 0.15% Al 0.2% Al 0.25% Al

40 30 20 10 0 200

700

λ, (nm)

1200

80

450nm

70 60

pure

ɳ%

50

0.05% Al

40

0.1% Al

30

0.15% Al

20

0.2% Al 0.25% Al

10 0 200

700

1200

λ, (nm)

80

500nm

70 60

pure

ɳ%

50

0.05% Al 40

0.1% Al

30

0.15% Al

20

0.2% Al 0.25% Al

10 0 200

700

1200

λ, (nm)

Fig. (4-46)The quantum efficiency for ZnTe/Si detectors with different Al doping ratio at different thickness. 121

Chapter four

Results And Discussion *

C. Specific Detectivity (D ): Fig.(4-47) show the change of the specific detectivity with the wavelength for ZnTe/Si heterojunction at different Al doping ratio and in different thickness (biased at 3V). It is clear from the figure that the all curves have similar behavior to the spectral responsivity curves according to the Eq.(2-57). The maximum value of specific detectivity was (3.191x1011 cm.Hz1/2.W-1) at about 746nm for thickness 500nm, where the Al ratio 0.2%, and the variation of quantum efficiency due to the same reason mentioned earlier for responsivity. It is obvious that the D* increases with increasing doping ratio until 0.2% then it decreases with increasing Al ratio, due to increase of the defect that leads to increase of the recombination centers which effect the value of D* .

3E+11

400nm

D*, (cm.Hz1/2 .W-1)

2.5E+11

pure

2E+11

0.05% Al 0.1% Al 1.5E+11

0.15% Al 0.2% Al 0.25% Al

1E+11

5E+10

0 200 300 400 500 600 700 800 900 1000

λ, (nm)

122

Chapter four

Results And Discussion

3E+11

450nm

D*, (cm.Hz1/2 .W-1)

2.5E+11

2E+11 pure 1.5E+11

0.05% Al 0.1% Al 0.15% Al

1E+11

0.2% Al 0.25% Al

5E+10

0 200 300 400 500 600 700 800 900 1000

λ, (nm) 3.5E+11

500nm

D*, (cm.Hz1/2 .W-1)

3E+11

2.5E+11 pure 0.05% Al

2E+11

0.1% Al 0.15% Al

1.5E+11

0.2% Al 0.25% Al 1E+11

5E+10

0 200 300 400 500 600 700 800 900 1000

λ, (nm)

Fig. (4-47)The specific detectivity for ZnTe/Si detectors with different Al doping ratio at different thickness.

123

Chapter four

4-8

Results And Discussion

Response Time and Carrier Life Time:

Fig (4-48) show effect of thickness and different Al doping ratio on the response time and carrier lifetime. It is clear that the response time decrease with increase thickness and doping ratio due to decreased the capacitance of the heterojunction with doping which may be leads to increase in the depletion layer, and that lead to improved electrical properties and increase response speed.

Also,

the

lower

response

time

can

show

for

500nm

{ZnTe:Al(0.2%)/Si}.

Fig. (4-48) The response time and carrier lifetime for different thickness.

124

in

Chapter four Results And Discussion The carrier lifetime can be calculated from photocurrent gain value , it can see from figures, increase carrier lifetime with increase doping ratio, where increase carrier lifetime lead to increase diffusion carrier length and increase llumination current, where photocurrent gain value increase. The maximum value of carrier lifetime for thickness 500nm and for (0.2%) Al ratio, the variation are attributed to the increasing in the absorption of incident radiation in thin film which is generating electron-hole pairs, and that lead to increase illumination current, the smaller behaver was also observed in the other studies[21].

4-9 Conclusions:  Structural Properties: 1- The results of structural properties showed that the structure of ZnTe alloys and films deposited in different thickness and doping with different Al ratio was polycrystalline with zinc blend crystal structure with (111) plane as preferential orientation. 2- By increasing the thickness the grain size of ZnTe films will be increased while the roughness decrease and with increasing doping ratio the grain size of ZnTe films decrease while the roughness increase.  Optical Properties: 1- The optical transitions in ZnTe films are direct and the value of absorption coefficient increases with increasing of thickness and doping ratio. 2- The optical energy gap decreases with increasing thickness and doping ratio. 3- We can take advantage in manufacturing photodetector where high absorption for (500nm film ratio 0.2%) is very much useful and the best in application for ZnTe detectors.

125

Chapter four

Results And Discussion

 Electrical Properties: 1- Two mechanisms of activation energy decreases with increasing thickness and doping ratio and the conductivity increases with increasing of thickness and doping ratio, the highest value of D.C conductivity for 500nm thickness in [ ZnTe: Al (0.2%)]. 2- Hall measurements showed that all the films are p-type and the mobility increases with increasing of thickness and doping ratio.  Photodetector characterization: 1-

C-V

measurements

revealed

an

abrupt

heterojunction

ZnTe

/Si

Heterojunction device and the capacitance decreases with increasing of the reverse bias voltage while values of the width of the depletion layer and the V bi increases. 2- I-V measurements for ZnTe/Si heterojunction showed decrease the dark current with increasing of thickness and doping except at 0.25% Al ratio and the ideality factor values decreases with increasing thickness and doping. 3- Under illumination, the photocurrent increases with increases thickness and doping and the highest illumination currents at 500nm thickness in {ZnTe: Al (0.2%)/Si} photodetector. 4- All the photodetectors have a good response in the visible region for different thickness and the response intensity is enhanced with Al doping ratio. 5- The highest value of the spectral response, quantum efficiency and specific detectivity was near infrared regions for{ZnTe: Al (0.2%)/Si} photodetector.

126

Chapter four

4-10

Results And Discussion

Suggestions for Future Works:

1- Preparing ZnTe films using DC magnetron sputtering technique and study the structural, optical and electrical properties with different thicknesses and comparing the results . 2- Studying the effect of annealing temperatures, substrate temperature and rate of deposition on properties of ZnTe/Si heterojunction. 3- Preparation of ZnTe heterojunction with different substrate such as GaAs, InSb and CdTe to study their optoelectronic properties for these heterojunction. 4- Fabricate ZnTe photodetector doping with Cu and Ag to study their optoelectronic properties for these heterojunction. 5- Fabricate ZnTe solar cell using thermal evaporation technique and study the effect thickness and annealing on solar cell characterization.

127

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136

Appendix Some Properties of Aluminum [20 - 22] Material

Aluminum

Chemistry Symbol

Al

Density (g/cm3)

2.7

Melting point (K)

660.3

Resistivity (Ω.m)

2.82x10 -8

Ionic Radius (pm)

53.5

Atomic Weight (pm)

143

Crystal structure

FCC - Cubic

Lattice constant A˚

4.05

Color

Silvery Gray

Thermal Conductivity (W/m.K)

236 at (25oC)

Dielectric Constant

1.7

Refractive Index

1.44

137

‫الخـالصـــة‬ ‫ُحضرت في هذا البحث سبيكة للمركب )‪ (ZnTe‬بإذابة عناصر السبيكة في أنبوب مفرغ من‬ ‫الكوارتز‪ ،‬وبإجراء فحوصات حيود األشعة السينية لمسحوق السبيكة تبين أنها تمتلك تركيب متعدد‬ ‫التبلور ومن النوع المكعـب‪ .‬تم ترسيب أغشية )‪ (ZnTe‬النقية والمطعمة باأللمنيوم )‪ (ZnTe:Al‬بطريقة‬ ‫التبخير الحراري في الفراغ بمعدل ترسيب ‪ .(1.2 ±0.1) nm sec-1‬وهذه األغشية تم تطعيمها‬ ‫باأللمنيوم )‪ (Al‬بنسب تطعيم ‪. (0.05, 0.1, 0.15,0. 2, 0.25)%‬‬ ‫أظهرت نتائج قياسات )‪ (XRD‬أن جميـع األغشية المحضّرة كانت ذات تركيب بلوري متعدد التبلور‬ ‫ومن النوع المكعـب مع هيمنة النمو باإلتجاه (‪ )111‬لألغشيـة المحضّرة كافة‪ ،‬وقد ازداد معـدل الحجم‬ ‫البلوري مع تناقص ‪ FWHM‬للقمم المميزة مع زيادة السمك‪ .‬كذلك إزاحة القمم المميزة نحو قيم ‪2θ‬‬ ‫الكبيرة وتنـاقص سريع في شدة كل القمم مع زيـادة نسـب التطعيم باأللمنيوم‪.‬‬ ‫باإلضافة الى نتائج ‪ ،XRD‬أظهرت نتائج مجهر القوة الذرية )‪ (AFM‬ان جميع االغشية المحضرة‬ ‫تمتلك توزيع متجانس للحبيبات وخشونة السطح تتناقص مع زيادة السمك تتناقص مع زيادة نسبة التطعيم‬ ‫باأللمنيوم‪ .‬أوضحت قياسات الخواص البصرية بان االنتقاالت البصرية كانت مباشرة مسموحة وان قيم‬ ‫النفاذية تتناقص مع زيادة نسبة التطعيم باأللمنيوم وبالتالي سوف تزداد االمتصاصية ‪ ،‬ان فجوة الطاقة‬ ‫البصرية لكل االغشية تتناقص مع زيادة السمك وأن قيم فجوة الطاقة البصرية يمكن التحكم بها عن طريق‬ ‫نسبة التطعيم باأللمنيوم ‪ .‬كذلك تم حساب معامل االمتصاص والثوابت البصرية بوصفه دالة لطاقة‬ ‫الفوتون‪.‬‬ ‫تضمنت دراسة الخصائص الكهربائية لألغشية المحضرة آليتين لالنتقال االلكتروني‪ ،‬أي طاقـتين‬ ‫تنشيط‪ .‬وأظهرت النتائج أن التوصيلية تزداد بزيادة ك ٌل من السمك ونسبة التطعيم‪ .‬وبينت نتائج تأثير هول‬ ‫أن األغشية كافة هي من النوع )‪ ،(P-type‬وان تركيز الحامالت والتحركية يزداد بزيادة كل من السمك‬ ‫صنع هو من النوع الحاد‪.‬‬ ‫و نسبة التطعيم باأللمنيوم‪ .‬وقد بينت نتائج قياسات (سعة ‪ -‬جهد) أن المفرق ال ُم ّ‬ ‫وأن جهد البناء الداخلي )‪ (Vbi‬وعرض منطقة النضوب يزداد بزيادة السمك ونسبة التطعيم‪.‬‬ ‫أظهرت نتائج قياسات (تيار‪-‬جهد) للمفرق‪ ZnTe /Si‬تيار الظالم في حالة االنحياز االمامي يتغير‬ ‫مع الفولطية المسلطة ويزداد بزيادة السمك بينما يتناقص تيار التشبع وعامل المثالية‪ ،‬وكذلك تحت شرط‬ ‫االنحياز العكسي يتناقص تيار الظالم للمفرق المطعم بشكل اقل من تيار الظالم للمفرق النقي‪ .‬كما‬ ‫أوضحت القياسات الكهروضوئية زيادة في تيار اإلضاءة للمفارق الهجينة مع زيادة ك ٌل من شدة اإلضاءة‬ ‫الساقطة ونسبة التطعيم‪ .‬وقد اظهرت قيم الكاشفية النوعية والكفاءة الكمية تزاح نحو الطول الموجي‬ ‫االعلى مع زيادة السمك ‪ ،‬وتحصل اعلى استجابة طيفية للكواشف الضوئية المطعمة بنسبة )‪ %)0.2‬من‬

‫االلمنيوم عند االطوال الموجية ‪ {666, 673 ,746}nm‬للسمك }‪500 , 450 ,400‬‬ ‫التوالي‪.‬‬

‫{ ‪ nm‬على‬

‫اظهرت نتائج االستجابة الطيفية ان الكواشف الضوئية تعمل ضمن المنطقة المرئية ومنطقة‬

‫األشعة تحت الحمراء القريبة‪ ،‬ونصل على افضل استجابة عند السمك ‪ 500nm‬و نسبة تطعيم )‪،%)0.2‬‬ ‫حيث بلغت أعلى قيمة لإلستجابة الطيفية (‪0.475A/W‬عند الطول الموجي (‪ ،)746nm‬وبذلك يمكن‬ ‫اعتبار قيم السمك والتطعيم اعاله هي الظروف االمثل لتصنيع كاشف ضوئي من مادة ‪.ZnTe‬‬

‫جمهورية العراق‬ ‫وزارة التعليم العالي والبحث العلمي‬ ‫جامعة بغداد‬ ‫كلية التربية للعلوم الصرفة ‪ /‬ابن الهيثم‬

‫دراسة خصائص كاشف ضوئي‬ ‫‪ ZnTe:Al/Si‬محضر بطريقة‬ ‫التبخير الحراري‬ ‫أطروحة مقدمة إلى‬ ‫كلية التربية للعلوم الصرفة‪ /‬ابن الهيثم‪ ،‬جامعة بغداد‬ ‫وهي جزء من متطلبات نيل درجة دكتوراه فلسفة في الفيزياء‬ ‫من قبل‬

‫حنان كاظم حسون‬ ‫(بكالوريوس علوم في الفيزياء ‪) 1997‬‬ ‫( ماجستير علوم في الفيزياء ‪) 2006‬‬ ‫بإشراف‬

‫أ‪ .‬د‪ .‬سمير عطا مكي‬ ‫‪1438‬هـ‬

‫‪ 2017‬م‬