brookite. [3,4] . The anatase phase is especially adequate for those applications due to its crystal structure and a higher band gap of 3.2 eV compared to the 3 eV ...
Ministry of Higher Education and Scientific Research University of Technology Applied Sciences Department
Characterization of Pure and dopant TiO2 thin films for gas sensors applications A thesis submitted By
Khaled Z.Yahya A thesis submitted to the University of Technology, department of Applied Sciences as a partial fulfillment for the requirement of Doctor of Philosophy degree In Laser and Opto-electronic Technique Supervised by
Prof.Dr. Adawiya J.Haider
Prof.Dr. Raad M.S.Al-Haddad
June 2010
ﻭﺯﺍﺭﺓ ﺍﻟﺘﻌﻠﻴﻡ ﺍﻟﻌﺎﻟﻲ ﻭ ﺍﻟﺒﺤﺙ ﺍﻟﻌﻠﻤﻲ ﺍﻟﺠﺎﻤﻌﺔ ﺍﻟﺘﻜﻨﻠﻭﺠﻴﺔ
ﻗﺴﻡ ﺍﻟﻌﻠﻭﻡ ﺍﻟﺘﻁﺒﻴﻘﻴﺔ
ﺨﺼﺎﺌﺹ ﺃﻏﺸﻴﺔ ﺃﻭﻜﺴﻴﺩ ﺍﻟﺘﻴﺘﺎﻨﻴﻭﻡ ﺍﻟﻨﻘﻲ ﻭ ﺍﻟﻤﺸﻭﺏ ﻟﺘﻁﺒﻴﻘﺎﺕ ﻤﺘﺤﺴﺴﺎﺕ ﺍﻟﻐﺎﺯ ﺍﻁﺭﻭﺤﺔ ﺘﻘﺩﻡ ﺒﻬﺎ
ﺨﺎﻟﺩ ﺯﻜﺭﻴﺎ ﻴﺤﻴﻰ ﺍﻟﻰ ﻗﺴﻡ ﺍﻟﻌﻠﻭﻡ ﺍﻟﺘﻁﺒﻴﻘﻴﺔ ﻓﻲ ﺍﻟﺠﺎﻤﻌﺔ ﺍﻟﺘﻜﻨﻭﻟﻭﺠﻴﺔ ﻭ ﻫﻲ ﺠﺯﺀ ﻤﻥ ﻤﺘﻁﻠﺒﺎﺕ ﻨﻴل ﺩﺭﺠﺔ ﺍﻟﺩﻜﺘﻭﺭﺍﻩ ﻓﻲ ﺘﻘﺎﻨﺎﺕ ﺍﻟﻠﻴﺯﺭ ﻭﺍﻟﻜﻬﺭﻭﺒﺼﺭﻴﺎﺕ
ﺃ .ﺩ .ﻋﺩﻭﻴﺔ ﺠﻤﻌﺔ ﺤﻴﺩﺭ
ﺒﺄﺸﺭﺍﻑ
ﺍ .ﺩ .ﺭﻋﺩ ﻤﺤﻤﺩ ﺼﺎﻟﺢ ﺍﻟﺤﺩﺍﺩ
ﺣﺰﯾﺮان 2010
CONTENT CHAPTER ONE
Introduction and Historical Review
Page
1- Introduction………………………………………..………………...…….
1
1-1 Historical Review …………………………………………...……...……
2
1-2 Aim of the work…………………………………………………………..
10
CHAPTER TWO
Fundamental Properties of Laser Deposition and TiO2
Thin Films 2. Introduction……………………………......................................................
11
2.1 Laser ablation mechanisms ……………………………………………...
11
2.1.1 Laser – Target interaction ......................................................................
11
2.1.2 Laser – plasma interaction …………………………………………….
14
2.1.3 Plasma plume expansion ………………………………………………
15
2.1.4 Plume- Substrate Interaction ……………………………………….
17
2.2 Lasers for PLD…………………………………………………...............
18
2.3 Advantages of PLD………………………………...……………………...
19
2.4 Titanium Dioxide ……………………………………….………………..
20
2.4.1 Crystalline Structure of TiO2 …….…………………………………....
21
2.4.1.1 TiO2 in Anatas Metastable Phase…………………………………….
21
2.4.1.2 TiO2 in Rutile stable Phase ……………………….……………….…
22
2.4.1.3 TiO2 in Brookite Structure ………………………………….……......
23
2.4.1.4 X-ray diffraction ……………………………………………..…….....
25
2.5 Surface Morphology ……………………….……………………………
29
2.6 Optical Properties ..………………………. …….……………….……..
32
2.7 Optical Absorption and Absorption Edge ……………………………….
36
2.8 Optical Constants …………………………………………………..…….
37
2.9 Doping TiO2…………..…………………………………………………..
38
2.10 Noble Metal Diffusion inside TiO2 Bulk………………………….…….
41
2.11 Gas sensors …………………………………………………………….…
43
2.12 Operation principles of the semiconductor gas sensor………………….
43
2.13 Thin film resistive gas sensors……………………………………….…..
44
2.14 Chemical Sensors…………………………………………………………
45
2.15 General Approach to Semiconductor Gas Sensors…………………..…
48
2.16 Semiconductor Sensing Materials for TiO2………………………….… 2-17 Bulk Conductance Effects ……………………………………………….
54 54
2.18 Carbon Monoxide …………………………………………………...…..
55
2.19 Other applications of TiO2 ………………………….………………….
56
CHAPTER THREE
Experimental Work and Techniques
3. Introduction …………………………...……………………………………
58
3.1 Deposition Equipment ……………………………………………..........
58
3.1.1 Nd: YAG Laser Source………………….………………………………
58
3.2 Vacuum System…………………..……………..………………………..
59
3-2-1 Vacuum pump………………………………………………………….
59
3-2-2 Pressure monitoring…………………………………………………..
59
3-2-3 Gas Supply System…………………………………………………….
59
3.2.4 Target holder …………………………..….………………………......
60
3.2.5 Substrate heater …………………………………….…………………..
61
3.3 Target preparation………………………………………………………...
61
3.4 Preparation of Substrate Surface for Thin Film Deposition ……………
62
3.5 Procedure of Thin Film Deposition by PLD……………………………..
63
3.6 Characterization measurements ……………………..…………………..
64
3.6.1 Electrodes Deposition…………………………………………………..
64
3.6.2 Thickness measurement …………………………………..................
64
3.6.3 Structural and morphological measurements.…………………………
65
A) X-Ray Diffraction spectra………………………………………….…….
65
B) Scanning electron microscopy (SEM)……. ……….………………….....
67
C) Atomic Force Microscopy (AFM) ………………………….…………….
68
D) X-ray Fluorescence (XRF) ……………………………………..............
68
3.6.4
Optical measurement ………………………………………………...
69
A)Transmission measurement………………………………………………..
69
B) Photoluminescence measurements……………………………………….
69
3.6.5 Gas sensors measurement……….……………………………………. CHAPTER FOURE
70
Results and Discussion
4 Introduction………………………………………………………………
71
4.1 Effect of Deposition Conditions on the Characteristic of Thin films
71
Grown Using PLD……………………………………………………. 4.1.1 substrates Temperatures effect …....... .……..……………………..……
71
4.1.2 Oxygen pressur effect …………………………………………..….…..
75
4.1.3 Laser Fluence effect …………….……………..……...….....................
77
4.1.4 The doping effect of noble metal (Ag, Pt, Pd and Ni)………….…….
78
4-2 X - ray Florescence ……………………………..……………….……….
84
4.3 Surface Morphology by SEM………………………….………….……..
86
4.3.1 Effect of substrate Temperatures……..………………………………
86
4.3. 2 Oxygen Pressure effect ……………………………………….………
86
4.3. 3 The doping effect of noble metal (Ag, Pt, Pd and Ni)………….…….
87
4.4 Atomic Force Microscopy (AFM)……………………..………………..
92
4.4.1 Substrate Temperatures effect ……………………………………….
92
4.4.2 Oxygen Pressure effect ……………………………….……………...
93
4.4.3 The doping effect of noble metal (Ag, Pt, Pd and Ni)………….…….
93
4-5 Film thickness measurement…………………………………………….
98
4.6 Optical Properties……………………………….………………………..
100
4.6.1 Transmission……………………………………………………………
100
4.6.2 Absorption………………………………………………………………
103
4.6.3 Optical Energy Gap…………………………………………………….
108
4.6.4
Refractive index………………………………………………………
113
4-6-5 Extinction Coefficient………………………………………….………
115
4.6.6 Dielectric Constant…………………………………………….……….
117
4.6.7 Photoluminescence (PL)…………………………………………....…..
120
4.6.7.1 Substrate Temperatures effect ………………………………..……...
120
4.6.7.2 The doping effect of noble metal (Ag, Pt, Pd and Ni)………….……..
122
4.7 Sensing properties: Chemical Sensing Measurements …………………
125
4.7.1 Operation time Effect on sensing properties…………………………..
126
4.7.2 Operation time Effect on resistance properties………………………..
127
4.7.3
Operation time Effect on current properties…………………………
128
4.8.4 Operation Substrate temperature Effect on sensing properties………
130
CHAPTER FIVE
Conclusion and Future work
5. Conclusion ………………………………………………………………..
137
5.1 Future Work……………………………………………………………..
138
References……………………………………………………………………
139
Khaled Z.Yahya . “Characterization of Pure and dopant TiO2 thin films for gas sensors applications" University of Technology Department of Applied Science .PH.D Supervisors : Dr. Adawiya J.Haider and Dr. Raad M.S.Al-Haddad. 2010 147p.
Abstract Titanium dioxide (TiO2) has been extensively studied and demonstrated to be suitable to detect toxic gases such as CO and NOx that effect the quality of life . There fore TiO2 thin films are good candidates in the gas sensor industry .In addition noble metal dopants to the titanium dioxide materials make them sensitive to CO gas. In this work, a double frequency Q-switching Nd:YAG laser beam (λ=532nm, repetition rate 6 Hz and the pulse duration 10ns)has been used, to deposit TiO2 thin films pure and doped with (Ag ,Pt ,Pd and Ni ) at various doping percentages (1 wt.%, 2 wt.% and 3 wt.%) on glass and Si (111) substrates to be activated by visible light irradiation as well as ultraviolet irradiation. Basic material characterization has been carried out. Many growth parameters have been considered to specify the optimum condition, namely substrate temperature (200-500˚C), oxygen pressure (10 -5×10-2 mbar) and laser fluence energy density (0.8, 1.2 and 1.8)J/cm2. The structure properties of TiO2 pure and doped with noble metal were investigated by means of x-ray diffraction. As a result, it has been found that film structure and properties strongly depended on substrate temperature and doping concentration. X-ray diffraction (XRD) showed that at substrate temperatures higher than 300 °C the structure of the deposited thin films changed from amorphous to crystalline corresponding to the tetragonal TiO2 anatase phase, and at substrate temperature Ts=500°C produced both rutile and anatase phases. It has been found that 3 % wt (Ag ,Pt ,Pd and Ni ) doped TiO2 thin film was the most sensitive element to CO gas. The surface morphology of the deposits materials have been studied by using scanning electron (SEM) and atomic force microscopes (AFM). The grain size of the nanoparticles observed at the surface depended on the substrate temperature, where 500°C was the best temperature and partial pressure of oxygen 5×10-1 mbar was the best pressure during the growth process . TiO2 doped with Pt metal has the
smallest grain size (١1nm ) , RMS roughness increased with increasing substrate temperature (Ts) which are (11.2nm) for thin films deposited at (500)ºC and the samples are very rough with RMS value of (28 nm) for TiO2 thin films doped with 3% (Pt). UV-VIS transmittance measurements have shown that our films are highly transparent in the visible wavelength region, with an average transmittance of ~90% which makes them suitable for sensor applications .Dopants such as Ag and Pt shifted the absorption edge of TiO2 into the visible region .The optical band gap of the films has been found to be 3.2 eV for indirect transition and 3.6 eV for direct transition at 400˚C. The refractive index (n) and
extinction coefficient (K)
decreased as the substrate temperature increased. In addition the photoluminescence spectrum analyses of all the thin films showed that there were two kinds of luminescence transitions involved when the thin film was excited with energy higher than the band gap value (3.2 eV) . The intensity of two peaks, A lied in the UV spectrum (375 nm) and B lied in the visible spectrum (525 nm). The sensitivity toward CO gas has been measured under 50 ppm concentrations . TiO2 doped with noble metal has a sensitivity higher than pure TiO2 where as TiO2 doped with Pt metal deposited on Si (111) has maximum sensitivity to CO gas with avalue of (23 %) with best annealing operation temperature at 250°C, and resistance decreased reached (109 Ω) with increasing doping concentration due to increase in the sensing current reached (10 nA) of the TiO2 films. Keywords :TiO2 PLD Technique.
اﻟﺨﻼﺻﺔ Ύ ѧ Ϭϟ ϲѧ Θ ϟ NOx وCO اﺛﺒﺘﺖ اﻟﺪراﺳﺎت اﻟﺸﺎﻣﻠﺔ ان اوﻛﺴﯿﺪ اﻟﺘﯿﺘﺎﻧﯿﻮم ﻟﮫ اﻟﻘﺎﺑﻠﯿﺔ ﻋﻠﻰ ﻛﺸﻒ اﻟﻐﺎزات اﻟﺴﺎﻣﺔ ﻣﺜﻞ ﻏﺎز ΔΑΎ ѧ η Ϊѧ ѧ Ϩ ϋϭ ϊѧ γ ϭ ϞϜѧ θΑ Ε ίΎ ѧ ϐϟ ΕΎ ѧ δδΤΘ ϣ ΔϋΎ Ϩ ѧ λ ϲѧ ѧ ϓ Δѧ Ϙϴ ϗήϟ ϡϮϴ ϧ Ύ ѧ Θ ϴ Θ ϟ Ϊϴ ѧ δϛϭ Δϴ ѧ θϏ ϞϤόΘ ѧ δΗ ϭ ΓΎ ѧ ϴ Τϟ ϰѧ Ϡ ϋ Ϣѧ Ϭϣ ήϴ ΛΎ ѧ Η . اﻟﺴﺎمCO اوﻛﺴﯿﺪ اﻟﺘﯿﺘﺎﻧﯿﻮم ﺑﺎﻟﻤﻌﺎدن اﻟﻨﺒﯿﻠﺔ ﯾﺠﻌﻠﮭﺎ اﻛﺜﺮ ﺣﺴﺎﺳﯿﺔ ﻟﻐﺎز ϭΫ Δѧ ϴ ϋϮϨ ϟ Ϟѧ ϣΎ ϋ Δѧ ϴ Ϩ ϘΘ Α Ϟѧ Ϥόϳ ϭ ˾˼˻ nm ϲΟϮѧ Ϥϟ ϝϮτϟ ϭΫ ϲπ ΒϨ ϟ ϙΎ ϳ ϡϮϴ ϣΪϴ ϧ έΰϴ ϟ ϝΎ ϤόΘ γ ϢΗ άϫ Ύ Ϩ Μ ΤΑ ϲϓ Ϧϴ ѧ Η ϼΑ ˬ Δѧ π ϓ ϥΩΎ ѧ όϤϟ Ύ Α ΔΑϮѧ θϤϟ ϭ Δϴ ϘϨ ϟ ϡϮϴ ϧ Ύ Θ ϴ Θ ϟ Ϊϴ δϛϭ Δϴ θϏ ΐϴ γ ήΘ ϟ Δϴ ϧ Ύ Λ Ϯϧ Ύ ϧ ˺˹ Δπ Βϧ Ϊϣ ϭ ΰΗ ήϫ ˿ Δϳ έ ήϜΗ ϝΪόϣ Ύ Ϭόϴ όѧ θΗ Ϊѧ όΑ Ύ Ϭτϴ ѧ θϨ Θ ϟ ˺˺˺ ϥϮϜϴ Ϡ ѧ δϟ ϭ ΝΎ ѧ Οΰϟ Ϧѧ ϣ Ϊѧ ϋ Ϯϗ ϰѧ Ϡ ϋ 1 , 2 , 3 % Δѧ ϔϠ Θ Ψϣ ΔΑΎ ѧ η ΰѧ ϴ ϛ ήΘ Α Ϟѧ Ϝϴ ϧ ϭ ϡ Ϯѧ ϳ ΩϼΑˬ έΎ ѧ ΒΘ ϋϷ ήѧ ψϨ Α Εάѧ Χ Δϴ ѧ θϏϷ ˯Ύ ѧ Ϥϧ Ϸ Ϟѧ ϣ Ϯϋ ΓΪѧ ϋ Ω Ϯѧ Ϥϟ ϩάϬϟ Δϴ γΎ γ ϻ κ Ύ μ Ψϟ Δγ έΩ ϢΗ ϭ Δϴ ΠδϔϨ Βϟ ϕϮϓϭ Δϴ ήϣ Δόη΄Α έΰѧ ϴ Ϡ ϟ Δѧ ϗΎ σ Δѧ ϓΎ Μ ϛˬ έΎ ѧ Α ϲѧ Ϡ ϣ (10-5*10-2 Ϧϴ Πѧ δϛϭϷ ςϐο ˬ 200-500 ºC) ﻟﺘﺤﺪﯾﺪ اﻟﺤﺎﻟﺔ اﻟﻤﺜﻠﻰ ﻣﺜﻞ درﺟﺔ ﺣﺮارة اﻟﻘﺎﻋﺪة ΏϮѧ θϤϟ ϭ ϲѧ ϘϨ ϟ ϡϮϴ ϧ Ύ ѧ Θ ϴ Θ ϟ Ϊϴ ѧ δϛϭϻ Δѧ ϴ Βϴ ϛήΘ ϟ κ Ύ ѧ μ Ψϟ Δϴ Ϩ ϴ ѧ δϟ Δόѧ ηϻ Ξ Ύ ѧ Θ ϧ Ζѧ Θ ΒΛ ϭ ( 0.8 1.8,1.2 ) J/cm2 ΓήΛΆѧ Ϥϟ Ύ ѧ Ϥϛ ΐ Ϯθϟ ΰϴ ϛήΗ ϭ ΓΪϋΎ Ϙϟ Γέ ήΣ ΔΟέΩ ϰϠ ϋ ΓΪθΑ ΪϤΘ όΗ Ύ Ϭμ Ύ μ Χϭ Δϴ θϏϻ ΐϴ ϛ ήΗ ϥ Ξ Ύ Θ Ϩ ϟ ΕήϬχ Κϴ Σ ΔϠ ϴ ΒϨ ϟ ϥΩΎ όϤϟ Ύ Α ϰѧ ϟ · ϲ Ϯѧ θϋ Ϧѧ ϣ Δϴ ѧ θϏϷ ΐѧ ϴ ϛήΗ ϝϮѧ ΤΘ ϳ 300 ºC Ϧѧ ϣ ήѧ Μ ϛ ΓΪѧ ϋΎ ϗ Γέ ήѧ Σ Δѧ ΟέΩ Ϊѧ Ϩ ϋ Ϫѧ ϧ Δϴ Ϩ ϴ ѧ δϟ Δόѧ ηϷ ΩϮϴ Σ Ξ Ύ Θ ϧ ΕήϬχ έϮѧ σ Ϧѧ ϣ Ϟѧ ϛ ϥϮϜϳ 500 ºC وﻋﻨﺪ رﻓﻊ درﺟﺔ ﺣﺮارة اﻟﻘﺎﻋﺪة اﻟﻰ، ﺑﻠﻮري ﻣﺴﺎوي إﻟﻰ ﻃﻮر اﻷﻧﺎﺗﺎس اﻟﺮﺑﺎﻋﻲ ﻻوﻛﺴﯿﺪ اﻟﺘﯿﺘﺎﻧﯿﻮم ϡϮѧ ϳ ΩϼΒϟ ˬ Ϧϴ ѧ Η ϼΒϟ ˬ Δѧ π ϔϟ Δѧ Ϡ ϴ ΒϨ ϟ ϥΩΎ ѧ όϤϟ Ϧѧ ϣ 3% ΐϳ Ϯѧ θΗ ΔΒѧ δϨ Α ΏϮѧ θϤϟ ϡϮϴ ϧ Ύ Θ ϴ Θ ϟ Ϊϴ δϛϭ ϥΎ Α ΪΟϭ αΎ Η Ύ ϧ ϻ ϭ Ϟϴ Η ϭήϟ .CO واﻟﻨﯿﻜﻞ ( اﻛﺜﺮ ﺗﺤﺴﺴﯿﺔ اﻟﻰ ﻏﺎز اﺣﺎدي اوﻛﺴﯿﺪ اﻟﻜﺎرﺑﻮن اﻟﺴﺎم . (AFM) ( وﻣﺠﮭﺮ اﻟﻘﻮى اﻟﺬرﯾﺔSEM ) ﺗﻢ دراﺳﺔ ﻃﺒﻮﻏﺮاﻓﯿﺔ اﻟﺴﻄﺢ ﺑﺎﺳﺘﺨﺪام اﻟﻤﺠﮭﺮ اﻻﻟﻜﺘﺮوﻧﻲ اﻟﻤﺎﺳﺢ ΓΪѧ ϋΎ ϗ Γέ ήѧ Σ Δѧ ΟέΩ Ϟѧ π ϓ Ζѧ ϧ Ύ ϛϭˬ ΓΪϋΎ Ϙϟ Γέ ήΣ ΔΟέΩ ϰϠ ϋ ΪϤΘ ϋ τδϟ ΪϨ ϋ ΕήϬχ ϲΘ ϟ Δϳ Ϯϧ Ύ Ϩ ϟ ΕΎ Ϥϴ δΠϠ ϟ ϲΒϴ ΒΤϟ ϢΠΤϟ ϭ Ϧϴ ѧ Η ϼΒϟ ϥΪѧ όϤ Α ΏϮѧ θϤϟ ϡϮϴ ϧ Ύ ѧ Θ ϴ Θ ϟ Ϊϴ ѧ δϛϭ Ϛѧ Ϡ Θ ϣ έΎ Α ϲϠ ϣ 10-1) واﻓﻀﻞ ﺿﻐﻂ اوﻛﺴﺠﯿﻦ ﻋﻨﺪ اﻧﻤﺎء اﻟﻐﺸﺎء ﻛﺎن500 °C ھﻲ Ύ ѧ ϬΘ Ϥϴ ϗ Ζѧ ϐϠ Βϓ τѧ δϟ Δϧ Ϯѧ θΧ ΓΩΎ ѧ ϳ ί ϰѧ ϟ ΓΪѧ ϋΎ Ϙϟ Γέ ήѧ Σ ΔΟέΩ ΓΩΎ ϳ ί ΕΩ Ύ Ϥϛ ήΘ ϣϮϧ Ύ ϧ ˺˺ ϪΘ Ϥϴ ϗ ΖϐϠ Α Κϴ Σ ϲΒϴ ΒΣ ϢΠΣ ήϐλ ϡϮϴ ϧ Ύ ѧ Θ ϴ Θ ϟ Ϊϴ ѧ δϛϭ ΔΑΎ ѧ η Ϊ ѧ Ϩ ϋ ήΘ ϣϮϧ Ύ ѧ ϧ ˻́ τѧ δϟ Δϧ Ϯѧ θΧ Δѧ Ϥϴ ϗ Ζѧ ϐϠ Αϭ 500 °C ΓΪѧ ϋΎ ϗ Γέ ήѧ Σ ΔΟέΩ ΪϨ ϋ ήΘ ϣϮϧ Ύ ϧ ˺˺ ˻ . ﻣﻦ ﻣﻌﺪن اﻟﺒﻼﺗﯿﻦ3% ﺑﻨﺴﺒﺔ ﺗﺮﻛﯿﺰ Ξ Ύ ѧ Θ ϧ Ζѧ ϧ Ύ ϛ Δϴ Πѧ δϔϨ Βϟ ϕϮѧ ϓϭ Δѧ ϴ ήϤϟ Δόѧ ηϸϟ Δѧ ϳ ΫΎ ϔϨ ϟ ϑ Ύ ѧ ϴ τϣ ΕΎ ѧ γΎ ϴ ϗ Δτѧ γ ϮΑ Δϳ ήѧ μ Βϟ κ Ύ ѧ μ Ψϟ Δγ έΩ ϢΗ Ϛϟ άϛ Δѧ π ϔϟ ϲϧ Ϊѧ ѧ όϤΑ ϡϮϴ ϧ Ύ ѧ Θ ϴ Θ ϟ Ϊϴ ѧ δϛϭ ΔΑΎ ѧ η Ϊѧ ѧ Ϩ ϋϭ Δϴ ѧ δδΤΘ ϟ ΕΎ ѧ Ϙϴ ΒτΘ ϟ Δѧ Ϥ ϼϣ Ύ ѧ ϬϠ όΠϳ Ύ ѧ Ϥϣ 90 % Ϧѧ ϣ ϰѧ Ϡ ϋ Δϳ ήѧ ѧ μ Βϟ Δѧ ϳ ΫΎ ϔϨ ϟ (3.2 eV Γήѧ ηΎ Βϣ ήϴ ϐϟ Δϳ ήμ Βϟ ΔϗΎ τϟ ΓϮΠϓ ΔϤϴ ϗ ΖϐϠ Α Δϴ ήϤϟ ΔϘτϨ Ϥϟ ϮΤϧ ι Ύ μΘ ϣϻ ΔϓΎ ΤΑ ΔΣ ί Ι ϭΪΣ φΣϼϧ Ϧϴ Η ϼΒϟ ϭ ϥΎ ѧ μ Ϙϧ ϰѧ ϟ ΓΪѧ ϋΎ Ϙϟ Γέ ήѧ Σ Δѧ ΟέΩ ΓΩΎ ѧ ϳ ί ϯΩ ϭ 500 °C ΓΪѧ ϋΎ ϗ Γέ ήѧ Σ Δѧ ΟέΩ Ϊѧ Ϩ ϋ 3.6 eV) Γήѧ ηΎ ΒϤϟ Δϳ ήμ Βϟ ΔϗΎ τϟ ΓϮΠϓ ϭ .(K) ( وﻣﻌﺎﻣﻞ اﻟﺨﻤﻮدn) ﺑﻘﯿﻤﺔ ﻣﻌﺎﻣﻞ اﻻﻧﻜﺴﺎر ˯Ύ ѧ θϐϟ Ξϴ ѧ ϬΗ Ϊѧ Ϩ ϋ ΕϻΎ ѧ ϘΘ ϧ ϻ Ϧѧ ϣ ϥΎ ѧ ϋϮϧ Εήѧ Ϭχ ΔόϨ ѧ μ Ϥϟ Δϴ ѧ θϏϼϟ Δѧ ϴ ϧ ϮΗ Ϯϔϟ Γ˯Ύ ѧ πΘ γ ϻ ΓΪѧ η Ξ Ύ Θ ϧ Ϛϟ Ϋ ϰϟ ΔϓΎ ο ϻΎ Α Δϴ Πѧ δϔϨ Βϟ ϕϮϓ ΔϘτϨ Ϥϟ ΪϨ ϋ ϊ ϘΗ A ﺷﺪة اﻟﻘﻤﺔ،(3.2 eV) اﻟﺮﻗﯿﻖ ﺑﻄﺎﻗﺔ اﻛﺒﺮ ﻣﻦ ﻓﺠﻮة ﻃﺎﻗﺔ ﻏﺸﺎء اوﻛﺴﯿﺪ اﻟﺘﯿﺘﺎﻧﯿﻮم اﻟﺘﻲ ﺑﻠﻐﺖ . ﻧﺎﻧﻮﻣﺘﺮ٥٢٥ ﺗﻘﻊ ﻋﻨﺪ اﻟﻤﻨﻄﻘﺔ اﻟﻤﺮﺋﯿﺔ ﻋﻨﺪ اﻟﻄﻮل اﻟﻤﻮﺟﻲB ﻧﺎﻧﻮﻣﺘﺮ وﺷﺪة اﻟﻘﻤﺔ٣٧٥ ﻋﻨﺪ اﻟﻄﻮل اﻟﻤﻮﺟﻲ Δѧ Ϡ ϴ ΒϨ ϟ ϥΩΎ ѧ όϤϟ Ύ Α ΏϮѧ θϤϟ ϡϮϴ ϧ Ύ ѧ Θ ϴ Θ ϟ Ϊϴ ѧ δϛϭ Ϛѧ Ϡ Θ ϣ ϥϮѧ ϴ Ϡ ϣ Ϟѧ Ϝϟ ˯ΰѧ Ο ˾˹ ΰѧ ѧ ϴ ϛήΗ Ζѧ ѧ ΤΗ CO ίΎ ѧ ϐϟ Δϴ ѧ δδΤΘ ϟ ΏΎ ѧ δΣ Ϣѧ Η ϥϮϜϴ Ϡ ѧ δϟ Ϧѧ ϣ ΓΪѧ ϋΎ ϗ ϰѧ Ϡ ϋ ΐѧ γ ήϤϟ ϭ Ϧϴ ѧ Η ϼΒϟ ϥΪѧ όϤΑ ΏϮѧ θϤϟ ϡϮϴ ϧ Ύ ѧ Θ ϴ Θ ϟ Ϊϴ ѧ δϛϭ ϥ ϭ ϲϘϨ ϟ ϡϮϴ ϧ Ύ Θ ϴ Θ ϟ Ϊϴ δϛϭ Ϧϣ ήΒϛ Ϫϴ δδΤΗ وادى زﯾﺎدة ﺗﺮﻛﯿﺰ، 250 ºC (ﻋﻨﺪ اﻓﻀﻞ درﺟﺔ ﺣﺮارة ﺗﺸﻐﯿﻞ ﺑﻠﻐﺖ23% ) ﺑﻘﯿﻤﺔCO ( اﻣﺘﻠﻚ أﻋﻠﻰ ﻗﯿﻤﺔ ﻟﻠﺘﺤﺴﺴﯿﺔ ﻟﻐﺎز١١١)
Ζѧ ϐϠ Α Κѧ ϴ Σ ϡϮϴ ϧ Ύ ѧ Θ ϴ Θ ϟ Ϊϴ ѧ δϛϭ ίΎ ϐϟ β δΤΘ ϟ έΎ ϴ Η ΔϤϴ ϘΑ ΓΩΎ ϳ ί ϭ ϡϭ اﻟﺸﻮاﺋﺐ اﻟﻰ اﻟﻨﻘﺼﺎن ﺑﻘﯿﻤﺔ اﻟﻤﻘﺎوﻣﺔ ﺣﯿﺚ ﺑﻠﻐﺖ ﻗﯿﻤﺘﮭﺎ ١٠٩ ﻗﯿﻤﺘﮫ ١٠ﻧﺎﻧﻮ أﻣﺒﯿﺮ .
Chapter One
Introduction and Historical Review
1- Introduction Titanium dioxide TiO2 (titania) is a cheap, non-toxic and one of the most efficient semiconductor photocatalysts for extensive environmental applications because of its strong oxidizing power, high photochemical corrosive resistance and cost effectiveness[1]. Due to these inherent properties, TiO2 is the most suitable candidate for degradation and complete mineralization of toxic organic pollutants in water[1,2]. It is well known that TiO2 exists in three crystalline structures: rutile, anatase and brookite
[3,4]
. The anatase phase is especially adequate for those
applications due to its crystal structure and a higher band gap of 3.2 eV compared to the 3 eV in rutile. Anatase and rutile have properties of interest for sensing applications[5] . Ni,Pt, and Ag has been found to be an efficient dopant for improving the gas sensor activity for CO gases
[6,7]
. In
principle, transition metal with proper oxidation state replace some of the Ti (IV) from lattice producing an impurity state that reduces the band gap of TiO2 . In particular the pd doping was found to increase the sensitivity of CO gas to 3 times from pure TiO2 [8]. It is already established that material properties depend strongly on precursors and synthesis methods in correlation with the thermodynamic process parameters. For the synthesis of nanoparticle systems the hydrothermal method was intensively utilised in the last decade [9] . Titanium dioxide( TiO2) has attracted much attention in recent years due to its great potential for applications in optical elements, electrical insulation, capacitors or gates in microelectronic devices , photovoltaic solar cells, anti reflection coatings , optical waveguides, photonic crystals
[3]
, devices
based on metal etc[10]. TiO2 films with specific crystal structure, orientation or morphology exhibit specific characteristics, which makes it important to control the phase structure of TiO2 films during the growth. The methods of sol–gel spin-coating, anodization, oxygen plasma assisted molecular beam epitaxy and pulsed laser deposition (PLD) have been used to fabricate TiO2 1
Chapter One
Introduction and Historical Review
films. Among these methods, PLD technique has been widely used for growing oxide films because it allows for stoichiometry of the synthesized material. And because Si substrate is widely used in semiconductor industry the growth of TiO2 films on Si substrates using PLD attracted much attention. TiO2 is a unique material in view of its versatile properties which comprise high refractive index, wide band gap, and resistance to chemical and physical impacts [11,12]. Gas sensors based on semiconductor metal oxide thin films focused numerous research efforts during the last few years. Among them, titanium dioxide (TiO2) has been investigated due to its sensing properties in front of hydrogen
[2]
, carbon monoxide and oxygen, hydrocarbons, or humidity
detectors[13]. Its sensing capability has been proved to improve with the addition of metal dopants such as Pt , Ni ,Pd ,Ag ,Cr,Fe and Co[14].
1.١ Historical Review The efforts toward using Lasers in depositing thin films started soon after the invention of reliable high power lasers. Early observations of the ease with which the material could be vaporized by the intense interaction of high power laser pulses with material surface demonstrated that the intense laser radiation could be successfully used to deposit thin films of that material .Titanium Oxide thin film has attracting attention as one of the promising material with wide applications. It has been prepared and characterized by many workers and using different technique, which greatly affected the obtained film characteristic. The first complete study about the electrical and optical properties for crystalline titanium dioxide type ( rutile ) was prepared as crystal pieces by researcher (D.C.Cronemeger 1952) [18]. In recent years, applications to environmental cleanup have been one of the most active areas in heterogeneous photocatalysis. This is 2
Chapter One
Introduction and Historical Review
inspired by the potential application of TiO 2 based photocatalysts for the
destruction
of
organic
compounds
in
polluted
air
and
wastewater[16]. In [1995] J. Osterwalderb et al
[19]
Deposited TiO2: Cr grown by
plasma-assisted molecular beam epitaxy. They studied the relationship between structural quality and magnetic ordering, using epitaxial Crdoped anatase TiO2 with excellent structural quality as a model system. Epitaxial films deposited slowly at 0.08 (A°/Sec) possess a perfect crystalline structure, whereas films deposited at 0.2 (A°/Sec) are found to have a highly defected crystalline structure, as characterized by X-ray diffraction (XRD). In [1997] G. Korotcenkov and Sang Do Han[20] prepared (Cu, Fe, Co, Ni)-doped Titanium dioxide films deposited by spray pyrolysis. The annealing at 850-1030 ◦C was carried out in the atmosphere of the air. For structural analysis of tested films they have been using X-ray diffraction, Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM) techniques. It was established that the doping did not improve thermal stability of both film morphology and the grain size. It was made a concluded that the increased contents of the fine dispersion phase of Titanium dioxide in the doped metal oxide films, and the coalescence of this phase during thermal treatment were the main factors, responsible for observed changes in the morphology of the doped TiO2 films. In [١٩٩٨] X. H. XU et al
[21]
studied the effect of calcinations
temperatures on photocatalytic activity of TiO2 films prepared by an electrophoretic deposition ( EPD) method. TiO2 films fabricated on transparent characterized
electro-conductive by
X-ray
glass
diffraction
substrates (XRD),
and X-ray
were
further
photoelectron
spectroscopy (XPS), field emission scanning electron microscope (FESEM), UV–vis diffuse reflectance spectra and Photoluminescence 3
Chapter One
Introduction and Historical Review
spectra (PL). FESEM images indicated that the TiO2 films had roughness surfaces, which consisted of nano-sized particles. In [١٩٩٩] Yanan Fu et al
[22]
studied transparent TiO2 thin films
with high photocatalytic activity prepared on glass substrates via the sol-gel method from tetra isopropyl titanium ethanol solution containing polyethylene glycol and diethylene glycol . The former was chosen to increase the surface area of the film and the later to stabilize the dipping solution. The dipping process with a pull-up speed of 1.5( mm/s) was used to obtain the thin films. The dipping process was carried out 10-40 times. The thin films were calcined at 450 °C for 1 hour after every ten dippings. The thickness of a 40x-dipped film was 0.6 mm and the apparent area of the TiO2 thin film was 12.7 cm2. Photocatalytic activity of the thin films was studied using the decomposition of gaseous acetaldehyde. Six 10 W fluorescent black light bulbs provided the irradiation. The concentration of the acetaldehyde vapor was 1000 ppm throughout the experiment. In [٢٠٠٠] E. Gyorgy and E. Axente
[23]
studied The
characterization and CO gas sensing properties of pure and doped TiO 2 with Pt thin films deposited on glass substrates by (PLD) technique, at laser energy densities of (1 J/cm2). Pure TiO2 thin film less sensitive to CO gas compared to the TiO 2 thin film doped with 4% Pt. In [2001] B. Farkas et al
[24]
prepared transparent TiO2:Ni thin
films with different Ni concentration 0.01, 0.015 and 0.03 at 600 °C on quartz substrates by (PLD) technique using Nd: YAG pulsed laser (λ=532 nm). .Ni doping thin films showed a shift towards the visible in the absorption edge of the UV-Vis absorption spectra of the thin film. The magnitude of this shift was found to increase with the amount of dopant. The values of band gap values for pure, 0.01, 0.015 and 0.03 4
Chapter One
Introduction and Historical Review
Ni concentration were determined to be 3.1, 2.76, 2.62, and 2.23 respectively. In [2002] D. Dzibrou et al
[25]
deposited TiO2 thin films on
quartz and silicon wafers, by PLD method using Nd: YAG pulsed laser (λ=355nm, 10 Hz) with laser energy density of 1.5 J/cm2. The thin films were thermally treated at temperatures of 300 °C, 400 and 500 °C in air for 1 hour. The coatings obtained were uniform, smooth with very good optical properties. The sample annealed at lower temperature had the characteristic appearance of an amorphous material. The samples treated at 400°C and 500 °C were crystallized. TiO2 had direct and indirect band gaps.The band gap values for both transitions were different in comparison to the well-known value of 3.03 eV for the indirect band gaps and 3.43 for the direct . In [2002] S.A. Tomas et al radio frequency
[26]
carried out an experiment using
reactive sputtering technique to prepare transparent,
nanocrystalline and photocatalytic TiO2 pure and doping 1 and 3% Ag thin film . Spectroscopic techniques have been used to study the optical and structural properties of the films. A gradual shift of the transmission spectrum towards longer wavelengths has been observed when TiO2 doped with an increased amount of Ag, which indicates a decrease in the band gap value of TiO2 upon Ag doping. The photoluminescence (PL) spectrum of the films, have been measured which
showed a gradual shift of the
emission peak towards the longer wavelength region and supported the lowering of the band gap with Ag doping. The band gap energies were calculated from transmittance and reflectance data. In[2003] H. Shinguu et al
[27]
studied the structural properties and
morphologies of TiO2 thin films, in which they were deposited on Si(100) and Si(111) substrates by using ArF excimer laser (operating with wavelength 248 nm at 500 ºC) .The films have been annealed for 10 hours at the temperature 600ºC, in oxygen and air flow.The TiO2 film deposited 5
Chapter One
Introduction and Historical Review
on (111)-oriented silicon exhibited a better anatase crystalline than that on (100)-oriented silicon. Whereas a higher annealing time needed to transform anatase structure into rutile structure for films deposited on Si(111) than on Si(100). The AFM images showed that the substrate orientation had no great effect on the surface morphologies for both anatase as-deposited films and rutile annealed films. In[2004] L.C.Tien et al
[28]
deposited TiO2 thin films on sapphire
by using ArF excimer laser (operating with wavelength 193 nm, pulse width 15 ns, repetition frequency 10 Hz and power 100 mJ ) at a substrate temperature of 500°C. The diagnostic of the ablation plume showed the interaction of the evaporated Ti particles with buffer O2 gas. The dependence of the buffer O2 gas pressure was studied by spectroscopy of ablation plume, thickness of films, morphology of the surface using SEM and AFM micrographs, XRD patterns and Raman spectra. The morphology showed the formation of nanostructure by interactions of evaporated Ti particles with the buffer O2 gas. The structures of the PLD thin films showed epitaxial growths in the high substrate temperature (500°C) and an appearance of anatase at high buffer O2 gas pressure owing to the contributions of the TiO molecules. In[2004] Yoshiaki Suda et al
[29]
prepared TiO2 films on
different substrate at different temperatures (100-400) ºC by using KrF Excimer laser ( =532nm, =3.5ns) at about 1 J/cm2 laser density. They found that all films showed (101) anatase phase at the optimized conditions. Photoluminescence (PL) results indicated that the thin films fabricated at the optimized conditions showed the intense near band PL emissions In[2005]
Tamiko and Ohshima
[30]
prepared TiO2 thin films by
PLD method using XeCl excimer laser 308 nm wavelength which was used to irradiate TiN (purity 99.9%) target and TiO2 (99.99%) target in nitrogen/oxygen gas mixture. The color of the film changed from 6
Chapter One
Introduction and Historical Review
TiO2(transparent) to TiN(dark brown) with increasing the nitrogen concentration ratio. The crystalline structure of the films prepared strongly depends on the nitrogen concentration ratio in the gas mixture and the target material. The anatase type TiO2 crystalline structure can be observed to be independent of the nitrogen concentration ratio in nitrogen/oxygen gas mixture. [31]
In[2005] A. P. Caricato et al
studied nanostructured TiO2 thin
films prepared by (PLD) KrF excimer pulsed laser system (wavelength = 248 nm) on indium-doped tin oxide (ITO) substrates under different substrate temperature and pressure conditions (T = 250 ,400,500 and 600 °
C, P = 10-2 and 10-1 Torr ) . AFM results showed the samples prepared at
400 °C have much more uniform surfaces and smaller particle size than that prepared at 600 °C. The XPS results indicated that the binding energy of the Ti core level system pressure was dependent on substrate temperature . However, under 10-1 Torr, only anatase phase was observed even at the temperature higher than the commonly reported anatase-to-rutile phase transition range (~ 600 °C). In[2006] Narumi Inoue et al
[32]
prepared thin films of pure and
TiO2 doped Pd using Nd: YAG pulsed laser (λ=355nm, 10 Hz) with laser energy density of 1.8 J/cm2. The gas sensing performance of these films for various gases was tested. Both pure and Pd -doped TiO2 based sensors showed highest responses to CO gas with poor sensitivity to H2 gas as compared to later doped Pd ( 3%).TiO2 thin films showed sensitivity to CO gas as high as 14% while pure TiO2 thin films showed poorsensitivity to CO gas(4%). The effects of microstructure and additive concentration on the gas response, selectivity, response time and recovery time of the sensor in the presence of H2 gas were studied and discussed. In[2006] Kishor Karki et al
[33]
deposited TiO2 thin films on Si
(111) substrate by using (PLD) ArF excimer laser (operating with wavelength 193 nm).The sensor detected different concentrations of CO 7
Chapter One
Introduction and Historical Review
gas from alterations in resistances of samples. The operation temperature varies from room temperature to 230°C. X-ray diffraction (XRD) and (AFM) were applied to characterize the structure and surface morphology of the deposited TiO2/Si films. In [2007] T. Nambara, K. Yoshida
[34]
studied the crystalline rutile
type titanium dioxide (TiO2) thin films which were prepared by (PLD) at substrate temperature 850 °C . The optical properties of the present rutile films were different from that of single crystal TiO2. UV-Vis spectra of PLD films showed a blue shift. The value of the gap was 3.30 eV, which was shifted from 3.02 eV as the bulk value. They considered quantum size and strain effects of PLD-TiO2 crystalline.They observed sensitivity trends with respect to thickness 250 nm of TiO2 thin film sensors This thickness is comparable to the depletion length Comparison of the sensitivity of TiO2 films toward 250 ppm of CO gas at 550 C. In [2008] Matthew et al [35] examined the growth of TiO2 thin films by (PLD) on SrTiO(STO), LaAlO3 (LAO), and fused silica with a KrF (248 nm, 2 Hz pulses). The laser output was maintained at 320 mJ per pulse, substrates at 550 °C The films grown on the fused silica substrates showed very small XRD peaks of the (200) and (110) oriented rutile TiO2, but are much less crystalline and conductivity σ∼1000 (Ω cm)-1 than the films grown on STO σ∼2500 (Ω cm)-1 and LAO σ∼2000 (Ω cm)-1 at the same conditions. The films on STO were not only crystalline, but appeared to grow epitaxially on the lattice-matched substrate. In [2008] M. Walczak et al [36] studied the effect of oxygen pressure on the structural and morphological characterization of TiO2 thin films deposited on Si (100) by using KrF Excimer laser operated at wavelength of 248 nm and repetition rate 5Hz . The laser energy density was about 2 J/cm2 ). They found that the decreasing of oxygen pressure from (10 -2 Torr to 10
-1
Torr) produced highly homogeneous nanostructured morphology 8
Chapter One
Introduction and Historical Review
with grain size as small as 40 nm and high quality nanostructure was observed at the 10 -1 Torr of oxygen . In [2009] Mikel Sanz et al
[37]
deposited TiO2 films on Si (100) by
PLD by using three different Nd:YAG laser wavelengths (266nm, 532nm and 355nm). They found that the films grown at λ=266 nm has smallest nanoparticles (with average diameter 25 nm) and the narrowest size distribution was obtained by ablation at 266 nm under 0.05 Pa of oxygen. The effect of temperature on the structural and optical properties of these films have been investigated systematically by XRD, SEM, FTIR, and PL spectra. In [2009], Mohammad Hafizuddin et al films
onto SiO2
[38]
prepared
via sol-gel technique .They studied
TiO2 thin gas sensing
properties and microstructures of TiO2 thin films. TiO2 thin film exhibit a satisfactory response towards ethanol and methanol vapor. However, the ability to select different type of gas remain the main issue as the detection pattern for both gases are similar where XRD investigation showed that the thin films were amorphous. In [2009], Dang Thi et al
[39]
prepared Titanium dioxide transparent thin
films (TiO2) sensors,with different thickness by (PLD) techniques (using XeCl Excimer Laser 308 nm wavelength )using ceramic targets onto glass substrates .Thick films were necessary to increase the sensitivity of TiO2 sensors to CO gas. Electrical measurements, x-ray diffraction and scanning electron microscopy have been used to study the (CO) gas sensitivity, structure and morphology of the sensors. In [2009] J. D. Fergusona et al films by PLD using
[40]
studied PL spectra of TiO2 thin
Nd:YAG laser (532nm). Thin films calcined at
different temperature with an excitation wavelength of 300 nm are shown in Fig (1-6) . A strong and wide PL signal at about 380–420 nm is attributed to the band–band PL phenomenon with the light of energy approximately equal to the band gap energy of the anatase and rutile. The 9
Chapter One
Introduction and Historical Review
band of anatase and rutile are 387.5 and 413.3 nm, due to the fact that their band gap energies are 3.2 and 3.0 eV, respectively .
1-2 Aim of the work The aim of this work is to reveal specific properties of TiO2 nanostructure prepared by pulsed laser deposition technique . 1. Characteristics of structural , microstructural and photoluminescence properties of TiO2 . 2.Studing the sensitivity and selectivity of thin films pure and doped with different noble metal deposited by PLD to CO gas.
10
Chapter Two Thin Films
Fundamental Properties of Laser Deposition and TiO2
2. Introduction Pulsed laser deposition (PLD) is a thin-film deposition method, which uses short and intensive laser pulses to evaporate target material. The ablated particles escape from the target and condense on the substrate. The deposition process occurs in vacuum chamber to minimize the scattering of the particles. In some cases, however, reactive gases are used to vary the stoichiometry of the deposit [41]. This chapter explains the main properties which make TiO2 a good candidate for certain application and how such properties change with deposition conditions according to what is already published .
2.1 Laser ablation mechanisms In PLD a pulsed high-energetic laser beam is focused on a target resulting in ablation of material. At the early stage of the laser pulse a dense layer of vapor is formed in front of the target. Energy absorption during the remainder of the laser pulse causes, both, pressure and temperature of this vapor to increase, resulting in partial ionization. This layer expands from the target surface due to the high pressure and forms the so-called plasma plume
[2]
. During this expansion, internal thermal and
ionization energies are converted into the kinetic energy (several hundred eV) of the ablated particles. Attenuation of the kinetic energy due to multiple collisions occurs during expansion into low- pressure background gas. Usually, the laser ablation process is divided in two stages, separated in time [19,16]: 1. Target evaporation and plasma formation 2. Plasma expansion.
2.1.1 Laser – Target interaction Ideally the plasma plume produced should have the same stoichiometry as the target if we hope to grow a film of the correct ١١
Chapter Two Thin Films
Fundamental Properties of Laser Deposition and TiO2
composition. For example, if the target surface was heated slowly, say by absorbing the light from a CW laser source, and then this would allow a significant amount of the incident power to be conducted into the bulk of the target. The subsequent melting and evaporation of the surface would essentially be thermal i.e. the difference between the melting points and vapor pressures of the target constituents would cause them to evaporate at different rates so that the composition of the evaporated material would change with time and would not represent that of the target. This incongruent evaporation leads to films with very different stoichiometry from the target
[45]
. To achieve congruent evaporation the energy from the
laser must be dumped into the target surface rapidly, to prevent a significant transport of heat into the subsurface material, so that the melting and vapor points of the target constituents are achieved near simultaneously. The high laser power density that this implies is most readily achieved with a pulsed or Q-switched source focused to a small spot on the target. If the energy density is below the ablation threshold for the material then no material will be removed at all, though some elements may segregate to the surface [43,44]. In general the interaction between the laser radiation and the solid material takes place through the absorption of photons by electrons of the atomic system. The absorbed energy causes electrons to be in excited states with high energy and as a result the material heats up to very high temperatures in a very short time. Then, the electron subsystem will transfer the energy to the lattice, by means of electron-phonon coupling
[45,16]
.When the
focused laser pulse arrives at the target surface the photons are absorbed by the surface and its temperature begins to rise. The rate of this surface heating, and therefore the actual peak temperature reached, depends on many factors: most importantly the actual volume of material being heated. This will depend not only upon how tightly the laser is focused but also on ١٢
Chapter Two Thin Films
Fundamental Properties of Laser Deposition and TiO2
the optical penetration depth of the material. If this depth is small then the laser energy is absorbed within a much smaller volume. This implies that we require a wavelength for which the target is essentially opaque and it is in general true that the absorption depth increases with wavelength. The rate of heating is also determined by the thermal diffusivity of the target and the laser pulse energy and duration. In a high vacuum chamber, elementary or alloy targets are struck at an angle of 45o by pulsed and focused laser beam. The atoms and ions ablated from the target are deposited on substrate, which is mostly attached with the surface parallel to the target surface at a target-to-substrate distance of typically 2-10 cm
[21]
. In PLD technique, the target materials are first
sputtered (or say ablated) into a plasma plume by a focused laser beam an angle of 45o. The materials ablated then flow (or fly) onto the substrate surface, on which the desired thin films are developed. Therefore, the interaction of intense laser which matters plays an important role in PLD process [38]. The thin film formation process in PLD generally can be divided into the following four stages (see figure 2-1) 1.
Laser radiation interaction with the target.
2.
Dynamics of the ablation materials.
3.
Deposition of the ablation materials with the substrate.
4.
Nucleation and growth of a thin film on the substrate surface.
١٣
Chapter Two Thin Films
Fundamental Properties of Laser Deposition and TiO2
Figure (2-1): Interaction between laser beam and matters [21].
The incident laser pulse induces extremely rapid heating of significant mass/volume of the target material. This may cause phase transition and introduce high amplitude stress in the solid target. The output of pulsed laser is focused onto a target material maintained in vacuum or with an ambient gas. The target is usually rotated in order to avoid repeated ablation from the same spot on the target. The lasers used in PLD studies range in output wavelengths from the ultraviolet (excimer laser which operates at different UV wavelengths) to the near- and mid-infrared (Nd-YAG and CO2 lasers) through the visible wavelengths, with fundamental and SHG laser output [21,47]. Figure (2-2) shows the theory of the Pulsed Laser Deposition (PLD) and Pulsed Laser Ablation (PLA).
Figure (2-2) typical pulsed laser deposition or ablation [47]
2.1.2 Laser – plasma interaction In the description of the laser–plasma interaction, the laser pulse duration plays a crucial role. Whereas in the case of nanosecond (ns) laser ١٤
Chapter Two Thin Films
Fundamental Properties of Laser Deposition and TiO2
pulse, the forming plasma interacts with the laser beam ”tail”. In the case of femtosecond (fs) laser pulse the previous mechanism doesn’t take place. Because of the formation of a plasma in front of the target, the laser beam will be partially absorbed before it reaches the target i.e. so called (plasma shielding effect)
[48]
and increases the plume ionization degree,
complicating the plume expansion mechanism. Due to the plasma-laser interaction, the temperatures of the evaporated material increases therefore rapidly to extremely high values and the electrons are further accelerated. The excited particles will emit photons, leading to a bright plasma plume, which is characteristic for the laser ablation process. The main absorption processes are the Inverse Bremsstrahlung (IB) and the direct single-photon processes, IB involves absorption of photons by free electrons which are accelerated during collision with neutral or ionized atoms. The cross-section for IB via electron-neutral collisions is much smaller than that via electron ion collisions, but can be important for the initial plume of a weakly ionized gas. Initially, there may be very few "seed" electrons present, produced by thermal emission from the solid or multi-photon ionization processes[49,50]. The contribution from multi-photon processes increases with decreasing wavelength, but it particularly important for ultra fast lasers [51].
2.1.3 Plasma plume expansion Since the onset of the material removal described in the previous sections takes place within a very short time after the pulse (1-100 ps), on the time scale of the plasma expansion (μs), the laser–target event can be regarded as a momentary release of energy. The spatial structure of the vapor plasma at the early stage of its expansion is well known to be a cloud strongly forwarded in the direction normal to the ablated target. The reason of this characteristic plasma elliptic shape, called plume, is in the strong difference in pressure gradients ١٥
Chapter Two Thin Films
Fundamental Properties of Laser Deposition and TiO2
in axial and radial directions: the plasma expands in the direction of maximum pressure gradient [52,53]. Another important characteristic of the ablation plume pertinent to PLD is the angular distribution of the ejected species in the plume or simply the plume angular distribution . In case of vacuum the plume angular distribution is determined by the collisions of the plume particles among themselves in the initial stage. When plume is small however in the presence of the ambient gas the plume angular distribution is modified due to collision between the plume species and background gas atoms. These collisions scatter the plume particles from their original trajectories and broaden the angular distribution. It is generally expected that for a given background gas these additional collisions will lead to wider angular distribution of lighter plume species and similarly a scattering ambient with high mass will more effectively disperse the plume species compared to a low mass scattering ambient [54,17]. Expansion the plume in vacuum is driven by the energy which is accumulated as thermal energy and energy which is stored as excitation and ionization in the initial layer. This energy is converted to kinetic energy of the atoms in the plume, and eventually all atoms will move with an asymptotic, constant velocity distribution. As soon as the laser pulse ends, there is little further transfer of energy and mass to the ablation plume, and the plume propagation can essentially be considered as an adiabatic expansion [55,56]. The initial expansion of an ablation plume in a background gas does not differ much from the expansion in vacuum, since the driving pressure (~ 1 kbar) usually is much higher than that of a low-pressure background gas (< 1 mbar).
١٦
Chapter Two Thin Films
Fundamental Properties of Laser Deposition and TiO2
Most of the other existing treatments have been performed for a specific choice of target and background gas and cannot readily be extended to other target-gas combinations[57, 58].. Laser ablation with ultra short pulses (
