structural studies of silicon, germanium, carbon alloy thin films

311 Pages·2012·12.32 MB·English

STRUCTURAL STUDIES OF
SILICON, GERMANIUM, CARBON
ALLOY THIN FILMS

A THESIS SUBMITTED FOR
THE Ph. D (SCIENCE) DEGREE
OF
JADAVPUR UNIVERSITY

AYANA BHADURI
ENERGY RESEARCH UNIT
INDIAN ASSOCIATION FOR THE CULTIVATION OF SCIENCE
JADAVPUR, KOLKATA-700032
OCTOBER, 2008
Dedicated to my parents
CERTIFICATE FROM THE SUPERVISOR

This is to certify that the thesis entitled “Structural studies of silicon,
germanium, carbon alloy thin films” submitted by Smt. AYANA BHADURI who got
her name registered on 30.07.2007 for the award of Ph.D (Science) degree of Jadavpur
Universuty, is absolutely based upon her own work under the supervision of Prof.
Partha Chaudhuri and that neither this thesis or any part of it has been submitted for
any degree/ diploma or any other academic award anywhere before.

Prof. Partha Chaudhuri
Date:
Signature of the Supervisor with official seal
Contents
Contents Page No.

1. Acknowledgement vii
2. Abstract xi
3. Chapter 1: Introduction 1
4. Chapter 2: Review of Past Works 7

2.1. Introduction 9
2.2.1. Structure of amorphous silicon 10
2.2.2. Structure of hydrogenated amorphous silicon 11
2.2.3. Microstructure 12
2.2.4. Incorporation of Hydrogen atoms in a-Si:H network and related
microstructure 13
2.2.5. Powder formation in a-Si:H 14
2.2.5.1. Nanocrystallites nucleation within the plasma 17
2.3. Alloying Si with C or Ge 20
2.3.1.Hydrogenated amorphous silicon-carbon (a-SiC:H) 21
2.3.1.1.Effect of C-content on the film a-SiC:H microstructure 23
2.3.1.2. Incorporation of Hydrogen atoms in a-SiC:H network and related
microstructure 25
2.3.2. Hydrogenated amorphous silicon germanium (a-SiGe:H) 25
2.3.2.1. Effect of Ge concentration on the film microstructure 27
2.3.2.2.Incorporation of Hydrogen atoms in a-SiGe:H network
and related microstructure 29
2.3.2.3. Powder formation in a-SiGe:H 29
2.4. Hydrogenated amorphous carbon (a-C:H) 30
2.4.1. Incorporation of Hydrogen atoms in a-C:H network 32
2.4.2. Effect of different bonding configurations [sp3, sp2] 33
2.4.3. Powder formation and Nanocrystallites formation in a-C:H 37
2.5. Bibliography 39

5. Chapter 3: Deposition Systems and Characterization techniques 55

3.1. Introduction 57
3.2. Deposition methods 57
3.2.1. Different techniques for thin film deposition 57
3.2.1.1. Thermal evaporation 58
3.2.1.2. Sputtering 58
i
Contents
3.2.1.3. Sol-gel method 59
3.2.1.4. Chemical vapour deposition 60
3.2.1.4.1. Plasma assisted methods 60
3.2.1.4.1.1. RF and DC PECVD 60
3.2.1.4.1.1.1. Square wave pulse modulation 65
3.2.1.4.1.2. Microwave CVD 67
3.2.1.4.1.3. Electron Cyclotron Resonance (ECR) CVD 67
3.2.1.4.1.4. Plasma Beam Deposition 68
3.2.1.4.2. Catalytic CVD (HWCVD) 69
3.2.1.4.3. Photo CVD 70
3.3. Material Characterization Methods 71
3.3.1. Plasma Diagnostic Techniques 71
3.3.1.1. Optical emission spectroscopy (OES) 71
3.3.1.2. Laser Light Scattering (LLS) 72
3.3.2. Film Thickness 73
3.3.3. Structural characterization 74
3.3.3.1. X-ray diffraction (XRD) technique 75
3.3.3.2. Small Angle X-ray Scattering (SAXS) 76
3.3.3.3. Transmission Electron Microscope (TEM) 77
3.3.3.4. Scanning Electron Microscope (SEM) 79
3.3.3.5. Atomic Force Microscope (AFM) 80
3.3.3.6. Vibrational Spectroscopy 83
3.3.3.6.1. Raman Spectroscopy 83
3.3.3.6.1.1. a-Si:H and nc-Si:H 84
3.3.3.6.1.2. a-Si:Ge 85
3.3.3.6.1.3. Multi-wavelength (M W) Raman spectroscopy for a-C:H films 86
3.3.3.6.2. Fourier Transform Infrared spectroscopy (FTIR) 90
3.3.3.6.2.1. a-Si:H 91
3.3.3.6.2.2. a-SiC:H 94
3.3.3.6.2.3. a-SiGe:H 95
3.3.3.6.2.4. a-C:H 97
3.3.3.7. Rutherford Back Scattering (RBS) 100
3.3.3.8. Electron Energy Loss Spectroscopy (EELS) 100
3.3.4. Mechanical Properties (Carbon samples) 102
3.3.4.1. Nanoindentation 102
3.3.5. Transport properties 104
3.3.5.1. Electrical conductivity 104
3.3.5.1.1. Photoconductivity 106
3.3.5.2. Steady state Photocarrier grating (SSPG) 108
3.3.5.3. Modulated Photocurrent method (MPC) 109
3.3.6. Optical Properties 111
3.3.6.1. UV/Vis/NIR Spectroscopy 114
3.3.6.2. Photoconductivity spectra 116
3.3.6.2.1. Constant photocurrent method (CPM) 116
3.3.6.2.2. Dual Beam Photoconductivity (DBP) 117
3.4. Bibliography 120
ii
Contents

6. Chapter 4: Characterization of Structural and Transport Properties of Amorphous
Silicon Thin Films Deposited Near Dusty Plasma Condition 131

4.1. Introduction 133
4.2. Experimental 134
4.3. Results 136
4.3.1. Intensity distribution of laser light scattering 136
4.3.2. HRTEM study of the powders 138
4.3.3. Structural and electronic properties of the deposited films 139
4.4. Discussion 142
4.5. Conclusion 146
4.6. Bibliography 147

7. Chapter 5: Structural Optimization of a-Si Ge :H Thin Films by Controlling
1-x x

Different Deposition Parameters 151

5.1. Introduction 153
5.2. Experimental 155
5.2.1. Dilution series 155
5.2.2. Power density series 156
5.2.3. Variation of the GeH to SiH flow ratio 156
4 4
5.2.4. Pulsed series samples 156
5.3. Film Characterization 158
5.4. Results 159
5.4.1. Ge-concentration 159
5.4.2. H-Bonding 160
5.4.2.1. H Diluted series 161
2
5.4.2.2. Ar Diluted series 162
5.4.2.3. H and Ar both dilution 163
2
5.4.2.4.Power density variation 164
5.4.2.5. Variation of the GeH to SiH flow ratio 165
4 4
5.4.2.6. Duty cycle series 167
5.4.2.7. T series keeping T at 100msec 169
off on
5.4.3. Microstructural study 170
5.4.3.1. Laser light scattering 170
5.4.3.2. SAXS measurements 170
5.4.3.3. X ray diffraction 173
5.4.3.4. TEM Study 175
5.4.3.5. Surface study by AFM 179
5.5. Discussion 183
5.6. Conclusion 195
5.7. Bibliography 196
iii
Contents

8. Chapter 6: Study of the Optoelectronic Properties of a-Si Ge :H Thin Films Prepared
1-x x
Under Various Deposition Conditions 203

6.1. Introduction 205
6.2. Experimental 205
6.3. Results 207
6.3.1. Optical properties 207
6.3.1.1. Continuous mode deposited samples 208
6.3.1.2. Pulsed mode deposited samples 212
6.3.2. Transport properties 214
6.3.2.1. Photosensitivity 214
6.3.2.2. Activation energy 218
6.3.2.3. Ambipolar diffusion length and mobility lifetime product 219
6.4. Discussion 222
6.5. Conclusion 229
6.6. Bibliography 229

9. Chapter 7: Effect of Different Diluents Gas On the Properties of the Amorphous
Carbon Films Deposited from Methane 233

7.1. Introduction 235
7.2. Samples and experiments 235
7.3. Results 236
7.3.1. Deposition 236
7.3.2. Structural properties 238
7.3.2.1. FTIR 238
7.3.2.2. Auger spectroscopy and Electron Energy Loss spectroscopy 239
7.3.2.3. RAMAN studies 239
7.3.3. Mechanical properties 241
7.3.4. Transport properties 241
7.3.5. Optical Absorption 244
7.4. Discussion 248
7.4.1. Model 1: distribution of DOS considering p-p* states and a
hypothetical energy gap 251
7.4.2. Model 2: distribution of DOS considering p-p* states and s-s*
Gaussian distributions 254
7.4.3. Role of argon during deposition 256
7.5. Conclusion 257
7.6. Bibliography 258

iv
Contents
10. Chapter 8: Formation of Nanocrystalline Diamond In Polymer Like Carbon Films
Deposited by Plasma CVD 259

8.1. Introduction 261
8.2. Experimental 261
8.3. Results 262
8.3.1. IR Study 262
8.3.2. OES study 263
8.3.3. HRTEM Study 265
8.4. Discussion 268
8.5. Conclusion 274
8.6. Bibliography 274

11. Chapter 9: Structural And Transport Properties of Hydrogenated Amorphous
Silicon–Carbon Alloys Obtained from Ar–SiH –CH Gas Mixture 277
4 4

9.1. Introduction 279
9.2. Experimental 279
9.3. Results 280
9.4. Discussion 285
9.5. Conclusion 287
9.6. Bibliography 288

12. List of Publications 291

v
Acknowledgements

vi
Acknowledgements

ACKNOWLEDGEMENTS
This is the only part in the thesis when I write for myself. Everywhere else you will
find that I say “we”. And that is because behind every measurement and every result there is
always somebody without whom it would not have been possible. Actually today’s research
involves the efforts of specialists with different background and different points of view. This
work would be impossible to accomplish without the collaboration of my colleagues from
various research centers of different Institutes. I would like to take this opportunity to publicly
show my gratitude to those who have contributed in one or another way to my work during the last
five years.
First of all I would like to thank my supervisor, Prof. Partha Chaudhuri for the
opportunity he has offered me to join his dynamic group, and of course for his unflinching support,
advice, direction and encouragement during the course of this research. I am grateful to him
for his mentorship in introducing me to the field of electronic materials. He has been an ideal
thesis supervisor who has not only put me on many interesting tracks but has also helped me
with many fruitful discussions. He has also provided opportunities for me to attend many
scientific conferences, where I could be further inspired. Besides his contagious excitement,
he was critical in providing unlimited ideas. He taught me that scientific research requires
perseverance, endurance and dedicated quest. He made me feel that there is no royal road to
science, and only those who do not dread the fatiguing climb of its steep paths have a chance
of gaining its luminous goal. Thank you for your help, your ideas, your support, and simply
for cheering me up as and when I needed it.
Prof. Chaudhuri has opened me the doors to the amorphous semiconductors like a-
SiGe:H ,a-C:H, a-Si:H, a-SiC:H world. I would like to thank him for the opportunity I have
had of sharing his wide expertise in many productive discussions.
I am thankful to Prof. A. K. Barua, Prof. S. Ray, Prof. P. Chatterjee and Prof. D. Das
for their cooperation and the various help they offered me.
I am also grateful to our collaborators for encouraging me to pursue these studies. I
am thankful for their collaboration and discussions and the measurements they have carried
out for our analysis. I also specially thank Dr. C. Longeaud of Laboratoire de Génie
Electrique de Paris (LGEP), CNRS UMR 8507, SUPELEC, Universités Paris VI et XI,
France; for our fruitful discussions, for his expertise and for the transport measurements
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