EFFECT OF HIGH-ENERGY ION BEAM IRRADIATION ON THE STRUCTURAL, OPTICAL AND GAS SENSING
PROPERTIES OF Sn02 THIN FILMS
SANJU RANI
DEPARTMENT OF PHYSICS
INDIAN INSTITUTE OF TECHNOLOGY DELHI NEW DELHI -110016
APRIL 2009
CERTIFICATE
I am satisfied that the thesis entitled "Effect of high-energy ion beam irradiation on the structural, optical and gas sensing properties of Sn02 thin films"
presented by Sanju Rani is worthy of consideration for the award of the degree of Doctor of Philosophy and is a record of the original bonafide research work carried out by her under my guidance and supervision and that the results contained in it have not been submitted in part or full to any other University or Institution for the award of any degree/diploma.
Date: Dr. M.C. Bhatnagar
Principal Scientific Officer Thin film Laboratory Department of Physics
Indian Institute of Technology Delhi New Delhi- 110016, INDIA
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Acknowledgements
First of all, I wish to thanks my thesis supervisor Dr. M. C. Bhatnagar for giving me an opportunity to work with him in the field of metal oxide gas sensors.
I would like to express my gratitude to Dr. Nirmalya Karar of NPL, Delhi, who not only helped me in paper related works but also carried out proofreading of my thesis and suggested finer points.
I am grateful to Dr. D. Kanjilal (IUAC, New Delhi) for giving me an opportunity to carry to ion beam experiments at IUAC under his supervision. He had been the source of constant encouragement and stimulating discussions that help me to shape my research in big way. At this point, I express my sincere thanks to Dr. N.K. Puri (T.S. Engineering College, Greater Noida, Uttar Pradesh, India) for presenting the beam time proposal along with Dr. Somnath C. Roy. I greatly appreciate their enthusiasm on the subject, constant encouragement and valuable advice. I would like to thanks Mr. Yaspal, Mr.
Sandeep of IUAC for their help, at the time of irradiation experiments.
It has been a matter of great pride and privilege in being a member of the Thin Film Lab at IIT Delhi. I express my profound gratitude to Prof. K. L. Chopra who founded this lab. I also express my best regards to the faculty members of our lab: Prof(s) L. K. Malhotra, D. K. Pandya, S. C. Kashyap, V. D. Vankar, B. R. Mehta, G. B. Reddy, V. Dutta and Dr(s) R. D. Tarey and S. Chowdhury.
I sincerely acknowledge the help of Dr. V.N. Singh and Mr. Khanna for HRTEM and TEM measurements respectively; Dr. D. Varandani for AFM studies; Mr. Mandeep and Mrs. Sangeeta for XRD measurements; Mr. Hardeep and Mr. SarabPreet for XPS
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experiments. I am also thankful to all the IDDC staff members for assisting in fabricating my gas sensing system.
It has been a great pleasure to have the company of my lab mates Dr. A. Ranga Rao, Dr. Suneet K. Arora, Mandeep, Sandeep and Mukesh. Frequent discussions with them over tea break have been both very refreshing and encouraging.
I also express my sincere thanks and gratitude to my senior's colleagues, Dr. (s) Aruna, Manoj, Priyanka, Kanwal, Gopinathan, Shubhra, Suchitra, Ajay, Gargi and Manish. I would also like to appreciate support of my other lab mates Manika, Himani, Bharti, Archna, Manjeet, Sushil, Vishakha and Sudesh.
I also want to express my warmest regards to Rupali for giving me company during thesis writing and for proof reading of my thesis.
My research work never has been successfully completed without constant support and encouragement from my husband, Dr. Somnath C. Roy (MRI, Pennsylvania State University, USA). I can not express my thanks to him in words.
I also want to thank my parents, Shri Prakash Chand and Smt. Droupdi Devi , who taught me the value of hard work. Their love, affection and relentless support have been inspiration me to reach this stage in my life. I would also like to share this moment of happiness with my brothers Girish and Yogesh, sisters Sujata and Lata, brother in-law Mr. Kishor Kumar, nephew Erthanshu , nice Aartha. They rendered me enormous support during the whole tenure of my research. In the final stage of research encouragement and support from my "in-laws" and their family is also gratefully acknowledged. Finally, the presence of God's abundant grace has made this endeavor a success.
(Sanju Rani) iii
Abstract
Tin oxide (Sn02) has been extensively studied in past and recent years due to its important applications in gas sensors, photovoltaic cells, batteries, photonic crystals, catalysis, and photo catalysts. High chemical and thermal stability, wide range of electrical conductivity and optical transparency (with various dopants) of its thin films, and sensitivity towards many oxidizing and reducing gases make Sn02 one of the most useful members of the metal-oxide family. In spite of many stabilized properties of SnO2, the continuous search of materials with novel properties has kept Sn02 under constant focus of the research community, and new methods to tailor its properties are being continually investigated.
Irradiation of materials with high-energy (in MeV range) ion beam or "swift heavy ion (SHI) irradiation" has been well established as a powerful tool for material modifications. High amount of energy deposited into the material in a very short interval of time (10-13 s) produces significant changes in crystallinity, microstructure, and defect chemistry of a material. Using SHI irradiation, various properties of a material can easily be tailored and the technique has been used on many metallic, semiconducting and insulating systems. In case of Sn02, however, very few studies on the effect of SHI irradiation have been reported. In this dissertation work, a detailed study on the effect of SHI irradiation on structural, optical, electrical and gas sensing properties of sol-gel derived Sn02 thin films is presented.
The Sn02 thin films were deposited by sol-gel dip coating process. Simplicity of the technique, ease of dopants incorporation, and flexibility of coating on large and uneven substrates are some of the advantages of the sol-gel technique. In this work, the
u'1
sol was prepared with SnC14 dissolved in propanol. Thin layers of Sn02 were coated onto coming glass and quartz substrates by dip coating technique at a pulling speed of 10 cm/sec. Each layer was dried at 200°C for 15 minutes before the deposition of the next layer. After attaining the required thickness, the samples were annealed at 600°C for 2 hours. Effect of iron doping was also studied by preparing Fe-doped Sn02 films.
Irradiation of the prepared samples were done with 75 MeV Nis+ and 100 MeV Ag8 ion beam at different fluences ranging from 1 x 1011 ions/cm2 to 3 X1013 ions/cm2 at Inter University Accelerator Center, New Delhi.
Structural characterization of the irradiated samples was carried out with X-ray diffractograms, whereas High Resolution Transmission Electron Microscopy (HRTEM) and Atomic Force Microscopy (AFM) were used for the characterization of microstructure and surface morphology respectively. XRD patterns of Nis+ irradiated un- doped Sn02 samples show that there is an increase in crystallinity up to a fluence of 1 x 1013 ions/cm2 beyond which amorphization occurs. For Ag8 irradiated samples, however, crystallinity decreases with increasing ion fluence. Such contrasting difference due to irradiation by Nis+ and Ag8 ion beams has been discussed and explained. HRTEM images show increase in average size of crystallites with initial increase in ion fluence for Nis+ irradiated samples, while grain fragmentation effect is observed in Ag8 irradiated samples. Corresponding changes in surface roughness of the samples were recorded through AFM. Fe-doped Sn02 samples, when irradiated with Nis+ beam show decrease in crystallinity with increase in ion fluence. Interestingly, at the highest fluence a new phase corresponding to Sn-Fe alloy also appears.
IYA
The optical properties of Nis+ and Ag8 irradiated samples were studied by UV- Vis spectroscopy and photoluminescence spectroscopy (PL). In both cases of Nis+ and Ag8 irradiation, optical absorption spectra showed a red shift with increase in ion fluence. These observations are discussed and explained on the basis of defect formation, dielectric confinement and Moss-Burstein effect.
The PL spectra of Nis+ irradiated un-doped Sn02 samples show broad peak in the yellow region corresponding to interstitial oxygen, the intensity of which increases with increase in ion fluence. On the other hand, PL spectra of Ag8 irradiated samples show peak in blue region corresponding to oxygen vacancies.
Gas sensing experiments on Nis+ irradiated un-doped Sn02 samples show p-type behaviour and selectivity enhancement towards NH3. The Ag8 irradiated samples, on the other hand, show enhanced n-type conductivity and selectivity towards CO. These gas sensing properties are explained on the basis of observed changes in defect-chemistry of the irradiated samples. An enhancement of sensitivity towards CO is also observed in irradiated Fe-doped samples.
Finally, the I-V measurements were performed in air and with the help of temperature dependent I-V data of different samples the effect of irradiation on Schottky barrier height is estimated.
Contents
Certificate i
Acknowledgement ii
Abstract iv
Contents iv
List of Figures xiii
List of Tables
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Chapter 1 Introduction
1.1 Tin oxide (Sn02) 1
1.2 Applications of Sn02 3
1.2.1 Transparent conductors 3
1.2.2 Oxidation catalyst 4
1.2.3 Solid state gas sensors 5
1.3 Sn02 gas sensors 5
1.3.1 Sensing mechanism 6
1.3.2 Catalytic mechanism 8
1.3.3 Sensors parameters 9
1.3.4 Parameters influencing the gas sensing properties 10
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1.4 Synthesis techniques of Sn02 thin films 11 (a) Powder
(b) Thick films (c) Thin films
1.5 Present status of Sn02 based gas sensors 14 1.6 Methods of modifying Sn02 thin films for gas sensors 15
1.6.1 Pre-deposition techniques 15
(a) Control of deposition parameters (b) Selection of substrate
(c) Chemical additives (doping)
1.6.2 Post-deposition techniques 17
(a) Heat treatments
(b) Surface treatments with over layer coating (c) Modification by electron and ion beams
1.7 Interaction of the ion beam with material 19 1.7.1 Interaction models of swift heavy ions with matter 20
(a) Thermal spike (b) Coulomb explosion
1.7.2 Modification in the films by high energy beam irradiation 23
1.8 Motivation for the present work 25
1.9 Objectives of the present work 25
1.10 Thesis Plan 26
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Chapter 2
Experimental techniques
2.1 Introduction 29
2.2 Thin film deposition techniques 29
2.2.1 Sol-gel process 30
2.2.2 Advantages of sol-gel technique 33
2.2.3 Limitations of sol-gel technique 34
2.2.4 Sol-gel dip coating technique 34
2.3 Experimental details of sol-gel process used in the present work 36
2.4 Ion irradiation experiments 38
2.4.1 Pelletron Accelerator Laboratory at IUAC 39
2.4.2 Material Science beam line 40
2.5 Characterization of Sn02 thin films 41
2.5.1 Glancing angle X-ray diffraction (GAXRD) 41
2.5.2 Atomic force microscopy (AFM) 44
2.5.3 Transmission electron microscopy (TEM) and High resolution
transmission microscopy (HRTEM) 46
2.5.4 X-ray photoelectron spectroscopy (XPS) 48 2.5.5 Optical absorption (UV-Vis NIR spectrophotometry) 49
2.5.6 Photoluminescence (PL) spectroscopy 51
2.5.7 Thickness measurement 53
2.6 Gas sensing experiments 53
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2.6.1 Gas response measurements 54 2.6.2 Barrier height measurements (Current-Voltage characteristics) 55
Chapter 3
Structural, microstructural and surface morphology studies
3.1 Introduction 57
3.2 Effect of ion irradiation on structure of Sn02 thin films 57 3.2.1 75 MeV Nis+ irradiated un-doped Sn02 films 57 3.2.2 100 MeV Ag8 irradiated un-doped Sn02 films 60 3.2.3 Pristine and irradiated (75 MeV Nis+ and 100 MeV Ag8 ) Fe- 63
doped Sn02 films
3.3 Effect of ion irradiation on microstructure of Sn02 thin films 67 3.3.1 75 MeV Nis+ irradiated un-doped Sn02 films 67 3.3.2 100 MeV Ag8 irradiated un-doped Sn02 films 69 3.3.3 Pristine and 75 MeV Nis+ irradiated on Fe-doped Sn02 films 71 3.4 Effect of ion irradiation on surface morphology of Sn02 thin
films 74
3.4.1 75 MeV Nis+ irradiated un-doped Sn02 films 74 3.4.2 100 MeV Ag8 irradiated un-doped Sn02 films 75 3.4.3 Pristine and 75 MeV Nis+ irradiated on Fe-doped Sn02 films 76
3.5 Summary 77
M
Chapter 4
Optical characterization
4.1 Introduction 79
4.2 Effect of ion irradiation on bandgap of Sn02 thin films 79 4.2.1 75 MeV Nis+ irradiated un-doped Sn02 films 79 4.2.2 100 MeV Ag8 irradiated un-doped Sn02 films 81 4.2.3 75 MeV Nis+ and 100 MeV Ag8 irradiated Fe-doped Sn02 films 84 4.3 Effect of ion irradiation on defect states of Sn02 thin films 88 4.3.1 75 MeV Nis+ irradiated un-doped Sn02 films 88 4.3.2 100 MeV Ag8 irradiated un-doped Sn02 films 90
4.3.3 Fe-doped Sn02 films 92
4.4 Summary 94
Chapter 5
Electrical measurements: Gas sensing experiments and Current-Voltage (I-V) characteristics
5.1 Introduction 95
5.2 Effect of ion irradiation on gas sensing properties of Sn02 thin
films 96
5.2.175 MeV Nis+ irradiation un-doped Sn02 films 96 5.2.2 100 MeV Ag8 irradiation un-doped Sn02 films 102 5.2.3 Fe-doped Sn02 (Pristine and irradiated) Sn02 films 105
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5.2.4 XPS studies of 75 MeV Nis+ and 100 MeV Ag8 irradiated Sn02 107 films
5.2.5 Schematic diagram depicting mechanism of p and n-type 113 conductivities
5.3 Effect of ion irradiation on Current- Voltage (I-V) characteristics 114 5.3.11-V characteristics of pristine Sn02 thin films 114 5.3.2 Effect of Nis+ and Ag8+ irradiated on I-V characteristics of Sn02
films 116
5.4 Summary 119
Chapter 6
Summary of results, conclusions and scope for future work
6.1 Summary of the results and conclusions 121
6.2 Scope for the future work 123
References 124
List of publications 139
Biography of the Author 142
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