DEFECT MEDIATED NOVEL STRUCTURAL, OPTICAL, ELECTRICAL AND MAGNETIC PROPERTIES IN Ti1-xTaxO2 THIN FILMS ARKAJIT ROY BARMAN (M.S., COLORADO STATE UNIVERSITY, U. S. A. M.Sc., INDIAN INSTITUTE OF TECHNOLOGY, BOMBAY, INDIA B.Sc., PRESIDENCY COLLEGE, CALCUTTA, INDIA) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN SCIENCE DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2011 In memory of the unconditional love bestowed on me by Late Smt. Naba Durga Debi (Thamma) and Late Sri Kisor Kanti Barman (Jethu) ACKNOWLEDGEMENTS The last four years which led to this thesis have been the most defining years of my life. I am grateful to a lot of people who have been instrumental in making them so. It humbles me to acknowledge them. If I have to name ONE man for whom I am writing this Thesis and this acknowledgement, he has to be my advisor, Prof. T. Venkatesan. Venky, as he is called by one and all has been one of the biggest influences in my life. I consider myself to be extremely fortunate to have known, worked together with and been supervised by Venky. He has encouraged me in all my efforts and endeavors. He has managed to keep me motivated in my research. I cannot thank him more for not giving up on me, even though, at times I was giving up on my research. Venky has been extremely patient with me. He has always been available to answer my doubts, even if that meant, long international calls at the wee hours or meetings extending till the middle of the night. Venky has had a tremendous contribution in my developing as an individual. Apart from all these, Venky has imparted me with profound knowledge and deep insights about Oxides and Defect Induced Magnetism and has provided me with every possible opportunity to develop myself as an experimental scientist. I will be ever indebted to Prof. T. Venkatesan. I also want to take this opportunity to acknowledge my co-supervisor, Prof. Chua Soo Jin. Prof. Chua has been extremely encouraging and had taken keen interest in my research activities. He has always helped me out with his invaluable inputs about my work. i Dr. Sankar Dhar, my mentor and colleague has worked most closely with me throughout my graduate student life. I have learnt the basics of most of the experimental techniques that I have used for my research from Sankar da. Whenever I had felt totally lost with my research, I had blindly turned to Sankar da for help. He has been of phenomenal help in managing my research work and giving direction to it. His critical inputs have definitely helped me in taking my work to the next level. He has shown great confidence in me throughout. I fondly remember the initial days in the lab when we toiled hard together to set up the PLD. I feel happy to thank him for all his help. I thank Prof. Ariando and Prof. A. Rusydi for the invaluable support. There is no doubt whatsoever, that my work would not have been possible without them. They have been of tremendous help with experiments as well as theoretical understandings of my subject. I will miss our paper writing sessions together and the occasional pizza parties. I also thank Prof. H. Yang and Prof. O. Barbaros for the many fruitful discussions and the opportunities to work together. I would like to extend my special gratitude to Dr. Daniel Lubrich. Dan has been a constant source of encouragement to me. We have had many interesting discussions on varied topics. I particularly enjoyed working with him on NanoSpark projects. It helped me a lot with building a different outlook which is hard to develop in a strictly academic environment. I would want to thank Dr. K. Gopinadhan. Gopi has been the epitome of sincerity whom all graduate students in our lab have tried to idolize. Gopi has helped me a lot with transport measurements and helped me understand the intricate physics related. ii I would also want to thank Dr. S. Saha. We have been good friends in the few days that we have known each other. Surajit is a focused individual with very sharp instincts of a researcher. He has helped me with Raman measurements and with understanding the data. I thank Dr. C.B. Tay. Chuan Beng is a very helpful individual and is always ready to PL measurements, even if it is on weekend nights. I definitely want to thank Dr. W. Lú. Weiming has helped me a lot with SQUID measurements and also with depositions. I have been fortunate enough to have some of the most wonderful, talented and helpful labmates. I want to thank Xiao Wang , Young Jun Shin, Mahdi Jamali, Mallikarjunarao Motapothula, Jae Sung Son, Jae Hyun Kwon, Anil Annadi, Liu Zhiqi, Yong Liang Zhao, Teguh Citra Asmara, Zhihua Yong, Amar Srivastava, Tarapada Sarkar, Naomi Nandakumar, Masoumeh Fazlali and last but not the least Michal Dykas. Over the years we have been more of good friends and less of colleagues. I guess we will always remember the night outs in the lab. I also warmly remember all the Summer Internship students who have worked with me during my stay at NUSNNI-NanoCore. It has been an honor to know and work with you all. I definitely want to thank all the people who has supported with running the lab smoothly throughout the period of my research. I want to thank Jason Lim, Syed Nizar, Malathi, Catherine Tai Guat Hoon and all the other staffs at the NUSNNI NanoCore office. I would like to take this opportunity to mention my friends in Singapore. Most importantly, I want to thank my room-mate, Pankaj for putting up with the weird schedules and habits of a graduate student. I feel great pleasure to mention Arun, Shruti, Kingshuk, Nibedita, Sahoo, Pradipto, Adeeb, Arpan, Deepal, Sandeep, Bhavesh, Trond, Hallgeir, Cecilia, Solveig, Marit, iii Heidi, Bård, Isaac, Cari, Sarah, and most importantly Heekyoung. I thank you all from the bottom of my heart for the much necessary distractions. It has been a pleasure knowing all of you. Just because I not want to get killed, I will mention Aritra and Anshuk. It makes no sense for me to thank you. I should rather thank my stars that I managed to finish my thesis and am writing the acknowledgement even with two guyz like you in my life for the past twenty-five years. It is always difficult to express love and gratitude to family members. It appears so futile. My parents and my sister – you are the source of my sustenance. I could not have asked for anything more from you. It is all because of you. iv TABLE OF CONTENTS ACKNOWLEDGEMENTS i TABLE OF CONTENTS . v ABSTRACT viii LIST OF PUBLICATIONS . x LIST OF TABLES . xiii LIST OF FIGURES . xiv LIST OF SYMBOLS xx Introduction . 1.1 Fundamental Physical Properties of TiO2 . 1.1.1 Crystalline Structure of TiO2 . 1.1.2 Electronic Band Structure of TiO2 . 1.2 Defects and Dopings in Semiconductors 1.2.1 Intrinsic and Extrinsic Defects in Semiconductors 1.2.2 Thermodynamics of Defect Formation and Compensation . 1.3 Applications of TiO2 . 11 Structural Analysis of Pulsed Laser Deposition grown Ti1-xTaxO2 Thin Films 12 2.1 Pulsed Laser Deposition Technique 12 2.2 Ti1-xTaxO2 Thin Film Preparation . 14 2.3 Structural Analysis of Ti1-xTaxO2 Thin Films . 15 2.3.1 X-Ray Diffraction Studies . 15 2.3.2 Raman Spectroscopy Studies . 21 2.3.3 Rutherford Backscattering – Ion Channeling Studies 27 2.3.4 Atomic Force Microscopy Studies . 31 2.4 Conclusion 33 Ti1-xTaxO2: A New Alloy with Transparent Conducting Properties . 34 3.1 Transparent Conducting Oxides . 34 3.1.1 Electrical Conductivity 35 3.1.2 Optical Properties . 38 3.2 Alloying Effect of Ti1-xTaxO2 Thin Films . 40 3.2.1 Ultra Violet-Visible Spectroscopy and Electrical Transport of Ti1-xTaxO2 . 41 3.2.2 High Energy Optical Reflectivity of Ti1-xTaxO2 46 3.3 Conclusion 49 Universal Kondo Effect in Ti1-xMxO2 (M=Nb, Ta) Thin Films 50 4.1 Brief History to Kondo Effect . 50 4.2 Low Temperature Transport Data on Ti0.94M0.06O2 (M = Nb, Ta) Thin Films . 53 4.3 Conclusion 67 v Role of Ta versus Magnetic Contaminants in Defect Mediated Ferromagnetism in Ti1-xTaxO2 films 70 5.1 Dilute Magnetic Semiconductors (DMS) . 70 5.1.1 Spintronic Devices . 70 5.1.2 Origin of Ferromagnetism in DMS 74 5.1.3 Dilute Magnetic Semiconducting Oxides 77 5.1.4 Defect Mediated Ferromagnetism 78 5.2 Magnetic Impurity Analysis in Ti1-xTaxO2 Thin Films . 80 5.2.1 Brief History and Motivation . 80 5.2.2 RBS and PIXE Results . 81 5.2.3 XAS Results . 82 5.2.4 TOF-SIMS Results . 85 5.2.5 APT Results . 85 5.3 Conclusion 89 Cationic Vacancy Induced Room Temperature Ferromagnetism in Transparent Conducting Anatase Ti1-xTaxO2 Thin Films . 90 6.1 Structural, chemical, electrical and optical properties 91 6.2 Magnetic Properties 91 6.3 Theoretical Calculation . 101 6.4 Origin of Ferromagnetism . 102 6.5 Conclusion 108 Interplay Between Carrier and Cationic Defect Concentration in Ferromagnetism of Anatase Ti1xTaxO2 Thin Films . 110 7.1 PLD Deposition of Ti1-xTaxO2 Thin Films 110 7.2 Experimental Results and Discussions . 111 7.2.1 Dependence of Magnetization on PLD Deposition Conditions . 112 7.2.2 Role of Ta in the Magnetism of Ti1-xTaxO2 Thin Films 115 7.3 Conclusion 120 Summary and Future Work . 122 8. Summary 122 8.1.1 Ti1-xTaxO2: A New Alloy System 122 8.1.2 Electrical Properties in Ti1-xTaxO2 Alloy . 123 8.1.3 Magnetic Properties in Ti1-xTaxO2 Alloy . 124 8. Future Work . 125 Appendix A.1 Pulsed Laser Deposition (PLD) . 127 Appendix A.2 A New Route to Graphene Layers by Selective Laser Ablation 129 vi Appendix A.3 X-Ray Diffraction (XRD) 142 Appendix A.4 Raman Spectroscopy . 144 Appendix A.5 Rutherford backscattering-Ion Channeling 146 Appendix A.6 Atomic Force Microscopy (AFM) . 149 Appendix A.7 Ultra Violet- Visible (UV-Vis) Spectroscopy 151 BIBLIOGRAPHY . 153 vii ABSTRACT The main objective of this thesis is to explore the defect mediated structural, optical, electrical and magnetic properties of titanium oxide (TiO2) based alloy thin films grown by pulsed laser deposition (PLD). Such properties can be harnessed for suitable applications in the field of optoelectronics and spintronics as transparent conducting oxides (TCO) and defect induced diluted magnetic semiconductors (DMS) respectively. Single crystal thin films of pure TiO2 and tantalum (Ta) incorporated TiO2 (Ti1-xTaxO2) were grown epitaxially on lattice matched substrates such as LaAlO3 and SrTiO3. By varying the deposition temperatures and the oxygen partial pressures in the PLD process, both anatase and rutile polymorphs of TiO2 were grown. By investigating the growth dependence of the different phases of TiO2 on the deposition parameters, an elaborate phase diagram was developed. Rutherford backscattering-Ion Channeling (RBS) spectroscopy was used to study the crystal structure of all the films deposited. RBS-Ion Channeling studies showed that the crystallinity of the thin films improved with increasing deposition temperature and increasing oxygen partial pressure. Films with higher Ta incorporation also showed higher crystallinity. X-Ray Diffraction studies showed a lattice expansion in TiO2 with Ta incorporation in the out-of-plane direction. This was further supported by the Raman spectroscopy data which showed the softening of the out-of-plane vibrational modes and the hardening of the in-plane vibrational mode. Ultra Violet-Visible (UV-Vis) Spectroscopy was done on both anatase and rutile samples to study the effect of Ta incorporation in TiO2. The band gap of both anatase and rutile samples showed a blue shift with increasing Ta concentration. Using electrical transport data, it was argued that the band structure of TiO2 undergoes a drastic change with Ta incorporation resulting in the formation of a new alloy system. High energy optical reflectivity measurements were done viii Figure A5.2: RBS ion channeling mode for (a) a perfect crystalline lattice; (b) a disordered lattice. measurable intensity of spectrum. This intensity relative to the random (unaligned) backscattered signal (the ratio is called minimum channeling yield χmin) is nearly proportional to the number of disordered atoms or defects and hence indicates the degree of crystallinity of the sample. 148 Appendix A.6 Atomic Force Microscopy (AFM) Atomic force microscopy (AFM) is a scanning probe microscopy for surface mapping with the resolution of fractions of a nanometer. As depicted in Fig. A6.1, a sharp tip at the end of a microscale cantilever is used to scan the sample surface. When the tip is brought into close proximity of a sample surface, forces between the tip and the sample cause a deflection of the cantilever. This deflection is measured using a laser spot reflected from the top of the cantilever into a photodiode. The AFM can be operated in several modes, such as contact mode and dynamic mode. The contact mode is simple, and used only when the force between the tip and the sample is repulsive. In this model, the pointed tip is brought into proximity to a sample surface. A feedback mechanism is employed to adjust the distance between tip and sample (z-direction) so as to maintain a constant repulsive force. The cantilever is scanned across the sample surface (in x, y directions), yielding a map of z-direction topography of the sample. Figure A6.1: Schematic of an atomic force microscopy system. 149 In a dynamic mode, the cantilever is oscillated at its mechanical resonance frequency. The variation of tip-sample-force modifies the amplitude, phase and resonance frequency of the oscillation. Thereafter, the changes in oscillation with respect to the external reference oscillation give information about the sample's surface characteristics. The root-mean-square (rms) average roughness is calculated by the following equation: rms  r  x, y  dxdy  A where A is the area of interest and r(x,y) is the roughness profile. 150 (A6.1) Appendix A.7 Ultra Violet- Visible (UV-Vis) Spectroscopy The Ultraviolet (UV)-Visible spectroscopy is a simple technique to study the fundamental optical properties, such as transmittance and absorption of a material in response to the light in the visible and near UV ranges. A spectrophotometer is mainly composed of: a light source, sample holders, a monochromator, and a detector (Fig. A7.1). A substrate, which is the same as the one used in the sample under investigation, is a reference for subtracting the absorption of substrate in the sample. When light is shining on a sample, if the photon energy is less than the energy gap of the material, light will pass through the material without being absorbed, leading to a high transmittance (T=Iout/Iin). 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Blueshift in the band gap for anatase and rutile Ti1- xTaxO2 thin films with Vegard‟s law fitting as a function of Ta concentration…………………………………………………….43 Figure 3.4: (a) Temperature dependence of resistivity for anatase Ti1- xTaxO2 (x = 0 – 0.08) thin films and rutile Ti1- xTaxO2 (x = 0.08) thin film; (b) Variation of carrier density and Hall mobility for anatase Ti1- xTaxO2 thin films as a function of Ta concentration……………………………... actual activated carrier density as a function of Ta incorporation in Ti1- xTaxO2 thin films (blue solid squares) Variation of calculated carrier density as a function of Ta incorporation in Ti1- xTaxO2 thin films with 100% carrier activation (red solid circles) Variation of inactivated carrier density as a function of Ta incorporation in Ti1- xTaxO2 thin films (green open triangle) … ………………….…………………………………………………... the anatase and the rutile structures are not that different their electrical transport, optical and magnetic properties appear to be quite distinct 1.2 Defects and Dopings in Semiconductors 1.2.1 Intrinsic and Extrinsic Defects in Semiconductors Defining classically, a semiconductor is a material with electrical conductivity lower than that of a metal (104 Scm-1) and higher than that of an insulator... the films and the substitution of Ta5+ in Ti4+ sites in the crystal lattice Ion channeling studies for films grown at different growth conditions show us the optimum condition for the growth of highly crystalline thin films Atomic Force Microscopy (AFM) has been used to measure the roughness and the surface topography of the films produced 2.2 Ti1- xTaxO2 Thin Film Preparation Thin films of Ti1- xTaxO2. .. devised for the defect mediated magnetism in Ti1- xTaxO2 thin films ix LIST OF PUBLICATIONS  AIP Advances 2, 012148 (2012), A Roy Barman, A Annadi, K Gopinadhan, W.M Lu, Ariando, S.Dhar, T Venkatesan; Interplay between carrier and cationic defect concentration in ferromagnetism of anatase Ti1- xTaxO2 thin films  Appl Phys Lett 99, 172103 (2011), W.M Lu, X Wang, Z.Q Liu, S.Dhar, A Annadi, K.Gopinadhan, A... increasing temperature, the electrons at the top of the VB are excited to the bottom of the CB, leaving a hole at the VB The electron in the CB and the hole in the VB are mobile and can thus induce conductance in the semiconductors In its native state, an intrinsic semiconductor contains defects arising during the various growth and fabrication processes There are many possibilities of such intrinsic defects... more interest to the research community in the recent times In its anatase phase, TiO2 has been shown to be metallic in nature, making it a possible replacement to tin doped indium oxide (ITO) Adding to that, the Kondo effect has also been observed in the anatase phase of TiO2, making it more interesting a system for studying defect mediated novel properties The anatase structure of TiO2, is shown in. .. one-dimensional point defects such as vacancies, interstitials and antisites In a vacancy, as the name suggests, an atom is missing from its site in the crystal lattice For interstitials, the atom is residing in the free space in the crystal instead of its actual position We have an anitsite type of defect, when a cation sits at the anion position in the crystal lattice and vice versa Intrinsic defects of... centers Such magnetic centers scatters electrons resulting in an up-turn of the resistivity curve as function of temperature This phenomenon, known as Kondo effect has been found in Ti1- xTaxO2 thin films Thorough and systematic Hall measurements have also been done on Ti1- xTaxO2 thin films to study the variation of carrier density and electron mobility with deposition conditions Defect mediated magnetism... varistors and may even find usage in MOSFETs as gate insulators or as spacer material in magnetic spin-valve systems Nanostructured TiO2 is used in Li-based batteries and electrochromic devices TiO2 is also used heavily in the medical industry and plays an important role in the biocompatibility of bone implants The findings in this thesis will hopefully clear some very fundamental questions regarding TiO2 . DEFECT MEDIATED NOVEL STRUCTURAL, OPTICAL, ELECTRICAL AND MAGNETIC PROPERTIES IN Ti 1-x Ta x O 2 THIN FILMS ARKAJIT ROY BARMAN (M.S., COLORADO. ABSTRACT The main objective of this thesis is to explore the defect mediated structural, optical, electrical and magnetic properties of titanium oxide (TiO 2 ) based alloy thin films grown by. Conducting Anatase Ti 1-x Ta x O 2 Thin Films 90 6.1 Structural, chemical, electrical and optical properties 91 6.2 Magnetic Properties 91 6.3 Theoretical Calculation 101 6.4 Origin of Ferromagnetism