Defect mediated novel structural, optical, electrical and magnetic properties in ti1 xtaxo2 thin films

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). However, if the photon energy is greater than the energy gap of the material, the electrons absorb the photon energy and jump to higher energy levels. In this case, the Figure A7.1: Schematic of an Ultraviolet (UV)-Visible spectrophotometer. 151 transmission of the material is low. The absorption α can be derived from transmittance using the following equation,    T  ln   d  1 R  (A7.1) where d is the thickness of the sample, T is the transmittance and R is the reflection. A typical UV-visible spectrum is a plot of transmittance or absorption versus light wavelength. From the UV-visible spectrum, one can calculate the optical band gap of the material under study. 152 BIBLIOGRAPHY [1] M. Imada, A. Fujimori, Y. Tokura, Metal-insulator transitions, Reviews of Modern Physics, 70 (1998) 1039-1263. [2] M. Batzill, U. Diebold, The surface and materials science of tin oxide, Progress in Surface Science, 79 (2005) 47-154. [3] E. Dagotto, CORRELATED ELECTRONS IN HIGH-TEMPERATURE SUPERCONDUCTORS, Reviews of Modern Physics, 66 (1994) 763-840. [4] D.U. Gubser, S.A. Wolf, R.J. Soulen, THE SUPERCONDUCTIVITY PARTY, Science, 245 (1989) 112-112. [5] G.D. Wilk, R.M. Wallace, J.M. Anthony, High-kappa gate dielectrics: Current status and materials properties considerations, Journal of Applied Physics, 89 (2001) 5243-5275. [6] A. Sawa, Resistive switching in transition metal oxides, Materials Today, 11 (2008) 28-36. [7] T. Minami, Transparent conducting oxide semiconductors for transparent electrodes, Semiconductor Science and Technology, 20 (2005) S35-S44. [8] R.B.H. Tahar, T. Ban, Y. Ohya, Y. Takahashi, Tin doped indium oxide thin films: Electrical properties, Journal of Applied Physics, 83 (1998) 2631-2645. [9] E. Fortunato, D. Ginley, H. Hosono, D.C. Paine, Transparent conducting oxides for photovoltaics, Mrs Bulletin, 32 (2007) 242-247. [10] D.E. Williams, Semiconducting oxides as gas-sensitive resistors, Sensors and Actuators BChemical, 57 (1999) 1-16. [11] Z.P. Shao, S.M. Haile, A high-performance cathode for the next generation of solid-oxide fuel cells, Nature, 431 (2004) 170-173. [12] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Formation of titanium oxide nanotube, Langmuir, 14 (1998) 3160-3163. [13] A. Trovarelli, Catalytic properties of ceria and CeO2-containing materials, Catalysis Reviews-Science and Engineering, 38 (1996) 439-520. [14] S.B. Ogale, Dilute doping, defects, and ferromagnetism in metal oxide systems, Advanced Materials, 22 (2010) 3125-3155. [15] U. Diebold, The surface science of titanium dioxide, Surface Science Reports, 48 (2003) 53229. 153 [16] F.A. Grant, Properties of Rutile (Titanium Dioxide), Reviews of Modern Physics, 31 (1959) 646. [17] V.E. Henrich, The surfaces of metal oxides, Reports on Progress in Physics, 48 (1985) 1481. [18] J.K. Burdett, T. Hughbanks, G.J. Miller, J.W. Richardson, J.V. Smith, STRUCTURAL ELECTRONIC RELATIONSHIPS IN INORGANIC SOLIDS - POWDER NEUTRONDIFFRACTION STUDIES OF THE RUTILE AND ANATASE POLYMORPHS OF TITANIUM-DIOXIDE AT 15 AND 295-K, Journal of the American Chemical Society, 109 (1987) 3639-3646. [19] C.J. Howard, T.M. Sabine, F. Dickson, STRUCTURAL AND THERMAL PARAMETERS FOR RUTILE AND ANATASE, Acta Crystallographica Section B-Structural Science, 47 (1991) 462-468. [20] J. Muscat, V. Swamy, N.M. Harrison, First-principles calculations of the phase stability of TiO2, Physical Review B, 65 (2002). [21] S.-D. Mo, W.Y. Ching, Electronic and optical properties of three phases of titanium dioxide: Rutile, anatase, and brookite, Physical Review B, 51 (1995) 13023. [22] F. Labat, P. Baranek, C. Domain, C. Minot, C. Adamo, Density functional theory analysis of the structural and electronic properties of TiO2 rutile and anatase polytypes: Performances of different exchange-correlation functionals, Journal of Chemical Physics, 126 (2007). [23] M.M. Islam, T. Bredow, A. Gerson, Electronic properties of oxygen-deficient and aluminum-doped rutile TiO_{2} from first principles, Physical Review B, 76 (2007) 045217. [24] L.A. Grunes, R.D. Leapman, C.N. Wilker, R. Hoffmann, A.B. Kunz, Oxygen K near-edge fine structure: An electron-energy-loss investigation with comparisons to new theory for selected 3d Transition-metal oxides, Physical Review B, 25 (1982) 7157. [25] K.M. Glassford, J.R. Chelikowsky, Structural and electronic properties of titanium dioxide, Physical Review B, 46 (1992) 1284. [26] M. Ramamoorthy, R.D. King-Smith, D. Vanderbilt, Defects on TiO_{2} (110) surfaces, Physical Review B, 49 (1994) 7709. [27] P.J.D. Lindan, N.M. Harrison, M.J. Gillan, J.A. White, First-principles spin-polarized calculations on the reduced and reconstructed TiO_{2} (110) surface, Physical Review B, 55 (1997) 15919. [28] J. Goniakowski, J.M. Holender, L.N. Kantorovich, M.J. Gillan, J.A. White, Influence of gradient corrections on the bulk and surface properties of TiO_{2} and SnO_{2}, Physical Review B, 53 (1996) 957. 154 [29] H. Tang, F. Lévy, H. Berger, P. Schmid, Urbach tail of anatase TiO2, Physical Review B, 52 (1995) 7771-7774. [30] G.A. Baraff, M. Schlüter, Electronic Structure, Total Energies, and Abundances of the Elementary Point Defects in GaAs, Physical Review Letters, 55 (1985) 1327. [31] G. Mandel, Self-Compensation Limited Conductivity in Binary Semiconductors. I. Theory, Physical Review, 134 (1964) A1073. [32] Y. Tsur, I. Riess, Self-compensation in semiconductors, Physical Review B, 60 (1999) 8138. [33] A. Zunger, Practical doping principles, Applied Physics Letters, 83 (2003) 57. [34] S.B. Zhang, S.H. Wei, A. Zunger, A phenomenological model for systematization and prediction of doping limits in II-VI and I-III-VI2 compounds, Journal of Applied Physics, 83 (1998) 3192-3196. [35] R.K. Singh, J. Narayan, Pulsed-laser evaporation technique for deposition of thin films: Physics and theoretical model, Physical Review B, 41 (1990) 8843. [36] A.R. Barman, M. Motapothula, A. Annadi, K. Gopinadhan, Y.L. Zhao, Z. Yong, I. Santoso, Ariando, M. Breese, A. Rusydi, S. Dhar, T. Venkatesan, Multifunctional Ti(1-x)Ta(x)O(2): Ta doping or alloying?, Applied Physics Letters, 98 (2011). [37] T. Ohsaka, F. Izumi, Y. Fujiki, RAMAN-SPECTRUM OF ANATASE, TIO2, Journal of Raman Spectroscopy, (1978) 321-324. [38] T. Ohsaka, TEMPERATURE-DEPENDENCE OF THE RAMAN-SPECTRUM IN ANATASE TIO2, Journal of the Physical Society of Japan, 48 (1980) 1661-1668. [39] S. Guha, K. Ghosh, J.G. Keeth, S.B. Ogale, S.R. Shinde, J.R. Simpson, H.D. Drew, T. Venkatesan, Temperature-dependent optical studies of Ti1-xCoxO2, Applied Physics Letters, 83 (2003) 3296-3298. [40] M. Ocana, J.V. Garciaramos, C.J. Serna, LOW-TEMPERATURE NUCLEATION OF RUTILE OBSERVED BY RAMAN-SPECTROSCOPY DURING CRYSTALLIZATION OF TIO2, Journal of the American Ceramic Society, 75 (1992) 2010-2012. [41] I. Hamberg, C.G. Granqvist, EVAPORATED SN-DOPED IN2O3 FILMS - BASIC OPTICAL-PROPERTIES AND APPLICATIONS TO ENERGY-EFFICIENT WINDOWS, Journal of Applied Physics, 60 (1986) R123-R159. [42] Y. Furubayashi, T. Hitosugi, Y. Yamamoto, K. Inaba, G. Kinoda, Y. Hirose, T. Shimada, T. Hasegawa, A transparent metal: Nb-doped anatase TiO2, Applied Physics Letters, 86 (2005). 155 [43] T. Hitosugi, Y. Furubayashi, A. Ueda, K. Itabashi, K. Inaba, Y. Hirose, G. Kinoda, Y. Yamamoto, T. Shimada, T. Hasegawa, Ta-doped anatase TiO2 epitaxial film as transparent conducting oxide, Japanese Journal of Applied Physics Part 2-Letters & Express Letters, 44 (2005) L1063-L1065. [44] C.G. Granqvist, A. Hultaker, Transparent and conducting ITO films: new developments and applications, Thin Solid Films, 411 (2002) 1-5. [45] T. Minami, New n-type transparent conducting oxides, Mrs Bulletin, 25 (2000) 38-44. [46] D.S. Ginley, C. Bright, Transparent conducting oxides, Mrs Bulletin, 25 (2000) 15-18. [47] R.G. Gordon, Criteria for choosing transparent conductors, Mrs Bulletin, 25 (2000) 52-57. [48] M. Tadatsugu, Transparent conducting oxide semiconductors for transparent electrodes, Semiconductor Science and Technology, 20 (2005) S35. [49] S.X. Zhang, S. Dhar, W. Yu, H. Xu, S.B. Ogale, T. Venkatesan, Growth parameter-property phase diagram for pulsed laser deposited transparent oxide conductor anatase Nb : TiO2, Applied Physics Letters, 91 (2007). [50] N.F. Mott, THE BASIS OF THE ELECTRON THEORY OF METALS, WITH SPECIAL REFERENCE TO THE TRANSITION METALS, Proceedings of the Physical Society of London Section A, 62 (1949) 416-422. [51] N.F. Mott, ELECTRONS IN DISORDERED STRUCTURES, Advances In Physics, 16 (1967) 49-&. [52] M.N. Alexander, D.F. Holcomb, Semiconductor-to-Metal Transition in n-Type Group IV Semiconductors, Reviews of Modern Physics, 40 (1968) 815. [53] E. Burstein, Anomalous Optical Absorption Limit in InSb, Physical Review, 93 (1954) 632. [54] T.S. Moss, The Interpretation of the Properties of Indium Antimonide, Proceedings of the Physical Society. Section B, 67 (1954) 775. [55] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma, Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide, Science, 291 (2001) 854-856. [56] S.A. Chambers, S. Thevuthasan, R.F.C. Farrow, R.F. Marks, J.U. Thiele, L. Folks, M.G. Samant, A.J. Kellock, N. Ruzycki, D.L. Ederer, U. Diebold, Epitaxial growth and properties of ferromagnetic co-doped TiO2 anatase, Applied Physics Letters, 79 (2001) 3467-3469. [57] S.A. Chambers, C.M. Wang, S. Thevuthasan, T. Droubay, D.E. McCready, A.S. Lea, V. Shutthanandan, C.F. Windisch, Epitaxial growth and properties of MBE-grown ferromagnetic 156 Co-doped TiO2 anatase films on SrTiO3(001) and LaAlO3(001), Thin Solid Films, 418 (2002) 197-210. [58] D.H. Kim, J.S. Yang, K.W. Lee, S.D. Bu, D.W. Kim, T.W. Noh, S.J. Oh, Y.W. Kim, J.S. Chung, H. Tanaka, H.Y. Lee, T. Kawai, J.Y. Won, S.H. Park, J.C. Lee, Investigations on the nature of observed ferromagnetism and possible spin polarization in Co-doped anatase TiO2 thin films, Journal of Applied Physics, 93 (2003) 6125-6132. [59] J. Osorio-Guillén, S. Lany, A. Zunger, Atomic Control of Conductivity Versus Ferromagnetism in Wide-Gap Oxides Via Selective Doping: V, Nb, Ta in Anatase TiO_{2}, Physical Review Letters, 100 (2008) 036601. [60] S.X. Zhang, S.B. Ogale, W.Q. Yu, X.Y. Gao, T. Liu, S. Ghosh, G.P. Das, A.T.S. Wee, R.L. Greene, T. Venkatesan, Electronic Manifestation of Cation-Vacancy-Induced Magnetic Moments in a Transparent Oxide Semiconductor: Anatase Nb:TiO(2), Advanced Materials, 21 (2009) 2282-+. [61] J. Domaradzki, D. Kaczmarek, E. Prociow, A. Borkowska, D. Schmeisser, G. Beuckert, Microstructure and optical properties of TiO2 thin films prepared by low pressure hot target reactive magnetron sputtering, Thin Solid Films, 513 (2006) 269-274. [62] A.E.J. Gonzalez, S.G. Santiago, Structural and optoelectronic characterization of TiO2 films prepared using the sol-gel technique, Semiconductor Science and Technology, 22 (2007) 709716. [63] D.H. Bao, X. Yao, N. Wakiya, K. Shinozaki, N. Mizutani, Band-gap energies of sol-gelderived SrTiO3 thin films, Applied Physics Letters, 79 (2001) 3767-3769. [64] Y.F. Gao, Y. Masuda, Z.F. Peng, T. Yonezawa, K. Koumoto, Room temperature deposition of a TiO2 thin film from aqueous peroxotitanate solution, Journal of Materials Chemistry, 13 (2003) 608-613. [65] A.R. Denton, N.W. Ashcroft, Vegard‟s law, Physical Review A, 43 (1991) 3161. [66] W. Li, M. Pessa, J. Likonen, Lattice parameter in GaNAs epilayers on GaAs: Deviation from Vegard's law, Applied Physics Letters, 78 (2001) 2864-2866. [67] J. Kondo, RESISTANCE MINIMUM IN DILUTE MAGNETIC ALLOYS, Progress of Theoretical Physics, 32 (1964) 37-&. [68] T.L. Bell, K.G. Wilson, Nonlinear renormalization groups, Physical Review B, 10 (1974) 3935. [69] K.G. Wilson, The renormalization group: Critical phenomena and the Kondo problem, Reviews of Modern Physics, 47 (1975) 773. 157 [70] H.R. Krishna-murthy, J.W. Wilkins, K.G. Wilson, Renormalization-group approach to the Anderson model of dilute magnetic alloys. II. Static properties for the asymmetric case, Physical Review B, 21 (1980) 1044. [71] H.R. Krishna-murthy, J.W. Wilkins, K.G. Wilson, Renormalization-group approach to the Anderson model of dilute magnetic alloys. I. Static properties for the symmetric case, Physical Review B, 21 (1980) 1003. [72] R. Ramaneti, J.C. Lodder, R. Jansen, Kondo effect and impurity band conduction in Co : TiO2 magnetic semiconductor, Physical Review B, 76 (2007). [73] S.X. Zhang, S.B. Ogale, W.Q. Yu, X.Y. Gao, T. Liu, S. Ghosh, G.P. Das, A.T.S. Wee, R.L. Greene, T. Venkatesan, Electronic Manifestation of Cation-Vacancy-Induced Magnetic Moments in a Transparent Oxide Semiconductor: Anatase Nb:TiO2, Advanced Materials, 21 (2009) 2282-+. [74] B.L. Altshuler, A.G. Aronov, P.A. Lee, INTERACTION EFFECTS IN DISORDERED FERMI SYSTEMS IN DIMENSIONS, Physical Review Letters, 44 (1980) 1288-1291. [75] Y. Nagaoka, SELF-CONSISTENT TREATMENT OF KONDOS EFFECT IN DILUTE ALLOYS, Physical Review, 138 (1965) 1112-&. [76] R. More, H. Suhl, MAGNETORESISTANCE OF DILUTE MAGNETIC ALLOYS, Physical Review Letters, 20 (1968) 500-&. [77] D.R. Hamann, NEW SOLUTION FOR EXCHANGE SCATTERING IN DILUTE ALLOYS, Physical Review, 158 (1967) 570-&. [78] D. Goldhaber-Gordon, H. Shtrikman, D. Mahalu, D. Abusch-Magder, U. Meirav, M.A. Kastner, Kondo effect in a single-electron transistor, Nature, 391 (1998) 156-159. [79] D. Goldhaber-Gordon, J. Gores, M.A. Kastner, H. Shtrikman, D. Mahalu, U. Meirav, From the Kondo regime to the mixed-valence regime in a single-electron transistor, Phys. Rev. Lett., 81 (1998) 5225. [80] J. Nygard, D.H. Cobden, P.E. Lindelof, Kondo physics in carbon nanotubes, Nature, 408 (2000) 342. [81] T.A. Costi, A.C. Hewson, V. Zlatic, TRANSPORT-COEFFICIENTS OF THE ANDERSON MODEL VIA THE NUMERICAL RENORMALIZATION-GROUP, J. Phys.:Condens. Matter, (1994) 2519. [82] H. Keiter, E. Mullerhartmann, J. Zittartz, MAGNETORESISTANCE OF DILUTE MAGNETIC ALLOYS, Solid State Communications, 16 (1975) 1247-1249. 158 [83] I. Zutic, J. Fabian, S. Das Sarma, Spintronics: Fundamentals and applications, Reviews of Modern Physics, 76 (2004) 323-410. [84] S. Das Sarma, Spintronics, American Scientist, 89 (2001) 516-523. [85] R.P. Feynman, QUANTUM-MECHANICAL COMPUTERS, Foundations of Physics, 16 (1986) 507-531. [86] S. Datta, B. Das, ELECTRONIC ANALOG OF THE ELECTROOPTIC MODULATOR, Applied Physics Letters, 56 (1990) 665-667. [87] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Spintronics: A spin-based electronics vision for the future, Science, 294 (2001) 1488-1495. [88] M. Julliere, TUNNELING BETWEEN FERROMAGNETIC-FILMS, Physics Letters A, 54 (1975) 225-226. [89] R. Fiederling, M. Keim, G. Reuscher, W. Ossau, G. Schmidt, A. Waag, L.W. Molenkamp, Injection and detection of a spin-polarized current in a light-emitting diode, Nature, 402 (1999) 787-790. [90] C. Zener, Interaction between the d-Shells in the Transition Metals. II. Ferromagnetic Compounds of Manganese with Perovskite Structure, Physical Review, 82 (1951) 403. [91] J.B. Torrance, M.W. Shafer, T.R. McGuire, Bound Magnetic Polarons and the InsulatorMetal Transition in EuO, Physical Review Letters, 29 (1972) 1168. [92] A.C. Durst, R.N. Bhatt, P.A. Wolff, Bound magnetic polaron interactions in insulating doped diluted magnetic semiconductors, Physical Review B, 65 (2002) 235205. [93] A.H. Macdonald, P. Schiffer, N. Samarth, Ferromagnetic semiconductors: moving beyond (Ga, Mn)As, Nature Materials, (2005) 195-202. [94] H. Ohno, Making nonmagnetic semiconductors ferromagnetic, Science, 281 (1998) 951956. [95] Y. Ohno, D.K. Young, B. Beschoten, F. Matsukura, H. Ohno, D.D. Awschalom, Electrical spin injection in a ferromagnetic semiconductor heterostructure, Nature, 402 (1999) 790-792. [96] S.R. Shinde, S.B. Ogale, S. Das Sarma, J.R. Simpson, H.D. Drew, S.E. Lofland, C. Lanci, J.P. Buban, N.D. Browning, V.N. Kulkarni, J. Higgins, R.P. Sharma, R.L. Greene, T. Venkatesan, Ferromagnetism in laser deposited anatase Ti_{1-x}Co_{x}O_{2-δ} films, Physical Review B, 67 (2003) 115211. 159 [97] D.C. Kundaliya, S.B. Ogale, S.E. Lofland, S. Dhar, C.J. Metting, S.R. Shinde, Z. Ma, B. Varughese, K.V. Ramanujachary, L. Salamanca-Riba, T. Venkatesan, On the origin of hightemperature ferromagnetism in the low-temperature-processed Mn-Zn-O system, Nature Materials, (2004) 709-714. [98] S.A. Chambers, Ferromagnetism in doped thin-film oxide and nitride semiconductors and dielectrics, Surface Science Reports, 61 (2006) 345-381. [99] H. Pan, J.B. Yi, L. Shen, R.Q. Wu, J.H. Yang, J.Y. Lin, Y.P. Feng, J. Ding, L.H. Van, J.H. Yin, Room-Temperature Ferromagnetism in Carbon-Doped ZnO, Physical Review Letters, 99 (2007) 127201. [100] I.S. Elfimov, A. Rusydi, S.I. Csiszar, Z. Hu, H.H. Hsieh, H.J. Lin, C.T. Chen, R. Liang, G.A. Sawatzky, Magnetizing Oxides by Substituting Nitrogen for Oxygen, Physical Review Letters, 98 (2007) 137202. [101] A. Oiwa, Y. Mitsumori, R. Moriya, T. Słupinski, H. Munekata, Effect of Optical Spin Injection on Ferromagnetically Coupled Mn Spins in the III-V Magnetic Alloy Semiconductor (Ga,Mn)As, Physical Review Letters, 88 (2002) 137202. [102] A. Korbecka, J.A. Majewski, On the origin of room-temperature ferromagnetism in widegap semiconductors, Low Temperature Physics, 35 (2009) 53-57. [103] T.C. Kaspar, S.M. Heald, C.M. Wang, J.D. Bryan, T. Droubay, V. Shutthanandan, S. Thevuthasan, D.E. McCready, A.J. Kellock, D.R. Gamelin, S.A. Chambers, Negligible Magnetism in Excellent Structural Quality Cr_{x}Ti_{1-x}O_{2} Anatase: Contrast with HighT_{C} Ferromagnetism in Structurally Defective Cr_{x}Ti_{1-x}O_{2}, Physical Review Letters, 95 (2005) 217203. [104] K. Ando, Seeking room-temperature ferromagnetic semiconductors, Science, 312 (2006) 1883-1885. [105] D.W. Abraham, M.M. Frank, S. Guha, Absence of magnetism in hafnium oxide films, Applied Physics Letters, 87 (2005). [106] T.S. Herng, D.C. Qi, T. Berlijn, J.B. Yi, K.S. Yang, Y. Dai, Y.P. Feng, I. Santoso, C. Sánchez-Hanke, X.Y. Gao, A.T.S. Wee, W. Ku, J. Ding, A. Rusydi, Room-Temperature Ferromagnetism of Cu-Doped ZnO Films Probed by Soft X-Ray Magnetic Circular Dichroism, Physical Review Letters, 105 (2010) 207201. [107] A. Sundaresan, R. Bhargavi, N. Rangarajan, U. Siddesh, C.N.R. Rao, Ferromagnetism as a universal feature of nanoparticles of the otherwise nonmagnetic oxides, Physical Review B, 74 (2006) 161306. [108] N.H. Hong, J. Sakai, N. Poirot, V. Brizé, Room-temperature ferromagnetism observed in undoped semiconducting and insulating oxide thin films, Physical Review B, 73 (2006) 132404. 160 [109] A. Rusydi, P. Abbamonte, H. Eisaki, Y. Fujimaki, S. Smadici, N. Motoyama, S. Uchida, Y.J. Kim, M. Rübhausen, G.A. Sawatzky, Strain Amplification of the 4k_{F} Chain Instability in Sr_{14}Cu_{24}O_{41}, Physical Review Letters, 100 (2008) 036403. [110] J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Donor impurity band exchange in dilute ferromagnetic oxides, Nature Materials, (2005) 173-179. [111] J.M.D. Coey, et al., Charge-transfer ferromagnetism in oxide nanoparticles, Journal of Physics D: Applied Physics, 41 (2008) 134012. [112] B.W. Wessels, Ferromagnetic semiconductors and the role of disorder, New Journal of Physics, 10 (2008) 055008. [113] I.S. Elfimov, S. Yunoki, G.A. Sawatzky, Possible Path to a New Class of Ferromagnetic and Half-Metallic Ferromagnetic Materials, Physical Review Letters, 89 (2002) 216403. [114] M. Venkatesan, C.B. Fitzgerald, J.M.D. Coey, Unexpected magnetism in a dielectric oxide, Nature, 430 (2004) 630-630. [115] J. Osorio-Guillén, S. Lany, S.V. Barabash, A. Zunger, Magnetism without Magnetic Ions: Percolation, Exchange, and Formation Energies of Magnetism-Promoting Intrinsic Defects in CaO, Physical Review Letters, 96 (2006) 107203. [116] J. Osorio-Guillén, S. Lany, A. Zunger, Atomic Control of Conductivity Versus Ferromagnetism in Wide-Gap Oxides Via Selective Doping: V, Nb, Ta in Anatase TiO2, Physical Review Letters, 100 (2008). [117] A.J. Behan, A. Mokhtari, H.J. Blythe, D. Score, X.H. Xu, J.R. Neal, A.M. Fox, G.A. Gehring, Two Magnetic Regimes in Doped ZnO Corresponding to a Dilute Magnetic Semiconductor and a Dilute Magnetic Insulator, Physical Review Letters, 100 (2008) 047206. [118] C.B. Fitzgerald, M. Venkatesan, J.G. Lunney, L.S. Dorneles, J.M.D. Coey, Cobalt-doped ZnO - a room temperature dilute magnetic semiconductor, Applied Surface Science, 247 (2005) 493-496. [119] J.M.D. Coey, M. Venkatesan, P. Stamenov, C.B. Fitzgerald, L.S. Dorneles, Magnetism in hafnium dioxide, Physical Review B, 72 (2005) 024450. [120] D.J. Larson, A. Cerezo, J. Juraszek, K. Hono, G. Schmitz, Atom-Probe Tomographic Studies of Thin Films and Multilayers, Mrs Bulletin, 34 (2009) 732-737. [121] M. Kodzuka, T. Ohkubo, K. Hono, F. Matsukura, H. Ohno, 3DAP analysis of (Ga,Mn)As diluted magnetic semiconductor thin film, Ultramicroscopy, 109 (2009) 644-648. 161 [122] Y.M. Chen, T. Ohkubo, M. Kodzuka, K. Morita, K. Hono, Laser-assisted atom probe analysis of zirconia/spinel nanocomposite ceramics, Scripta Materialia, 61 (2009) 693-696. [123] Y. He, O. Dulub, H. Cheng, A. Selloni, U. Diebold, Evidence for the Predominance of Subsurface Defects on Reduced Anatase TiO_{2}(101), Physical Review Letters, 102 (2009) 106105. [124] S. Hashimoto, A. Tanaka, A. Murata, T. Sakurada, Formulation for XPS spectral change of oxides by ion bombardment as a function of sputtering time, Surface Science, 556 (2004) 22-32. [125] X.J. Yu, O. Wilhelmi, H.O. Moser, S.V. Vidyaraj, X.Y. Gao, A.T.S. Wee, T. Nyunt, H.J. Qian, H.W. Zheng, New soft X-ray facility SINS for surface and nanoscale science at SSLS, Journal of Electron Spectroscopy and Related Phenomena, 144 (2005) 1031-1034. [126] B.T. Thole, P. Carra, F. Sette, G. van der Laan, X-ray circular dichroism as a probe of orbital magnetization, Physical Review Letters, 68 (1992) 1943. [127] P. Carra, B.T. Thole, M. Altarelli, X. Wang, X-ray circular dichroism and local magnetic fields, Physical Review Letters, 70 (1993) 694. [128] J. Stohr, Exploring the microscopic origin of magnetic anisotropies with X-ray magnetic circular dichroism (XMCD) spectroscopy, Journal of Magnetism and Magnetic Materials, 200 (1999) 470-497. [129] B.L. Henke, E.M. Gullikson, J.C. Davis, X-RAY INTERACTIONS PHOTOABSORPTION, SCATTERING, TRANSMISSION, AND REFLECTION AT E=5030,000 EV, Z=1-92, Atomic Data and Nuclear Data Tables, 54 (1993) 181-342. [130] P. Abbamonte, L. Venema, A. Rusydi, G.A. Sawatzky, G. Logvenov, I. Bozovic, A structural probe of the doped holes in cuprate superconductors, Science, 297 (2002) 581-584. [131] P. Abbamonte, A. Rusydi, S. Smadici, G.D. Gu, G.A. Sawatzky, D.L. Feng, Spatially modulated 'Mottness' in La2-xBaxCuO4, Nature Physics, (2005) 155-158. [132] F.M.F. de Groot, M. Grioni, J.C. Fuggle, J. Ghijsen, G.A. Sawatzky, H. Petersen, Oxygen 1s x-ray-absorption edges of transition-metal oxides, Physical Review B, 40 (1989) 5715. [133] F.M.F. de Groot, J. Faber, J.J.M. Michiels, M.T. Czyżyk, M. Abbate, J.C. Fuggle, Oxygen 1s x-ray absorption of tetravalent titanium oxides: A comparison with single-particle calculations, Physical Review B, 48 (1993) 2074. [134] R. Rauer, G. Neuber, J. Kunze, J. Backstrom, M. Rubhausen, Temperature-dependent spectral generalized magneto-optical ellipsometry for ferromagnetic compounds, Review of Scientific Instruments, 76 (2005). 162 [135] P.J. Hay, J.C. Thibeault, R. Hoffmann, ORBITAL INTERACTIONS IN METAL DIMER COMPLEXES, Journal of the American Chemical Society, 97 (1975) 4884-4899. [136] J. Garcia-Barriocanal, J.C. Cezar, F.Y. Bruno, P. Thakur, N.B. Brookes, C. Utfeld, A. Rivera-Calzada, S.R. Giblin, J.W. Taylor, J.A. Duffy, S.B. Dugdale, T. Nakamura, K. Kodama, C. Leon, S. Okamoto, J. Santamaria, Spin and orbital Ti magnetism at LaMnO(3)/SrTiO(3) interfaces, Nature Communications, (2010). 163 [...]... 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