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Unusual Electronic Transport and Magnetism in Titanium Oxide Based Semiconductors and Metals

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Unusual Electronic Transport and Magnetism in Titanium Oxide Based Semiconductors and Metals ABSTRACT Title of dissertation: UNUSUAL ELECTRONIC TRANSPORT AND MAGNETISM IN TITANIUM OXIDE BASED SEMICONDUCTORS AND METALS Shixiong Zhang, Doctor of Philosophy, 2007 Dissertation directed by: Professor T. Venky Venkatesan Department of Physics The main objective of this thesis was to explore the structural, electrical, mag- netic and optical properties of titanium based novel oxide thin ¯lms, such as trans- parent conducting oxides (TCOs) and diluted magnetic semiconductors (DMSs), so as to be able to realize optoelectronics and spintronics applications. I demonstrated that niobium doped titanium dioxide (TiO2) in its epitax- ial anatase phase grown at certain condition is an intrinsic transparent conducting oxide, with both its conductivity and transparency comparable to that of the com- mercial transparent electrode In-Sn-O being widely used in current optoelectronic devices. I investigated the growth parameter dependence of structure and conductivity of this material. It was found that the growth temperature is a crucial parameter for the structural quality as well as the electron mobility, while the oxygen par- tial pressure is essential for the conduction electron concentration. The excellent conductivity of niobium doped TiO2 should be attributed to the extremely high sol- ubility of niobium in the TiO2 matrix as well as a very shallow donor level created in the TiO2 band gap. I investigated several important oxide based DMS systems, such as niobium and cobalt dual doped TiO2, transition metal (TM) element doped SrTiO3 etc. I found that niobium dual doping is an e®ective way to introduce carriers into the classical Co: TiO2 system, which provides the feasibility of studying the RKKY interaction in this system by chemical doping. Our detailed characterization of TM doped SrTiO3 questioned the intrinsic nature of the ferromagnetism observed by other groups. By a systematic study of Hall e®ect on superparamagnetic Co-(La,Sr)TiO3 thin ¯lms, I was able to demonstrate that the magnitude of the anomalous Hall e®ect is a way to distinguish between intrinsic and extrinsic DMS. A Kondo e®ect was observed in niobium doped TiO2 grown at certain con- dition. The origin of magnetic moments in this system was suggested to be from the cation vacancy defects. This observation of defect magnetism in conventional non-magnetic TiO2 may shed light on the occurrence of ferromagnetism in oxide diluted magnetic semiconductors. UNUSUAL ELECTRONIC TRANSPORT AND MAGNETISM IN TITANIUM OXIDE BASED SEMICONDUCTORS AND METALS by Shixiong Zhang Dissertation submitted to the Faculty of the Graduate School of the University of Maryland, College Park in partial ful¯llment of the requirements for the degree of Doctor of Philosophy 2007 Advisory Committee: Professor T. Venky Venkatesan, Chair/Advisor Professor Richard L. Greene Professor Lourdes G. Salamanca-Riba Professor Christopher Lobb Professor J. Robert Anderson °c Copyright by Shixiong Zhang 2007 Acknowledgement Many people have helped me during the entire process of graduate school. I would like to extend my gratitude to all of them. First and foremost, I would like to thank my advisor Prof. T. Venky Venkate- san. With his outstanding guidance and encouragement, and always valuable sug- gestions, I had great opportunities to investigate the physical properties of functional oxide thin ¯lm systems. In addition to physics, Venky has taught me many things about life, which I think will be very useful to my future career. Dr. Satish B. Ogale is my initial mentor who I worked the most closely with. He is an essential part of my education who has advised and guided my scienti¯c growth in these three years. He has been an invaluable resource for me. Prof. Richard L. Greene is the one who initially introduced me into Venky's group. He is a very nice person and has given me almost unlimited access to his lab facilities. Without his help, many of my projects could not have been ¯nished on time. I thank Professors Anderson, Lobb and Salamanca-riba for taking interest in my work and serving on my committee. I would also like to thank the entire faculty and sta® of the CSR (now as CNAM) at UMD. I highly acknowledge CSR for the ¯nancial support. I owe a lot to my lab-mates, Sanjay Shinde, Darshan Kundaliya, Sankar Dhar, Wegdan Ramadan, Betsy Pugel, Greg Glangham and Arun Lukyx. Sanjay taught me a lot of physics and material science. He gave me many useful suggestions on my ii work even after he left Venky's group. Darshan is the one who I also worked closely with. I learnt many technique skills from him. I thank Sankar for his assistance in RBS measurements. I would like to thank Betsy for sharing her thesis writing experience with me. I would like to thank my collaborators, Darren Young and Prof. Salamanca- riba at material science, Lianfeng Fu and Prof. Nigel Browning at University of California, Davis for their help on TEM and EELS meausurements. I thank Xingyu Gao and Prof. Lee at National University of Singapore for XPS, UPS and XAS measurements. I would also like to acknowledge several Chinese fellows in the center. Weiqiang Yu helped me a lot on the electronic transport measurements, and the discussion with him is a very important source to understand physics. Bing Liang gave me much of her assistance on SQUID measurements. I could easily get help from Pengcheng Li, Hua Xu and Zhengli Li whenever I had trouble with the experiment techniques. I enjoyed my discussion on material growth with several graduate fellows, Shengqiang Ren, Sung Hwan Lim, Yi Qi, Shige Fujimoto, and Makoto Murakami, all from material science department. I thank Josh Higgins for helping me with PPMS measurements and sharing his thesis with me. I thank James Tse for helping me understand the physics behind spin Hall e®ect as well as anomalous Hall e®ect. I thank my internship manager Eric Granstrom, mentor Konstantin R. Niko- laev at seagate for giving me the opportunity to work in the industrial environment. I thank my friend Jenny Gao and Paul Henny for helping me understand the physics and applications of CPP-spin valve and TMR stacks. I thank Wei Tong (one of my iii best friends) currently at Colorado State University for his continuous help. I would like to thank my girlfriend Meijuan and my family. Their love is the most important support in my life. iv Table of Contents List of Figures viii List of Abbreviations xiv 1 Introduction 1 1.1 A Brief Introduction to Semiconductor Physics . 1 1.2 Fundamental Physical Properties of TiO2 ................ 5 1.2.1 Crystalline Structural Polymorphs . 5 1.2.2 Electronic Structure . 6 1.3 Potential Applications of TiO2 ...................... 10 2 Niobium doped TiO2: Transparent Conducting Anatase vs Highly Resistive Rutile 14 2.1 Introduction . 14 2.1.1 Transparent Conducting Oxides . 14 2.1.2 Mott Transition . 17 2.1.3 Optical Transitions . 18 2.1.4 Motivation . 21 2.2 Sample Preparation . 21 2.3 Structural Characterization . 22 2.3.1 X-ray Di®raction . 22 2.3.2 Rutherford-back Scattering Channeling . 23 2.3.3 Atomic Force microscopy . 23 2.3.4 Transmission Electron Microscopy . 26 2.4 Conductivity . 27 2.5 Optical properties . 33 2.6 Summary . 35 3 The Growth Parameter - Property Phase Diagram of Anatase Nb: TiO2 37 3.1 Practical Doping Rules . 37 3.2 Basic Growth Parameters . 40 3.3 Substrate Temperature E®ects . 41 3.3.1 Niobium Substitutional Fraction . 41 3.3.2 Carrier Concentration and Hall Mobility . 42 3.4 Oxygen E®ects . 46 3.4.1 Valence States of Niobium and Titanium . 46 3.4.2 Carrier Concentration, Hall Mobility and Niobium Substitu- tional Fraction . 48 3.5 Summary . 49 v 4 Introduction to Oxide based Diluted Magnetic Semiconductors (DMSs) 51 4.1 Introduction to Ferromagnetism . 51 4.1.1 Magnetic Moment & Exchange Interaction . 51 4.1.2 Mean Field Approximation . 53 4.1.3 Magnetic Domains & Hysteresis Curve . 55 4.2 Spintronics . 57 4.2.1 Spin-based Electronic Devices . 58 4.2.2 Diluted Magnetic Semiconductors . 61 4.2.3 Theoretical Models on the Origin of Ferromagnetism . 65 4.2.3.1 RKKY Interaction . 65 4.2.3.2 Bound Magnetic Polarons . 67 5 Niobium and Cobalt Dual Doped TiO2: an RKKY Motivation 70 5.1 Motivation . 70 5.2 Experimental Results . 71 5.2.1 Sample Preparation . 71 5.2.2 Structure and Chemical Distribution . 71 5.2.3 Magnetism and Conductivity . 76 5.3 Summary . 80 6 SrTiO3 (STO)-based DMS 82 6.1 Overview . 82 6.2 Search for FM in Nb: SrTiO3 with Transition Metal Dopants . 83 6.2.1 Motivation . 83 6.2.2 Sample Preparation and Microstructure Characterization . 84 6.2.3 Magnetism and Conductivity . 87 6.2.4 Discussions . 90 6.2.5 Summary . 93 7 Magnetism and Anomalous Hall E®ect (AHE) in Co-(La,Sr)TiO3 94 7.1 Magnetism in Co-(La,Sr)TiO3 ...................... 94 7.1.1 Sample Preparation and Structural Characterization . 94 7.1.2 Magnetism . 95 7.1.2.1 Introduction to Superparamagnetism . 95 7.1.2.2 General Magnetic Properties . 100 7.1.2.3 Discussion . 104 7.2 Anomalous Hall E®ect in Co-(La,Sr)TiO3 . 104 7.2.1 Introduction to Anomalous Hall E®ect . 104 7.2.2 The Motivation for Hall E®ect Study in Co-(La,Sr)TiO3 . 107 7.2.3 Electronic Transport and Hall E®ect in Co-(La,Sr)TiO3 Thin Films . 108 7.2.4 The Origin of AHE in Superparamagnetic DMS . 109 7.2.5 A New Parameter to Test Intrinsic DMS by AHE . 112 7.3 Summary . 114 vi 8 Magnetic E®ect in Anatase Nb: TiO2 Thin Films 115 8.1 Introduction .
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