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Theoretical Investigation of Novel Spin-Polarized Materials for Spintronic Applications

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Theoretical Investigation of Novel Spin-Polarized Materials for Spintronic Applications The University of New South Wales Faculty of Science School of Materials Science and Engineering Theoretical investigation of novel spin-polarized materials for spintronic applications A Thesis in Materials Science and Engineering By Anh Pham Submitted in Partial Fulfilment of the Requirement for the Degree of Doctor of Philosophy October 2014 i ORIGINALITY STATEMENT ‘I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged.’ Signed Date i ACKNOWLEDGEMENT I would like to thank my supervisor Prof. Sean Li for all the supports that he has given me since I joined his group. Without his strong support I would not have completed this thesis. I also want to express my appreciation to Dr. Hussein Assadi for introducing me to the density functional theory technique as well as guiding me in the early stages of my study. I also acknowledge the financial support from the Australian Research Council during my research. Finally I want to dedicate this thesis to my partner for always being there during my long scientific journey. ii ABSTRACT Development of spin-polarized materials is important for the realization of spintronic devices. Two approaches are taken: (i) investigating magnetism in diluted magnetic semiconductors (DMSs), and (ii) developing novel 2D materials with intrinsic spin currents. The focus of this thesis is to study these two classes of materials using the density functional (DFT) theory. The first part focuses on ZnO based DMS. The systematic study of C/N doped ZnO with defects shows that ZnO:C exhibits ferromagnetism, but dopant complex C2 tends to inhibit this property. Most importantly, the computational method plays a key role in studying DMS. Due to the popularity of hydrogenated ZnO:Co, this system is tested using the different methods. The DFT+U method shows a very specific configuration of hydrogenated Co complex can generate ferromagnetism. The inadequacies of traditional method are further exposed when the dopant geometry is compared between the hybrid functional method and normal DFT. The hybrid method includes the Fock exchange which significantly improves the description of long-range inter-atomic forces in the geometry relaxation yielding a more accurate description. The second part focuses on the study of novel 2D materials with. The studies are devoted to increase the band gap of of novel 2D materials. In silicene and germanene, it is shown that the adsorption and substitution of Tl can increase the bandgaps to 0.29 eV at a small doping limit of ~2%. In addition, Tl can also tune the conductivity of silicene from n to p types depending on the doping sites, and preserving the high mobility of the undoped structure. Furthermore, the author also predicts several new wide bandgap quantum spin Hall insulator (QSHI). QSHI is a novel material with an insulating bulk state and spin-polarized metallic edges. By modifying the surfaces through hydrogenation, the orbital interactions of Bi, Pb, Cr, Mo, and W in the low energy level are confined on two dimensional resulting in novel QSHI with giant gap and strongly correlated property. These results represent important contribution to the study of spintronic materials. First, the understanding between the theoretical method and magnetic properties in semiconductors can help to accurately predict new spintronic materials. Most importantly, the study in orbital engineering of 2D materials with hydrogenation provides new insights into the nature topological materials which have practical applications for the next generation of electronic devices. iii List of Publications Included as Part of the thesis 1. Anh Pham, M. H. N. Assadi, Y. B. Zhang, A. B. Yu, and S. Li “Weak d0 magnetism in C and N doped ZnO” Journal of Applied Physics, 110 art. no. 123917 (2011). 2. Anh Pham, Y B Zhang, M H N Assadi, A B Yu and S Li “Ferromagnetism in ZnO:Co originating from a hydrogenated Co–O–Co complex” Journal of Physics: Condensed Matter 25, 116002 (2013). 3. Anh Pham, M H N Assadi, A B Yu and Sean Li “Critical role of Fock exchange in characterizing dopant geometry and magnetic interaction in magnetic semiconductors” Physical Review B 89, 155110 (2014). 4. Anh Pham, Carmen J Gil, and Sean Li “Orbital engineering of 2D materials with hydrogenation: a realization of giant gap and strongly correlated topological insulators”, Being Reviewed in Physical Review Letters (2014) 5. Anh Pham, Carmen J Gil, and Sean Li “Engineering silicene and germanene’s band gap and conductivity with diluted doping of heavy elements: a first principle study”, in preparation. 6. Carmen J Gil, A Pham, A Yu and Sean Li “An ab-initio study of transition metals doped WSe2 for long-range room-temperature ferromagnetism in two-dimensional transition metal dichalcogenide” Journal of Physics: Condensed Matter 26, 306004 (2014). iv Statement of Contribution of Others I, Anh Pham, was the person who mainly conducted the calculations including analyses as well as writing the manuscripts in all publication presented in this thesis. Signature I as a co-author endorse that this level of contribution by the candidate indicated above is appropriate. Dr. Mohammad H. N. Al Assadi Dr. Yuebin Zhang Prof. Aibing Yu Prof. Sean Li Ms. Carmen J Gil. v List of Abbreviation 2D: Two-dimensional 3D: Three-dimensional BMP: Bound Magnetic Polaron CVD: Chemical Vapour Deposition DFT: Density Functional Theory DMS: Diluted Magnetic Semiconductor GKA: Goodenough-Anderson-Kanokori QH: Quantum Hall QSH: Quantum Spin Hall QSHI: Quantum Spin Hall Insulator RKKY: Ruderman–Kittel–Kasuya–Yosida TKKN: Thouless, Kohmoto, Nightingale and den Nijs TM: Transition metal vi TABLE OF CONTENTS Certificate of Originality……………………………………………………………………….i Acknowledgement…………………………………………………………………………….ii Abstract……………………………………………………………………………………….iii List of Publications…………………………………………………………………………...iv Statement of Contribution of Others…………………………………………………………..v List of Abbreviation…………………………………………………………………………..vi Table of Contents…………………………………………………………………………….vii CHAPTER 1 Motivation and Outlines……………………………………………………….1 CHAPTER 2 Literature Review………………………………………………………………9 2.1 Diluted Magnetic Semiconductors…………………………………………...10 2.2 Novel Two Dimensional Materials…………………………………………..19 CHAPTER 3 Computational Method……………………………………………………….36 CHAPTER 4 Weak d0 Magnetism in C and N doped ZnO……………………………........45 CHAPTER 5 Ferromagnetism in ZnO:Co Originating from a Hydrogenated Co–O–Co Complex……………………………………………………………………………………...53 CHAPTER 6 Critical Role of Fock Exchange in Characterizing Dopant Geometry and Magnetic Interaction in Magnetic Semiconductors…………………………………………..62 CHAPTER 7 Engineering Silicene and Germanene’s Band Gap and Conductivity with Diluted Doping of Heavy Elements: a First Principle Study………………………………...69 CHAPTER 8 Orbital Engineering of 2D Materials with Hydrogenation: A Realization of Giant Gap and Strongly Correlated Topological Insulators………………………………….86 CHAPTER 9 Conclusions…………………………………………………………………105 APPENDEX An Ab-Initio Study of Ttransition Metals Doped with WSe2 for Long-Range Room Temperature Ferromagnetism in Two-Dimensional Transition Metal Dichalcogenide……………………………………………………………………………...109 vii Chapter 1: Introduction Spintronic materials represent a novel platform in which information can be controlled rapidly through the manipulation of the quantum mechanical properties of the electrons through an external electric or magnetic field. One of the earliest discoveries of such materials was the discovery of the colossal magneto resistance in metallic hetero- structures [1, 2]. Since then, the field of spintronics has expanded at a rapid rate with the discoveries of several exotic materials for practical applications [3-6]. These materials can be classified into two types based on different scientific approaches. The first type of materials are known as diluted magnetic semiconductors (DMSs) in which the interplay between charge and spin properties are manipulated through the introduction of impurities. These impurities can be either magnetic/non-magnetic but their introductions in the host materials generate a ferromagnetic interaction between the dopants making the doped materials magnetic. Various host materials have been studied ranging from II-VI semiconductors [7-9], III-V materials [10-12], IV-VI chalcogenides [13-15] based materials to wide band gap oxides [16-20] since the 1980s. Subsequently, the prediction of room temperature of ferromagnetism in oxide DMS materials by Dietl et al. [21] sparked off a new chapter in the field of DMS research with increasing focus in the II-VI oxide materials. Several experiment studies were conducted in doping II-VI material like ZnO with different dopants to generate ferromagnetism [22-24]. However, these studies have generated several contradicting results which question the nature of magnetism in doped semiconductor
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