ABSTRACT Title of Document: ELECTRONIC TRANSPORT IN BISMUTH SELENIDE IN THE TOPOLOGICAL INSULATOR REGIME Dohun Kim, Doctor of Philosophy, 2013 Directed By: Professor, Michael S. Fuhrer, Department of Physics The 3D topological insulators (TIs) have an insulating bulk but spin-momentum coupled metallic surface states stemming from band inversion due to strong spin-orbit interaction, whose existence is guaranteed by the topology of the band structure of the insulator. While the STI surface state has been studied spectroscopically by e.g. photoemission and scanned probes, transport experiments have failed to demonstrate clear signature of the STI due to high level of bulk conduction. In this thesis, I present experimental results on the transport properties of TI material Bi2Se3 in the absence of bulk conduction (TI regime), achieved by applying novel p-type doping methods. Field effect transistors consisting of thin (thickness: 5-17 nm) Bi2Se3 are fabricated by mechanical exfoliation of single crystals, and a combination of conventional dielectric (300 nm thick SiO2) and electrochemical or chemical gating methods are used to move the Fermi energy through the surface Dirac point inside bulk band gap, revealing the ambipolar gapless nature of transport in the Bi2Se3 surface states. The minimum conductivity of the topological surface state is understood within the self-consistent theory of Dirac electrons in the presence of charged impurities. The intrinsic finite- temperature resistivity of the topological surface state due to electron-acoustic phonon scattering is measured to be 60 times larger than that of graphene largely due to the smaller Fermi and sound velocities in Bi2Se3, which will have implications for topological electronic devices operating at room temperature. Along with semi-classical Boltzmann transport, I also discuss 2D weak anti-localization (WAL) behavior of the topological surface states. By investigating gate-tuned WAL behavior in thin (5-17 nm) TI films, I show that WAL in the TI regime is extraordinarily sensitive to the hybridization induced quantum mechanical tunneling between top and bottom topological surfaces, and interplay of phase coherence time and inter-surface tunneling time results in a crossover from two decoupled (top and bottom) symplectic 2D metal surfaces to a coherently coupled single channel. Furthermore, a complete suppression of WAL is observed in the 5 nm thick Bi2Se3 film which was found to occur when the hybridization gap becomes comparable to the disorder strength. ELECTRONIC TRANSPORT IN BISMUTH SELENIDE IN THE TOPOLOGICAL INSULATOR REGIME By Dohun Kim Dissertation submitted to the Faculty of the Graduate School of the University of Maryland, College Park, in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2013 Advisory Committee: Professor Michael S. Fuhrer, Chair Assistant Professor Johnpierre Paglione Associate Professor Min Ouyang Associate Research Scientist William G. Cullen Professor Ichiro Takeuchi © Copyright Dohun Kim 2013 To my parents 김 건 태, 이 증 균 ii Acknowledgements On writing this thesis organizing my past works in Ph.D. study, I feel strongly more than ever that my journey so far would not be possible without supports from people around me. First and foremost, my parents, 김건태 and 이증균, have always encouraged my path as a scholar. All of my rational abilities are inherited from them. Mentorship is an essential part in any higher level study, and doctorate course is not an exception. I express my sincere gratitude to my advisor Prof. Michael S. Fuhrer. As a student having an engineering background, diverting from what I have learned in undergraduate was not an easy task especially when it comes to entering the field of physics research. He is the first person who acknowledged my motivation in studying physics. He is always open to discussing with people and guided us by first knowing exactly what one’s strength is. I particularly admire his comprehensive knowledge in general subjects of condensed matter physics and ability to see the big picture. He taught me by showing how academic insight can lead to consistent interpretation on the problem at hand. It is mutual faith between him and I which made fruitful accomplishments possible. Most definitely I will be a mentor someday, and it is my hope that I become proud of myself by putting lessons from him into practice. I would like to thank the members of my dissertation committee, Prof. Johnpierre Paglione, Prof. Min Ouyang, Dr. William G. Cullen and Prof. Ichiro Takauchi, for their guidance that leads to the completion of this thesis. I would like to thank Dr. Nicholas Butch, Paul Syers, and Kevin Kirshenbaum for providing me high quality Bi2Se3 starting iii material. My academic accomplishments during Ph.D study were possible entirely because I fortuitously had colleagues who are experts in single crystal growth techniques. I would like to thank all the people who have helped me in finishing the work in this thesis: Dr. Chaun Jang, Dr. SungJae Cho, Dr. Claudia Ojeda, Dr. Wenzhong Bao, Dr. Daniel Lenski, Dr. Enrique Cobas, Dr. Alexandra Curtin, Xinghan (Harold) Cai, Jack Hellerstedt, Liang Li, Jinglei Ping, Jacob Tosado, Dr. Yilin Wang, Kristen Burson, Michelle Zimmermann and Mahito Yamamoto. I would like to thank Prof. Sankar Das Sarma, Prof. Shaffique Adam, Prof. Euyheon Hwang and Qiuzi Li for sharing of theoretical insights. My gratitude is also extended to all my friends. I do realize that completing Ph.D study is only the starting point of my future career and I am sure that unprecedented adversities will come as I move on. I appreciate what they have done for me during my Ph.D. program and cherish all the wonderful memories. With helpful communication and supports that I have enjoyed, I feel that I am already seeing the solution. I hope that their memories with me are equally joyful as mine. 2013. 3. 10. Late night in my office, Dohun Kim iv Table of Contents Acknowledgements ............................................................................................................ iii Table of Contents ................................................................................................................ v List of Figures ................................................................................................................... vii Chapter 1 Introduction to topological insulators ................................................................ 1 1.1 Introduction ............................................................................................................... 1 1.2. Bi2Se3 : toward realization of topological transport regime .................................... 9 1.3 Outline of the thesis ................................................................................................ 12 Chapter 2 Charge transport in gapless Dirac surface states .............................................. 14 2.1 Transport theory of surface charge transport in Dirac system ................................ 14 2.2 Minimum conductivity and Self-consistent theory ................................................. 18 Chapter 3 Experimental methods ...................................................................................... 21 3.1 Bi2Se3 field effect transistor fabrication ................................................................. 21 3.1.1 Mechanical exfoliation of bulk Bi2Se3 single crystals .................................... 21 3.1.2 Field effect transistor fabrication process ........................................................ 25 3.2 Novel p-type doping methods for achieving topological transport regime ............ 28 3.2.1 Charge transfer doping with organic molecules .............................................. 28 3.2.2 Polymer electrolyte gating ............................................................................... 29 3.3 Measurement setup ................................................................................................. 30 3.3.1 Four probe AC lock-in technique .................................................................... 30 3.3.2 Variable temperature 4He cryostat ................................................................... 32 Chapter 4 Surface conduction of topological Dirac electrons in bulk insulating Bi2Se3 . 35 4.1 Electronic transport in the topological insulator regime ......................................... 36 4.1.1 p-type doping effects near room temperature .................................................. 36 4.1.2 Gapless ambipolar field effect ......................................................................... 39 4.1.3 Single band conduction in the topological transport regime ........................... 42 4.1.4 Absence of band bending effect : the role of bulk capacitance ....................... 44 4.2 Charge inhomogeneity driven minimum conductivity in Dirac surface states ....... 51 4.2.1 Charged-impurity limited transport in the Boltzmann transport regime ......... 51 4.2.2 Charge inhomogeneity driven electron hole puddles and minimum conductivity............................................................................................................... 55 4.3 Conclusion .............................................................................................................. 59 v Chapter 5 Intrinsic electron-acoustic phonon resistivity of Bi2Se3 in the topological insulator regime ...............................................................................................................