Bismuth Telluride and Antimony Telluride Based Co-evaporated Thermoelectric Thin Films: Technology, Characterization, and Optimization by Niloufar Ghafouri A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Electrical Engineering) in The University of Michigan 2012 Doctoral Committee: Professor Khalil Najafi , Co-Chair Assistant Research Scientist Rebecca L. Peterson, Co-Chair Professor Yogesh B. Gianchandani Professor Massoud Kaviany Professor Ctirad Uher © Niloufar Ghafouri 2012 All Rights Reserved Dedication To my parents. ii Acknowledgements First, I would like to thank my advisor, Prof. Khalil Najafi, for his continuous guidance and support. His encouragement, enthusiasm, and patience have always inspired me during the critical moments of my graduate studies. I have learned so many great lessons by his commitment and dedication to high quality, challenging and innovative research. I am also thankful to Dr. Rebecca Peterson who was my co-advisor during the past three years of my graduate school. She has been very kind and patient to provide useful suggestions, feedback and help on all aspects of this work. Research direction and new ideas through this thesis have improved significantly as a result of incorporating her vision and knowledge. I would also like to acknowledge my dissertation committee, Prof. Ctirad Uher, Prof. Massoud Kaviany and Prof. Yogesh Gianchandani, who provided valuable guidance with regards to my research. I have learned a lot from Prof. Uher, who kindly invited me to his laboratory, introduced me to the challenges of thermoelectric material deposition and taught me all about the characterization of thermoelectric films in person. I am grateful to Prof. Kaviany who was always supportive of this work and helped me with constructive discussion and suggestions. I also thank Prof. Gianchandani for providing valuable comments on this dissertation. Being a member of Najafi’s research group, I was truly fortunate to work with so many talented people. I should thank my former colleagues: Andy Gross and Sangwoo Lee who iii walked me through the basic fabrication steps in cleanroom with dedication and patience. I am truly grateful to Erkan Aktakka for the insightful discussions on my research, Tzeno Galchev for his unique and sarcastic way of guidance, Seow Yuen Yee for her sincere friendship, Mahdi for the enjoyable teatime talks, Ali for his help, friendship, and non- stop talking and Amir for his constant support and encouraging conversations. I also have to thank Yi Yuan for helping me with the SEM, AFM and XRD characterization and cleanroom processing. He began to work with me as an undergraduate mentee, but shortly developed as a strong researcher and joined our group for his graduate studies. I have also enjoyed working with other group members over the years: Jae Yoong Cho, Daniel Egert, Jeff Gregory, James McCullagh, Kevin Owen, Stacey Tang, Sang Won Yoon, Sang-Hyun Lee, and Jay Mitchell. I would like to take this opportunity to thank the graduate students and research fellows in Prof. Uher’s Lab, who helped me with thermoelectric thin film characterization and measurements. I must also thank my other friends at EECS, who made my graduate school experience much less boring and in fact quite interesting and educational: Angelique Johnson, Razi Haque, Christine Eun, Ning Gulari, Gayatri Perlin, Ruba Borno, Scott Wright, Allan Evans, Naveen Gupta, Karthik Visvanathan and Trushal Chokshi. Much gratitude goes to the LNF staff members, especially Robert Gordenker, Dennis Schweiger, James Kulman, and Pilar Herrera-Fierro. I am also truly grateful to Dennis Grimard, who helped me with bringing up and calibrating the Lab 18 evaporator tool despite his extremely busy schedule. I also should thank the staff members at EMAL, who helped with the characterization of the thermoelectric thin films and all of the SSEL staff members and EECS administrative assistants, especially Trasa Burkhardt, Deb iv Swartz, Laura Jarels, Karen Richardson, Jonathan Plummer and Beth Stalnaker. Thanks go to my former and current Iranian friends in Michigan: Nazanin, Mehrdad, Shirin, Hossein, Elnaz, Armin, Parisa, Mehrzad, Amir, Mona and Mojtaba, who each, in their own special way, made my life in Michigan pleasant during these years. I like to specially thank Nazanin for bringing her livelihood and kindness to my life, Mona for her support and companionship during the long nights of my graduate studies and also for proofreading my thesis, while she was preparing for her oral defense. I owe my deepest gratitude to my dear friend, Mojtaba, who not only helped extensively with writing and proofreading my thesis, but also stood by me, motivated and comforted me during the difficult days of my life in Ann Arbor. I should also thank my friends from high school and college: Sara, Maryam K., Bahar, Sanaz, Maryam M., Azadeh, Leili and Aram. Even though destiny has spread us apart around the world, looking forward to gatherings, hearing their voices, sharing laughs and great memories have always given me pleasure and energy. Last but not least, I have to thank my family. My uncle, Ebi, and his wife, Barb, who have always been encouraging and compassionate. My grandmother, Atefeh, who is the sunshine of my childhood memories. Her selfless, compassionate and strong spirit and especially her sweet and energetic voice have always encouraged me to be a better person in life. I am truly grateful to my brother, Sohrab, for being a great friend and for always being calm and understanding with my complaints and worries about life and family. I am also thankful to my beautiful sister-in-law, Maral, for her kindness and support. I am truly blessed to have the unconditional love and support of my amazing parents, Maryam and Esfandiar, who have always been present in my life despite the distance. Their v encouragement and trust always makes me more confident about pursuing my goals in life. The regular phone conversations with my mom have been the highlight of my nights and the continuing enthusiasm and interest of my dad in my research work is one of the main reasons I have been able to complete this dissertation. vi Table of Contents Dedication ................................................................................................... ii Acknowledgements ................................................................................... iii List of Figures .......................................................................................... xii List of Tables ......................................................................................... xviii Abstract .....................................................................................................xx Chapter 1 Introduction .............................................................................. 1 1.1. Thermoelectric Microsystems Applications ............................................................ 4 1.1.1. Thermoelectric Micro Coolers .......................................................................... 4 1.1.2. Thermoelectric Micro Generators ..................................................................... 5 1.2. Dissertation Contributions and Outline ................................................................... 6 Chapter 2 Introduction to Thermoelectric Materials and Thermoelectric Thin Film Deposition ....................................................... 9 2.1. Principles of Thermoelectrics .................................................................................. 9 2.1.1. Seebeck, Peltier and Thomson Effects.............................................................. 9 2.1.2. Thermoelectric Figure of Merit ...................................................................... 12 2.1.3. Seebeck Coefficient ........................................................................................ 14 2.1.4. Electrical Resistivity ....................................................................................... 15 2.1.5. Thermal Conductivity ..................................................................................... 16 2.2. Thermoelectric Applications .................................................................................. 16 2.2.1. Thermoelectric Cooling .................................................................................. 17 2.2.2. Thermoelectric Power Generation .................................................................. 18 vii 2.3. Thermoelectric Materials ....................................................................................... 19 2.3.1. Bulk Thermoelectric Materials ....................................................................... 19 2.3.1.1. Silicon ...................................................................................................... 20 2.3.1.2. Lead Chalcogenides ................................................................................. 21 2.3.1.3. Skutterudites ............................................................................................ 21 2.3.1.4. Bismuth-Antimony Alloys ....................................................................... 22 2.3.1.5. Bismuth Telluride .................................................................................... 22 2.3.1.6. Antimony Telluride .................................................................................. 24 2.3.1.7. (Bi,Sb)Te-based Ternary Alloys .............................................................