Vertical Self-Defined Thermoelectric Legs for Use in Thin- Film Micro Thermo Electric Generators (µTEG) by Yi Yuan A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Electrical Engineering) in The University of Michigan 2019 Doctoral Committee: Professor Khalil Najafi, Chair Dr. Baoxing Chen Professor Rebecca L. Peterson Professor Jamie Phillips Professor Citrad Uher Yi Yuan [email protected] ORCID iD: 0000-0003-1975-8455 © Yi Yuan 2019 Dedication To my parents. ii Acknowledgements There are a countless number of people who made this work possible; this section is by no means a comprehensive list. My advisor, Professor Khalil Najafi has provided vital guidance and support throughout my graduate studies. To my parents, Chunhui Pan and Zhe Yuan, who have provided a different, but equally vital, source of guidance and support. I would like to thank Dr. Baoxing Chen of Analog Devices for his support of thermoelectric research at Najafi group, and my thesis committee in general. I would also like to thank the numerous staff and engineers at the Lurie Nanofabrication facility and in the Electronical Engineering department. Their consummate professionalism kept the entire place running, allowing the work of every graduate student. I would also like to thank my lab mates, both in Najafi group and the clean room. They have been an immeasurable source of help throughout the years. I would like to especially thank Niloufar, Guohong, and Zongliang, who all provided the initial mentorship necessary for a successful thesis project. To my friends, who reminded me life existed outside the lab, and to Yemin Tang for her company and love. I am happy to have met her in Ann Arbor. iii Table of Contents Dedication………………………………………………………………ii Acknowledgements……………………………….…………………...iii List of Figures………………………………………………………….x List of Tables……………………………………………………...…..xx Abstract………………………………………………….……….....xxiii Chapter 1 Introduction ........................................................................... 1 1.1 Micro-Thermoelectric Generators and Coolers ..................................................... 3 Micro-Thermoelectric Generators ........................................................................................... 3 Micro-Thermoelectric Coolers ................................................................................................ 5 1.2 Emerging μTEG Applications .................................................................................... 6 1.3 Goals and Motivation ................................................................................................. 6 1.4 Thesis Contributions .................................................................................................. 8 1.5 Outline ...................................................................................................................... 10 Chapter 2 Background on Thermoelectrics ....................................... 12 2.1 Thermoelectric Phenomenon ................................................................................... 12 The Seebeck effect ................................................................................................................ 12 Peltier effect........................................................................................................................... 14 Thomson effect ...................................................................................................................... 15 Nernst–Ettingshausen effect .................................................................................................. 16 iv 2.2 Thermoelectric Materials ......................................................................................... 16 Material Figure of Merits ...................................................................................................... 16 Seebeck Coefficient ............................................................................................................... 17 Electrical Conductivity .......................................................................................................... 19 Thermal Conductivity ............................................................................................................ 20 2.3 Thermoelectric Materials ......................................................................................... 21 Chalcogenides........................................................................................................................ 22 Bismuth telluride and Antimony Telluride............................................................................ 23 TAGS ..................................................................................................................................... 25 Traditional Semiconductors ................................................................................................... 25 2.4 Nanostructured Materials ......................................................................................... 26 Quantum Confinement .......................................................................................................... 27 Chapter 3 Micro-Thermoelectric Generators .................................... 28 3.1 Ideal Generator Power Output ................................................................................. 29 3.2 Source of Loss .......................................................................................................... 32 Thermal Losses ...................................................................................................................... 32 Electrical Resistance .............................................................................................................. 34 3.3 Optimal μTEG Leg Dimensions .............................................................................. 36 3.4 The Need for Longer Thermocouples (TE legs) ...................................................... 37 3.5 High-thermal resistance designs .............................................................................. 38 Lateral Designs ...................................................................................................................... 39 Alternative Deposition Methods............................................................................................ 41 3.6 Reported μTEGs ....................................................................................................... 43 3.7 Commercial Devices ................................................................................................ 45 3.8 Conclusion ................................................................................................................ 47 Chapter 4 High-Aspect Vertical Thermocouples for use in μTEGs 48 v 4.1 Improving Micro-Thermoelectric Generator Performance ...................................... 48 4.2 High-Aspect Vertical Thermocouples .................................................................. 49 4.3 Modeling .................................................................................................................. 50 Model Parameters .................................................................................................................. 53 Modeled Output ..................................................................................................................... 54 Optimal Leg Height and Film Thickness .............................................................................. 56 4.4 Comparison with Traditional Thin Film Designs .................................................... 58 Contacts and Interconnects .................................................................................................... 59 4.5 Composite BiSbTe Material ..................................................................................... 61 4.6 Other Design Considerations and Effects ................................................................ 62 Film Matching ....................................................................................................................... 62 Peltier Effect .......................................................................................................................... 64 Thermal Considerations......................................................................................................... 64 Scaffold Conduction .............................................................................................................. 64 Bond Ring .............................................................................................................................. 65 Si Thinning ............................................................................................................................ 66 4.7 Conclusion ................................................................................................................ 67 Chapter 5 Thermal Co-Evaporation of Bi2Te3/Sb2Te3 ...................... 68 5.1 Deposition System.................................................................................................... 71 5.2 Evaporation Process ................................................................................................. 73 Reproducibility ...................................................................................................................... 74 Film Thickness Limitations ..................................................................................................