GALLIUM NITRIDE INTEGRATED MICROSYSTEMS FOR RADIO FREQUENCY APPLICATIONS by Azadeh Ansari A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Electrical Engineering) in the University of Michigan 2016 Doctoral Committee: Professor Mina Rais-Zadeh, Chair Professor Pei-cheng Ku Professor Kevin Pipe Professor Jasprit Singh © Azadeh Ansari All rights reserved 2016 To My Parents for their endless love ii Acknowledgement First of all, I would like to dearly thank my advisor, Professor Mina Rais-Zadeh, for her constant guidance and support. None of this work would have been possible without her help and patience. During my PhD years, she was not only a great mentor, but also a strong motivator and role model for me and I shall always remain thankful for her believing in me and providing me the opportunity to conduct research in this field. I am also thankful to my research committee, Prof. Jasprit Singh, a wonderful teacher of not only semiconductor physics but also wellness and life; Prof. Kevin Pipe and his research group for sharing their expertise in acoustic waves and thermal modeling of AlGaN/GaN HEMTs; and Prof. Peicheng Ku and his research group for their wisdom on GaN growth and material characterization. In the past five years, the resonant MEMS group has been a second home to me where I lived with my lab-mates and am grateful to them for making working insightful and fun, from when I first started, Yonghyun, Vikram, Vikrant, and Zhengzheng and in subsequent years Adam, Muzhi, Cesar, Mohsen and Milad. I would like to take the opportunity to particularly thank Vikrant who was my clean room mentor and taught me all the intricacies of micro/nano fabrication, which I had absolutely no background in; Lei Shao from Prof. Pipe group kindly shared his fabrication recipes of GaN HEMTs with me and was the person to reach out for when anything would go wrong with the HEMT fabrication; and Adam who simply made our lab more fun and was the helping hand when needed. Special thanks go to the postdoc members in our lab Dr. Roozbeh Tabrizian and Dr. Haoshen Zhu iii for their expertise in acoustics and all the technical discussions. I will be forever grateful for the help and support of all my colleagues for providing a friendly and productive research environment in the lab. It goes without saying that none of this research would have been accomplished was it not the help and assistance of the staff at the Lurie Nanofabrication Facility (LNF) at University of Michigan. I am very grateful that I had the opportunity to work at LNF and WIMS (center of wireless integrated micro-systems) at University of Michigan, where a very significant portion of the MEMS history has been shaped throughout the years. I would also like to express my gratitude to my dear friends in Ann Arbor, who made Ann Arbor home away from home for me. I cannot imagine how my life would have been if I did not share all the good moments and memories with them. Above all, I would like to express my deepest appreciation to my parents, Parvin and Mojtaba for their endless love and support. They are the ones who encouraged me to reach for my goals and aim high. I certainly owe all my accomplishments to them. A big thank you goes to my dearest grandmother, Tooran, who brought a smile on my face each and every time I would see her through Skype during the last five years. I dedicate this thesis to my family for their unconditional love. iv Table of Contents Dedication………………………………………………………………………………...ii Acknowledgement ............................................................................................................ iii List of Figures ................................................................................................................. viii List of Tables ............................................................................................................... xxvii Abstract ....................................................................................................................... xxviii Introduction ................................................................................................. 1 1.1 Motivation and Background ...................................................................................... 1 1.2 Piezoelectric Transduction ........................................................................................ 5 1.3 Quality Factor and 𝒇 ×𝑸 limits ............................................................................... 8 1.4 Coupling Efficiency and the 𝒌𝒆𝒇𝒇𝟐 ×𝑸 Metric ................................................... 11 1.5 Temperature Coefficient of Frequency ................................................................... 12 1.6 Power Handling Capability ..................................................................................... 14 1.7 Research Objectives & Contributions ..................................................................... 14 5.1 Organization of Thesis ....................................................................................... 16 Electromechanical Material Properties and Process Technology of Gallium Nitride Thin Films............................................................................................ 21 2.1 Motivation, background and Challenges ................................................................. 22 2.2 Electro-mechanical Properties of GaN .................................................................... 24 2.3 Crystallinity of GaN on Si ....................................................................................... 28 2.4 GaN-on-SOI Epitaxial Growth................................................................................ 32 2.5 Fabrication of GaN MEMS ..................................................................................... 33 2.6 GaN Resonators with Embedded Meshed Metal Bottom Electrode ....................... 37 2.7 Conclusion ............................................................................................................... 42 Gallium Nitride MEMS Devices .............................................................. 46 3.1 GaN-on-Si thickness-mode MEMS resonators and filters ...................................... 47 3.2 GaN Thickness-mode MEMS Resonators .............................................................. 54 3.3 GaN Contour-mode MEMS Resonators ................................................................. 58 v 3.4 GaN Radial (Breathing)-mode Coupled Ring Resonators ...................................... 59 3.5 Conclusion ............................................................................................................... 59 Integration of GaN MEMS, HEMTs and Resonant Body HEMTs ..... 62 1.1 Intimate Integration of GaN MEMS Resonators and HEMTs: Two Approaches .. 63 1.2 GaN Resonator/HEMT Device Modelling .............................................................. 66 1.3 HEMT Characterization .......................................................................................... 66 1.4 Integrated GaN Resonators/HEMT Characterization ............................................. 68 1.5 Temperature Compensation of GaN MEMS Resonators ........................................ 70 1.6 The Resonant Body Transistor ................................................................................ 74 1.7 RB-HEMT Modeling .............................................................................................. 77 1.8 RB-HEMT Measurement Results ........................................................................... 80 1.9 RB-HEMT Analytical Modelling............................................................................ 87 1.10 Conclusion ............................................................................................................. 92 Depletion-mediated AlGaN/GaN Piezoelectric Resonators and Resonant HEMTs ............................................................................................................ 96 5.1 A Brief History ................................................................................................... 97 5.2 Design and Characterization .............................................................................. 98 5.3 Effect of Electric Field on Acoustic Properties .................................................. 99 5.4 Q Enhancement ................................................................................................ 108 5.5 Standard GaN piezoelectric resonators with lateral electric field excitation ... 112 5.6 Optimization of the geometry of acoustic cavity ............................................. 114 5.7 Phonon Trap Design ......................................................................................... 115 5.8 Device Characterization ................................................................................... 117 5.9 Forward-biased Schottky IDTs ........................................................................ 121 5.10 Embedded Resonant AlGaN/GaN HEMTs ...................................................... 124 5.11 Resonant HEMTs: Design and Characterization ............................................. 126 5.12 Q Enhancement Mechanism in Resonant HEMTs ........................................... 129 5.13 Thermal Modelling ........................................................................................... 130 5.14 Conclusion ........................................................................................................ 134 vi Chapter 5: References ................................................................................................. 135 Big Picture & Future Directions ............................................................ 137 vii List of Figures Figure 1.1 The combined need for digital and non-digital functionalities in an integrated system is translated as a dual trend in the International Technology Roadmap for Semiconductors (ITRS): miniaturization of the digital functions (“More Moore”) and functional diversification (“More-than-Moore”). Image taken from ITRS [1.1]. .............. 2 Figure 1.2 GaN HEMTs and power transistors find diverse applications in RF, microwave and satellite communication. Image taken from Cree (Wolf Speed) [1.3]. ........................ 3 Figure 1.3 (a) A simplified schematic of a front-end transmitter based on all-GaN modules. (b) A schematic of a GaN MEMS/HEMT-based pierce oscillator. .................................... 4 Figure 1.4 COMSOL simulation of displacement of an exemplary resonator showing (a) fundamental length-extensional mode, (b) cross section of a thickness-extensional resonance mode. .................................................................................................................. 7 Figure 1.5 (a) Vertical electric field applied between the top and the bottom electrode, and (b) lateral electric field between two adjacent electrodes. (c) This work: electric field applied between the top electrode and an embedded bottom electrode. ............................. 8 Figure 1.6 Theoretical 𝑓×𝑄 limits of GaN showing Akhieser and L-R regime for phonon- phonon loss limit (black lines), phonon-electron loss limit at different carrier density levels (blue lines) and TED limit (red line) [1.15]. ..................................................................... 10 viii Figure 1.7 A simple schematic of an RF transceiver module. GaN power amplifiers (PAs) are already being used in base stations. Future direction of research on GaN includes realization of novel electroacoustic devices (e.g. acoustic circulators), and integration of all GaN components to build an integrated GaN MMIC transceiver module. ................. 15 Figure 1.8 A simplified cross section schematic of my thesis organization with a single device representing the main idea in each chapter. Chapter 2 involves the growth and fabrication of such structures. ........................................................................................... 18 Figure 2.3 GaN stiffness matrix components vs. temperature. Temperature coefficient of stiffness components are extracted for C , C , C , C and C for GaN Wurtzite 11 33 12 13 44 crystalline structure. .......................................................................................................... 27 Figure 2.1 Ga-face and N-face crystal structure of GaN.GaN thin films grown by MOCVD yield Ga-face, whereas MBE-grown GaN yields a N-face crystalline structure [2.9]. .... 29 Figure 2.2 XRD spectroscopy on a GaN thin film grown on a Si (111) substrate using metal-organic chemical vapor deposition (MOCVD). Inset: Rocking curve of the (0002) GaN plane exhibits a very clear peak and a FWHM of 1296 arcsec [2.4]. ...................... 30 Figure 2.4 Schematic of the epi-stack of MOCVD-grown AlGaN/GaN heterostructure on Si (111) by Nitronex Corporation. The thickness of the transition and buffer layers are indicated. 2DEG is induced at the AlGaN/GaN interface due to spontaneous and piezoelectric polarization. The peak charge concentration of 2DEG measured with Hg ix