gallium nitride resonators for infrared detector arrays and resonant acoustoelectric amplifiers

196 Pages·2014·9.91 MB·English

GALLIUM NITRIDE RESONATORS FOR INFRARED DETECTOR
ARRAYS AND RESONANT ACOUSTOELECTRIC AMPLIFIERS

by

Vikrant Jayant Gokhale

A dissertation submitted in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy
(Electrical Engineering)
in the University of Michigan
2014

Doctoral Committee:

Assistant Professor Mina Rais-Zadeh, Chair
Professor Yogesh Gianchandani
Professor Katsuo Kurabayashi
Professor Euisik Yoon
© Vikrant Jayant Gokhale
All rights reserved
2014
To
Family, Friends, and Teachers

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Acknowledgements
First and foremost I would like to thank my advisor, Professor Mina Rais-Zadeh,
for providing me the opportunity to pursue this research and supporting me through
the years. Her help and advice have been instrumental in achieving the results I write
about in this dissertation. I also thank my thesis committee members, Professor
Yogesh Gianchandani, Professor Katsuo Kurabayashi, and Professor Euisik Yoon for
their help and support as teachers and for the insightful comments I received from
them regarding my research.
I would like to take this opportunity to thank my group members in the Resonant
MEMS group. From the day we started our research, Yonghyun, Zhengzheng and
Vikram were the best of colleagues and friends. In subsequent years, Azadeh, Adam
and more recently Muzhi, Cesar, and Feng have continued that tradition. I was ably
supported by Yu Sui and Paul Myers, who worked with me for more than a year each.
I will be forever grateful for their help and support and for providing a friendly and
conducive research environment in the lab.
WIMS, the umbrella organization for research in Microsystems at the University
of Michigan, has played a strong role in my development as a MEMS researcher. This
work has benefited profusely due to the ancillary support provided through WIMS to
student members. Interactions with other WIMS students at official and semi-official
events have broadened the scope of my technical knowledge.
This work would not have been complete without the support, guidance and the
patience of the Lurie Nanofabrication Facility staff. I would like to thank them for
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bearing with me through the years and supporting my research efforts in the
cleanroom. I would also like to thank my colleagues from other research groups for
providing company, moral support, and process training in the cleanroom. The long
nights spent in the cleanroom were enjoyable to a large part due to their support and
friendship.
My friends at Michigan, many who started at the same time as I, have been a
constant source of support and camaraderie. Without them I would not have made it
through the long years. My friends from my time at the Vellore Institute of
Technology, in India, where I did undergraduate studies have remained my best
friends; extending their support for over ten years and from half a world away.
I would like to thank my extended family in Chicago who provided me a home
away from India whenever I wanted.
My girlfriend Lindsay has supported me, encouraged me, and helped me through
the tougher times for the last three years. Meeting her was one of the best things that
happened to me in Michigan. I cannot express how grateful I am for her being there
for me. Last and definitely not the least, I want to express my deepest gratitude and
love to my parents and brother who have always encouraged me to pursue my goals
and had the fullest confidence that I would succeed in my endeavors.

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Table of Contents

Dedication ................................................................................................................... ii
Acknowledgements ............................................................................................................ iii
List of Figures ................................................................................................................. ix
List of Tables ............................................................................................................ xxviii
List of Abbreviations ..................................................................................................... xxix
ABSTRACT .............................................................................................................. xxxi

Chapter 1 Introduction ............................................................................................... 1
1.1. Infrared Detectors: A Brief History ......................................................................... 1
1.2. Uncooled Infrared Detectors .................................................................................... 2
1.3. Uncooled Resonant Infrared Detectors .................................................................... 4
1.4. Comparison of Resonant Detectors with Contemporary Technology ..................... 5
1.5. Basic Principles of Operation of Devices in This Work .......................................... 7
1.5.1. Resonant Detector.............................................................................................. 8
1.5.2. IR Absorbing Coatings and Structures ............................................................ 10
1.5.3. Differential Operation and Detector Arrays .................................................... 12
1.6. Characterization of GaN Resonators ...................................................................... 14
1.7. Phonon-Electron Loss in GaN Resonators ............................................................. 15
1.8. Acoustoelectric Amplification in GaN BAW Resonators ...................................... 17
1.9. Contributions .......................................................................................................... 18
1.10. Organization of the Thesis ................................................................................... 19

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Chapter 2 Resonant IR Detectors and Arrays .......................................................... 21
2.1. Theoretical Model and Resonator Design .............................................................. 21
2.1.1. Thermal Dependencies of Resonator Frequency ............................................. 23
2.1.2. Design for Low NETD .................................................................................... 27
2.1.3. Thermal Design of the Resonator Element...................................................... 31
2.2. Prototype Detectors and Sense-Reference Pairs .................................................... 34
2.2.1. Fabrication Process .......................................................................................... 34
2.2.2. Prototype Measured Results ............................................................................ 35
2.2.3. Differential Sensing ......................................................................................... 37
2.3. Resonant IR Detector Arrays ................................................................................. 41
2.3.1. Measured Results of Small Formal Arrays ...................................................... 43
2.3.2. Response Time ................................................................................................ 46
2.4. IR Detector Arrays using Other Materials ............................................................. 48
2.4.1. AlN .................................................................................................................. 49
2.4.2. AlN-on-Si ........................................................................................................ 50
4.4.3. PZT-on-SiO .................................................................................................... 51
2
2.5. Expected Performance for Various Material Combinations .................................. 52
2.5.1. Dynamic Range ............................................................................................... 52
2.5.2. Temperature Coefficients of Offset & Sensitivity ........................................... 54
2.5.3. Packaging of Imaging Cores............................................................................ 54
2.5.4. Expected Performance for Various Material Combinations ............................ 55
2.6. Broadband CNT-ND Nanocomposite Absorbers ................................................... 58
2.6.1. Motivation: Thin, Broadband IR Absorbers .................................................... 58
2.6.2. Fabrication of CNT/DND/Polymer Nanocomposite ....................................... 59
2.6.3. Measured IR Absorption ................................................................................. 62
2.7. Narrowband Spectrally-Selective Plasmonic Metamaterial Absorbers ................. 65
2.7.1. Motivation ....................................................................................................... 65
2.7.2. Theory and Design........................................................................................... 66
2.7.3. Fabrication and Characterization ..................................................................... 67

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Chapter 3 Characterization of GaN Resonators ....................................................... 70
3.1. Need for Electromechanical Characterization of GaN ........................................... 70
3.2. Epitaxial GaN films ................................................................................................ 71
3.2.1. Epitaxial Growth.............................................................................................. 71
3.2.2. Crystal Quality ................................................................................................. 72
3.2.3. Dry Etching of GaN ......................................................................................... 73
3.3. Electromechanical Properties of GaN .................................................................... 74
3.3.1. Elastic and Hardness Moduli ........................................................................... 74
3.3.2. Acoustic Properties .......................................................................................... 75
3.4. Performance Metrics for GaN Resonant Devices .................................................. 77
3.4.1. Quality Factor and f (cid:3400)Q Limits...................................................................... 77
3.4.2. Coupling Efficiency and the keff2 (cid:3400)Q Metric ............................................... 79
3.4.3. Temperature Coefficient of Frequency ............................................................ 81
3.4.4. Power Handling and IIP ................................................................................. 82
3

Chapter 4 Phonon-Electron Interactions in Piezoelectric-Semiconductor BAW
Resonators ................................................................................................................. 84
4.1. Theory of Acoustoelectric Amplification in Resonators ........................................ 86
4.1.1. Standing Waves in Resonators ........................................................................ 86
4.1.2. Frequency Dependence of Gain and Loss ....................................................... 93
4.1.3. Other Loss Mechanisms in Piezoelectric Semiconductor BAW Resonators .. 96
4.1.4. Influence on Total Intrinsic Q ......................................................................... 98
4.1.5. Limiting Values of f(cid:3400)Q product .................................................................... 99
4.2. Experimental Findings ......................................................................................... 102
4.2.1. Fabrication ..................................................................................................... 102
4.2.2. Measurement Procedures and Controls ......................................................... 102
4.2.3. Analytical Extraction of Qm .......................................................................... 105
4.2.4. Equivalent Electrical Model Fitting .............................................................. 107
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4.3. Comparison of Experimental Data and Models ................................................... 109
4.4. Material Dependencies for Acoustoelectric Loss/Gain ........................................ 111
4.5. Experimental Controls .......................................................................................... 117
4.5.1. Effect of Contact Non-Linearity .................................................................... 117
4.5.2. Effect of RF power ........................................................................................ 119
4.5.3. Effect of Temperature .................................................................................... 120
4.6. Potential Materials for Acoustoelectric Amplification ........................................ 122
4.7. Limits of Acoustoelectric Amplification .............................................................. 124
4.8. Discussion ............................................................................................................ 126

Chapter 5 Summary and Future Work ................................................................... 128
5.1. Summary .............................................................................................................. 128
5.2. Future Research Directions .................................................................................. 129
5.2.1. Resonant IR Detector Arrays ......................................................................... 129
5.2.2. AlN-on-SiO with SiO Tethers ..................................................................... 131
2 2
5.2.3. Readout Circuits ............................................................................................ 133
5.2.4. Characterization using a Blackbody Source .................................................. 134
5.2.5. Quality Factor Improvement and its Effect on IR Sensitivity ....................... 135
5.2.6. Loss Mechanisms in Resonators.................................................................... 136
Appendix A- Derivation of Phonon-Electron Interactions for Travelling Waves in
Piezoelectric Semiconductors ......................................................................................... 138
Bibliography ............................................................................................................... 149

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List of Figures
Fig. 1.1: (a) Schematic of a photonic infrared detector using a mercury cadmium telluride
material system. Detection of IR radiation depends on the bandgap transition in the
semiconductor upon irradiation. For best results, the device must be cooled to low
temperatures. (b) Schematic of a microbolometer with silicon nitride membrane coated
with a vanadium dioxide layer. Vanadium dioxide changes its resistance as a function of
temperature, thus imparting this structure the ability to sense heat absorbed due to
infrared radiation. This is a popular design, and has proven performance as a low cost
uncooled detector. Images from [1]. ................................................................................... 3 
Fig. 1.2 : Comparison of contemporary, commercially available IR imagers using
uncooled bolometer arrays. Table from a review paper by Niklaus et al [6]. .................... 6 
Fig. 1.3 : (a) The resonant IR detector is a thin-film micromechanical resonator
mechanically suspended by thin tethers. The resonator body accounts for most of the
thermal mass of the device, while the tethers isolate the resonator thermally. The
resonator is coated with an IR absorber layer that efficiently absorbs incoming radiation.
(b) The transduction mechanism can be separated as the absorption of IR radiation
resulting in a temperature shift, resulting in a proportional frequency change for the
resonator. (c) A depiction of the reduction of frequency of a resonator due to IR
illumination. For most materials, increased temperature causes a reduction in frequency. 8 
Fig. 1.4 : Schematic showing the absorption, transmission and reflection of infrared
radiation by an IR absorber coating on the resonator substrate. The absorption can be
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