This work presents the first comprehensive utilization of Gallium Nitride (GaN) in high-performance, high-frequency micromechanical resonators. It presents characterization of critical electromechanical properties of GaN and validation of high-performance designs.
The primary motivation behind this project is the use of GaN resonators as sensitive, low-noise, uncooled infrared (IR) detectors. IR response of micromechanical resonators is based on radiative absorption and a consequent shift in its resonant frequency. Mechanical resonators are expected to perform better than contemporary uncooled IR detectors as the noise equivalent temperature difference (NETD) is primarily limited by each resonator’s thermomechanical noise, which is smaller than resistive bolometers. GaN is an ideal material for resonant IR detection as it combines piezoelectric, pyroelectric, and electrostrictive properties that lead to a high IR sensitivity up to -2000 ppm/K (~ 100× higher than other materials). To further improve IR absorption efficiency, we developed two thin-film absorbers: a carbon nanotube (CNT)-polymer nanocomposite material with broad-spectrum absorption efficiency (> 95%) and a plasmonic absorber with narrow-spectrum absorption (> 45% for a select wavelength) integrated on the resonator. Designs have also been successfully implemented using GaN-on-Si, aluminum nitride (AlN), AlN-on-Si, and lead-zirconate-titanate (PZT), and fabricated both in-house and using commercial foundry processes. Resonant IR detectors, sense-reference pairs, and small-format arrays (16 elements) are successfully implemented with NETD values of 10 mK, and ~1 ms-10 ms response times.
This work also presents the first measurements and analysis of an exciting, fairly unexplored phenomenon: the amplification of acoustic standing waves in GaN resonators using electrical energy, boosting the quality factor (Q) and reducing energy losses in the resonator. This phenomenon is based on phonon-electron interactions in piezoelectric semiconductors. Under normal conditions this interaction is a loss mechanism for acoustic energy, but as we discovered and consistently demonstrated, it can be reversed to provide acoustoelectric amplification (resulting in Q-amplification of up to 35%). We present corroborated analytical and experimental results that describe the phonon-electron loss/gain in context with other loss mechanisms in piezoelectric semiconductor resonators. Research into this effect can potentially yield insights into fundamental solid-state physics and lead to a new class of acoustoelectric resonant amplifiers.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/108759/1/vikrantg_1.pd
