Gallium Nitride Resonators for Infrared Detector Arrays and Resonant Acoustoelectric Amplifiers.

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

Gallium nitride-on-silicon micromechanical overtone resonators and filters

In this paper, for the first time, we report on high-performance GaN-on-silicon micromechanical resonators and filters. A GaN-on-silicon resonator is reported which exhibits a quality factor of 1850 at 802.5 MHz, resulting in an f×Q value twice the highest reported for GaN-based resonators to date. The effective coupling coefficient for the GaN resonator is extracted to be 1.7%, which is among the best reported in the literature

A thin-film infrared absorber using CNT/nanodiamond nanocomposite

This paper reports on the fabrication and characterization of thin-film nanocomposites comprised of tangled carbon nanotubes in a polymer matrix. The density of nanotubes in the polymer was significantly increased using detonation nanodiamonds. Nanodiamonds reduce the surface forces between the polymer and the nanotubes and mitigate the agglomeration problem of nanotubes in polymer. This resulted in thinner, more uniform networks that serve as efficient absorbers of infrared energy over a broad spectrum, ranging from the visible to the mid-wavelength infrared. An infrared absorbance of 97 % was achieved for a 1.6 µm thick nanocomposite film across the spectral range of 714 nm to 5 µm. The films are electrically insulating, mechanically and thermally stable up to 300 ˚C, and can be integrated with microbolometers to enhance their responsivity

Effects of growth temperature on electrical properties of GaN/AlN based resonant tunneling diodes with peak current density up to 1.01 MA/cm2

Identical GaN/AlN resonant tunneling diode structures were grown on free-standing bulk GaN at substrate temperatures of 760 °C, 810 °C, 860 °C, and 900 °C via plasma-assisted molecular beam epitaxy. Each sample displayed negative differential resistance (NDR) at room temperature. The figures-of-merit quantified were peak-to-valley current ratio (PVCR), yield of the device with room-temperature NDR, and peak current density (Jp). The figures-of-merit demonstrate an inverse relationship between PVCR/yield and Jp over this growth temperature series. X-ray diffraction and transmission electron microscopy were used to determine the growth rates, and layer thicknesses were used to explain the varying figures-of-merit. Due to the high yield of devices grown at 760 °C and 810 °C, the PVCR, peak voltage (Vp), and Jp were plotted vs device area, which demonstrated high uniformity and application tunability. Peak current densities of up to 1.01 MA/cm2 were observed for the sample grown at 900 °C.publishedVersionPeer reviewe