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This report presents the first demonstration of passive RF comb filters made using epitaxial GaN/NbN/SiC high overtone bulk acoustic resonators (epi-HBARs). The two-port device is fabricated on electronic-grade GaN, electrically transduced, and acoustically coupled. The multi-mode epi-HBAR comb filter demonstrated here has 158 sharp filter passbands periodically distributed between 1 and 4 GHz (L–S-bands) with a free spectral range (FSR) of 17 MHz. The individual passbands of the epi-HBAR comb filter demonstrate transmission bandwidths (BWs) up to 800 kHz, ${f} {\times } {Q}$ values of up to $7\times 10^{{14}}$ Hz, and an average ${k}_{ {\text {eff}}}^{ {{2}}} {\times } {Q}$ figure of merit of 41.2 at room temperature. The GaN/NbN/SiC epi-HBAR comb filter is capable of operating at high RF power levels, with linear and distortion-free performance seen up to at least 1 W of continuous wave (CW) power and up to at least 10 W of pulsed power. The compact epi-HBAR comb filters can be co-fabricated with GaN-based electronics and could potentially replace larger, off-chip or discrete-component comb filters. They can be used for spectrum sensing and as signal processing elements for remote sensing and pulsed radar.

We present analytic and experimental evidence highlighting the importance of acoustic impedance matching for efficient power transfer in RF-MEMS composite resonators such as high-overtone bulk acoustic mode resonators (HBARs) and thin-film piezoelectric on substrate (TPoS) resonators. We show that materials used for the piezoelectric film and the bottom metal electrode in a composite resonator can be chosen or tailored for specific low-loss substrates, resulting in efficient acoustic power transmission across the interfaces of the acoustic source (piezoelectric transducer), intermediate layers including the bottom electrode, and into the acoustic cavity (substrate). We find that a composite resonator with good interfacial acoustic matching exhibits characteristic free spectral range (FSR) variations that are not well modeled in the literature, clearly differentiating it from resonators with poor acoustic matching. We verify this model by comparing the FSR spectra of the first experimentally demonstrated epitaxially grown Sc0.18Al0.82N/AlN/TaN/SiC HBARs (with a mismatched TaN bottom electrode) with epitaxial GaN/AlN/NbN/SiC HBARs where all constituent layers are acoustically matched to the substrate. Historically, the choice and quality of materials used for composite resonators has been limited by process constraints, but advances in epitaxial growth and heterogeneous integration techniques allow us to integrate multiple high quality, acoustically matched layers to form multi-functional composite resonators. [2020-0247]

The optical knife-edge displacement measurement technique can be used to quantify in-plane vibrations of microstructures at radio frequencies. This paper presents an analytical model and experimental results for this technique that demonstrate precise displacement measurements for electrostatic microelectromechanical resonators at frequencies ranging from 13 MHz to 895 MHz. It is also shown that high-resolution spatial mapping of displacement mode shapes for fundamental and higher order vibration modes can be achieved. The optical knife-edge measurements have a resolution as low as 455 fm/ $\surd$ Hz at 13.6 MHz, and under 1 pm/ $\surd$ Hz up to 1.4 GHz. This paper expands the capabilities of the knife-edge technique by working with all types of in-plane microelectromechanical resonators, improving the resolution by at least a factor of 2, and increasing the frequency range by a factor of 60. [2018-0094]

Gallium nitride (GaN) is a wide bandgap semiconductor material and is the most popular material after silicon in the semiconductor industry. The prime movers behind this trend are LEDs, microwave, and more recently, power electronics. New areas of research also include spintronics and nanoribbon transistors, which leverage some of the unique properties of GaN. GaN has electron mobility comparable with silicon, but with a bandgap that is three times larger, making it an excellent candidate for high-power applications and high-temperature operation. The ability to form thin-AlGaN/GaN heterostructures, which exhibit the 2-D electron gas phenomenon leads to high-electron mobility transistors, which exhibit high Johnson's figure of merit. Another interesting direction for GaN research, which is largely unexplored, is GaN-based micromechanical devices or GaN microelectromechanical systems (MEMS). To fully unlock the potential of GaN and realize new advanced all-GaN integrated circuits, it is essential to cointegrate passive devices (such as resonators and filters), sensors (such as temperature and gas sensors), and other more than Moore functional devices with GaN active electronics. Therefore, there is a growing interest in the use of GaN as a mechanical material. This paper reviews the electromechanical, thermal, acoustic, and piezoelectric properties of GaN, and describes the working principle of some of the reported high-performance GaN-based microelectromechanical components. It also provides an outlook for possible research directions in GaN MEMS.

This paper presents the analysis, design, fabrication, and the first measured results demonstrating the use of gallium nitride (GaN)-based micromechanical resonator arrays as high-sensitivity, low-noise infrared (IR) detectors. The IR sensing mechanism is based on monitoring the change in the resonance frequency of the resonators upon near IR radiation. The resonators are characterized for their RF and thermal performance and exhibit a radiant responsivity of 1.68%/W, thermal time constant on the order of 556 μs, and an average IR responsivity of -1.5% when compared with a reference resonator, for a 100 mK radiation-induced temperature rise. An analysis of the design of the devices is presented as a path toward better design, specifically, for low thermal noise equivalent temperature difference in the long wavelength IR spectrum.

We report on the characterization of thin-film near and short wavelength infrared absorbers comprised of carbon nanotubes dispersed in a polymer. Charged nanodiamond particles are used to effectively and uniformly disperse the carbon nanotubes in the polymer matrix, leading to a very homogenous film. Using this new technique, we demonstrate an infrared absorption of up to 95% in films with thicknesses . This remarkably high absorption is the result of low reflection off the surface and high absorption across the film thickness. The complex refractive index of the films is extracted using an effective media approximation. Calculations show the film has a wide angle for high absorption and is polarization independent. These films are easy to fabricate, robust and damage-resistant, and are compatible with post-processing techniques. These films can be used as the coating layer to boost the efficiency of uncooled infrared sensors and solar-thermal energy harvesters.