Vikrant J. Gokhale

Gallium Nitride as an Electromechanical Material

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 Johnsons 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.

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.

Uncooled Infrared Detectors Using Gallium Nitride on Silicon Micromechanical Resonators

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.

Infrared Absorption Properties of Carbon Nanotube/Nanodiamond Based Thin Film Coatings

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.

Phonon-Electron Interactions in Piezoelectric Semiconductor Bulk Acoustic Wave Resonators

This work presents the first comprehensive investigation of phonon-electron interactions in bulk acoustic standing wave (BAW) resonators made from piezoelectric semiconductor (PS) materials. We show that these interactions constitute a significant energy loss mechanism and can set practical loss limits lower than anharmonic phonon scattering limits or thermoelastic damping limits. Secondly, we theoretically and experimentally demonstrate that phonon-electron interactions, under appropriate conditions, can result in a significant acoustic gain manifested as an improved quality factor (Q). Measurements on GaN resonators are consistent with the presented interaction model and demonstrate up to 35% dynamic improvement in Q. The strong dependencies of electron-mediated acoustic loss/gain on resonance frequency and material properties are investigated. Piezoelectric semiconductors are an extremely important class of electromechanical materials, and this work provides crucial insights for material choice, material properties, and device design to achieve low-loss PS-BAW resonators along with the unprecedented ability to dynamically tune resonator Q.

Monolithic integration of GaN-based micromechanical resonators and HEMTs for timing applications

A platform for intimate integration of high-frequency gallium nitride (GaN) micromechanical resonators and AlGaN/GaN high electron mobility transistors (HEMTs) is reported. For the first time, cascade of a two-port GaN bulk acoustic resonator and AlGaN/GaN HEMT was co-fabricated on a silicon substrate. A high quality factor (Q) of 7413 is reported for a GaN contour-mode resonator at the resonance frequency of 119.8 MHz. More than 30 dB of signal tuning was achieved by using integrated HEMT for signal readout and amplification at the resonator output.

Novel uncooled detector based on gallium nitride micromechanical resonators

This work presents measured results demonstrating an uncooled infrared (IR) detector based on gallium nitride (GaN) micromechanical resonators. GaN-based photonic detectors are typically designed to operate in the ultraviolet (UV) regime as the absorption spectrum of wide-band gap GaN peaks at a wavelength of ~360 nm. In contrast, the transduction mechanism of the device presented in this work is the pyroelectric perturbation of a GaN micromechanical resonator, allowing the detection of radiation in the IR regime. IR radiation within the absorption spectrum of the resonating stack material (mainly the IR absorber) is converted into heat causing pyroelectric charge release, which in turn shifts the resonant frequency via changes in the acoustic velocity of GaN. A thin-film IR absorber based on carbon-nanotube nanocomposite is proposed, which offers IR absorptivity of more than 95%. As a proof of concept, we demonstrate a GaN resonant detector operated at 119 MHz, which exhibits an IR sensitivity of ~4 Hz/10nW.

Observation of the acoustoelectric effect in gallium nitride micromechanical bulk acoustic filters

We report on the experimental verification of the acoustoelectric effect in gallium nitride (GaN) and present a model to describe this effect in GaN thickness-mode bulk acoustic filters. Filters are fabricated using 2.2 µm thick n-type GaN on high resistivity silicon epiwafers obtained from SOITEC. Acoustoelectric effect was observed by applying an electric field parallel to c-axis, the direction of acoustic wave propagation. Improvement in the insertion loss and out-of-band rejection was observed and Q amplifications exceeding 240% was achieved. Acoustoelectric effect makes it possible to dynamically tune the frequency response of GaN resonators and filters.

Subwavelength plasmonic absorbers for spectrally selective resonant infrared detectors

This work presents the first resonant infrared (IR) detectors with integrated nanostructured subwavelength plasmonic gratings designed to selectively absorb long wavelength infrared (LWIR) radiation. The resonant detectors are the smallest in size demonstrated so far. The absorbers are optimized for a spectral wavelength of 10.15 μm and experimentally demonstrate an absorbance of 46% with a Full Width at Half Maximum (FWHM) of 1.7 μm. The absorbed thermal energy causes a fast (sub-millisecond) proportional change in the frequency of the resonator. The combination of resonant IR detectors with integrated plasmonic absorbers enables spectrally selective IR detectors. Each detector in the array can be optimized for different wavelengths, thus enabling a multi-spectral array for IR spectroscopy and multispectral thermal imaging.

Approaching the intrinsic quality factor limit for micromechanical bulk acoustic resonators using phononic crystal tethers

We systematically demonstrate that one-dimensional phononic crystal (1-D PnC) tethers can significantly reduce tether loss in micromechanical resonators to a point where the total energy loss is dominated by intrinsic mechanisms, particularly phonon damping. Multiple silicon resonators are designed, fabricated, and tested to provide comparisons in terms of the number of periods in the PnC and the resonance frequency, as well as a comparison with conventional straight-beam tethers. The product of resonance frequency and measured quality factor (f×Q) is the critical figure of merit, as it is inversely related to the total energy dissipation in a resonator. For a wide range of frequencies, devices with PnC tethers consistently demonstrate higher f×Q values than the best conventional straight-beam tether designs. The f×Q product improves with increasing number of PnC periods, and at a maximum value of 1.2 × 1013 Hz, approaches limiting values set by intrinsic material loss mechanisms.