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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]

We report on phonon diffraction limited performance of GaN/AlN/NbN/SiC epitaxial HBARs (epi-HBARs) that act as Fabry-Perot acoustic cavities with parallel reflectors. In a series of devices with varying transducer diameter (∅), we find that the smallest epi-HBARs (∅=22.5 μm on a 350 μm thick substrate) are indeed limited by diffraction, yet achieve f×Q ≈1012 Hz at 7.2 K. Near the diffraction limit, f×Q≈ 1014 Hz for ∅ >50 μm, and for larger devices that are not diffraction limited, f×Q≈ 2×1016 Hz. We discuss the practical tradeoffs involved in using such high performance electrically transduced epi-HBARs compared to confocal HBARs with an engineered curved surface that are popular for phonon-qubit coupling in quantum acoustodynamic (QAD) systems.

Integrating femtosecond (fs) lasers to electron microscopies has enabled direct imaging of transient structures and morphologies of materials in real time and space, namely, ultrafast electron microscopy (UEM). Here we report the development of a laser-free UEM offering the same capability of real-time imaging with high spatiotemporal resolutions but without requiring expensive fs lasers and intricate instrumental modifications. We create picosecond electron pulses for probing dynamic events by chopping a continuous beam with a radiofrequency (RF)-driven pulser, where the repetition rate of the electron pulses is tunable from 100 MHz to 12 GHz. A same broadband of electromagnetic wave is enabled for sample excitation. As a first application, we studied the GHz electromagnetic wave propagation dynamics in an interdigitated comb structure which is one of the basic building blocks for RF micro-electromechanical systems. A series of pump-probe images reveals, on nanometer space and picosecond time scales, the transient oscillating electromagnetic field around the tines of the combs, and time-resolved polarization, amplitude, and nonlinear local field enhancement. The success of this study demonstrates the feasibility of the low-cost laser-free UEM in real-space visualizing of dynamics for many research fields, especially the electrodynamics in devices associated with information processing technology., 25 pages, 4 figures

Heterogeneous integration of complementary materials and device technologies is a demonstrated pathway for meeting the demand for next generation RF and mixed-signal circuits and has historically been accomplished via chip or circuit level wafer bonding and through-substrate vias [1] . A more intimate approach is integration at the device level via a micro-assembly technique such as micro-transfer printing [2] , which uses a polymer stamp to pick-and-place individual devices released from a source substrate to a multi-technology target substrate with micron-level alignment accuracy. This approach decouples the device technology from the growth substrate and enables technology agnostic circuit design and application-specific substrate choice. Here we demonstrate micro-transfer printing of GaN high-electron-mobility transistors (HEMTs) released from SiC growth substrates to other technologically relevant substrates such as Si and diamond. We show that there is no significant degradation in DC electrical characteristics after transfer printing, improved thermal performance can be achieved when the devices are transferred to single crystal diamond, and that post-transfer processing, such as interconnect metallization is possible with standard 2D lithographic techniques.

A 300 keV transmission electron microscope was modified to produce broadband pulsed beams that can be, in principle, between 40 MHz and 12 GHz, corresponding to temporal resolution in the nanosecond to picosecond range without an excitation laser. The key enabling technology is a pair of phase-matched modulating and de-modulating traveling wave metallic comb striplines (pulsers). An initial temporal resolution of 30 ps was achieved with a strobe frequency of 6.0 GHz. The placement of the pulsers, mounted immediately below the gun, allows for preservation of all optical configurations, otherwise available to the unmodified instrument, and therefore makes such a post-modified instrument for dual-use, i.e., both pulsed-beam mode (i.e., stroboscopic time-resolved) and conventional continuous waveform mode. In this article, we describe the elements inserted into the beam path, challenges encountered during integration with an in-service microscope, and early results from an electric-field-driven pump–probe experiment. We conclude with ideas for making this class of instruments broadly applicable for examining cyclical and repeatable phenomena.

This report presents some of the first experimental characterization of surface acoustic wave (SAW) devices using single-crystal ScAlN epitaxially grown on SiC. Due to the excellent wave guiding provided by the ScAlN/SiC heterostructure, SAW phase velocities greater than 12,000 m/s are measured, higher than comparable ScAlN SAW devices on other substrates. The phase velocity dispersion for measured devices compares well with simulated values. We observe up to $k^{2}=0.52{\%}$ even for very small thickness to wavelength ratios ( $t/\lambda ). We show that epitaxial ScAlN/SiC can achieve extremely low SAW propagation loss ( $\alpha ), comparable to state-of-the-art piezoelectric/diamond SAW devices, and are linear at CW RF power levels up to ≈30 dBm (1W), with 1 dB gain compression at 34 dBm and an IIP3 of 45 dBm.

Solid-state quantum acoustodynamic (QAD) systems provide a compact platform for quantum information storage and processing by coupling acoustic phonon sources with superconducting or spin qubits. The multi-mode composite high-overtone bulk acoustic wave resonator (HBAR) is a popular phonon source well suited for QAD. However, scattering from defects, grain boundaries, and interfacial/surface roughness in the composite transducer severely limits the phonon relaxation time in sputter-deposited devices. Here, we grow an epitaxial-HBAR, consisting of a metallic NbN bottom electrode and a piezoelectric GaN film on a SiC substrate. The acoustic impedance-matched epi-HBAR has a power injection efficiency >99% from transducer to phonon cavity. The smooth interfaces and low defect density reduce phonon losses, yielding (f × Q) and phonon lifetimes up to 1.36 × 1017 Hz and 500 µs respectively. The GaN/NbN/SiC epi-HBAR is an electrically actuated, multi-mode phonon source that can be directly interfaced with NbN-based superconducting qubits or SiC-based spin qubits., Acoustic resonators may find application for qubit coupling in compact quantum information and processing systems. Here the authors show a multi-phonon source with high quality factors and long phonon lifetimes via epitaxial high-overtone bulk acoustic resonators.

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]

This work demonstrates the first on-chip integration of a high overtone bulk acoustic resonator (HBAR) with a high electron mobility transistor (HEMT) using an epitaxial AlGaN/GaN/NbN heterostructure on a SiC substrate. This pairing combines the robust structure, periodic mode spacing, high mode density, and high quality factor (Q) of the HBAR with the amplification and nonreciprocal characteristics of the HEMT. From L-band to X-band, the HBAR exhibits among the highest Q and f×Q products reported with Q > 104 and f×Q > 1014 at 295 K, and Q > 106 and f×Q > 1015 at 20K. The drain-connected HEMT+HBAR pair demonstrates 34 dB On/Off ratio and 16 dB directional contrast (both at 3 GHz), while retaining the signature periodic spacing and mode density of the HBAR. HEMT+HBAR devices can be building blocks for comb filters, circulators, and sparse spectrum front-ends. The TMN/III-N heterostructure (with superconducting, semiconducting, and piezoelectric properties) and excellent cryogenic performance also indicates enormous potential as an integrated quantum platform for computation, communications, sensing, and metrology.

Epitaxial integration of perovskite oxide materials with GaN has unlocked the potential to improve functionality and performance in high-power RF and power-switching applications. In this work, we demonstrate structural and electrical properties of high dielectric constant Sr1−xCaxTiO3 epitaxial layers grown on AlGaN/GaN/4H-SiC high-electron-mobility transistor structures with compositions ranging from x = 0 to x = 0.53 and oxide film thicknesses ranging from 7 to 126 nm. We show (111) orientation in the SrCaTiO3 (SCTO) thin films using a 1 nm (100) TiO2 buffer layer grown by RF-plasma-assisted oxide molecular beam epitaxy. Current–voltage measurements show up to 5 orders of magnitude reduced leakage with SCTO films when compared to Schottky contacted samples. Capacitance–voltage measurements show minimal hysteresis, an extracted dielectric constant (κ) as high as 290, and a fixed positive interface charge density of 2.38 × 1013 cm−2 at the SCTO/AlGaN interface. The direct integration of the SCTO layer does not significantly affect the two-dimensional electron gas (2DEG) density or the channel mobility with the 2DEG density as a function of SCTO thickness having good agreement with 1D Poisson–Schrodinger simulations. RF characterization of interdigitated capacitors using the SCTO films on unintentionally doped GaN/SiC shows that the films maintain their high κ into microwave frequencies and only exhibit a slight reduction in κ with increased lateral electric fields. These results demonstrate that the integration of a high-κ oxide with GaN can potentially improve electric field management in RF high-electron-mobility transistors and increase the device breakdown voltage without significant degradation to channel transport properties.

Ga2O3 has emerged as a promising material for next generation power electronics. While β-Ga2O3 (monoclinic) is the most stable and studied of six Ga2O3 polymorphs, the slightly less energetically favorable α-, ε-, and κ-Ga2O3 phases have unique characteristics that can be exploited such as larger bandgaps, alloying for dopant control, or polarization beneficial to the formation of two-dimensional electron gas (2DEG) channels. Specifically, α-Ga2O3 (rhombohedral, corundum) has the largest bandgap of ~5.3 eV and can be alloyed with α-Al2O3 (8.8 eV) and α-In2O3 (3.7 eV) for bandgap engineering. Both ε-Ga2O3 (hexagonal, P63mc) and κ-Ga2O3 (orthorhombic, Pna21 ) phases are polar, with a predicted spontaneous polarization strength up to 10 times larger than GaN and 3 times larger than AlN. Like the III-N system, polarization induced charges can lead to higher charge densities and mobilities in 2DEGs formed at heterojunctions, which would improve the viability of Ga2O3 electronic devices. Plasma-enhanced atomic layer deposition (PEALD) is a popular, conformal, energy-enhanced synthesis method for thin films due to its many advantages, including: deposition at reduced growth temperatures, access to metastable phases, improved crystallinity, and increased growth rates. In this work, we use PEALD to produce high-quality heteroepitaxial Ga2O3 and (AlxGa1-x)2O3 (AlGO) films and investigate materials properties such as phase selectivity, ternary solubility limits, and electrical and optical performance. All Ga2O3 films were deposited in a Veeco Fiji G2 reactor equipped with a load lock and turbo pump using trimethygallium, trimethylaluminum, and O2 plasma. Initial studies on c-plane sapphire substrates at 350°C and 8 mTorr show the phase could be altered from β to α by a varying the pure O2 flow during plasma pulse from 5-40 sccm [1]. Optical emission spectroscopy indicate that the changes in the relative concentration of atomic oxygen is crucial for phase selectivity while the high ion flux to the surface can contribute to the crystallinity at low Tg [2]. To grow ε(κ)-Ga2O3 on c-plane sapphire required going to a much higher temperature (500°C), pressure (100’s mTorr), and O2 flow (100sccm) [1]. Without modifications to the current ALD system, pure ε(κ)-Ga2O3 on sapphire was not achieved under any conditions. Using optimum growth conditions for the three phases on sapphire, films were deposited on GaN and diamond to determine the effect of substrate structure. Transmission electron microscopy was conducted to determine the specific phases (β, ε, and κ) present in each case, and showed the amount of each phase varied with PEALD parameters. While films on diamond resulted in mixed β/ε(κ) phases, pure ε(κ)-phase films were attained on GaN and the strain varied with pressure and Tg. Vertical breakdown measurements were taken for both β- and ε(κ)-Ga2O3/n+ GaN substrates. Breakdown fields varied between 3.8-7.0 MV/cm dependent on the phase and strained state of the Ga2O3 films. β-Ga2O3 films showed less variability in breakdown field from device to device than ε(κ)-Ga2O3 films, but neither showed a dependence on device size. While PEALD is beneficial for depositing thin films of metastable phases, practical devices often require much thicker barrier and active layers. For this reason, we investigated integrating PEALD metastable Ga2O3 films with traditional semiconductor deposition techniques, such as molecular beam epitaxy (MBE), capable of extending these layers beyond 100 nm in thickness. The same MBE conditions were used to deposit Ga2O3 films on GaN substrates with and without PEALD ε(κ)-Ga2O3 nucleation layers. Those deposited without the PEALD metastable nucleation layer resulted in stable β-phase films, while those with nucleation layers resulted in pure ε(κ)-phase films. This shows importance of PEALD for realizing practical device structures using metastable phases. Finally, to investigate heterojunctions for 2DEG formation, AlxGa1-xO films were developed. While the full stoichiometric range could be reached using a PEALD digital alloying method, crystallinity was lost above x = 0.2 for the β phase, x = 0.35 for the ε(κ) phase, and x=0.6 for the α phase. Initial device structures will be shown in order to establish the feasibility of these films in device applications. [1] Wheeler, et al. Chem. Mater. 2020, 32, 1140-1152 [2] Boris, et al. JVST A 2019, 37(6), 060909 Figure 1

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 optical measurements of the dynamic strain profiles along the tethers of microelectromechanical resonators and relates them to mechanical quality factor (Q). Such measurements allow for the quantification of tether dissipation and fair comparison between various tether designs. Our experiments present the first systematic comparison between the best-performing conventional tethers with one-dimensional phononic crystal (PnC) tethers for silicon bulk acoustic resonators, and demonstrate more than 3× improvement in Q when the PnC tethers are used. The spatial decay rate of the mechanical strain profile along the tethers correlates well with the measured Q. This work is the first to demonstrate one-dimensional PnC tethers for electrostatic bulk acoustic 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.

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.

This work presents the first measured results for resonant AlN-based infrared (IR) detectors fabricated using a proprietary InvenSense AlN MEMS process. Resonators fabricated in the first fabrication run achieved high electromechanical performance with a Q of ∼1400 at 115 MHz, insertion loss of 17.9 dB, and a motional impedance of 670 Ω. The detectors are coated with an IR absorber layer (SiN x ), and tested for response to IR radiation using a calibrated, traceable black body source. The estimated responsivity of the device is 210ppb/µW for the longwave-infrared (LWIR) spectrum. The detectors are expected to have low noise, with estimated NEDT and NEP of 51 mK and 52.7 pW/Hz0.5, respectively. The resonators are fabricated in a hybrid MEMS/CMOS wafer level packaged die, allowing for CMOS-based routing and readout.

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. Keywords—micromechanical resonator, IR detector, plasmonics, metamaterial.

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.

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.

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.

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.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

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.

In this paper, measurements and characterization results of several micromechanical bulk-mode resonators and filters fabricated from single crystalline gallium nitride are presented. A 167.6 MHz length-extensional mode resonator is demonstrated that exhibits an unloaded quality factor of 1370 and motional impedance of 485 Ω at atmospheric pressure and 300 K. The ƒ×Q values of the resonators presented in this work measured under ambient conditions are significantly higher than prior work and prove that GaN is a suitable material as a micromechanical resonating element for high-power applications. The relevant material properties of GaN are also characterized.

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.

This paper presents the theory and measured results of a lownoise un-cooled infrared (IR) detector, which uses a combination of piezoelectric, pyroelectric, electrostrictive, and resonant effects to achieve high sensitivity. The sensor consists of a high-Q gallium nitride (GaN) micro-mechanical resonator coated with an IR absorber layer. The IR absorber converts the IR energy into heat. The generated heat causes a shift in the frequency characteristics of the GaN resonators. IR detection is achieved by sensing the shift in the resonance frequency of the exposed GaN resonator as compared to a reference resonator. A prototype GaN-based IR sensor is implemented, showing a frequency shift of 400 Hz per μW of absorbed power. The change in the beat frequency upon IR radiation is 1830 ppm/μW, making it possible to sense IR radiation in nano watts range. INTRODUCTION The detection of infrared radiation is generally accomplished using either photonic or thermal detectors, both of which have generated a large amount of research over the last two decades [1]. The relative disadvantages of thermal detectors in terms of detectivity and response time are balanced by the fact that they do not require cooling systems, reducing the system weight, power consumption, and form factor. Furthermore, thermal detectors are better suited for radiation extending into the far wavelength IR, which is extremely useful for optical astronomy. While bolometric devices have been the most popular thermal sensors commercially, there is an increasing interest in resonant IR detectors [2]-[4]. This work presents a GaN-based resonant IR sensor that utilizes the unique material properties of GaN to achieve high sensitivity. A reference resonator is included in the system for self-calibration (Fig. 1). Fig. 2 shows a representative scanning electron micrograph (SEM) of a GaN detector. Figure 1: Schematic of a resonant sensor and a reference pair with interdigitated finger electrodes. The sensor is coated with an absorber layer. The reference resonator is either not coated with the absorber or the absorber layer on the reference resonator is blocked using the top electrode.