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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

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.

This Letter reports measured cryogenic temperature trends for over 300 longitudinal phonon modes spanning >10 GHz in an epitaxial GaN/NbN/SiC high overtone bulk acoustic resonator (epi-HBAR). We present temperature profiles from 7.2 K to 200 K for the mode frequency ( f), the quality factor (Q), the figure of merit ( f × Q), and the phonon loss or attenuation coefficient ( α). We show that for all m phonon modes, f m T follows an identical parabolic trend, with a zero-slope turnover temperature of 35 K. Thus, the epi-HBAR comb spectrum can be considered an ensemble of modes with the same temperature dependencies, potentially enabling the design of precise multi-mode temperature-stable RF oscillators and clocks operating at GHz frequencies. Using temperature trends for ( f × Q ) m and α m, we provide strong evidence that the epi-HBARs are fundamentally limited by anharmonic phonon scattering in the materials that make up the epi-HBAR. Crucially, we unambiguously demonstrate the evolution of this anharmonic phonon scattering from the low frequency Akhiezer scattering regime α m ∝ T 1 to the high frequency Landau–Rumer scattering regime α m ∝ T 4, using hundreds of phonon modes in the same device. Finally, we show that at extremely low temperatures, other emergent loss mechanisms overshadow anharmonic phonon scattering. This finding motivates further investigation into the root causes of these limiting mechanisms for precision RF signal processing, quantum acoustodynamics, and other applications that require extremely low loss micromechanical devices.

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. publishedVersion

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.

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.