Metal-Free High-Overtone Bulk Acoustic Resonators With Outstanding Acoustic Match and Thermal Stability

This letter presents a novel type of epitaxial gallium nitride (GaN) on semi-insulating silicon carbide (4H-SiC)-based high-overtone bulk acoustic resonators (HBARs) that eliminates the need for metal electrodes. The device is piezoelectrically transduced through a quantum well induced two-dimensional electron gas (2DEG) and a heavily-doped GaN layer as the top and bottom electrodes, respectively. Room to cryogenic temperature evolution for 50 HBAR phonon modes near the fundamental transduction envelope (2.032 GHz) is demonstrated. It shows an exceptionally low temperature coefficient of frequency (TCF) at −5.709 ppm/K, a converged free spectral range (FSR) at 17.9 MHz, and a figure of merit ${f} \times {Q}$ at $6.7\times 10^{{13}}$ Hz at 15 K ( $1.7\times 10^{{13}}$ Hz at 298 K). This is the result of complete acoustic and thermal impedance match due to a metal-free design, which is promising for sensing, microwave signal processing, and quantum acoustodynamics (QAD).