Fully Impedance-Matched High-Overtone Bulk Acoustic Wave Resonators Using 2DEG Electrodes

This paper presents for the first time a kind of metal-free high-overtone bulk acoustic wave resonators (HBARs), which consists of a thin piezoelectric gallium nitride (GaN) film epitaxially grown on a thick semi-insulating hexagonal silicon carbide (4H-SiC). Ultrasonic waves and resonances are excited in the top transducer by applying RF power through the active area with a two-dimensional electron gas (2DEG) sheet and injected into the substrate cavity through a highly doped GaN. We find that the free spectral range (FSR) of the composite structure demonstrated here converges to a constant, indicating that the interfacial impedance is well-matched. In addition, the measured Q values of the metal-free devices are 100.7% - 282.1% greater than that of metal-covered devices, opening up a new possibility for developing piezoelectric ultrasonic transducers with ultralow energy dissipation and better operational performance.