High-overtone Bulk Acoustic Resonators and Comb Filters using Epitaxial ε-Ga₂O₃ Films on 4H-SiC

This work demonstrates novel high-overtone bulk acoustic resonators (HBARs) with only top electrodes using an epitaxial ɛ-Ga2O3 piezoelectric film grown on conductive 4H-SiC substrates. The device exhibits a broad frequency response spanning 1 to 8 GHz, with a free spectral range (FSR) of 18.6 MHz between adjacent modes. Key performance metrics include an f∙Q product exceeding 1.2×1014 Hz at 70 K and over 1.5×1013 Hz at 300 K, along with excellent temperature stability characterized by a low temperature coefficient of frequency (TCF) of -15.46 ppm/°C. The acoustic parameters of ε-Ga2O3 are extracted, including density of 5001.7 kg/m3, elastic constant CD33 of 2.82×1011 N/m2, longitudinal acoustic wave velocity of 7596 m/s and intrinsic electromechanical coupling coefficient k2t of 7.9%. Evaluation of the theoretical f∙Q limit and acoustic impedance mismatch reveals substantial potential for further performance enhancement. Additionally, a comb filter was demonstrated by laterally coupling two ɛ-Ga₂O₃ HBARs, achieving over 275 equidistant passbands across an over 5 GHz bandwidth. These results highlight the promise of ɛ-Ga₂O₃-based HBARs for advanced RF applications. Leveraging its excellent piezoelectric and electronic properties, ɛ-Ga₂O₃ enables monolithic integration of acoustic devices with on-chip electronics, paving the way for compact, high-performance RF systems.