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 ε-Ga2O3 piezoelectric film grown on conductive 4H-SiC substrates. The device exhibits a broad frequency response spanning 1-8 GHz, with a free spectral range (FSR) of 18.6 MHz between adjacent modes. Key performance metrics include an f⋅Q product exceeding $1.2\times 10^{{14}}$ Hz at 70 K and over $1.5\times 10^{{13}}$ 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 a density of 5001.7 kg/m3, an elastic constant ${ C}_{{33}}^{D} $ of $2.82\times 10^{{11}}$ N/m2, a longitudinal acoustic wave velocity of 7596 m/s, and an intrinsic electromechanical coupling coefficient ${k}_{t}^{{2}}$ of 7.9%. Evaluation of the theoretical f⋅Q limit and acoustic impedance mismatch reveals substantial potential for further performance enhancement. In addition, a comb filter was demonstrated by laterally coupling two ε-Ga2O3 HBARs, achieving over 275 equidistant passbands across an over 5 GHz bandwidth. These results highlight the promise of ε-Ga2O3-based HBARs for advanced radio frequency (RF) applications. Leveraging its excellent piezoelectric and electronic properties, ε-Ga2O3 enables monolithic integration of acoustic devices with on-chip electronics, paving the way for compact, high-performance RF systems.