Phonon Diffraction Limited Performance of Fabry-Pérot Cavities in Piezoelectric epi – Hbars

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.