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

We report on phonon diffraction limited performance of GaN/AIN/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 (Ø), We find that the smallest epi-HBARS (Ø=22.5 um on a 350 um thick substrate) are indeed limited by diffraction, yet achieve f × Q ≈ 10 ¹² Hz at 7.2 K. Near the diffraction limit, f × Q ≈10 ¹⁴ Hz for Ø>50 um, and for larger devices that are not diffraction limited, f × Q ≈ 2×10 ¹⁶ 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.