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Solid-state quantum acoustodynamic (QAD) systems provide a compact platform for quantum information storage and processing by coupling acoustic phonon sources with superconducting or spin qubits. The multi-mode composite high-overtone bulk acoustic wave resonator (HBAR) is a popular phonon source well suited for QAD. However, scattering from defects, grain boundaries, and interfacial/surface roughness in the composite transducer severely limits the phonon relaxation time in sputter-deposited devices. Here, we grow an epitaxial-HBAR, consisting of a metallic NbN bottom electrode and a piezoelectric GaN film on a SiC substrate. The acoustic impedance-matched epi-HBAR has a power injection efficiency >99% from transducer to phonon cavity. The smooth interfaces and low defect density reduce phonon losses, yielding (f × Q) and phonon lifetimes up to 1.36 × 10 17 Hz and 500 µs respectively. The GaN/NbN/SiC epi-HBAR is an electrically actuated, multi-mode phonon source that can be directly interfaced with NbN-based superconducting qubits or SiC-based spin qubits.

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ScAlN is a promising material for applications spanning wide-bandwidth filters, high-electron-mobility transistors, and ferroelectric memory. We investigate conditions influencing wurtzite phase purity for heteroepitaxial ScAlN, and present methods to rapidly identify phase purity degradation. Even for N-rich samples, phase purity is sensitive to the III/V ratio near the N-rich to metal-rich transition. Epitaxial Sc x Al 1−x N samples can be grown at 700 °C with x = 0.06-0.22, although the phase purity degrades for x = 0.32. By reducing the substrate temperature to 390 °C, we demonstrate 200 nm Sc 0.32 Al 0.68 N heteroepitaxial films with a record low rocking curve full-width at half-maximum of 1840 arcsec.