  A high-overtone bulk acoustic resonator (HBAR), in which a piezoelectric
transducer is set on an acoustic cavity, has been attracting attention in both
fundamental research and RF applications due to its scalability, high
frequency, and high quality factor. The acoustic impedance matching in HBARs is
crucial for efficient acoustic power transfer from the piezoelectric transducer
to the cavity. However, impedance mismatch remains in most HBARs due to the
metal layer insertion between the piezoelectric layer and cavity substrate. In
this study, we fabricated a nearly impedance-matched high-quality HBAR using an
epitaxial AlN piezoelectric layer directly grown on a conductive SiC cavity
substrate with no metal layer insertion. The small impedance mismatch was
verified from the variation in the free spectral range (FSR), which is
comparable to the best value in previously reported HBARs. The experimentally
obtained FSR spectra was greatly reproduced by using the Mason model. Broadband
phonon cavity modes up to the K-band (26.5 GHz) were achieved by reducing the
thickness of the AlN layer from 800 to 200 nm. The high figure of merit of
$f\times\text{Q} \sim 1.3\times 10^{13}\ \textrm{Hz}$ at 10 GHz was also
obtained. Our nearly impedance-matched high-quality HBAR will enable the
development of RF applications, such as low-phase noise oscillators and
acoustic filters, as well as research on high-frequency acoustic systems
hybridized with electric, optical, and magnetic systems.
