X–Ka Band Epitaxial ScAlN/AlN/NbN/SiC High-Overtone Bulk Acoustic Resonators

This letter presents the first demonstration of epitaxial scandium aluminum nitride (ScAlN) based high-overtone bulk acoustic resonators (epi-HBARs) with over 1600 acoustic cavity resonance modes spanning the X – Ka bands (8 GHz – 40 GHz). We present data up to the
$2150^{\text {th}}$
overtone (39.99 GHz). This is an unprecedented result even for HBARs, which often exhibit hundreds of overtones. The measurements demonstrate the successful combination of multiple innovations, namely: a) the growth of ultra-thin, high quality Sc0.33Al0.67N (high Sc fraction), b) on an epitaxial heterostructure with lattice- and acoustic impedance-matched layers, and c) the selective etching of ScAlN on NbN. Key metrics for the ScAlN epi-HBARs include
${Q} >7000$
,
$\text{f}\times \text{Q}\,\,>\,\,10^{{14}}$
Hz, and
$k^{{2}}_{\textit {eff}}\,\,\times \text{Q}_{\textit {BV}{D}} >0.22$
at cryogenic temperatures for frequencies as high as 40 GHz (>500,
${6}\times {10} ^{{12 }}$
Hz, >0.1 at room temperature). Temperature trends motivate future investigation of unquantified high frequency acoustic loss mechanisms. Such robust RF MEMS epi-HBARs with piezoelectric drive and readout are promising candidates for compact microwave/millimeter wave signal processing elements.