Single Crystalline ScAlN Surface Acoustic Wave Resonators with Large Figure of Merit (Q × kₜ²)

Surface acoustic wave (SAW) resonators based on single crystalline aluminum nitride (AlN) and scandium aluminum nitride (ScAlN) grown by molecular beam epitaxy (MBE) demonstrated substantial improvement in acoustic performance compared to the state-of-art devices with sputtered piezoelectric layers on silicon substrates. High coupling coefficient $\left( {k_t^2} \right)$ up to 5.0% and 7.8% were found in resonators with AlN and ScAlN device layers, respectively, due to better crystallinity and scandium doping. This resulted in high figure of merits $\left( {Q \times k_t^2} \right)$ among SAW devices on silicon up to 5.4, which is about twice larger than the previous work using sputtered ScAlN on silicon and comparable to those on non-silicon substrates.