Non-Polar a-plane AlScN($11\overline{2}0$) Thin Film Based SAW Resonators with Significantly Improved Electromechanical Coupling

1 μm thick non-polar a-plane AlScN(112̅0) thin films were prepared by magnetron sputter epitaxy on r-plane Al
₂
O
₃
(11̅02) substrates. X-ray diffraction (XRD) measurements confirmed it as a single-phase wurtzite a-plane AlScN(112̅0). Surface acoustic wave (SAW) resonators with wavelengths λ = {2, 2.5, 3, 4, 5, 6, 7, 8, 10} μm were fabricated on a-plane Al
_0.77
Sc
_0.23
N(112̅0), a-plane Al
_0.68
Sc
_0.32
N(112̅0), and c-plane Al
_0.77
Sc
_0.23
N(0001) thin films. Compared to c-plane Al
_0.77
Sc
_0.23
N-based resonators, 1.85-10 times higher effective electromechanical coupling k
_eff
²
was observed in the whole investigated resonator wavelength range. 50% higher figure of merit was obtained from a-plane Al
_0.77
Sc
_0.23
N-based highest frequency (1.71 GHz) SAW resonators and as high as 4.1% coupling was achieved for a-plane Al
_0.68
Sc
_0.32
N-based resonators (1.39 GHz). Moreover, SAW propagation properties of AlScN(112̅0)/Al
₂
O
₃
(11̅02) and AlScN(0001)/ Al
₂
O
₃
(0001) were analyzed using finite element method (FEM) simulations, the results were compared and they confirmed the experimentally observed trends.