Enhanced electromechanical coupling in SAW resonators based on sputtered non-polar Al0.77Sc0.23N (1120) thin films

Non-polar a-plane Al0.77Sc0.23N(1120)thin films were prepared by magnetron sputter epitaxy on r-plane Al2O3 (1102) substrates. Different substrate off-cut angles were compared, and the off-cut angle of 3 resulted in the best structural quality of the AlScN layer. Structural characterization by x-ray diffraction confirmed that single phase, wurtzite-type, a-plane AlScN (1120), surface acoustic wave resonators were fabricated with wavelengths l = 2-10 lm (central frequency up to 1.7 GHz) with two orthogonal in-plane propagation directions. A strong dependence of electromechanical coupling on the in-plane orientation was observed. Compared to conventional c-plane AlScN based resonators, an increase of 185-1000% in the effective electromechanical coupling was achieved with only a fractional decrease of <10.5% in series resonance frequency.