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In this work, surface acoustic wave (SAW) modes and their dependence on propagation directions in epitaxial Al 0.68 Sc 0.32 N(0001) films on Al 2 O 3 (0001) substrates were studied using numerical and experimental methods. In order to find optimal propagation directions for higher-order SAW modes, phase velocity dispersion branches of Al 0.68 Sc 0.32 N on Al 2 O 3 with Pt mass loading were computed for the propagation directions <11 2 ¯ 0> and <1 1 ¯ 00> with respect to the substrate. Experimental investigations of phase velocities and electromechanical coupling were performed for comparison with the numerical results. Simulations carried out with the finite element method and a Green function approach allowed identification of each wave type, including Rayleigh, Sezawa, and shear-horizontal wave modes. For the propagation direction <1 1 ¯ 00>, significantly increased wave guidance of the Sezawa mode compared to other directions was observed, resulting in enhanced electromechanical coupling (k eff 2 = 1.6 %) and phase velocity (v phase = 6 km / s). We demonstrated that selecting wave propagation in <1 1 ¯ 00> with high mass density electrodes results in increased electromechanical coupling without significant reduction in phase velocities for the Sezawa wave mode. An improved combination of metallization, Sc concentration x , and SAW propagation direction is suggested that exhibits both high electromechanical coupling (k eff 2 > --> 6 %) and high velocity (v phase = 5.5 km / s) for the Sezawa mode. [ABSTRACT FROM AUTHOR]

This work reports on the growth of 1 µm nonpolar a-plane Al0.7Sc0.3N(11 2 ¯ 0) thin films on an r-plane sapphire Al2O3(1 1 ¯ 02) via magnetron sputter epitaxy. The electro-acoustic properties of the film structures were characterized using surface acoustic wave (SAW) resonators. Measured electrical responses were found to be strongly anisotropic in terms of the wave propagation direction. We identified a sagittal polarized Rayleigh wave mode with large coupling ( k eff 2 = 3.7%), increased phase velocity (v = 4825 m/s), as well as high quality factor (Q > 1000) for SAW propagation along the c-axis [0001] and normalized thicknesses h / λ = 0.2. Finite element method simulations using electro-acoustic properties of Al0.7Sc0.3N obtained from the density functional theory reproduce our experimental results. [ABSTRACT FROM AUTHOR]