Properties of higher-order surface acoustic wave modes in Al1−xScxN/sapphire structures

In this work, surface acoustic wave (SAW) modes and their dependence on propagation directions in epitaxial ${\mathrm{Al}}_{0.68}{\mathrm{Sc}}_{0.32}$ N(0001) films on ${\mathrm{Al}}_2{\mathrm{O}}_3$ (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 ${\mathrm{Al}}_{0.68}{\mathrm{Sc}}_{0.32}$ N on ${\mathrm{Al}}_2{\mathrm{O}}_3$ with Pt mass loading were computed for the propagation directions <11$\overline{2}$ 0> and <1$\overline{1}$ 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$\overline{1}$ 00>, significantly increased wave guidance of the Sezawa mode compared to other directions was observed, resulting in enhanced electromechanical coupling ($k_{\mathrm{eff}}^2=1.6\text{\%}$ ) and phase velocity ($v_{\mathrm{phase}}=6\mathrm{km}/\mathrm{s}$ ). We demonstrated that selecting wave propagation in <1$\overline{1}$ 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 ($k_{\mathrm{eff}}^2>6\text{\%}$ ) and high velocity ($v_{\mathrm{phase}}=5.5\mathrm{km}/\mathrm{s}$ ) for the Sezawa mode.