N. Feil

This work reports on the growth of 1 µm nonpolar a-plane Al_0.7Sc_0.3N(11$\overline{2}$ 0) thin films on an r-plane sapphire Al₂O₃(1$\overline{1}$ 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 ($k_{\text{eff}}^2=$ 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 $h/\lambda =0.2$ . Finite element method simulations using electro-acoustic properties of Al_0.7Sc_0.3N obtained from the density functional theory reproduce our experimental results.

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.

We present detailed investigations of the structural, elastic, dielectric, and piezoelectric properties of scandium aluminum nitride (Sc_xAl_1−xN) with the wurtzite crystal structure by means of first-principles calculations based on density functional theory in order to enable a detailed comparison to the corresponding physical properties of GaAlN and InAlN. The goal of our approach is to use atomistic simulations to extract the novel solid state characteristics of Sc_xAl_1−xN crystals by the determination of complete sets of coefficients for the elastic, compliance, and piezoelectric tensor and to confirm the theoretical predictions by experimental measurements of selected tensor coefficients. The calculation of the tensor components is accompanied by a detailed analysis of the crystal structures, e.g., average bond length, bond angles, lattice parameters, and mass density in dependence on alloy composition of Sc_xAl_1−xN. If an increasing number of Al atoms of up to x = 0.5 are replaced by Sc atoms, we observe a nonlinear change of the ratio of lattice parameter $\frac{c(x)}{a(x)}$ and average bond angles of about 10% and 5%, respectively, which give an indication of an increasing deviation of the crystal structure of Sc_xAl_1−xN from an ideal hexagonal lattice. As a consequence of the deformed crystal structure and the iconicity of the Sc–N bond, we predict a change in value of the elastic coefficient $C_{33}^{ScAlN}(x)$ , the piezoelectric coefficient $e_{33}^{ScAlN}(x)$ , and the value of spontaneous polarization $P_{SP}^{ScAlN}(x)$ of up to 65%, 150%, and 230%, respectively. Based, on these simulation results, physical features of practical use are derived, like the average bulk, shear, and the Young modulus as well as the reciprocal Young's modulus and Poisson ratio. Furthermore, the spontaneous polarization of Sc_xAl_1−xN is approximated, taking nonlinear effects into account as well as the piezoelectric polarization caused by uniaxial, biaxial, and hydrostatic stresses in dependency on alloy composition and strain. A detailed comparison of the structural and polarization related properties of GaAlN and InAlN allows us to point out the peculiarity of wurtzite ScAlN crystals within the group III-nitrides.