In this work, all electroacoustic material parameters, i.e., the elastic, piezoelectric, and dielectric coefficients, as well as the mass density, were determined experimentally for wurtzite aluminum scandium nitride (Al1−xScxN) for a wide range of Sc concentrations of up to x = 0.32 from the same material source for the first time. Additionally, the mass density and piezoelectric coefficient were determined even up to x=0.42. Two sets of 1 μm-thick AlScN(0001) thin films were deposited on Si(001) using reactive pulsed-DC magnetron cosputtering. One set of thin films was used to determine the a- and c- lattice parameters and the effective relative dielectric coefficient ε33,f, using X-ray diffraction and capacitive measurements, respectively. Lattice parameters were then used to extract average internal parameter u, bond length, and bond angle, as well as mass density, as a function of Sc concentration. Density functional theory calculations were performed to provide the equilibrium lattice parameters a, c, and u, as well as the bond angle and the bond lengths for wurtzite-AlN and layered hexagonal-ScN. The second set of films was used to fabricate surface acoustic wave (SAW) resonators with wavelengths λ from 2 up to 24 μm. The SAW dispersion in conjunction with finite element modeling fitting was used to extract the elastic stiffness as well as the piezoelectric coefficients. The overall evolution of the material parameters and the change of the crystal structure as a function of Sc concentration is discussed in order to provide a possible explanation of the observed behavior.

We present detailed investigations of the structural, elastic, dielectric, and piezoelectric properties of scandium aluminum nitride (ScxAl1−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 ScxAl1−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 ScxAl1−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 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 ScxAl1−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 C33ScAlN(x), the piezoelectric coefficient e33ScAlN(x), and the value of spontaneous polarization PSPScAlN(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 ScxAl1−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.

Non-polar a-plane Al0.77Sc0.23N 112¯0 thin films were prepared by magnetron sputter epitaxy on r-plane Al2O3(11¯02) 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 112¯0, surface acoustic wave resonators were fabricated with wavelengths λ = 2–10 μm (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.

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A‐plane Al0.7Sc0.3N ( 11 2 false¯ 0 ) thin films are grown on r‐plane Al2O3( 1 1 false¯ 02 ) substrates using reactive pulsed‐DC magnetron sputter epitaxy. This is the first report of successful synthesis of nonpolar epitaxial AlScN films with a high scandium concentration (30%). The influence of different sputtering conditions, such as magnetron power, temperature, and process gas flow rates, is investigated. The film characteristics are also compared on different substrate offcuts. Controlling the diffusion of adatoms on surface of the substrate is found to have the highest influence on film quality. The X‐ray diffraction measurements confirm in‐plane oriented AlScN ( 11 2 false¯ 0 ) layers and the final optimized films show significant improvement in rocking curve full width at half maximum (ω‐FWHM) of 11 2 false¯ 0 reflection. Corresponding atomic force microscopy (AFM) measurements show mean root square surface roughness (R q < 0.4 nm) nearing atomically smooth levels. The optimized films also exhibit anisotropic growth characteristics. A growth model for a‐plane AlScN has been proposed based on the growth parameters of the film.