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.

Alloys of scandium with AlN exhibit an enhanced piezoelectric coefficient that can boost the performance of nitride‐based electronic and optoelectronic devices such as high electron mobility transistors (HEMTs). Consequently, there is increasing interest in the epitaxial growth of high‐quality AlScN/GaN heterostructures. So far, only very recent reports on AlScN HEMT structures grown by molecular beam epitaxy (MBE) have been published. Herein, the motivation for depositing AlScN epitaxial layers by metal‐organic chemical vapor deposition (MOCVD) as well as the challenges associated with this approach are explained. For the first time, the successful deposition of epitaxial layers with a Sc content up to 30% (Al_0.7Sc_0.3N) is reported. It is shown that the deposited films consist of wurtzite‐type AlScN with high crystalline quality, demonstrating that MOCVD is suitable for the growth of HEMT structures with Sc‐based ternary nitrides.

Aluminum scandium nitride (Al_1−xSc_xN with x = 0–0.41) thin films were deposited by reactive pulsed-DC magnetron sputtering on Si(001) and Al₂O₃(0001) substrates. X-ray diffraction indicated high degree of c-axis orientation in all the films, and based on pole figure measurements, epitaxial relationship could be defined as $\left[10\overline{1}0\right]$ _AlScN//$\left[11\overline{2}0\right]$ _sapphire and (0001)_AlScN//(0001)_sapphire in films deposited on Al₂O₃. Piezoelectric coefficient increased up to d₃₃ = 31.6 pC/N in Al_0.59Sc_0.41N, which is 550% higher than for AlN. The biaxial elastic modulus and the in-plane coefficient of thermal expansion (CTE) as a function of Sc concentration were determined by thermal cycling method: biaxial elastic modulus decreased from 535 GPa in pure AlN to 269 GPa in Al_0.59Sc_0.41N and CTE was 4.65 × 10⁻⁶ K⁻¹ for AlN and 4.29 × 10⁻⁶ K⁻¹ for Al_0.59Sc_0.41N. Additionally, we observed an increase in CTE from 4.18 × 10⁻⁶ K⁻¹ at 65 °C to up to 6.38 × 10⁻⁶ K⁻¹ at 375 °C for Al_0.68Sc_0.32N. The experimentally determined CTE and elastic modulus allow a more precise design of Al_1−xSc_xN-based frequency filters which are used in mobile communications and are important parameters for the prediction of device performance at elevated temperatures.

Wurtzite Al_1−xSc_xN thin films with scandium Sc concentrations up to x = 0.41 were prepared by reactive pulsed DC magnetron co-sputtering at heater temperatures between 300 °C and 400 °C on Al₂O₃ substrates. Spectroscopic ellipsometry was used to determine the dielectric functions of wurtzite Al_1−xSc_xN by modeling the spectra with a three-layer model involving parametric oscillator functions for the Al_1−xSc_xN layers. By combining ellipsometry with transmission spectroscopy, we determined the composition dependence of the optical band gap E_g. For x < 0.25, the experimentally determined band gap follows the theoretical prediction with an offset, which arises from residual sub-band gap absorption. For higher Sc concentrations, for which the band gap is expected to become indirect, the experimental data deviate from the theoretical values for the direct band gap. No absorption that can be attributed to cubic ScN crystallites was observed up to x = 0.41, indicating a high phase purity in line with X-ray diffraction data.

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 (Al${}_{1-\mathrm{x}}$ Sc${}_{\mathrm{x}}$ N) 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 $\mathrm{x}=0.42$ . Two sets of 1 $\mu$ m-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 ${\epsilon }_{33,f}$ , 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 $\lambda$ from 2 up to 24 $\mu$ m. 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.