Sc-based III-nitride alloys were studied using Density Functional Theory with special quasi-random structures and were found to retain wide band gaps which stay direct up to x = 0.125 (Sc x Al 1-x N) and x = 0.375 (Sc x Ga 1-x N). Epitaxial strain stabilization prevents spinodal decomposition up to x = 0.3 (Scx Al 1-x N on GaN) and x = 0.24 (Sc x Ga 1-x N on GaN), with critical thicknesses for strain relaxation ranging from 3 nm to near-infinity. The increase in Sc content introduces compressive in-plane stress with respect to AlN and GaN, and leads to composition-and stress-tunable band gaps and polarization, and ultimately introduces ferroelectric functionality in Sc x Ga 1-x N at x ≈ 0.625.

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The ability to increase the thermal stability of protective coatings under work load gives rise to scientific and industrial interest in age hardening of complex nitride coating systems such as ceramic-like Ti1−xAlxN. However, the decomposition pathway of these systems from single-phase cubic to the thermodynamically stable binary nitrides (cubic TiN and wurtzite AlN), which are essential for age hardening, are not yet fully understood. In particular, the role of decomposition kinetics still requires more detailed investigation. In the present work, the combined effect of annealing time and temperature upon the nano-structural development of Ti0.46Al0.54N thin films is studied, with a thermal exposure of either 1 min or 120 min in 100 °C steps from 500 °C to 1400 °C. The impact of chemical changes at the atomic scale on the development of micro-strain and mechanical properties is studied by post-annealing investigations using X-ray diffraction, nanoindentation, 3D-atom probe tomography and high-resolution transmission electron microscopy. The results clearly demonstrate that the spinodal decomposition process, triggering the increase of micro-strain and hardness, although taking place throughout the entire volume, is enhanced at high diffusivity paths such as grain or column boundaries and followed within the grains. Ab initio calculations further show that the early stages of wurtzite AlN precipitation are connected with increased strain formation, which is in excellent agreement with experimental observations.

Elastic constants of hexagonal ScxGa1−xN and ScxAl1−xN up to x = 0.375 were calculated using a stress-strain approach. C11, C33, C44, and C66 decreased while C12 and C13 increased slightly with increasing x. The biaxial [0001] Poisson ratios increased from 0.21 for GaN to 0.38 for Sc0.375Ga0.625 N and from 0.22 for AlN to 0.40 for Sc0.375Al0.625N, due to greater u values, in-plane bond lengths and bond ionicities. Subsequently, critical thicknesses for stress relaxation were calculated for ScxAl1−xN/AlN, ScxGa1−xN/GaN, and ScxAl1−xN/GaN heterostructures using an energy balance model. These range from 2 nm for Sc0.375Al0.625N/AlN and Sc0.375Ga0.625N/GaN to infinity for lattice-matched Sc0.18Al0.82N/GaN.

Motivated by an increasing demand for coherent data that can be used for selecting materials with properties tailored for specific application requirements, we studied elastic response of nine binary early transition metal nitrides (ScN, TiN, VN, YN, ZrN, NbN, LaN, HfN, and TaN) and AlN. In particular, single crystal elastic constants, Young's modulus in different crystallographic directions, polycrystalline values of shear and Young's moduli, and the elastic anisotropy factor were calculated. Additionally, we provide estimates of the third order elastic constants for the ten binary nitrides.Comment: 10 pages, 7 figure