Interface-induced electronic structure toughening of nitride superlattices

We report on first-principles study of uniaxial strength for brittle cleavage of AlN/VN, AlN/TiN and VN/TiN systems. In agreement with previous studies we predict that the VN/TiN exhibits interface induced toughening of VN as compared to bulk values, and a similar effect is predicted also to occur in the VN/AlN system. However, a more detailed insight reveals, that the theoretical critical stress for brittle cleavage largely oscillates (even below the critical stress for bulk) with the distance from the interface inside the VN layer, a phenomenon not present (or hugely reduced) in TiN and AlN layers. The oscillating values for critical stress well correlate with the same behavior of interplanar distances and charge density. The origin of these unexpected properties was pinpointed to structural instabilities of cubic VN.