"Computational discovery, metastability, defects, and disorder in ternary nitride semiconductors"
Materials discovery is increasingly going beyond the prediction of thermodynamically stable materials at Daltonian compositions and their crystallographic primitive cells, including, e.g., metastable compounds, solid solutions, as well as defect- and disorder-enabled materials. Due to their relative instability against N2, nitrides are often grown as thin films via non-equilibrium deposition at relatively low temperatures, leading to atomic disorder. As a part of a broader nitrides discovery effort , we have predicted the crystal structures and evaluated the thermodynamic stability ranges (phase diagrams) on the extended scale of nonequilibrium chemical potentials. These efforts identified the previously unknown nitrides Sn2N2, Ti3N4 , as well as the ternaries Zn3MoN4, ZnMoN2 , and Zn2SbN3. The effects of disorder on structure selection and electronic structure properties are studied by Monte-Carlo sampling. Disorder is also the inherent driving force for the formation of solid solutions. In ZnSnN2, which is of interest for photovoltaics, unintentional oxygen incorporation leads to a highly off-stoichiometric, dual-sublattice mixed solid solution. By combining defect supercell and Monte-Carlo simulations with a Motif-based model Hamiltonian, we described a defect phase diagram for this material, explaining the unexpected non-monotonic doping behavior . This phase can be viewed as an isostructural alloy between ZnSnN2 and ZnO, where the mixing induces disorder on both the long and short range. The Monte-Carlo simulations identified a "magic composition" at which the mixing enthalpy can be strongly reduced due to perfect short-range order, while the residual entropy is sufficient to stabilize this phase at experimentally accessible temperatures. This unusual "disordered line compound" is electronically pristine as seen in the absence of any charge localization effects that are otherwise prominent when the short range order is imperfect.