Modeling the electronic structure and stability of three aluminum nitride phases

Phase transitions in aluminum nitride (AlN) were investigated by first principles total energy calculations. Three AlN crystal structures were considered: rock salt (NaCl), zinc blende and wurtzite. The cohesion energy was calculated within both GGA and LDA formalisms. According to the cohesion energy results, the ground state corresponds to the hexagonal wurtzite phase, in agreement with experimental evidence. However, the zinc blende and NaCl phases may be formed as metastable structures. To determine the energy gap the modified Becke-Johnson pseudopotential was applied, with results showing good agreement with the experimental data. The ground state structure exhibits direct electronic transitions. However, the zinc blende and NaCl phases show indirect band gap. Provided that external pressures may induce transitions from wurtzite to zinc blende or rock salt, these transitions were also investigated. Estimation of the pressure at the phase transition indicates that small pressures are needed to achieve such transitions.