Microscopic mechanism of the wurtzite-to-rocksalt phase transition of the group-III nitrides from first principles

There are mainly two kinds of microscopic mechanisms previously proposed for the structural phase transition from wurtzite to rocksalt, that is, the "hexagonal" and "tetragonal" paths. The present work gives a comparative study of these two mechanisms in group-III nitrides (AlN, GaN, and InN) from the energetic point of view based on first-principles calculations. The calculated results indicate that the energy-favored mechanism is dependent not only on the internal compositions but also on the external pressures. AlN and GaN prefer the hexagonal and the tetragonal paths, respectively, in a large pressure range investigated; however, in the case of InN, the tetragonal path is favored under lower pressure but the hexagonal one under higher pressure. In addition, a real-time measurement of the radial distribution function or the axial ratio c/a is suggested to distinguish these two transition paths in experiment. We also propose a simple model to make a rough estimate of the hysteresis cycle of the wurtzite-rocksalt transition and obtain good agreement with the experimental results for AlN and InN.