First-principles study of structural phase transformation and dynamical stability of cubic AlN semiconductors

Phase transformation and stability of cubic aluminium nitride (AlN) phases such as zinc-blende and rock-salt have been investigated using first-principles calculations based on density functional perturbation theory (DFPT) within quasi-harmonic approximation (QHA). The phonon dispersion relations of both the cubic phases have been calculated at various high-symmetry points of the Brillouin Zone. The pressure and volume dependence of phonon frequencies have been investigated. The application of pressure results in opposite trend of transverse acoustic (TA) phonon frequencies for rock-salt and zinc-blende AlN phases. The TA frequencies found to increase for the former one and decrease for the latter one with the increase in pressure. The dynamical instability results in a volume expansion of rock-salt AlN close to the equilibrium volume of zinc-blende AlN. Phase transformation of these cubic phases is further investigated by computing an equilibrium pressure-temperature phase diagram within QHA. The cubic rock-salt AlN is found to form at high pressures and temperatures than zinc-blende AlN. The temperature dependence of lattice constant and the corresponding volume thermal expansion coefficient of both the cubic phases have been investigated.