Phonons in cubic GaN/AlN semiconductor superlattices

Rigid-ion model calculations are reported for the lattice dynamics of cubic GaN, AlN, GaAlN and GaN/AlN superlattices. The short-range forces in the rigid-ion model are optimized by incorporating phonons at critical-points as an input and using the existing elastic constants as constraints on the values of the parameters. The long-range Coulomb forces are obtained exactly by using the Ewalds transformation method. For ultrathin superlattices, the dependence of phonons on wavevectors both parallel and perpendicular to the growth direction [001] are investigated. Theoretical results for the GaN/AlN superlattices are compared and discussed with the GaAs/AlAs system as well as with the model calculations of Grille and Bechstedt. Unlike GaAs/AlAs, the larger optical phonon values and partially changed confinement characteristics in GaN/AlN superlattices are believed to have significant affects on the relaxation properties of electrons in group III-nitride material systems.