Confinement of polar optical phonons in AlN/GaN superlattices

We study the optical-phonon spectra in heterojunctions fabricated from III to V nitride materials (GaN and AlN). We are concerned with the quaternary superlattice structure, namely, /substrate/AlN/AlxGa1−xN/GaN/AlxGa1−xN/…/, where the substrate is here considered to be a transparent dielectric medium like sapphire. We make use of a model based on the Frölich Hamiltonian, taking into account the macroscopic theory developed by Loudon, known as the continuum dielectric model. The optical-phonon modes are modelled considering only the electromagnetic boundary conditions (including retardation effects), in the absence of charge transfer between ions. Numerical results of the confined optical-phonon spectra are presented, characterizing three distinct optical-phonon classes designated as propagate (PR), interface (IF) and half-space (HS) modes. Furthermore, due to the dielectric anisotropy presented in the nitrides, some additional peculiarities will be presented, like dispersive confined modes.