Ab initio calculation of local vibrational modes. Application to GaAs : C and cubic GaN : As

Ab initio density-functional theory is used to calculate interatomic forces and the dynamical matrix of cubic GaN and GaAs with and without point defe cts. The Green's function method and the Dyson equation is applied to obtai n the change of the density of phonon states due to the defect, from which we find not only the local vibrational modes and their isotope shifts of Ga As: C outside the host-phonon bands, but also localized modes at the edges of the phonon bands and resonances inside the phonon bands. Our results ind icate that recently observed low-energy Raman peaks at a layer of cubic GaN on a GaAs substrate can be interpreted as resonances inside the acoustical phonon bands of GaN or GaAs. The energetic positions of these resonances a re not characteristic for the defect but occur at specific energies of the density of the host-phonon states. (C) 1999 Elsevier Science B.V. All right s reserved.