Resonant Raman study of intrinsic defect modes in electron- and neutron-irradiated GaAs

We report the results of a resonant Raman study of GaAs samples which contain {~}10¹⁷--10¹⁸ cm^-3 intrinsic defects that have been introduced by either electron or neutron irradiation. We observe several defect-related modes, two of which we identify as local modes of an intrinsic defect, most likely an As vacancy. In the vicinity of the E₀ transition we observe enhancements of these modes that are more dispersive than the enhancement of the TO-phonon scattering. We explain these observations phenomenologically with a fourth-order perturbation theory involving elastic scattering of either an electron or a hole by a defect. The theory predicts that strongly dispersive enhancement can occur for vibrational modes that involve enhanced motion of atoms in the immediate vicinity of a defect.