Resonant Raman scattering on free and bound excitons in GaN

Abstract
We comprehensively investigated the resonant Raman scattering effect in GaN at low temperatures applying a frequency-doubled titan-sapphire laser as quasicontinuous excitation source. The scattering cross sections of the E2(high), A1(LO), and 2A1(LO) phonon modes exhibit a resonance enhancement at the donor- and acceptor-bound excitons. The enhancement of the A1(LO) modes is even stronger than for the nonpolar E2(high) mode due to a Fröhlich interaction. Furthermore, the temporal behavior of the phonons near the resonance was studied using time-resolved spectroscopy. We found that the resonance process below the bound excitons is likely to proceed via the free exciton as intermediate state.