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 Fr\"ohlich 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.
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Kaschner et al. (2001) studied this question.
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