All of the possible types of phonon-assisted optical processes involving bound or extended initial, final and virtual electron states were classified, for the first time, in terms of site symmetry and selection rules for corresponding transitions. This theory was applied to phonon-assisted transitions, in hexagonal GaN, involving substitutional impurities and vacancies with C3v site-symmetry, as well as interstitial impurities and molecular point defects (paired impurities, double vacancies, vacancy-impurity complexes) occupying sites with C3v, Cs, and C1 symmetry. It was shown that phonon-assisted optical recombination was allowed in any polarization for free and bound carriers and excitons whatever the number of phonons involved. The nature of the virtual states and phonons could depend upon the polarization of the emitted light. In particular, the case of excitons bound to neutral donors or acceptors was considered. The predictions were in good agreement with published experimental optical spectra, which exhibited numerous lines that were attributed to one- and multi-phonon-assisted transitions.

Phonon-Assisted Optical Transitions in GaN with Impurities and Defects. P.Tronc, Y.E.Kitaev, G.Wang, M.F.Limonov, G.Neu: Physica B, 2001, 302-303, 291-8