The selection rules of phonon-assisted optical transitions of bound excitons in bulk wurtzite GaN are studied both experimentally and theoretically. The linearly polarized photoluminescence is detected within the phonon replicas of the lines of impurity bound excitons in the geometry, when the light wave vector is normal to the hexagonal axis, and the electric field vector is either perpendicular or parallel to it. The degree and even sign of the linear polarization is found to depend on the symmetry of the involved optical phonon. To explain these data, a group-theory approach is applied to derive the selection rules for zero-phonon and phonon-assisted transitions involving excitons bound to either substitutional (C3v symmetry) or interstitial (C3v, Cs, or C1 symmetries) impurities. The obtained theoretical selection rules are in an agreement with the experimental results, provided the exciton is bound to the impurities with the C3v symmetry. © 2008 The American Physical Society.

We report on a strong delay in light propagation through bulk GaN, detected by time-of-flight spectroscopy. The delay increases resonantly as the photon energy approaches the energy of a neutral-donor bound exciton (BX), resulting in a velocity of light as low as 2100 km/s. In the close vicinity of the BX resonance, the transmitted light contains both ballistic and diffusive components. This phenomenon is quantitatively explained in terms of optical dispersion in a medium where resonant light scattering by the BX resonance takes place in addition to the polariton propagation.