Polarized photoluminescence study of free and bound excitons in free-standing GaN

A study of the polarization properties of the exciton emission in $\mathrm{GaN}$ is presented. Photoluminescence measurements are performed for light propagation perpendicular to the $c$ axis of a free standing layer grown by hydride vapor phase epitaxy. Emission from different polariton branches of the ${\ensuremath{\Gamma}}_{5}$ and ${\ensuremath{\Gamma}}_{1}$ free exciton states are identified for the $\mathbf{E}\ensuremath{\perp}\mathbf{c}$ and $\mathbf{E}\ensuremath{\Vert}\mathbf{c}$ polarizations, respectively. The mixed-mode transverse-longitudinal state of the A exciton is also observed in the $\mathbf{E}\ensuremath{\Vert}\mathbf{c}$ polarized spectra. Donor-bond excitons involving a hole from the A and B valence bands are clearly distinguished and are found to follow the optical selection rules of the free excitons. The temperature dependence of the emission intensities is also investigated and the exciton thermalization processes for both polarizations are discussed.

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