Polarized microphotoluminescence and reflectance spectroscopy of GaN with k perpendicular to c: Strongly π-polarized line near theAexciton

Polarized microphotoluminescence ({μ}-PL) and reflectivity studies of thick wurtzite GaN grown by hydride vapor phase epitaxy on c sapphire are performed with the light vector k both normal to the c axis (k{⊥}c) and parallel to it. A strong PL peak is found in the vicinity of the A exciton in {π} polarization (k{⊥}c, E parallel to the c axis), in apparent contradiction to the selection rules. The {π}-polarized component exceeds in intensity the {σ}-polarized one up to {~}50 K. Power- and temperature-dependent measurements of both no-phonon and longitudinal optical phonon-assisted (monitoring the real density of exciton states) {μ}-PL reveal the complex nature of the {π}-polarized PL line near the A exciton. At low temperatures the {π}-component involves a bound B exciton contribution, while at higher temperatures contributions of scattered A exciton states become appreciable. The enhancement of the {π}-polarized component is attributed to the complex structure of the exciton-polariton branches for k{⊥}c. Temperature-dependent {μ}-PL and reflectance spectroscopies reveal additionally a difference in the optical properties between the sample regions at the top surface and the cleaved edges, tentatively explained as induced by different strains in these regions.