Band-edge photoluminescence and reflectivity of nonpolar ( 11 2 ¯ 0 ) and semipolar ( 11 2 ¯ 2 ) GaN formed by epitaxial lateral overgrowth on sapphire

Coalesced nonpolar $(11\overline{2}0)$ and semipolar $(11\overline{2}2)$ GaN layers obtained by epitaxial lateral overgrowth on sapphire have been studied by photoluminescence and reflectivity under normal incidence as a function of temperature and light polarization. In nonpolar GaN, two strongly allowed excitonic levels $A$ and $C$ are evidenced by reflectivity and are also observed in the luminescence spectra. The third exciton level $(B)$ is only observed in the luminescence spectra, showing that its oscillator strength is mainly concentrated in transitions involving light polarized along the growth axis. The lower energy $A$ exciton state is allowed for light polarized perpendicularly to the $c$ axis. Therefore, for light polarized along the $c$ axis, the thermal quenching of the luminescence of $A$ results in a strong increase of the higher energy exciton emission, with an activation energy corresponding to their energetic separation. We observe a similar temperature induced increase of luminescence for transitions involving basal stacking faults and donor-acceptor pairs. In the latter case, our observation is evidence of an acceptor excited state due to the valence band splitting. Careful examination of the energy differences between the $A$ exciton luminescence spectra recorded in both polarizations leads to an exchange splitting of $1\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, in good agreement with previous determinations. Our experimental results, exciton energies, and oscillator strengths are in good agreement with those calculated from a $\mathbf{k}=\mathbf{0}$ strained wurtzite valence band Hamiltonian using strain values measured by x-ray diffraction. Finally, the results for semipolar GaN are qualitatively similar to those obtained from nonpolar GaN. However, the optical selection rules are not as severe, owing to the fact that normal incidence light is never fully polarized perpendicular to the wurtzite basal plane.