Emission properties of a-plane GaN grown by metal-organic chemical-vapor deposition

We report on the emission properties of nonpolar $a$ -plane GaN layers grown on $r$ -plane sapphire. Temperature-, excitation-density-, and polarization-dependent photoluminescences and spatially resolved microphotoluminescence and cathodoluminescence are employed in order to clarify the nature of the different emission bands in the $3.0–3.5\mathrm{eV}$ spectral range. In the near band-edge region the emission lines of the donor-bound excitons $\left(3.472\mathrm{eV}\right)$ and free excitons $\left(3.478\mathrm{eV}\right)$ are resolved in the polarized low-temperature spectra, indicating a good quality of the layers. At low energies two other emissions bands with intensity and shape varying with the excited area are observed. The $3.42\mathrm{eV}$ emission commonly attributed to the excitons bound to basal plane stacking faults shows thermal quenching with two activation energies (7 and $30\mathrm{meV}$ ) and an S-shaped temperature dependence of the peak position. This behavior is analyzed in terms of hole localization in the vicinity of the stacking faults. The emission band that peaked at $3.29\mathrm{eV}$ is found to blueshift and saturate with increasing excitation intensity. The spatially resolved cathodoluminesence measurements show that the emission is asymmetrically distributed around the triangular-shaped pits occurring at the surface. The $3.29\mathrm{eV}$ emission is suggested to involve impurities, which decorate the partial dislocation terminating the basal stacking faults.