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

Abstract

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 3.0–3.5eV spectral range. In the near band-edge region the emission lines of the donor-bound excitons (3.472eV) and free excitons (3.478eV) 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 3.42eV emission commonly attributed to the excitons bound to basal plane stacking faults shows thermal quenching with two activation energies (7 and 30meV) 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 3.29eV 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 3.29eV emission is suggested to involve impurities, which decorate the partial dislocation terminating the basal stacking faults.