Excitons bound to structural defects in GaN

We analyzed the photoluminescence (PL) spectra of undoped GaN films grown by molecular beam epitaxy on sapphire substrates. While the PL spectra from high-quality samples contain free and bound exciton peaks only, the spectra from some samples involve sharp unidentified peaks in the energy range of 3.0 – 3.45 eV, specifically at 3.21, 3.32, 3.36, and 3.42 eV. We attribute these peaks to excitons bound to defects because of the linear and sometimes superlinear increase in their intensity with excitation density without saturation up to 100 W/cm2. With increasing temperature these peaks quench in a well-known fashion similar to that for excitons. In order to relate the observed peaks to the structural defects, we etched selected samples in hot H3PO4 acid or, alternatively, with photo-electrochemical (PEC) etching at room temperature in the presence of UV-illumination in a dilute KOH solution. In the former case the dislocations were etched leaving etched pits on the surface, while in the latter case the dislocations remained unetched due to a deficit of photogenerated holes at dislocation sites. We found that the 3.42 eV peak disappeared after both hot wet and PEC etching suggesting that the associated defect is at the GaN surface. Peaks at 3.21 and 3.36 eV could be enhanced greatly by PEC etching, which were correlated to bulk dislocations.