Photoluminescence of undoped GaN grown on c-plane Al2O3 by electron cyclotron resonance molecular beam epitaxy

We report results on the 77 K photoluminescence (PL) of undoped GaN epilayers grown on c-plane sapphire substrate by electron cyclotron resonance molecular beam epitaxy (ECR-MBE). It was found that the PL properties of undoped GaN epilayers are strongly affected by the growth conditions such as nitrogen pressure, growth temperature and ECR plasma power. The intensities of the donor-to-acceptor (D-A) pair emission peak at about 3.2 eV and of the deep-level emission peak at around 2.3 eV change with the variation of these three growth conditions. Assuming that the D-A peak and deep-level emission peak are related to native defects and carbon impurities, respectively, the systematic change of the PL spectra with growth conditions can be reasonably interpreted. At the optimum growth conditions, the PL spectra at 77 K are dominated by excitonic peak at 3.47 eV and the intensity of deep-level emission peak is one order of magnitude less than that of the excitonic peak. However, since the region regarded as optimum is relatively narrow, it is important to control the growth conditions precisely.