Temperature dependence of excitonic photoluminescence and residual shallow donors in high-purity GaN/Al2O3

The temperature dependence of free and bound exciton lines in high-purity, undoped wurtzite GaN layers has been studied by photoluminescence (PL) between 2 and 300 K. Below 30 K the neutral donor bound exciton D deg X produces the strongest near band gap PL signal whereas free A and B excitons dominate the spectrum at room temperature. A deconvolution of the asymmetric D deg X line shape provides strong evidence for two residual shallow donors differing in ionisation energies by a factor of 1.5. The origin of a PL line occurring at Eg = 116 meV is discussed in two alternative models.