Spatially resolved investigations of the excitonic luminescence in GaN

We performed spatially-resolved investigations on thick GaN layers using cathodoluminescence and micro-Raman experiments. Our measurements reveal that the peak position of the excitonic transition lines strongly depends on the distance to the substrate interface. The luminescence is shifted continuously to lower energies with decreasing distance, however, a strong blue shift occurs directly at the interface. We correlate these effects with bandgap renormalization and band filling effects induced by a strong gradient of free-carrier concentration in addition to a strain gradient found by our Raman experiments.