Free Excitonic Emission in Homoepitaxial Layers Grown on Bulk GaN Substrates

Variable temperature studies of free excitonic and phonon-assisted excitonic transitions in homoepitaxial gallium nitride (GaN) layers grown on bulk substrates are presented. Photoluminescence measurements were performed in the temperature range between 4.2 K and 100 K in order to compare epilayers grown on substrates with different polarities. Surprisingly, one of the investigated samples exhibits unique properties which have not been reported for homoepitaxial GaN before - the low temperature free excitonic emission is more intense than neutral donor bound excitonic transitions. As expected for high quality homoepitaxial GaN layers, the excitonic spectrum of this sample consists of narrow spectral lines. Moreover, the shape of a longitudinal optical phonon replica of the free excitonic emission suggests a very low concentration of point defects involved in exciton scattering. This hypothesis concerning a high purity of the sample, is also corroborated by a very low signal of donor bound excitons. However, an observed overall optical inferiority in terms of photoluminescence intensity also suggests the appearance of non-radiative recombination channels which selectively affect radiative exciton recombination.