Free and bound excitons in thin wurtzite GaN layers on sapphire
Free and bound excitons have been studied by photoluminescence in thin () wurtzite-undoped GaN, n-type GaN:Si as well as p-type GaN:Mg and GaN:Zn layers grown by metal-organic chemical vapour phase deposition (MOCVD). An accurate value for the free A exciton binding energy and an estimate for the isotropically averaged hole mass of the uppermost valence band are deduced from the data on undoped samples. The acceptor-doped samples reveal recombination lines which are attributed to excitons bound to and respectively. These lines are spectrally clearly separated and the exciton localization energies are in line with Haynes' rule. Whenever a comparison is possible, it is found that the exciton lines in these thin MOCVD layers are ultraviolet-shifted by 20 to 25 meV as compared to quasi-bulk () samples. This effect is interpreted in terms of the compressive hydrostatic stress component which thin GaN layers experience when grown on sapphire with an AlN buffer layer.