FREE AND BOUND EXCITONS IN THIN WURTZITE GAN LAYERS ON SAPPHIRE

Free and bound excitons have been studied by photoluminescence in thin (1-10 mu m) 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 dep osition (MOCVD). An accurate value for the free A exciton binding ener gy and an estimate for the isotropically averaged hole mass of the upp ermost Gamma(9) valence band are deduced from the data on undoped samp les. The acceptor-doped samples reveal recombination lines which are a ttributed to excitons bound to Mg-0 and Zn-0 respectively. These lines are spectrally clearly separated and the exciton localization energie s are in line with Haynes' rule. Whenever a comparison is possible, it is found that the exciton lines in these thin MOCVD layers are ultrav iolet-shifted by 20 to 25 meV as compared to quasi-bulk (greater than or equal to 100 mu m) samples. This effect is interpreted in terms of the compressive hydrostatic stress component which thin GaN layers exp erience when grown on sapphire with an AIN buffer layer.