THE EXCITONIC BANDGAP OF GAN - DEPENDENCE ON SUBSTRATE

The large variation in the bandgap and exciton energies reported in th e literature for GaN is discussed. We compare our photoluminescence da ta for unstrained bulk GaN with thin GaN layers grown on sapphire and SiC, respectively. It is demonstrated that the built-in strain has a s trong effect on the spectral positions of the intrinsic excitons, and consequently also on the bandgap. GaN layers on sapphire have an incre ased bandgap, while growth on SiC leads to a lower bandgap, compared t o unstrained bulk GaN. The temperature dependence of the free excitons has been studied for all three cases, and remarkably strong differenc es are reported. (C) 1997 Elsevier Science Ltd.