OPTICAL-PROPERTIES NEAR THE BAND-GAP ON HEXAGONAL AND CUBIC GAN

We have measured the photoluminescence and optical reflection spectra of cubic and hexagonal GaN films epitaxially grown on GaAs and 3C-SiC substrates by gas-source molecular beam epitaxy using microwave-activated NH3 source. The crystalline quality of the epilayers was improved by using an activated NH3 beam and 3C-SiC substrates, which enabled us to obtain intense optical signals. The measured spectra suggest that the band gap of cubic GaN is around 190 meV smaller than that of hexagonal GaN.