SOME PROPERTIES OF GALLIUM NITRIDE FILMS GROWN ON (0001) ORIENTED SAPPHIRE SUBSTRATES BY GAS-SOURCE MOLECULAR-BEAM EPITAXY

The Raman and photoreflectivity spectra of gallium nitride (GaN) films grown on (0001) oriented sapphire substrates by gas source molecular beam epitaxy (GSMBE) have been investigated. The Raman spectra showed the presence of the E-2(high) mode and a shift in the wavenumber of th is mode with respect to the GaN epilayer thickness. The Raman scatteri ng results suggest the presence of stress due to lattice and thermal e xpansion misfit in the films, and also indicate that the buffer layer play an important role in the deposition of high quality GaN layers. T he residual stress changes from tensile to compressive as the epilayer thickness increases. Samples subjected to anneal cycles showed an inc rease in the mobility due probably to stress relaxation as suggested b y an observed shift in the E-2(high) mode in the Raman spectra after a nnealing.