Raman scattering study of GaN films

Raman spectra of GaN films grown by molecular-beam epitaxy and hydride vapor-phase epitaxy on GaAs and Al2O3 substrates have been studied. It was found that longitudinal phonon modes disappear from the spectra of n(+) films due to screening by free carriers, but coupled plasmon phonon modes of the higher-energy branch are not observed because of strong damping of plasmons. Precise values for phonon frequencies and linewidths are presented. No differences in phonon frequencies for the films of different thicknesses grown on different substrates have been found which indicates that the strain due to lattice and thermal-expansion mismatch is relaxed by the formation of the dislocations very close to the substrate-film interface. (C) 1996 American Institute of Physics.