Influence of heteroepitaxy on the width and frequency of the E-2 (high)-phonon line in GaN studied by Raman spectroscopy

Wurtzite GaN layers are commonly grown heteroepitaxially on 6H-SiC or Al2O3 substrates, because of the lack of lattice-matched substrates. We study th e influence of these substrates mainly on the E-2(high)-phonon Raman line b y temperature dependent Raman spectroscopy. We find that the line broadenin g with sample heating is predominantly caused by intrinsic phonon-phonon sc attering in GaN. The small three-phonon contribution as well as the small i ntrinsic linewidth at low temperature are due to the rather low two-phonon density of states at the E-2(high)-phonon energy. Substrates with large lat tice mismatch cause inhomogeneous strain and defects in the layers, which l ead to a large, temperature independent, line broadening. We show that the temperature shift of the E-2(high)-phonon frequency is dominated by the GaN lattice expansion. The lattice of epilayers is strongly modified by the th ermal in-plane expansion of the substrate. The degree of relaxation at the growth temperature is reflected by deviation of the E-2(high)-line from the intrinsic phonon frequency. (C) 2001 American Institute of Physics.