Raman spectroscopy of disorder effects in AlxGa1-xN solid solutions

The results of Raman spectroscopic studies of the disorder effects in hexag onal AlxGa1-xN epitaxial layers grown by MBE and HVPE on different substrat es for a large range of Al concentrations are presented. The width of the n onpolar phonon line with E-2 symmetry results from the inhomogeneous broade ning due to spatial fluctuations in the Al content. The abnormally small br oadening of the A(1)(TO) polar phonon mode for x or (1 - x) << 1 and the la rge broadening for x congruent to 0.5-0.7 are attributed to the specific fr equency dependence of the density of states for the branch with the directi onal dispersion in pure crystals. Thus the Raman spectrum is found to be hi ghly sensitive to the composition of AlxGa1-xN epitaxial layers and its inh omogeneity. It is shown that in the estimation of the crystal composition, on the basis of Raman data, the influence of the homogeneous strain effects could be excluded via measuring a linear combination of two Raman line fre quencies. (C) 1999 Elsevier Science S.A. All rights reserved.