Raman spectroscopy of disorder effects in AlxGa1−xN solid solutions

The results of Raman spectroscopic studies of the disorder effects in hexagonal AlxGa1−xN epitaxial layers grown by MBE and HVPE on different substrates for a large range of Al concentrations are presented. The width of the nonpolar phonon line with E2 symmetry results from the inhomogeneous broadening due to spatial fluctuations in the Al content. The abnormally small broadening of the A1(TO) polar phonon mode for x or (1−x)≪1 and the large broadening for x≅0.5–0.7 are attributed to the specific frequency dependence of the density of states for the branch with the directional dispersion in pure crystals. Thus the Raman spectrum is found to be highly sensitive to the composition of AlxGa1−xN epitaxial layers and its inhomogeneity. 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 frequencies.