Statistical Ga clusters and A(1)(TO) gap mode in AlxGa1-xN alloys

Results of a comprehensive study of the behavior of the A(1)(TO) phonon mod e in hexagonal AlxGa1-xN alloys in the entire compositional range are descr ibed. It has been found that the Raman spectrum of AlxGa1-xN, with a Ga con tent (1-x)<0.3, exhibits a large broadening with a complex structure. We at tribute this structure to a manifestation of the phonon density of states i n the region of vibrations of the optical A(1)(TO) branch, and to the appea rance of a gap mode in AlN. Both effects are due to the substitution of hea vier ea atoms in the cation sublattice of,AlN. A theoretical approach is su ggested which describes changes in the vibrational spectrum at a sufficient ly strong perturbation resulting from isoelectron substitution. In the fram ework of the developed model, the dependence of the intensity and band shap e of the gap mode on Ga content are calculated and compared with the experi mental Raman data. The experimental and theoretical dependences are shown t o be in good agreement in the region (1-x)<0.12-0.15. The obtained results indicate that the formation of an A(1)(TO) gap mode in the regions of low a nd intermediate Ga contents is caused by statistical Ga clusters in the cat ion sublattice of the solid solution. In a limited range of Ga contents, th e behavior of the A(1)(TO) phonon mode in AlxGa1-xN can be considered as a two-mode behavior.