Raman characterization of the optical phonons in Al_xGa _1-xN layers grown by MBE and MOCVD

We present the results of Raman measurements performed on Al _xGa_1-xN layers grown by MBE and MOCVD. The films were deposited on (0001) c-sapphire substrates, and the aluminum content covered the whole composition range for x from 0 (GaN) to 1 (AIN). It is shown that the energies of both A₁(TO) and A₁(LO) phonon modes smoothly increase with increasing x, indicating a one-mode behavior. The E₂ phonon mode, however, presents a different behavior. Its energy increases very slowly with aluminum content and, for x≈0.4, a new phonon mode shows up which is shifted to higher energies by 50 cm^-1. This new line leads to the E₂ AIN mode for increasing aluminum content. The linewidths and intensities of these modes strongly depend on composition. These results are compared with recent theoretical calculations. Finally, the Raman selection rules in the MBE and MOCVD samples are compared and conclusions about the quality of the layers are drawn.