Raman study of the optical phonons in Al_xGa_1-xN alloys

We have studied the Raman spectra of Al_xGa_1-xN layers grown by molecular beam epitaxy on sapphire substrates. The aluminum content x of the samples covered the whole composition range for x from 0 (GaN) to 1 (AlN). The energy of the optical modes and the broadening of the Raman peaks are determined as a function of x. Both one and two-mode behavior are simultaneously observed in the III-V semiconductor for different phonon modes. While the energy of the A₁(LO) phonon mode continuously increases with increasing x (one-mode behavior), the E₂ phonon mode presents two modes with energies that slowly increase with aluminum content. The frequencies of these modes are near those of pure GaN and pure AlN, and their intensities and linewidths strongly depend on composition. The behavior of the A₁(TO) phonon mode could not be clearly established. The broadening of the peaks shows a maximum for an aluminum content around 80%, due to the increasing structural disorder of the alloy.