RAMAN-STUDY OF THE OPTICAL PHONONS IN ALXGA1-XN ALLOYS

We have studied the Raman spectra of AlxGa1-xN layers grown by molecul ar beam epitaxy on sapphire substrates. The aluminum content x of the samples covered the whole composition range for x from 0 (GaN) to 1 (A lN). The energy of the optical modes and the broadening of the Raman p eaks 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 A(1)(LO) phonon mode continuousl y increases with increasing x (one-mode behavior), the E-2 phonon mode presents two modes with energies that slowly increase with aluminum c ontent. The frequencies of these modes are near those of pure GaN and pure AIN, and their intensities and linewidths strongly depend on comp osition. The behavior of the A(1)(TO) phonon mode could not be clearly established. The broadening of the peaks shows a maximum for an alumi num content around 80%, due to the increasing structural disorder of t he alloy. (C) 1997 Elsevier Science Ltd.