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 E 2 AlN mode for increasing aluminum content. The linewidths and intensities of these modes strongly depend on composition. These results are compared with recent theoretical calculations. The knowledge of these properties is important to determine the limits of their performance in device applications.
N are, to a large extent, unknown.
In the one-mode class, the frequencies of the different optic modes vary continuosly and aproximately linearly when changing the concentration of the alloy. On the other hand, in the two-mode type behavior two sets of optic modes are observed, each set corresponding to one of the two pure crystals which compose the alloy. The modes occuring in the composition region where the materials are almost pure have been atributed to either a local or a gap mode arising from a residual concentration of the minority component. 1-x N films in the compositional range 0 x 0.15 has been already performed. Consequently, they concluded that Al x Ga 1-x N presents a one mode behavior.