Composition dependence of optical phonon energies and Raman line broadening in hexagonal Al x Ga 1 − x N alloys

Studies of first- and second-order Raman scattering in hexagonal Al x Ga 1 - x N alloys are reported. The dependences of frequencies of all Raman-allowed optical phonons versus Al content are traced in detail in the entire composition range. The one-mode behavior of LO phonons and the two-mode behavior of the other phonons is established. It is shown that the composition dependences of A 1 (TO), A 1 (LO), E 1 (LO), and E 2 (low) phonon energies are convenient tools for the quantitative characterization of the Al content in Al x Ga 1 - x N alloys. The energy position of the B 1 (high) silent mode is proposed. A narrow gap separating the dispersion regions of transverse and longitudinal optical phonons is revealed in the phonon density-of-state function. The composition dependence of the phonon line broadening is investigated experimentally and theoretically. It is shown that the broadening is due to elastic phonon scattering by the composition fluctuations. A theoretical approach is used where the statistical and dynamical aspects of the phonon scattering are treated separately. The type, size, and number of the fluctuations responsible for the phonon line broadening are estimated. The theory is qualitatively consistent with the observed composition dependences.