We present the results of room- and low-temperature measurements of second-order Raman scattering for perfect GaN and AlN crystals as well as the Raman-scattering data for strongly disordered samples. A complete group-theory analysis of phonon symmetry throughout the Brillouin zone and symmetry behavior of phonon branches, including the analysis of critical points, has been performed. The combined treatment of these results and the lattice dynamical calculations based on the phenomenological interatomic potential model allowed us to obtain the reliable data on the phonon dispersion curves and phonon density-of-states functions in bulk GaN and AlN.

The first- and second-order Raman scattering and IR reflection have been studied for hexagonal InN layers grown on (0001) and $\text{(11̄02)}$ sapphire substrates. All six Raman-active optical phonons were observed and assigned: $E_2(\mathrm{low})$ at $\text{87\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}},$ $E_2(\mathrm{high})$ at $\text{488\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}},$ $A_1(\mathrm{TO})$ at $\text{447\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}},$ $E_1(\mathrm{TO})$ at $\text{476\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}},$ $A_1(\mathrm{LO})$ at $\text{586\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}},$ and $E_1(\mathrm{LO})$ at $\text{593\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}}.$ The ratio between the InN static dielectric constants for the ordinary and extraordinary directions was found to be ${\epsilon }_{\text{⊥0}}{\mathrm{/\epsilon }}_{\text{∥0}}\text{=0.91.}$ The phonon dispersion curves, phonon density-of-state function, and lattice specific heat were calculated. The Debye temperature at 0 K for hexagonal InN was estimated to be 370 K.

Studies of first- and second-order Raman scattering in hexagonal ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ 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 $A_1\left(\mathrm{TO}\right),$ $A_1\left(\mathrm{LO}\right),$ $E_1\left(\mathrm{LO}\right),$ and $E_2\left(\mathrm{low}\right)$ phonon energies are convenient tools for the quantitative characterization of the Al content in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ alloys. The energy position of the $B_1\left(\mathrm{high}\right)$ 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.