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.

Densities of a- and a+c-type threading dislocations for a series of GaN films grown in different modes by metalorganic vapor phase epitaxy are determined from the x-ray diffraction profiles in skew geometry. The reciprocal space maps are also studied. Theory of x-ray scattering from crystals with dislocations is extended in order to take into account contribution from both threading and misfit dislocations. The broadening of the reciprocal space maps along the surface normal and the rotation of the intensity distribution ellipse is attributed to misfit dislocations at the interface. We find that the presence of a sharp AlN/GaN interface leads to an ordering of misfit dislocations and reduces strain inhomogeneity in GaN films.

Hydrogen diffusion studies were performed in GaN samples with different Fermi level positions. It is shown that, at 350 °C, hydrogen diffusion is quite fast in heavily Mg doped p-type material with the Fermi level close to $E_v\text{+0.15\hspace{0.05em}}\mathrm{eV},$ considerably slower in high-resistivity $\mathrm{p-}\mathrm{GaN(Zn)}$ with the Fermi level $E_v\text{+0.9\hspace{0.05em}}\mathrm{eV},$ while for conducting and semi-insulating $\mathrm{n-}\mathrm{GaN}$ samples with the Fermi level in the upper half of the band gap no measurable hydrogen diffusion could be detected. For these latter samples it is shown that higher diffusion temperature of 500 °C and longer times (50 h) are necessary to incorporate hydrogen to appreciable depth. These findings are in line with previously published theoretical predictions of the dependence of hydrogen interstitials formation in GaN on the Fermi level position.