  The influence of a microwave treatment (MWT) on the optical properties of
hexagonal GaN films has been studied. To estimate the internal mechanical
strains and the degree of structural perfection in a thin near-surface layer of
the film, the electroreflectance (ER) method is used. The ER spectra are
measured in the interval of the first direct interband transitions. It has been
shown that the MWT results in the relaxation of internal mechanical strains in
the irradiated films. In addition, the structural perfection in the thin
near-surface layer of the irradiated film became higher. A mechanism that
includes resonance effects and the local heating of the film defect regions is
proposed to explain the effects observed.
