Near-bandgap optical properties of Al _{1− <i>x</i>} In _<i>x</i> N thin films grown on a <i>c</i> -plane freestanding GaN substrate

We investigated the relationship between the optical constants and localized states near the band-edge in high-quality crystalline Al 1− x In x N alloys, with an indium content x ranging from 0.12 to 0.22, grown on a c -plane freestanding GaN substrate. Optical constants were obtained by spectroscopic ellipsometry. The tanΨ and cosΔ spectra were fitted by the Adachi’s critical-point (AC) model. The effects of the near-band-edge localized states on the optical constants were characterized by the spectral broadening factor γ , which was obtained by the AC model. The γ increased with increasing x and this tendency also confirmed by the γ obtained by the photoluminescence excitation (PLE). The bandgap energies obtained by the AC model agree well with those obtained by the PLE. It is suggested that the indium-related near-band-edge localized states cause the spectral broadening of the refractive index and extinction coefficient spectra in Al 1− x In x N alloys.