Near-bandgap optical properties of Al1-x In x N thin films grown on a c-plane freestanding GaN substrate

We investigated the relationship between the optical constants and localized states near the band-edge in high-quality crystalline Al1-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 tan psi and cos Delta 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 gamma, which was obtained by the AC model. The gamma increased with increasing x and this tendency also confirmed by the gamma 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 Al1-x In x N alloys.