Optical Constants of Cubic GaN, AlN, and AlGaN Alloys

We have used spectroscopic ellipsometry to investigate, for the first time, the optical constants of cubic GaN, AlN, and AlGaN alloy epitaxial layers grown by molecular beam epitaxy. The refractive indices of cubic AlGaN were found to decrease with increasing Al content, as expected from the empirical observation that the refractive index decreases with increasing direct bandgap energy. In the transparent wavelength region, the refractive indices of cubic AlGaN with lower Al contents are somewhat larger than those of hexagonal modifications, while with higher Al contents, the refractive indices of cubic AlGaN and hexagonal AlGaN are almost equal. We also found that the direct bandgap energies of cubic AlGaN show a parabolic dependence on the Al content.