Dielectric function and critical points of the band structure for AlGaN alloys

The ordinary complex dielectric function (DF) of AlxGa1-xN alloys with 0 <= x <= 0.53 is determined by fitting spectroscopic ellipsometry data from the infrared to the vacuum ultraviolet spectral region (0.74 eV <= E <= 9.8 eV). The dispersion of the real part of the DF below the band gap is found to be in excellent agreement with previously published data. The obtained band gap energies are verified by photoluminescence and photoreflectance spectroscopy. In the high-energy range, three critical points of the band structure are clearly resolved. By applying a third-derivative based DF line shape analysis, the corresponding transition energies are determined. Their compositional dependences can be described on the basis of small bowing parameters.