We have studied the dependence of the absorption edge and the refractive index of wurtzite Al_xGa_1-xN films on composition using transmission, ellipsometry and photothermal deflection spectroscopy. The Al molar fraction of the Al_xGa_1-xN films grown by plasma-induced molecular beam epitaxy was varied through the entire range of composition (0  x  1). We determined the absorption edges of Al_xGa_1-xN films and a bowing parameter of 1.3 ± 0.2 eV. The refractive index below the bandgap was deduced from the interference fringes, the dielectric function between 2.5 and 25 eV from ellipsometry measurements. The measured absorption coefficients and refractive indices were used to calculate the design and reflectivity of AlGaN-based Bragg reflectors working in the blue and near-ultraviolet spectral region.

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