Optical characteristics of a europium-doped barium titanate inhomogeneous layer

An algorithm for solving the inverse problem of multiangle spectrophotometry of an inhomogeneous absorbing film is developed, which is based on polynomial representations of the density of the film material and its complex refraction coefficient. The algorithm is applied to determine the spectral and spatial distributions of the refractive index and absorption coefficient of a barium titanate film doped with europium. The film is deposited on a silicon dioxide layer on a silicon substrate using a sol–gel technology. The correctness of the obtained solution is verified by comparing the calculated and measured spectra of ellipsometric angles and the spectra of reflectance of the structure at normal incidence of light. Doping of barium titanate with europium leads to a decrease in the wavelength corresponding to the intrinsic absorption band edge, an increase in the band gap width of the material in the near-surface layer of the film, and a decrease in the concentration of europium deep into the film.