Análisis de las dependencias composicionales de las propiedades opticas de láminas semiconductoras amorfas del sistema As-S-Se

Amorphous As 40 S 60-x Se x (x = 0, 20, 30, 40 and 60 at. %) films have been prepared by vacuum thermal evaporation. Values for the refractive index and the absorption coefficient of these films have been determined from their transmission spectra, measured at normal incidence, in the spectral range from 400 to 2200 nm. It has been found that the refractive index of the samples increases with increasing Se content, over the entire spectral range. This behaviour can be explained, according to the Lorentz-Lorenz relationship, by the increased polarizability of the larger Se atoms, in comparison with S atoms. The values of the As effective coordination number, N c , have been estimated from the analysis of the dispersion of the refractive index, in terms of a model based on the single-oscillator formula. The increase in N s with increasing Se content, from around 3.0 to 3.4, which has been inferred from this analysis, suggests an increase in the structural compactness of the films when substituting S by Se. This conclusion is supported by the decrease observed in the intensity of the first sharp diffraction peak, in the X-ray patterns. Finally, it has been found a decrease in the optical bandgap with increasing Se content, from 2.38 to 1.79 eV, which can be plausibly explained considering the lower bonding energy of As-Se bonds compared with that of As-S bonds.