Optical properties and structure of amorphous (As0.33S0.67)100-xTex and GexSb40-xS60 chalcogenide semiconducting alloy films deposited by vacuum thermal evaporation

Amorphous films with compositions (AS(0.33)S(0.67))(100-x)Te-x (x = 0, 1, 5 and 10 at.%) and GexSb40-xS60 (X = 10, 20 and 30 at.%) have been prepared by thermal evaporation. The compositional dependences of their optical properties, with increasing Te and Ge content, respectively, are explained in terms of the modifications occurring in the film structure: Te joins the glassy network through the substitution of S atoms in the AsS3 pyramidal units, to form new AsS2Te mixed pyramidal units, on the one hand, and, on the other hand, there is a presence of GeS4 and S3Ge-GeS3 structural units, when the Ge content is increased. The refractive-index dispersion has been analysed on the basis of the Wemple-DiDomenico single-oscillator approach. It has been found that the refractive index increases in the As-S-Te system, with increasing, Te content, while it decreases instead, in the Ge-Sb-S system, with increasing Ge content. The behaviour of the Tauc gap, when the composition of the material is varied, shows, as expected, just the opposite trends.