Physical and optical properties of amorphous GexAs20S80-x thin films

We report the effect of replacement of sulfur by germanium on the optical constants and some other physical parameters of chalcogenide GexAs20S80-x (where x 0, 5, 10, 15 and 20 at%) thin films. Increasing germanium content affected the average heat of atomization, average coordination number, number of constraints and the cohesive energy. Films with thicknesses 800-820 nm of GexAs20S80-x were prepared by thermal evaporation of bulk samples. Transmission spectra, T(lambda), of the films at normal incidence were obtained in the region from 400 to 2500 nm. A straightforward analysis proposed by Swanepoel [J. Phys. E Sci. Instrum. 16 (1983) p 1214], based on the use of maxima and minima of the interference fringes, has been applied to derive the real and imaginary parts of the complex index of refraction and also the film thickness. Optical absorption measurements showed that the fundamental absorption edge is a function of composition. Optical absorption is due to allowed non-direct transition and the energy gap decreases while the refractive index increases with increasing germanium content. The chemical-bond approach has been applied to obtain the excess of S-S homopolar bonds and the cohesive energy of the GexAs20S80-x system.