Optical properties of thermally evaporated amorphous As40S60-xSex films

Compositional dependencies of the optical properties of as-deposited amorphous As40S60-xSex films (x = 0, 20, 30, 40 and 60 at.%), prepared by thermal evaporation, have been studied. The refractive index, n, and absorption coefficient, alpha, have been determined from the upper and lower envelopes of the transmission spectra, measured at normal incidence, in the spectral range from 400 to 2200 nm. An improved optical characterisation method for non-uniform films based on Swanepoel's ideas, which takes into account the weak absorption in the substrate, has been successfully employed, and it has allowed us to determine both the average thickness, (d) over bar, and the refractive index, n, of the films, with accuracies better than 1%. It has been found that the refractive index of the As40S60-xSex samples increases with increasing x, over the entire spectral range, which is related to the increased polarizability, alpha(p), of the larger Se atoms, in comparison with S atoms. The values of the As effective coordination number, N-c, have been estimated from an analysis of the dispersion of the refractive index, and an increase in N-c with increasing Se content, from around 3.0 to 3.4, has been inferred. (C) 2003 Elsevier Science B.V. All rights reserved.