Effect of compositional variations on the optical properties of Sb–Ge–Se thin films

Compositional dependences of the optical and physical properties of as-deposited amorphous SbxGe25-xSe75 films (x = 0, 5, 10, 15 and 20 at%), prepared by thermal evaporation, have been studied. The refractive index, n, and film thickness, d, have been determined from the upper and lower envelopes of the transmission spectra, measured at normal incidence, in the spectral range from 400 to 2500 nm. An optical characterization method for uniform films based on Swanepoel's ideas has been employed, and it has allowed us to determine the average thickness, , and the refractive index, n, of the films, with accuracies better than 1%. It has been found that the refractive index of the SbxGe25-xSe7 samples increases with increasing x, over the entire spectral range, which is related to the increased polarizability of the larger Sb atomic radius 1.38 A compared with the Ge atomic radius 1.22 A. The dispersion parameters such as E0 (single-oscillator energy) and Ed (dispersive energy) were discussed in terms of the single-oscillator Wemple-DiDomenico model. The absorption coefficient, alpha, and therefore the extinction coefficient, k, have been determined from the transmittance and reflectance spectra in the strong absorption region. The optical energy gap is derived from Tauc's extrapolation and a decrease in with increasing Se content. Finally, in terms of the chemical bond approach, cohesive energy has been applied to interpret the decrease in the glass optical gap with increasing Sb content in SbxGe25-xSe75.