Optical characterization of thermally evaporated thin films of As40S40Se20 chalcogenide glass by reflectance measurements
Optical reflection spectra, at normal incidence, of ternary chalcogenide thin films of chemical composition As40S40Se20, deposited by thermal evaporation, were obtained in the 400 nm to 2200 nm spectral region. The optical constants of this amorphous material were computed using an optical characterization method based mainly on the ideas of Minkov and Swanepoel of utilising the upper and lower envelopes of the spectrum, which allows us to obtain both the real and imaginary parts of the complex refractive index, and the film thickness. Thickness measurements made by a surface-profiling stylus have been carried out to cross-check the results obtained by the optical method. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple-DiDomenico model. The optical band gap has been determined from absorption coefficient data by Tauc's procedure. Finally, the photo-induced and thermally induced changes in the optical properties of a-As40S40Se20 thin films were also studied, using both transmission and reflection spectra.