Amorphous chalcogenide Ge20Se76Sn4 thin films of six different thicknesses (50–350 nm) are prepared by
the thermal evaporation technique. Optical transmission and reflection spectra, in the wavelength range
of the incident photons from 250 to 2500 nm, are used to study the effect of the film thickness on some
optical properties. It is found that the effect of film thickness leads to increase in the absorption coefficient,
refractive index, extinction coefficient and the width of the tails of localized states in the gap
region. The decrease in optical band gap energy with increasing the film thickness is attributed to the formation
of a band tail which narrows down the band gap. Dispersion analyses of refractive index reveal a
decrease in the single-oscillator energy and an increase in the dispersion energy with increase in film
thickness.