On the photo- and thermally-induced darkening phenomena in As40S40Se20 amorphous chalcogenide thin films

Annealing (in vacuum) at a temperature near the glass transition temperatur e and exposure (in air) with bandgap light of thermally-evaporated As40S40S e20 amorphous chalcogenide thin films, were found to be accompanied by stru ctural changes, which lead to changes in the refractive index and shifts in the optical absorption edge. Indications of photo-oxidation were found aft er light exposure in air. An optical characterization method, based on the transmission spectra at normal incidence of uniform thickness thin films, h as been used to obtain the optical constants and the film thicknesses corre sponding to the virgin, annealed and exposed As40S40Se20 samples. The dispe rsion of the refractive index is discussed in terms of the single-oscillato r Wemple-DiDomenico model. The optical absorption edges are described using the 'non-direct transition' model proposed by Tauc.