Interdependence of optical parameter changes in Ge-As-S films

The presence or absence of correlation between irreversible and reversible photo- and thermoinduced changes of optical band gap (Eg) and refractive index (n) is analyzed in films (∼700 nm thick) from compositions of the Ge2S3-AsS3 glassy system. We have shown that the large irreversible photoinduced changes of Eg and n agree qualitatively with the Moss relation (no4Eg=const). Moreover, the thermoinduced changes of Eg also correlate with that of n. However, this relation is not fulfilled in some cases when annealing is performed on the virgin films illuminated in vacuum. The irreversible changes of Eg after illumination in vacuum are smaller than that after illumination in air, where photooxidation could have an influence. It is found that the component with increased Eg, which is related to photooxidation, does not govern changes in n responsible for Moss relation. In the annealed films from some compositions the photodarkening fully disappears with increase in the illumination time. The Moss relation is not observed in some stages of this photoinduced process. The quantitative deviations from Moss relation are discussed based on formulas where thickness changes, which usually show no one-to-one correspondence with ΔEg, are taken into account.