On the thickness dependence of both the optical band gap and reversible photodarkening in amorphous Ge-Se films

Amorphous Ge30.5Se69.5 and Ge39.5Se60.5 films were prepared by thermal evaporation from previously synthetized bulk glasses. We did not observe a shift in the optical band gap values associated with the change in film thickness for the virgin and annealed states of the thin films. Certain minor changes in the optical band gap values were explained by the change in the chemical composition and by the change in the sample disorder. Both ultraviolet and visible and far infrared spectra indicate that annealed Ge39.5Se60.5 thin films are practically insensitive to the illumination while Ge30.5Se69.5 thin films show a reversible photodarkening. The observed reversible changes of the optical band gap for Ge30.5Se69.5 thin films were also not dependent on the film thickness. The network structural rigidity, which can be combined with a quick relaxation of the light induced excitation directly back to the ground state, was suggested as the reason for the absence of reversible photodarkening in Ge39.5Se60.5 thin films.