Light-induced oxidation and band-edge shifts in thermally evaporated films of germanium chalcogenide glasses

Films of amorphous ${\mathrm{GeSe}}_2$, ${\mathrm{GeSe}}_3$, and ${\mathrm{GeS}}_2$ have been deposited at various angles of incidence onto appropriate substrates and the nature of the photoinduced oxidation (as monitored by the Ge-O peak at ∼800 ${\mathrm{cm}}^{\mathrm{-}1}$ in the infrared absorption spectrum) and optical-absorption band-edge shifts has been studied. We have identified several processes which occur on illumination and which are manifested as shifts in the optical absorption. All normally deposited films exhibit bleaching, whether or not illumination takes place in air. This behavior is ascribed to a photoinduced reconstruction of the network, resulting in a more chemically ordered structure. Although ${\mathrm{GeS}}_2$ films exhibit photobleaching irrespective of the angle of incidence, selenide films can show photodarkening, particularly in the case of obliquely evaporated films. Illumination of the films in air invariably produces a larger change in the saturation value of the optical-absorption shift (irrespective of its sign), but this can lead to a crossover behavior from bleaching to darkening for small illumination doses. The photo-oxidation which occurs on illumination in air would be expected to lead to increased bleaching. However, the photodarkening which occurs in obliquely deposited selenide films cannot be understood in this way and, although not fully understood, this behavior may be related to the greater facility for photodensification (void collapse) exhibited by selenide films compared with sulfide films.