Photo darkening in As50Se50 thin films by 532Â nm laser irradiation

The photo darkening phenomena in As50Se50 chalcogenide thin film is studied under the influence of 532 nm laser irradiation. The optical absorption edge shifts towards lower energy side with irradiation at different interval of time. The refractive index and absorption coefficient of the film was determined, both before and after exposure, by analyzing the material's transmission spectrum. The value of the absorption coefficient of the As50Se50 thin film decreases on exposing the film to laser irradiation. The change in refractive index makes the material as useful candidate for optical switching. A microscopic model in which heteropolar bonds are broken by absorption of high energy photons and new homopolar bonds are formed simultaneously has been used to describe the effect. The photo induced effects in this case were caused by heteropolar chemical bonds switching in homopolar ones, as well as additional channel of bridge heteropolar bonds switching in short-layer ones. The both processes were associated with formation of anomalously coordinated defect pairs and accompanying atomic displacements at the level of medium-range ordering. X-ray photoelectron spectroscopy (XPS) is used to investigate the chemical alternations in the bonding. The proposed model was supported by the XPS data. The films were purely amorphous in nature as revealed from X-ray diffraction and composition of the film was determined from energy dispersive X-ray analysis.