Reversible and athermal photo-vitrification of As50Se50 thin films deposited onto silicon wafer and glass substrates

Photo-vitrification of As50Se50 thin films deposited onto silicon wafer and glass substrates has been studied using X-ray diffraction, far-infrared, and differential infrared spectroscopies. The optical study of this photo-amorphization effect has been carried out by two different methods enabling the determination of the average thickness and refractive index of a wedge-shaped thin film. The refractive-index behaviour of the as-evaporated, crystallized, and photo-vitrified As50Se50 films is analyzed within the single-oscillator approach. The optical-absorption edge is described using the non-direct transition model, and the optical energy gap is calculated. In the course of the vitrification of an As50Se50 thin film deposited on a silicon substrate the photo-oxidation of the film has been additionally detected and arsenic trioxide micro-crystals were formed on the surface of the film. Such oxidation has not been observed with As50Se50 films deposited on glass substrates, which demonstrates that the photo-vitrification phenomenon depends also on the type of substrate. Finally, it is concluded from the optical study that a reversible photo-darkening effect accompanies the photo-induced vitrification phenomenon.