Photo-induced changes of the optical constants of chalcodenide Ge_(19)Sb_1Te_(80) films

摘要

In this work a preliminary study of the influence of light illumination on the optical constants of chalcogenide thin films with non-stoichiometric Ge_(19)Sb_1Te_(80) composition is presented. Films of Ge_(19)Sb_1Te_(80) with a thickness of ～1 μm were deposited onto glass substrates by vacuum thermal evaporation of parent glasses. The as-deposited films were exposed to light by using a 500 W HBO mercury lamp. The optical constants of the films, before and after their illumination, were evaluated from the ellipsometric measurements in the spectral range of 300-820 nm. It has been established that illumination of the films leads to a decrease of the refractive index in the studied spectral region, while a slight increase of the extinction coefficient in the 600-800 nm range is observed. For the given film composition the optical bandgap energy of 0.76 eV reduces to 0.62 eV. The film structure shows a very slow recovery expressed in the increase of bandgap energy toward its initial value. The observed decrease of the bandgap energy after illumination is indicative that the film structure undergoes rearrangements upon exposure to light leading to the effect of photodarkening.