Influence of heat treatment on the optical properties and structure of Ge 20 Te 80 films

Ge20Te80 films were thermally evaporated onto ultrasonically cleaned glass substrates kept at room temperature. The as-deposited films showed an amorphous structure. An X-ray technique, a scanning electron microscope, and a transmission electron microscope were used to follow up the structural transformations that resulted from heat treatment. From the spectral dependence of the absorption coefficient, a direct electronic transition was mainly responsible for the photon absorption inside the films. The optical data indicated that the width of the localized states’ tails increases while the optical gap decreases with an increase of the annealing temperature of the investigated films. The optical results have been interpreted on the basis of amorphous–crystalline transformations.