Optical properties and structural changes of thermally co-evaporated CuInSe films

Homogeneous Cu20InxSe80−x (where x=10, 15, 18 and 20 at.%) films have been prepared using the co-evaporation technique. The optical measurements indicated that the electronic transition responsible for the optical absorption of the incident photons is the allowed direct transition. Increasing In content was behind the increase of the steepness of the band edges and the decrease of the optical gap of the investigated film. The optical gaps of the as-deposited Cu20In10Se70 and Cu20In20Se60 films are increased on increasing the time and/or temperature of the annealing process. Tetragonal CuInSe2 and hexagonal Cu2Se crystalline phases, resulted from heat treatment and identified using X-ray diffractometry and transmission electron microscope, have decreased the optical gap of the as-deposited films.