Amorphous to crystalline transition of selenium thin films of different thicknesses

Thin films of amorphous selenium obtained by vacuum evaporation display an increase of “optical gap” E_g^opt with an increase of thickness of the film. From the observed dependence of E_g^opt on the thickness of the film, the influence of the thickness on the gap states is interpreted in terms of the density of states model proposed by Mott and Davis. The amorphous to crystalline transition obtained by heat treatment of the specimen is also investigated. The minimum temperature for an appreciable change in crystallisation determined by the transmission of light through selenium films is also a function of the thickness and binding energy of the films. The crystalline structures resulting from heat treatment at different temperatures have been identified by scanning electron microscopy. The generation of different crystalline structures is reported in terms of the thickness and preparation conditions of the amorphous films.