Effect of heat treatment on the optical and electrical transport properties of Ge15Sb10Se75 and Ge25Sb10Se65 thin films

The effect of heat treatment on the optical and electrical properties of Ge15Sb10Se75 and Ge25Sb10Se65 thin films in the range of annealing temperature 373-723 K has been investigated. Analysis of the optical absorption data indicates that Tauc's relation for the allowed non-direct transition successfully describes the optical processes in these films. The optical band gap (E-g(opt.)) as well as the activation energy for the electrical conduction (Delta E) increase with the increase of annealing temperature (T-a) up to the glass transition temperature (T-g). Then a remarkable decrease in both the E-g(opt.) and Delta E values occurred with a further increase of the annealing temperature (T-a>T-g). The obtained results were explained in terms of the Mott and Davis model for amorphous materials and amorphous to crystalline structure transformations. Furthermore, the deduced value of E-g(opt.) for the Ge25Sb10Se65 thin film is higher than that observed for the Ge15Sb10Se75 thin film. This behavior was discussed on the basis of the chemical ordered network model (CONM) and the average value for the overall mean bond energy < E > of the amorphous system GexSb10Se90-x with x = 15 and 25 at%. The annealing process at T-a>T-g results in the formation of some crystalline phases GeSe, GeSe2 and Sb2Se3 as revealed in XRD patterns, which confirms our discussion of the obtained results. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.