Effect of heat treatment on the optical and electrical transport properties of Ge 15Sb 10Se 75 and Ge 25Sb 10Se 65 thin films

Abstract

The effect of heat treatment on the optical and electrical properties of Ge 15Sb 10Se 75 and Ge 25Sb 10Se 65 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 (Δ 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 Δ 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 Ge 25Sb 10Se 65 thin film is higher than that observed for the Ge 15Sb 10Se 75 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 Ge x Sb 10Se 90− x with x=15 and 25 at%. The annealing process at T a> T g results in the formation of some crystalline phases GeSe, GeSe 2 and Sb 2Se 3 as revealed in XRD patterns, which confirms our discussion of the obtained results.