Crystallization kinetics and effect of thermal annealing on optical constants in a-Ge25Se75−xTex glasses

Abstract

Differential scanning calorimetric (DSC) measurements have been made in glassy Ge25Se75−xTex alloys. DSC thermograms of the samples have been recorded at five different heating rates 5, 10, 15, 20 and 25K/min. From the heating rate dependences of the glass transition temperature (Tg) and crystallization peak temperature (Tc); the activation energy for crystallization (ΔEc) and the activation energy for glass transition (ΔEg) have been derived. The present investigation indicates that both the glass transition and the crystallization processes occur in a single stage. The amorphous Ge25Se75−xTex chalcogenide thin films have been prepared by thermal evaporation onto chemically cleaned glass/Si wafer (100) substrates. Thin films have been thermally annealed at three different temperatures 333, 343 and 353K for 2h. As-prepared and thermally annealed thin films were analyzed by X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and UV–VIS–NIR absorption measurements. It is found that the mechanism of the optical absorption follows the rule of non-direct transition. The optical energy gap decreases with increasing Te content and also by thermal annealing. The decrease in the optical gap is discussed on the basis of amorphous-crystalline transformations.