Optical and calorimetric studies of Ge–Sb–Se glasses

Optical properties of amorphous GexSb40 − xSe60 (x=22.5, 25, 27.5, 30 and 32.5 at.%) thin films prepared by electron beam evaporation have been measured in the visible and near-infrared spectral region. The variations of the electronic energy gap (Eg) with composition are discussed on the basis of both the overall mean bond energy and the topological model based on the structural dimensionality considerations. A pronounced peculiarity (maximum or minimum) appeared at around the same value of the average coordination number (Z)=2.65 for all the compositional dependencies of electronic energy gap, high frequency dielectric constant, optical relaxation time and the dispersion parameters, is attributed to a topological phase transition from two-dimensional (2D) layer type to three-dimensional (3D) cross-linked network structures in the present glassy thin films. This confirms the suitability of structural topological model to explain the compositional dependencies of the physical parameters, not only for bulk glasses but also for thin films evaporated from them. These results are supported by volumetric and the calorimetric measurements.