Optical and structural properties of Ge-Sb-Se thin films fabricated by sputtering and thermal evaporation

We deposited Ge₂₈Sb₁₂Se₆₀ chalcogenide films using either thermal evaporation or radio-frequency magnetron sputtering techniques, and then measured their structural and optical properties using various diagnosis tools. The refractive indices of the films were obtained from the transmission spectra based on Swanepoel method, and the optical band gaps were derived from optical absorption spectra using the Tauc plot. Raman and X-ray photoelectron spectra (XPS) were measured and decomposed into several peaks that correspond to the different structural units, and then the evolution of the relative concentrations of the structural units was investigated in order to build up the correlation between the structure and optical properties of the films. It was found that, the films generally possess a large number of Ge ₂Se_6/2, Sb₂Se_4/2 units and Se-Se bonds, and therefore are relatively higher defective compared with their bulk counterpart. This together with the modification of the chemical compositions results in a higher refractive index as well as a smaller optical band gap in the films. Thermal annealing of the deposited films could convert the optical and structure properties of the films close to those of bulk glass.