Physical and optical properties of thermally deposited Ge-S-Ga thin films

Abstract

Optical properties of Ge20S80−xGax thin films, using well established Swanepoel's method, have been reported. Thin films of Ge20S80−xGax glassy alloy were deposited on glass substrates by thermal evaporation technique under high vacuum conditions (~10−5Torr). FE-SEM images show that the films are amorphous and surface smoothness improves with an increase in Ga concentration. Presence of excess number of lone pair's electrons (L) and increase in average coordination number ⟨Z⟩, show that the material under study is a good glass former. Refractive index (n1) is found to increase whereas the extinction coefficient (K) decreases with an increase in Ga content. Increase in refractive index is a consequence of increased polarization and density of the glassy alloy. Addition of Ga augments the non-linear response of GeS binary alloy. Obtained values of non-linear refractive index (nnl) are higher than the values reported in the literature for other chalcogenide glasses. Optical band gap decreases with an increase in Ga concentration. Decrease in cohesive energy and formation of enough localized states in the band gap is liable for the decrease in optical band gap. Correlation between the refractive index dispersion and the microstructure of films has been elucidated by the single effective oscillator model. Obtained optical properties are consistent with predicted physical properties of Ge20S80−xGax glassy alloy.