Compositional dependence of the structural and optical properties of chalcogenide thin films

Thin films of GexGa5S(95 − x) (x = 25, 30, 35) prepared by melt quenching technique were deposited on glass slides by thermal evaporation technique under vacuum. A surface profiler was employed to characterize the morphology of the thin films. Optical transmission spectra measured by a UV/VIS/NIR spectrometer, in the spectral range from 400 to 2500 nm, have been used to determine the refractive index, with Swanepoelʹs method. The thicknesses of the films were calculated with good accuracies better than 1.06%. It has been found that the refractive index of the GexGa5S(95 − x) samples increases with increasing ratio of Ge/S. Optical band gaps were calculated from Taucʹs extrapolation procedure and were found to decrease from 2.8 eV to 2.59 eV with x increasing from 25 to 35. The structural units of the films were characterized by Raman spectroscopy, indicating that in addition to the basic structural units of edge-shared and corner-shared Ge(Ga)S4 tetrahedra, there are S–S homopolar bonds in S-rich films, and the main band of the films slightly shifted to higher frequency and broadened as the ratio of Ge/S increased. The behavior of refractive index and optical band gap were interpreted in terms of structural change of atoms involved in the thin films.