Optical and structural properties of Ge–Se bulk glasses and Ag–Ge–Se thin films

The optical and structural properties of Ge20Se80, Ge25Se75 and Ge30Se70 bulk glasses and Agx(Ge0.20Se0.80)100−x thin films, where x = 0, 6, 11, 16, 20 and 23 at.% were studied. All samples were confirmed as amorphous according to XRD. The Raman spectra showed increase in 260 cm−1 and 237 cm−1 and decrease in 198 cm−1 and 216 cm−1 bands with different Se content in the bulk samples. The optical bandgap energy of bulk samples decreased (2.17–2.08 eV) and refractive index increased (2.389–2.426 at 1550 nm) with increasing Se content in bulk glasses. The Ge20Se80 thin films were prepared by vacuum thermal evaporation from Ge30Se70 bulk glasses. The Raman spectra of the films showed that peaks at 260 cm−1 and 216 cm−1 decreased their intensities with increasing Ag content in the thin films. The significant red shift of bandgap energy occurred upon different Ag content. The optically induced dissolution and diffusion resulted in graded refractive index profile along the film thickness caused by different Ag concentration. The refractive index increased from the substrate side to the top of thin films. The graded profile was getting more uniform with increasing content of silver in the thin film.