Photo-induced surface metal-semiconductor transition driven by silver diffusion into amorphous chalcogenide

The bandgap light illumination of a Ag/ amorphous chalcogenide bilayer film causes Ag diffusion into the chalcogenide layer. Investigating how the electronic states and the bond configurations change is important to gain an understanding of Ag photo-diffusion on a microscopic scale. In this study, we perform the optical reflectivity, X-ray photoelectron spectroscopy, and extended X-ray absorption fine-structure spectroscopy measurements on Ag/ amorphous Ge₂₀S₈₀ films and examine the changes in measurements with bandgap illumination. The results showed that metallic reflectivity, which was caused by Ag 5s delocalized electrons, was lost and the electronic density of states at the Fermi energy disappeared upon light illumination, indicating a surface metal-semiconductor transition. Changes in the chemical bonds were clearly observed. The Ag ions were captured by the S atoms, but not by the Ge atoms. With increasing light-exposure time, both the S-S and Ge-S bonds break, and Ag-S bonds are formed. A model of the Ag photo-diffusion at the microscopic scale is proposed based on these experimental results.