Raman spectra and photostructural changes in the short-range order of amorphous As-S chalcogenides

Photoinduced changes of optical transmission, diffuse reflectance, refractive index and Raman spectra of AsxS100-x stoichiometric (x = 40) and non-stoichiometric (x = 38, 42) films, bulk glasses and glassy powders were measured. The photoinduced changes of the absorption edge, diffuse reflectance and refractive index were accompanied by changes in Raman band intensities and positions and, therefore, by changes of structure. The Raman spectra of freshly evaporated films, and spectra of As-rich, contain some bands of As4S4 molecules whose content (e.g. greater than or equal to 20 mol% in As42S58 freshly evaporated films, similar to 10 mol% for As42S58 glasses) depends on the stoichiometry and state of the films (freshly evaporated, exposed or annealed films) or upon the state of samples (bulk, powdered glass, exposed or annealed powder). The As4S4 content decreased by light exposure and also by annealing, while irradiation increases the content of As-As bonds. A model is proposed in which the photoinduced changes of the optical properties in As-S glasses are connected with photostructural changes, which change the short-range order, namely the densities of individual chemical bonds. (C) 1999 Elsevier Science B.V. All rights reserved.