Competitive photostructural effects in Ge-Se glass

A detailed calorimetric study of photostructural changes in glasses of the Ge-Se system is presented. The photomodified structure corresponds to a distinct entropy state intermediate between a fully thermally relaxed glass and a fresh glass. This intermediate photoinduced structure appears to result from the competitive effects of photorelaxation (negative entropic contribution) and photoexpansion (positive entropic contribution) whose simultaneous contribution leads to an equilibrium state after extensive irradiation. The compositional dependence of photorelaxation and photoexpansion reveals that strong glass formers are resilient to photostructural change relative to fragile glass formers. This observation can be explained by the presence of fewer minima on the energy landscape of strong glass former, therefore not allowing the structure to sample many configurational states and resulting in lesser photostructural changes. A comparison of the power dependence and kinetics of photorelaxation, photoexpansion, and photodarkening during subbandgap irradiation suggest that the three effects are individual and distinct components of the overall photosensitive process.