Studies of Silver Photodiffusion Dynamics in Ag/Ge_xS_1−x (x = 0.2 and 0.4) Films Using Neutron Reflectometry

To better understand the dynamics of silver photodiffusion into amorphous chalcogenide (Ch) films, it is informative to probe the time-dependent distribution of silver in the films while they are simultaneously exposed to visible light. Time-resolved neutron reflectometry is particularly well-suited to this purpose because it can follow time-dependent changes in the multilayer structure (Ag/Ag–Ch/Ch) while excluding the possibility of probe beam induced changes. This paper reports the results of time-resolved neutron reflectivity measurements of two Ag/Ge _x S _{1− x} ( x = 0.2 and 0.4) films as they are exposed to a visible light source. Analysis showed that silver diffusion occurs via two distinct processes: a fast diffusion that takes place during the first 2 and 10 min of sample illumination for the x = 0.2 and 0.4 films, respectively; and a subsequent slower change that is observed over the next 18 min ( x = 0.2 film) and 107 min ( x = 0.4 film). These results suggest the formation of a relatively stable Ag-rich phase in the reaction layer followed by slower diffusion at the interface between Ag-rich and Ag-poor layers. Fourier transform analysis shows that the position of the interface is essentially fixed — a conclusion that contradicts the “diffusion front” model that has been previously postulated.