Kinetics of Silver Photodiffusion Into Amorphous Ge₂₀S₈₀ Films: Case of Pre‐Reaction

Abstract

Silver photodiffusion into amorphous chalcogenide has attracted much attention because of its potential applications for example in memory devices. For its development, it is important to know how Ag ions diffuse in chalcogenide layers upon light exposure. In this paper, the photo-induced effect on "prereacted" films before light exposure, originating from Ag/Ge 20 S 80 /Si substrate and Ge 20 S 80 /Ag/Si substrate is investigated, using neutron reflectivity, X-ray reflectivity, and X-ray diffraction. Two types of "pre-reacted" films according to the original stacking order are obtained. In both cases, a pure Ag layer almost disappeared, and there is a small amount of monoclinic Ag 2 S. The reaction time for the photodiffusion is shorter, in both cases, than that in Ag/Ge 20 S 80 with a pure Ag layer, indicating a different reaction process. After prolonged light exposure, a uniform amorphous reaction layer is produced in the films originating from the Ag/Ge 20 S 80 /Si substrate, while both an amorphous reaction product and the Ag 2 S fragments exist in the films originating from Ge 20 S 80 /Ag/Si substrate. The mechanism of the specific photo-reaction is discussed in terms of the role of the Ag 2 S fragments.

Excitation Light Energy Dependence of Silver Photodiffusion into Amorphous Germanium Sulfide: Neutron and X‐Ray Reflectivity and X‐Ray Diffraction

“…The transient changes during the light illumination are shown in elsewhere. [ 18 ] Figure 2 shows the neutron scattering length density (SLD; see Section 8) depth profiles, which are the model depth profiles of the fits to the neutron reflectivity profiles in Figure 1. For 2.38 eV light excitation, the Ag layer remains intact and a reaction layer is formed between the Ag layer and the chalcogenide layer with step‐like Ag distribution.…”

“…[ 10–12 ] Recently, we focused on neutron beams, which do not induce silver diffusion into amorphous chalcogenides, and performed neutron reflectivity measurements for Ag/amorphous Ge chalcogenide under light illumination. [ 13–18 ] It was found from the measurements that the kinetics markedly depended on the composition of the amorphous chalcogenide film. When the film is S‐rich composition from the Ge–S system, a metastable Ag‐reaction layer is formed between the Ag and the germanium sulfide film first, and then the next Ag diffusion occurs from the metastable reaction layer to the chalcogenide host layer.…”

“…The experiment revealed that the reaction kinetics depended on the excitation light energy. [ 18 ] In this article, we investigate more details of the light energy dependence of silver photodiffusion combining neutron reflectivity, X‐ray reflectivity, and X‐ray diffraction. It is well known that a model depth profile of compositional layers resulted from a curve fit to neutron/X‐ray reflectivity profile is not a unique solution because of the phase problem.…”