Stability of thin-film Cu–As$_{2}$S$_{3}$ and Ag–As$_{2}$S$_{3}$ structures

Thin-film Cu–As$_{2}$S$_{3}$ and Ag–As$_{2}$S$_{3}$ structures obtained by successively evaporating Cu(Ag) and As$_{2}$S$_{3}$ in vacuum on glass substrates have been studied. Samples of these structures have been kept in air at room temperature in the dark for 6 months, and transmission spectra have been periodically taken of these samples. The logarithm of inverse transmission coefficient in the transparency domain of As$_{2}$S$_{3}$ has been used as a measure that is proportional to the thickness of the metal film. It has been found that for Ag–As$_{2}$S$_{3}$ the thickness of the metal film varies with storage time linearly, whereas for Cu–As$_{2}$S$_{3}$ this dependence can be described by two different linear sections. From the decrease in metal film thickness, it has been concluded that the Ag–As$_{2}$S$_{3}$ structure is more stable than Cu–As$_{2}$S$_{3}$. It has been supposed that silver, unlike copper, does not react with As$_{2}$S$_{3}$ when the structures are stored in the dark, although it permanently diffuses into As$_{2}$S$_{3}$ during storage.