TEM study of Ag 2Se developed by the reaction of polycrystalline silver films and selenium

Abstract

The formation of Ag 2Se phase has been studied in the case of high-temperature successive deposition of polycrystalline silver thin films and selenium. It is the first time that the formation of very large Ag 2Se single crystals of (001) orientation on polycrystalline Ag films are observed and discussed. Continuous 35 nm thick silver films prepared on amorphous substrates have been reacted with various amounts of selenium at different substrate temperatures above and below the bulk polymorphic phase transition temperature of Ag 2Se ( T c=133°C). All structural investigations were carried out on films transformed to or formed directly in the low temperature orthorhombic phase. TEM investigations revealed that the continuous Ag 2Se layers undergoing phase transformation during cooling to room temperature consisted of large (100–300 μm) (001) oriented orthorhombic crystals of various azimuthal orientations. No preferred orientation was found in discontinuous Ag 2Se films. Without phase transformation the orientation remained random with grain sizes below 1 μm. Strong thickness fluctuations showing up in the form of island–channel-like morphology within the Ag 2Se crystallites has been observed in all continuous layers. The formation of large (001) oriented single crystals is proposed to be related to the polymorphic phase transition from body-centred cubic to orthorhombic phase taking place during the cooling of the film after sample preparation.