The formation of (001) oriented Ag2Se films on amorphous and polycrystalline substrates via polymorphic-phase transition

Silver selenide thin films have been prepared on amorphous and polycrystalline substrates by the solid-vapour-phase reaction of vacuum deposited polycrystalline silver films and selenium. We have found earlier that at given conditions (0 0 1) oriented giant (100-300 mu m) grained Ag2Se layers develop. The details of formation of these peculiar films have been now studied by ex situ and in situ transmission electron microscopy. By quenching the samples from the reaction temperatures to room temperature earlier unknown intermediate stages of the formation of oriented giant grains were revealed. We observed, that during the reaction of polycrystalline Ag and selenium polycrystalline Ag2Se film develops, which can then transform to a (0 0 1) oriented giant grained one. The formation of these oriented films occurred always only on slowly cooling during the polymorphic transition from the high-temperature cubic - to the low-temperature orthorhombic phase as a result of the selective appearance and fast growth of (0 0 1) nuclei of the orthorhombic Ag2Se at the free upper film surface. The enthalpy difference between the HT and LT phases and the low surface energy of the (0 0 1)Ag2Se is supposed to be the driving force of this process. The fundamental role of the polymorphic-phase transition in the formation of (0 0 1) oriented, giant grained Ag2Se films will for the first time be shown in the present paper. (C) 1999 Elsevier Science B.V. All rights reserved.