Silver Selenide Thin Films Research Articles

The formation of (0 0 1) oriented Ag 2Se 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 μm) grained Ag 2Se 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 Ag 2Se 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 Ag 2Se 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)Ag 2Se 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 Ag 2Se films will for the first time be shown in the present paper.

Temperature dependence of the carrier mobility in Ag2Se layers grown on NaCl and SiOx substrates

Electrical propertics of monocrystalline and polycrystalline silver selenide thin films have been investigated in the temperature range of −196 to +150°C. Epitaxial and randomly oriented Ag2Se layers have been grown by vacuum deposition on NaCl and SiOx substrates, respectively. Special attention was paid to changes of the “mobility” (n-type conductivity) with temperature including the polymorphic phase transformation region (from 100 to 140 °C). The measurements have been performed in both directions of the temperature variation and a difference was found in the temperature of the phase transformation (hysteresis) depending upon the heating or cooling down conditions. A typical mobility of about 1000 cm2/Vs was found at room temperature. Shape and differences in the slopes of the mobility variations have been discussed.