The Optical Properties, Electronic Structure, and Photoconductivity of Arsenic Chalcogenide Layer Crystals

Abstract

Among the many semiconducting layer crystals, As2S3 and As2Se3 are distinguished from several points of view. The two-dimensionally-extended network which forms the molecular building block in these crystals is held together by nearly purely-covalent bonding of the lowest connectivity (threefold coordination) compatible with a layer structure. Unlike graphite (with its π-bonding admixture), the bonding is entirely σ-type; unlike the transition-metal dichalcogenides (with their complicating d electrons), only s and p valence electrons are involved; and unlike the gallium chalcogenides (with their Ga—Ga bonds), only a single bond type occurs. Thus from a chemical-bonding and bonding-topology viewpoint, the arsenic chalcogenides are perhaps the simplest of layer crystals. From a crystallographic viewpoint, however, they are more complex than the other cases mentioned; their layer and crystal symmetries are low, and their unit cells contain relatively many atoms. (The number of atoms per layer unit cell in As2S3 is 10, compared to 2 for graphite, 3 for MoS2, and 4 for GaSe.) This combination of structural complexity and chemical simplicity in the arsenic chalcogenides provides a rare opportunity for the observation and elucidation of interlayer-interaction effects, and has been exploited in recent lattice-vibrational studies of the layer-layer coupling in these crystals [1,2].