Influence of copper on the electronic properties of amorphous chalcogenides

We have studied the influence of alloying copper with amorphous arsenic sulfide on the electronic properties of this material. In our computer-generated models, copper is found in twofold near-linear and fourfold square-planar configurations, which apparently correspond to Cu(I) and Cu(II) oxidation states. The number of overcoordinated atoms, both arsenic and sulfur, grows with increasing concentration of copper. Overcoordinated sulfur is found in trigonal planar [S₃]⁺ configuration, and overcoordinated (fourfold) arsenic is in tetrahedral As₄⁺ configuration. Addition of copper suppresses the localization of lone-pair electrons on chalcogen atoms, and localized states at the top of the valence band are due to Cu 3d orbitals. Evidently, these additional Cu states, which are positioned at the same energies as the states due to [As₄]⁻-[S₃]⁺ pairs, found in our amorphous As₂S₃ models, are responsible for masking photodarkening in Cu chalcogenides.