Optical and transport properties and electronic structure of nickel doped arsenic chalcogenides

The electronic structure of monoclinic As2X3 (X = S, Se) is investigated using full potential linearized augmented plane wave method in the framework of density functional theory. From energy bands and the density of states it is seen that the lone pair p-states of sulfur/selenium contribute closest to Fermi energy level. Introduction of transition metal impurities such as Nickel modifies the semiconducting gap in As2S3 and As2Se3. The crystal field splits the Ni 3d bands with t2g electrons in the valence band and eg electrons in conduction band. We compute optical properties like the complex dielectric functions, refractive indices, absorption coefficient, reflectance, etc. The low symmetry chalcogenides exhibit optical dichroism. On doping, As2S3 and As2Se3 show additional losses in the IR regions, which indicates allowed interband transitions due to available 3d-states in the valence and conduction bands. A narrow transparent window in the visible region is available in Ni0.5As1.5Se3 crystals. Important transport properties such as Seebeck and Hall coefficients, and carrier concentration are computed. It is seen that these high resistivity chalcogenides are n-type semiconductors.