Optical properties of Co silicides: Experiment and density functional theory

Abstract

The dielectric functions of Co2Si, CoSi, and CoSi2 compounds have been measured and compared with the calculated ones. All three compounds have very common features in their density of states, especially near the Fermi level: Co 3d-dominated characteristics with very small amounts of Co and Si sp characteristics. Consequently, the features observed in the optical conductivity (OC) spectra of 0.5–5.0 eV for all the compounds also have very similar characteristics. Since in this energy range both the occupied and empty states involved in the interband-absorption peak formation have a 3d character in an energy range, the major factor contributing to the features observed in the OC spectra is the joint-density-of-states (JDOS) effects, and the optical-transition-matrix elements do not play an important role. Co2Si, whose crystal structure is orthorhombic, has a strong optical anisotropy, as expected. The structural dependence of the electronic band structure for Co silicides has also been evaluated by examining the optical properties of the disordered and the crystalline Co-Si alloy films of the CoSi and CoSi2 stoichiometry. The change in the OC spectra can be explained by “simple” JDOS effects in the DOS curves.