V. L. Shaposhnikov

Electronic and related properties of crystalline semiconducting iron disilicide

Band structure calculations for β‐FeSi2 have been performed by the linear muffin‐tin orbital method within the local density approximation scheme including exchange and correlation effects. A detailed analysis of the conduction and valence band structure around high‐symmetry points has shown the existence of a quasidirect band gap structure in the material. It is experimentally confirmed that between the threshold energy of optical interband transition of 0.73 eV and the first direct gap transition with appreciable oscillator strength at about 0.87 eV there is a region in which direct transition of low oscillator strength and indirect transitions overlap. That explains the tricky behavior of β‐FeSi2 in experimental investigations demonstrating it to be either a direct or indirect gap semiconductor.

Tungsten oxides. I. Effects of oxygen vacancies and doping on electronic and optical properties of different phases of WO3

In this part we present results of our ab initio calculations indicating that dispersion of the bands near the gap region for different phases of WO3 (namely, e-WO3, δ-WO3, γ-WO3, β-WO3, orth-WO3, α-WO3, and hex-WO3) is rather close. The rapid increase in the absorption coefficient starts at the lower energy range for α-WO3 and hex-WO3 than for the other phases in accordance with the calculated band gaps. An oxygen vacancy has turned out to decrease the gap by 0.50 eV and to shift the absorption coefficient to the lower energy range in the room temperature γ-WO3 phase. We have also traced changes caused by molybdenum and sulfur doping of γ-WO3. Only sulfur doped γ-WO3 has been revealed to display the formation of the impurity band along with a sizable reduction in the gap and the shift in the absorption coefficient to the lower energy range.

Tungsten oxides. II. The metallic nature of Magnéli phases

In the first part [D. B. Migas et al., J. Appl. Phys. 108, 093713 (2010)] electronic and optical properties of different phases of WO3 have been considered. In this part we present results of our ab initio calculations which clearly show that all Magneli phases of tungsten oxides WOx (namely, W32O84, W3O8, W18O49, W17O47, W5O14, W20O58, and W25O73) are characterized by metal-like properties. Their band structures display an energy gap in the valence band just below the Fermi level. We discuss how addition (removal) of oxygen atoms to (from) the unit cell of W18O49 affects the position of the Fermi level with respect to the energy gap and the charge carrier concentration. A possible mechanism has been suggested in order to switch from metallic to semiconducting properties for W18O49 and to explain experimental observations.

Theoretical and experimental study of interband optical transitions in semiconducting iron disilicide

The interband optical spectra of the semiconducting β phase of iron disilicide (β-FeSi2) were investigated in the energy range from 0.5 to 5.0 eV. The dielectric function and other optical functions were deduced from ellipsometric experiments and calculated within the local-density approximation by using the semirelativistic linear muffin-tin orbital method. Reasonable agreement between the calculated and measured data has been obtained.

Structural, electronic and optical properties of Ru2Si3, Ru2Ge3, Os2Si3 and Os2Ge3

We have performed a comparative study of structural, electronic and optical properties of Ru 2 Si 3 , Ru 2 Ge 3 , Os 2 Si 3 and Os 2 Ge 3 by means of ultrasoft pseudopotential and full-potential linearized augmented plane wave methods. The estimated difference in the cohesion energy between the low-temperature orthorhombic phase and the high temperature tetragonal one for all these compounds indicates that the former phase is lower in energy with respect to the latter one. All materials in the orthorhombic structure are found to be direct band-gap semiconductors, still some of them in the tetragonal structure display an indirect nature (Os 2 Si 3 ) or a competitive direct-indirect character (Ru 2 Ge 3 ) of the gap. Optical properties are discussed by analyzing the imaginary part of the dielectric function and the dipole matrix elements corresponding to different interband transitions indicating for osmium silicide and germanide the presence of low-energy transitions with an appreciable value of the oscillator strength.

Transport properties of semiconducting Rhenium silicide

Transport properties of semiconducting rhenium silicide ReSi1.75 were studied both experimentally and theoretically. For this high quality ReSi1.75 single crystals were grown by the zone melting technique with radiation heating and their resistivity and Hall coefficient were investigated. The room temperature carrier concentration in undoped material is appreciably high, about 1019 cm-3. The Hall mobility at room temperature is 30 cm2/V.s, that is considerably lower than previously reported values for single crystals. P-type conductivity persists across the entire interval investigated. Calculation of the charge carrier mobility involved the carrier effective masses, which were obtained from the ab initio electronic band structure and classical scattering mechanisms. A reasonable explanation of the hole mobility behavior is proposed and a strong anisotropy in hole effective masses and mobilities is predicted.

Electronic properties of osmium disilicide

Electronic property calculation of OsSi2 performed by the linear muffin-tin orbital method within the local density approximation scheme has shown the material to be an indirect gap semiconductor with a gap value of 0.95 eV. A direct transition with appreciable oscillator strength at 1.14 eV is predicted.

Electronic structure of stressed CrSi2

Abstract We present electronic properties of CrSi 2 under isotropic and anisotropic stress. Theoretical calculations were performed using the full-potential linearized-augmented-plane-wave method. The isotropic stress of the crystal leads to an almost linear variation of the direct and indirect transitions as a function of the lattice parameter, whereas anisotropic deformations result in more complicated dependencies. Uniaxial stretching of the lattice up to 106% converts chromium disilicide into a direct-gap semiconductor with a fundamental gap of about 0.3 eV. The compression of the lattice up to 94% changes the symmetry of the transitions.

Theoretical study of defect impact on two-dimensional MoS2

Our theoretical findings demonstrate for the first time a possibility of band-gap engineering of monolayer MoS2 crystals by oxygen and the presence of vacancies. Oxygen atoms are revealed to substitute sulfur ones, forming stable MoS2−xOx ternary compounds, or adsorb on top of the sulfur atoms. The substituting oxygen provides a decrease of the band gap from 1.86 to 1.64 eV and transforms the material from a direct-gap to an indirect-gap semiconductor. The surface adsorbed oxygen atoms decrease the band gap up to 0.98 eV depending on their location tending to the metallic character of the electron energy bands at a high concentration of the adsorbed atoms. Oxygen plasma processing is proposed as an effective technology for such band-gap modifications.

Structural, electronic and optical properties of semiconducting rhenium silicide

Structural, electronic and optical properties of semiconducting rhenium silicide (ReSi1.75) with various distributions of the silicon vacancies have been theoretically studied by means of ultrasoft pseudopotential and full-potential linearized augmented plane wave methods. We have found that the band dispersion is affected by vacancy positions, while the dielectric function and reflectivity display similar shapes for all considered variants, that can explain the rather scattered available experimental data on the gap value. Comparison between the calculated and ellipsometrically measured dielectric function and reflectivity on ReSi1.8 polycrystals grown by the Czochralski technique shows a good agreement.