Beta-of Fesi <sub> 2 </ Sub> Band Structure And Optoelectronic Properties Calculated

In this thesis, electronic structures, optical properties of bulkβ-FeSi₂, epitaxialβ-FeSi₂ on Si substrate and (Mn,Cr,Ni,Co)-dopedβ-FeSi₂ are investigated in detail by using first principles pseudo-potential methods based on the density function theory.Firstly, we calculated the bulk properties such as the geometric structure, band structure, density of states, partial density of states and optical properties in detail. The calculated results show thatβ-FeSi₂ is a quasidirect band gap semiconductor material and the band gap is 0.74eV; the density of states is mainly composed of Fe 3d and Si 3p; the calculation of dielectric function reveals thatβ-FeSi₂ is anisotropic, the biggest peak value of absorption is 2.67×10⁵cm^-1, the refractive index no is 4.2.Secondly, we calculated the geometric structure, electronic structure and optical properties of epitaxialβ-FeSi₂ on Si substrate in detail. The results indicate that the gap nature inβ-FeSi₂ turn from indirect to direct when a suitable strain field is induced in the structure, and the optical properties change with the change of the gap nature. Especially, the maximum absorption peak, the refractive index n, the reflectivity region and the peak of electronic energy loss function of type-C (β-FeSi₂(100)//Si(001)) are larger than the other epitaxial situations and bulk material's.Finally, the geometric structure, electronic structure and optical properties of doped(Mn,Cr,Ni,Co)β-FeSi₂ are calculated in detail. The results show that the volume ofβ-FeSi₂ cell increase with doping with Mn,Cr,Ni,Co; the total energy calculations for substitution of dopants at the FeI and the FeII sites reveal that Mn prefers the FeI site, whereas Cr, Co, and Ni prefer the FeII sites; p-type conduction is found with doping with Mn, Cr, and n-type with Co, Ni doping. We calculated the optical properties of impurityβ-FeSi₂ for the first time and found that the optical properties change with the different dopants. The spectrums of dielectric function, absorption, reflectivity, refractive index and conductivity index of dopedβ-FeSi₂ shift to the lower-energy region. Mn-, Cr-doped make the peak of energy loss function shift to the lower-energy region, and the maximum absorption peak value decreases; Co-, Ni-doped make the peak of energy loss function shift to the higher-energy region, and the maximum absorption peak value increases; Mn-, Co- and Ni-doped increase the refractive index n₀ of theβ-FeSi₂; the reflectivity region at higher energy and the conductivity also increase because of doping with Co or Ni.