Structural, electronic, and optical properties of beta-(Fe1-xCox)Si-2

Optimized crystal structure, electronic bands, and density of states near the band gap, and the dielectric function of beta-(Fe1-xCox)Si-2 with x equal to 0.0625 and 0.125 were obtained by means of total energy ultrasoft pseudopotential and full-potential linearized augmented plane-wave calculations. We address a preferential occupation of the Fe-1 and Fe-2 sites by the Co atoms at different concentrations indicating the Fe-2 site to be energetically favorable. We also discuss a variation in the energy gap and in the position of the donor level in beta-(Fe1-xCox)Si-2 with increasing Co content. Calculated imaginary and real parts of the dielectric function show a good agreement with results of ellipsometric measurements of samples grown by molecular-beam epitaxy.