Structural, electronic, and optical properties of β-(Fe1-xCox)Si2

Optimized crystal structure, electronic bands, and density of states near the band gap, and the dielectric function of β-(Fe 1 - x Co x )Si 2 with x equal to 0.0625 and 0.125 were obtained by means of total energy ultrasoftpseudopotential 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 β-(Fe 1 - x Co x )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.