Optical properties ofβ−FeSi2under pressure

Abstract

We have investigated the high-pressure optical absorption of Iron disilicide, $\ensuremath{\beta}\ensuremath{-}{\mathrm{FeSi}}_{2},$ thin films(90 nm in thickness) prepared from Si/Fe multilayers on Si (001) with template and ${\mathrm{SiO}}_{2}$ capping. It is found that the experimental absorption coefficient in the range of photon energy of about 0.3 eV beyond the band gap is a few orders of magnitude larger than the first-principles calculated absorption coefficient. A possible explanation for this large absorption coefficient is the saddle-point exciton effect by the calculated band structure. No critical points with negative hydrostatic pressure coefficients such as those of Si and GaAs are observed in $\ensuremath{\beta}\ensuremath{-}{\mathrm{FeSi}}_{2}$ near the band gap. The pressure coefficient for the direct band gap of $\ensuremath{\beta}\ensuremath{-}{\mathrm{FeSi}}_{2}$ is determined to be 15.9 meV/GPa. This small coefficient is due to the negative deformation potential of the valence-band maximum, and the large bulk modulus of $\ensuremath{\beta}\ensuremath{-}{\mathrm{FeSi}}_{2}.$