Bremsstrahlung-isochromat-spectroscopy and x-ray-photoelectron-spectroscopy investigation of the electronic structure of β-FeSi2and the Fe/Si(111) interface

High-quality thin films of β-${\mathrm{FeSi}}_2$ have been obtained by annealing at 600 °C Fe layers of thicknesses 50–100 Å deposited in ultrahigh vacuum on clean Si(111) substrates. The semiconducting nature of the orthorhombic β phase has been checked by monitoring the empty states above ${\mathit{E}}_{\mathit{F}}$ with bremsstrahlung isochromat spectroscopy and the occupied states with x-ray-photoemission and Auger-electron spectroscopies. We have found that the β-${\mathrm{FeSi}}_2$ phase is semiconducting with a gap of about 1.0±0.2 eV. Recent local-density-approximation calculations yielded two different values for the energy gap, i.e., 0.44 and 0.8 eV. Both calculations reproduce fairly well the experimental energy position of the valence states but only qualitatively that of the unoccupied states. The Fe/Si interface formation at room temperature has also been investigated. At very low coverages (Θ=1–2 Å) Fe and Si interdiffuse to form the β-${\mathrm{FeSi}}_2$-like phase at the interface, while for increasing coverages (Θ≥3 Å) the interfacial region is buried by the growth of a metallic Fe film in the form of clusters or discontinuous islands.