Bonding state of silicon segregated toα-iron surfaces and on iron silicide surfaces studied by electron spectroscopy

Combined (x-ray, ultraviolet) photoelectron, electron- and bremsstrahlung-excited Auger-electron, and electron-energy-loss (EELS) spectroscopic investigations yield insight into the bonding state of silicon (i) as segregated onto Fe(100) forming a $c(2\mathrm{}\times 2)$ superstructure at equilibrium, and (ii) on ${\mathrm{Fe}}_3$Si(100), FeSi(100), and Fe${\mathrm{Si}}_2$(poly) surfaces. The slight binding-energy shifts of the $\mathrm{Si}2s$, $\mathrm{Si}2p$, and $\mathrm{Fe}3p$ core levels as compared with the pure elements ($\alpha$-iron and silicon) indicate a small charge transfer from iron to silicon atoms in the silicides. For silicon in the segregated state, the bonding exhibits a predominantly homopolar character. The iron silicide valence bands show an invariable nonbonding $\mathrm{Fe}3d$—derived feature and bonding iron states about 2 eV below $E_F$. Independent of the Si bulk content, the density of states near $E_F$ is always high, reflecting the intermetallic character of the iron-silicon compounds. The Si- and Fe-induced valence states and interatomic features in the $\mathrm{Si}\left(L_{23}VV\right)$ Auger transitions are evidence of the prevailing iron-silicon interaction in ${\mathrm{Fe}}_3$Si and localized silicon bonding in the monosilicide and disilicide. Segregated Si on Fe(100) at surface saturation interacts for the most part laterally, but generates some slight modification in the iron electronic structure in this configuration. The EELS spectra essentially reflect the increasing degree of valence-electron delocalization and the diminishing number of oscillating electrons in going from Fe-Si(6 at.%) to Fe${\mathrm{Si}}_2$. The results are discussed in comparison with other transition-metal silicides and with related iron compounds as well as on the basis of crystallographic data.