Iron environment in pseudomorphic iron silicides epitaxially grown on Si(111)

Pseudomorphic iron-silicide phases grown on Si(111) have been studied by means of low-energy electron-diffraction, x-ray photoelectron-diffraction, and surface extended x-ray-absorption fine-structure experiments at the Fe K edge (7110 eV). These silicides have been epitaxially grown by codeposition of Fe and Si onto a room-temperature Si(111) substrate with silicide stoichiometry ranging from FeSi to ${\mathrm{FeSi}}_{\mathit{x}}$ (x∼2). In all cases, they were found to be epitaxial as attested to by a (1×1) low-energy electron-diffraction pattern observed after room-temperature deposition. X-ray-absorption measurements reveal that Fe atoms are coordinated with eight Si atoms for all metastable silicides with bond lengths of 2.34–2.37 Å and with Fe atoms with bond lengths in the 2.68–2.75-Å range. Fe-Si and Fe-Fe bond lengths as well as coordination numbers are found to increase with Fe contents within the silicide. These experiments confirm the formation of an epitaxial cubic and metastable CsCl-type FeSi upon deposition of Fe and Si in the 1:1 ratio onto a room-temperature Si(111) substrate. Furthermore, all data recorded from ${\mathrm{FeSi}}_2$ grown at room temperature or annealed at high temperature, are consistent with CsCl-derived or α-derived ${\mathrm{FeSi}}_2$ structures. Finally, the present data are inconsistent with the formation of a ${\mathrm{CaF}}_2$-type ${\mathrm{FeSi}}_2$ structure.