Electron states of interface iron silicides on Si(111)7×7

Solid-phase epitaxy of iron silicides on Si(111)7×7 substrates is studied by comparing photoemission spectroscopy of the extended states and core levels, and Fe ${\mathit{L}}_{2,3}$ x-ray-absorption spectroscopy, for the interface phases and the bulk stoichiometric silicides. Epitaxial growth favors the low-temperature formation of metallic tetragonal α-${\mathrm{FeSi}}_2$ which starts decomposing irreversibly into semiconducting orthorhombic β-${\mathrm{FeSi}}_2$ at above 600 °C. This interface chemistry is opposite to the bulk phase diagram where the α phase is the high-temperature stable disilicide which reversibly transforms into the β phase when the temperature is lowered to 950 °C. No evidence of metastable interface phases other than the bulk phases is found. The coexistence of bulk epitaxial phases over an extended temperature range indicates that local properties of the interface strongly influence the silicide phase transitions.