Epitaxy ofCoSix(1<x<2) silicides on Si(111) studied by photoemission and extended x-ray-absorption fine-structure techniques

Electronic and structural properties of epitaxial ${\mathrm{CoSi}}_{\mathit{x}}$ layers have been investigated by means of core-level and valence-band photoemission, x-ray photoelectron diffraction, and extended x-ray-absorption fine-structure (EXAFS) experiments. ${\mathrm{CoSi}}_{\mathit{x}}$ layers of various x compositions have been grown on silicon by low rate Co and Si co-deposition onto room-temperature Si(111) substrates, with film thicknesses ranging from 30 to 100 Å. Photoemission shows substantial differences in valence and core-level spectra with respect to those of stable fluorite-type ${\mathrm{CoSi}}_2$ and ɛ-CoSi and indicate that well-defined metastable phases are formed. In particular, core-level photoemission experiments performed with a monochromatized x-ray source show large Si 2p binding-energy shifts (∼0.4 eV) in the room-temperature deposited ${\mathrm{CoSi}}_{\mathit{x}}$ (1<x<2), with respect to stable ɛ-CoSi and ${\mathrm{CaF}}_2$-type ${\mathrm{CoSi}}_2$. X-ray photoelectron diffraction as well as extended x-ray-absorption fine-structure measurements suggest that these pseudomorphic phases have a cubic structure, over a wide composition range. EXAFS measurements reveal that Co atoms are coordinated with eight Si atoms with a bond length of ∼2.33 Å and with Co atoms with bond lengths in the 2.67–2.68 Å range. Such short Co-Co bond lengths show that the structure is definitively different from the stable ${\mathrm{CaF}}_2$-type ${\mathrm{CoSi}}_2$, even at the ${\mathrm{CoSi}}_2$ composition. All experimental data indicate that ${\mathrm{CoSi}}_{\mathit{x}}$ silicides crystallize in a cubic lattice close to that of pseudomorphic ${\mathrm{FeSi}}_{\mathit{x}}$ silicides, namely, a CsCl-type derived structure. © 1996 The American Physical Society.