EPITAXY OF COSIX (1-LESS-THAN-X-LESS-THAN-2) SILICIDES ON SI(111) STUDIED BY PHOTOEMISSION AND EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE TECHNIQUES

Electronic and structural properties of epitaxial CoSix layers have be en investigated by means of core-level and valence-band photoemission, x-ray photoelectron diffraction, and extended x-ray-absorption fine-s tructure (EXAFS) experiments. CoSix layers of various x compositions h ave been grown on silicon by low rate Co and Si co-deposition onto roo m-temperature Si(111) substrates, with film thicknesses ranging from 3 0 to 100 Angstrom. Photoemission shows substantial differences in vale nce and core-level spectra with respect to those of stable fluorite-ty pe CoSi2 and epsilon-CoSi and indicate that well-defined metastable ph ases are formed. In particular, core-level photoemission experiments p erformed with a monochromatized x-ray source show large Si 2p binding- energy shifts (similar to 0.4 eV) in the room-temperature deposited Co Si, (1<x<2), with respect to stable epsilon-CoSi and CaF2-type CoSi2. X-ray photoelectron diffraction as well as extended x-ray-absorption f inestructure 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 l ength of similar to 2.33 Angstrom and with Co atoms with bond lengths in the 2.67-2.68 Angstrom range. Such short Co-Co bond lengths show th at the structure is definitively different from the stable CaF2-type C oSi2, even at the CoSi2 composition. All experimental data indicate th at CoSix silicides crystallize In a cubic lattice close to that of pse udomorphic FeSix silicides, namely, a CsCl-type derived structure.