SYNTHESIS OF EPITAXIAL TERNARY CO1-XFEXSI2 SILICIDES WITH CSCL- AND CAF2-TYPE CUBIC STRUCTURES ON SI(111) BY CODEPOSITION TECHNIQUES

摘要

We have grown ternary Co1-xFexSi2 silicide films, about 100 Angstrom thick, by codeposition onto Si(111) held at room temperature in the whole composition range 0 less than or equal to x less than or equal to 1. Low-energy electron diffraction, inelastic medium-energy electron diffraction, x-ray diffraction, x-ray photoelectron diffraction, and photoemission spectroscopy techniques are used to investigate the atomic and electronic structure of these ternary silicides, It is found that the as-deposited films are metallic and adopt an ordered cubic structure of CsCl type with essentially random vacancies, except in Go-rich films (x less than or equal to 0.25) where a tendency toward an ordered vacancy arrangement of the CaF2 type is already visible. Upon annealing at 650 degrees C, these silicides are found to be metastable, but phase separation does not take place. Fe-rich (x greater than or equal to 0.85) films invariably convert into a semiconducting phase with a structure similar to the orthorhombic beta-FeSi2 one. Yet, most interestingly, the cubic structure is preserved for x less than or equal to 0.85, i.e., stabilized when the Co content exceeds about 15%. X-ray diffraction reveals that these phases exhibit partial CaF2 long-range order. The order parameter is close to zero for x=0.85 and increases rapidly with increasing Co content. The data indicate that Fe does not merely substitute for Co atoms in a perfect CaF2-type CoSi2 structure. Several of the Fe and possibly Co atoms preferentially occupy the interstitial octahedral sites of this latter structure even for x less than or equal to 0.50, while for x greater than or equal to 0.75 a local environment essentially similar to that in defected CsCl-type structure is observed for both Fe and Co sites. [References: 33]