The initial stages of ultrathin epitaxial COSi2 formation on Si(111) a
nd Si(100) surfaces were studied by core-level photoelectron spectrosc
opy with synchrotron radiation. A high surface sensitivity was obtaine
d for electrons with kinetic energy of approximately 30 eV, where the
mean free path is estimated to be less than 2.5 angstrom. Our Si2p dat
a show that on Si(111) a part of the Co atoms are in a CoSi2-like envi
ronment at room temperature, the rest forms a Co-Si solid solution. Fo
r a complete reaction, annealing to approximately 350-degrees-C is nee
ded. In this way we obtain the Co-rich CoSi2(111)-C surface terminatio
n free of pinholes which is stable up to at least 460-degrees-C. After
annealing to approximately 550-degrees-C the Si-rich CoSi2(111)-S sur
face termination containing some pinholes appears. Contrary to the Si(
111), Co deposition on Si(100) surface at room temperature does not le
ad to CoSi2 but only to a Co-Si solid solution. For coverages below 2.
6 ML heating to approximately 460-degrees-C is necessary to obtain CoS
i2, whereas for higher coverages heating to approximately 300-degrees-
C is sufficient. This effect is attributed to a change of the Co diffu
sion mechanism at room temperature. The CoSi2 films on the Si(100) sur
face produced in this way always contain pinholes.