Fe and Si were codeposited on Si(lll) with an understoichiometric atom
ic ratio of 1:3 at substrate temperatures between 580 and 720 degrees
C in order to investigate the nucleation and growth of silicide precip
itates under growth conditions similar to molecular beam allotaxy. Med
ium energy ion scattering and transmission electron microscopy were us
ed to determine cluster sizes, phases and shapes. The surface density
of clusters decreases and the average cluster size increases with incr
easing substrate temperature. Using Venables' theory on surface diffus
ion, we found an activation energy for surface diffusion of 0.76 +/- 0
.10 eV for Fe on Si(111) during coevaporation. Our results show that a
FeSi2 cluster containing more than three Fe atoms may be regarded as
stable on the surface. Clusters of the high-temperature alpha- and of
the low-temperature beta-phase were found side by side on the same sam
ples.