Unusual Ti adsorption on Si(001) and subsequent activation of Si ejection

The epitaxy of Ti on Si(001) exhibits a profound intermixing of Ti and Si atoms giving rise to the formation of titanium silicide. This phenomenon differs considerably from typical epitaxial growth and is not understood. Using first-principles total-energy calculations we examined the reaction of a Ti adatom with a Si(001) surface. We found that the penetration of the Ti adatom into a near-surface interstitial site and the subsequent ejection of its neighboring surface Si atoms onto a terrace is kinematically favored with respect to the "normal" hopping diffusion on a Si surface. These reactive processes provide the microscopic mechanism of an initial stage of transition-metal silicidation.