Diffusion and dimer exchange in surfactant-mediated epitaxial growth

We propose a new dimer-exchange mechanism for surfactant-mediated epitaxial growth of Ge on Si(001) surfaces based on first-principles total-energy calculations. We find that, on Si(001) covered by As surfactants, Ge dimers are formed not on As dimers but between As-dimer rows, and then substitute for the As atoms at subsurface sites. This exchange process leads to As-capped needlelike growth of Ge along {11̄0} direction which prevents islanding at high Ge coverage.