Surface segregation of Ge at SiGe(001) by concerted exchange pathways.

The segregation of Ge during growth on SiGe(001) surfaces was investigated by ab initio calculations. Four processes involving adatoms rather than ad-dimers were considered. The two most efficient channels proceed by the concerted exchange mechanism and involve a swap between an incorporated Ge and a Si adatom, or between Si and Ge in the first and the second surface layers, respectively. The calculated activation energies of approximately 1.5 eV explain well the high-temperature experimental data. Segregation mechanisms involving step edges are much less efficient.