Ab initio study of the mixed dimer formation in Ge growth on Si(100)

We investigate the mechanism of mixed Ge-Si dimer formation in the growth of submonolayer Ge on Si(100) through first-principles total-energy calculations. Calculated surface energies show that the Ge-Si and Ge-Ge dimer growth modes are almost equally favorable. Displacive adsorption of individual Ge adatoms to form substitutional mixed Ge-Si dimers is found to need a large activation energy due to the generation of a Si dangling bond during the reaction. We propose a mechanism, in which two Ge-Si dimers result via an atomic exchange during adatom dimerization process. This exchange dimerization process requires an activation energy of 0.1 eV, and it is more favorable than the normal Ge-Ge dimerization process in consideration of both kinetics and energetics and, thus, may explain predominant mixed Ge-Si dimers observed at submonolayer coverages of Ge. Finally, we compare calculated atomic structures of mixed Ge-Si and pure Ge-Ge dimers on Si(100) with previous theoretical and experimental results.