Si magic clusters on Si(111) surfaces: Atomic structure, stability, and diffusion

Atomic and electronic structures of Si magic clusters formed during annealing/quenching of the Si(111)-7 × 7 surface are explored by means of first-principles calculations. We propose a buckled atomic chain (BAC) model of the clusters, which, although it does not show the lowest formation energy among all candidates considered, agrees with scanning tunneling microscopy data and has the lowest number of constituent atoms, thus favoring its formation kinetics upon quenching from high temperature. The BAC model consists of a buckled Si chain with alternating sp3- and sp2-like bond configurations of atoms, with half of them coming from an external source, while the other half are adatoms from the 7 × 7 structure. According to the BAC model, the cluster is stabilized on reconstructed 5 × 5, 7 × 7, 9 × 9, … Si(111) surfaces through formation of strong covalent bonds that anchor it to the substrate. Based on analysis of cluster structures, we demonstrate that they cannot diffuse without losing their integrity, but rather disassemble into smaller mobile species, which may later reassemble at different surface sites. The picture presented sheds light on our understanding of Si transport during crystal growth.