During a scanning tunnelling microscopic study of (111) surfaces, at temperatures ranging from ambient to 600C, a special type of cluster was found which was not only stable with respect to surface diffusion, but was also the fundamental unit in mass transport phenomena, step fluctuations in detachment and attachment of Si atoms at step edges, and in epitaxial growth. On (111), it was unfavorable for the so-called magic clusters to grow in size by adding one or a few adatoms. Further deposition would increase the number density of the magic clusters, rather than lead to the formation of larger clusters. The interaction between magic clusters was attractive but weak. Scanning tunnelling microscopic images suggested that growth occurred via the destruction of magic clusters and almost simultaneous local surface reconstruction; followed by re-ordering of the atoms into a crystalline bilayer island. As much bond-breaking was involved, this so-called concerted reaction process had to occur at high temperatures.

Dynamic Behavior of Si Magic Clusters on Si(111) Surfaces I.S.Hwang, M.S.Ho, T.T.Tsong: Physical Review Letters, 1999, 83[1], 120-3