Formation of Ge clusters at a Si(111)-Bi-$\sqrt{3}\times\sqrt{3}$ surface

The possibility of the formation of a dense array of size-uniform Ge clusters on the Si(111) surface in the presence of the Bi surfactant has been demonstrated using scanning tunneling microscopy. It has been shown that the deposition of germanium at room temperature leads to the formation of two types of clusters. Clusters containing two to four atoms are one monolayer thick and are mobile on the Bi layer. The second type, bilayer clusters, containing eight to ten atoms, transform into epitaxial islands after annealing at $400$–$500{}^\circ$C. Models of possible atomic structures of bilayer clusters have been considered taking into account their positions relative to Bi trimers. It has been found that the probability of the substitution of Si atoms for Ge atoms during cluster formation does not exceed $20\%$.