Direct Observation of Two Dimensional Magic Clusters

Two dimensional magic clusters have been directly observed on the √3×√3R30^∘ reconstructed Ga/Si(111) surface using scanning tunneling microscopy. The magic numbers are $n(n+1)/2$, where n (2, 3, 4, or 5) is the number of atoms on each side of these triangular clusters with preferred orientation with respect to the substrate. The √3×√3R30^∘ adatom lattice surrounding the magic clusters exhibits characteristic vacancies. A structural model is proposed to account for the cluster orientation and lattice vacancies as well as the extraordinary abundance and stability of the decamers (n0ex0ex=0ex0ex4).