STM study of nucleation of Ag on Si(111)-(7 x 7) surface

Initial stages of Ag on Si(111)-(7 x 7) surface nucleation were studied at submonolayer coverage. Samples were prepared by thermal evaporation of Ag from tungsten wire under UHV conditions (p < 2.5x10(-8) Pa). Various deposition rates (0.002-0.1 ML s(-1)) were used to prepare Ag island films with coverages (0.002-2) ML (1 ML approximate to 7.58 x 10(14) atoms cm(-2)) at room temperature. We observed preferential growth on faulted half unit cells (F cells). At constant coverage both the island density and ratio of occupied F and U (unfaulted) cells are independent of the deposition rate, which is an evidence for dominant influence of substrate structure. The preference of nucleation in the F cells against U cells decreases with the coverage until the ratio is 1:1 for 1 ML Ag film. We have observed that presence of an Ag island in any type of the half unit cell (F or U) considerably reduces nucleation probability in neighbouring cells. This results in forming of structural patterns observed among randomly grown Ag-islands which is a new feature found for Ag/Si(111)-(7 x 7) system.