On the role of mobile nanoclusters in 2D island nucleation on Si(111)-(7 × 7) surface

Two-dimensional (2D) Si island nucleation has been studied by in situ reflection electron microscopy within a wide temperature range (650–1090 °С) on large-scale (∼10–100 µm) terraces to exclude the impact of step permeability and adatom sink to steps. The dependence of 2D island concentration N_2D on substrate temperature T and Si deposition rate R displays N_2D∝R^χexp (E_2D/kT) scaling which parameters change from χ≈0.81, E_2D≈1.02 eV to χ≈0.5, E_2D≈1.8 eV when Si(111) surface converts from (1 × 1) structure to (7 × 7) reconstruction. We propose that this strong E_2D rise accompanied by χ reduction is caused by the change of dominating diffusing particles from adatoms to reconstruction induced nanoclusters. Using a rate-equation model developed to account the dynamics of both diffusing species on the Si(111)-(7 × 7) surface, we show that a stable nucleus of a 2D island appears when two mobile nanoclusters merge together while nucleation kinetics is limited by their attachment to island edges.