  We present a combined experimental and theoretical study of submonolayer
heteroepitaxial growth of Ag on Si(111)-7x7 at temperatures from 420 K to 550 K
when Ag atoms can easily diffuse on the surface and the reconstruction 7x7
remains stable. STM measurements for coverages from 0.05 ML to 0.6 ML show that
there is an excess of smallest islands (each of them fills up just one
half-unit cell - HUC) in all stages of growth. Formation of 2D wetting layer
proceeds by continuous nucleation of the smallest islands in the proximity of
larger 2D islands (extended over several HUCs) and following coalescence with
them. Such a growth scenario is verified by kinetic Monte Carlo simulation
which uses a coarse-grained model based on a limited capacity of HUC and a
mechanism which increases nucleation probability in a neighbourhood of already
saturated HUCs (correlated nucleation). The model provides a good fit for
experimental dependences of the relative number of Ag-occupied HUCs and the
preference in occupation of faulted HUCs on temperature and amount of deposited
Ag. Parameters obtained for the hopping of Ag adatoms between HUCs agree with
those reported earlier for initial stages of growth. The model provides two new
parameters - maximum number of Ag atoms inside HUC, and on HUC boundary.
