Nucleation and growth of Ag islands on the phase of Ag on Si(111)

Abstract

We use scanning tunneling microscopy to measure densities and characteristics of Agislands that form on the -Ag phase on Si(111), as a function of deposition temperature. Nucleation theory predictsthat the logarithm of island density varies linearly with inverse deposition temperature.The data show two linear regimes. At 50–125 K, islands are relatively small, andisland density decreases only slightly with increasing temperature. At 180–250 K,islands are larger and polycrystalline, and island density decreases strongly withincreasing temperature. At 300 K, Ag atoms can travel for distances of the order of1 µm. Assuming that Ag diffusion occurs via thermally activated motion of single atomsbetween adjacent sites, the data can be explained as follows. At 50–125 K, the islanddensity does not follow conventional Arrhenius scaling due to limited mobility and aconsequent breakdown of the steady-state condition for the adatom density. At ∼ 115–125 K, a transition to conventional Arrhenius scaling with critical nucleus size (i = 1) begins, andat 180–250 K, i > 1 prevails. The transition points indicate a diffusion barrier of 0.20–0.23 eV and apairwise Ag–Ag bond strength of 0.14 eV. These energy values lead to an estimate ofi≈3–4 in the regime 180–250 K, where island density varies strongly with temperature.