Adsorption and dynamics of Si atoms at the monolayer Pb/Si(111) surface

In this work, we studied the adsorption behavior of deposited Si atoms along with their diffusion and other dynamic processes on a Pb monolayer-covered Si(111) surface from 125 to 230 K using a variable-temperature scanning tunneling microscope. The Pb-covered Si(111) surface forms a low-symmetry rowlike (root 7 x root 3) structure in this temperature range and the Si atoms bind favorably to two specific on-top sites (T-1A and T-1B) on the trimer row after deposition at the sample temperature of similar to 125 K. The Si atoms were immobile at low temperatures and started to switch between the two neighboring T1A and T1B sites within the same trimer when the temperature was raised to similar to 150 K. When the temperature was raised above similar to 160 K, the adsorbed Si atoms could hop to other trimers along the same trimer row. Below similar to 170 K, short hops to adjacent trimers dominated, but long hops dominated at temperatures above similar to 170 K. The activation energy and prefactor for the Si atoms diffusion were derived through analysis of continuous-time imaging at temperatures from 160 to 174 K. In addition, irreversible aggregation of single Si atoms into Si clusters started to occur at the phase boundaries or defective sites at temperatures above similar to 170 K. At temperature above similar to 180 K, nearly all Si atoms aggregated into clusters, which may have important implications for the atomic mechanism of epitaxial growth of Si on the Pb-covered Si(111) surface. In addition, our study provides strong evidence for breaking in the mirror symmetry in the (root 7 x root 3)-Pb structure, which has implications for the atomic model of this controversial structure.