Formation of Si clusters and their role in homoepitaxial growth on Si(1 1 1)-7 × 7 surfaces

With scanning tunneling microscopy (STM), we observe the dynamic behavior of Si atoms on Si(1 1 1)-7 × 7 surfaces. At temperatures below ∼200 °C, Si atoms are found to aggregate into small clusters that show interesting structures on surfaces. When the substrate temperature is raised above 200 °C, only a new type of Si magic clusters is discovered. These clusters dictate many dynamic phenomena on Si(1 1 1) surfaces, including mass transport, step fluctuations, epitaxial growth, and the decay of non-equilibrium structures. At temperatures above ∼400 °C, the magic clusters become mobile. Most of the clusters are confined within the half-cells, but some of them move out of the half-cells and re-appear at a distance of about a few hundred angströms (Å) away. Using Arrhenius analysis, the activation energies and pre-exponential factors for different moving pathways are derived. The jump-length distribution exhibits two maxima; one at the origin and another at ∼500 Å away. The growth of islands can occur either at the step edges or by a concerted reaction at the terraces as the cluster density fluctuates to reach a threshold value.