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The diffusion of single Pb atoms and the formation of stable clusters containing two and three Pb atoms on a Si(111)−(7×7) reconstructed surface was studied with variable temperature scanning tunneling microscopy (STM). The numbers of single Pb atoms and Pb atoms in clusters were monitored as a function of time. A new and simple statistical model describing diffusion was developed. This model enables us to estimate the diffusion coefficient and the activation energy of diffusion. The main advantage is the avoidance of questionable counting of jump events in successive STM images. For substrate temperatures of 308, 311, and 313 K we obtained diffusion coefficients of 8.1×10−18, 1.2×10−17, and 1.9×10−17 cm2 s−1, respectively, and the activation energy of diffusion equaled E=0.56±0.07 eV. The energy difference ΔE between the binding energies of single Pb atoms at faulted and unfaulted halves of the Si(111)−(7×7) unit cells was estimated to be 27±9 meV.

We extend the theoretical model of recrystallization of a-Si films induced by pulsed laser irradiation and determine c-Si cluster distribution in the solid phase and we discuss the possibility of nanocluster formation in the a-Si thin layer. Our theory is based on the description of nucleation and growth of the crystalline phase in molten Si during fast heating and cooling processes. We are able to describe nonequilibrium solidification, including the distribution of nanocrystals as a function of laser pulse energy. Calculations of temperature distribution during recrystallization processes and the kinetics of crystalline phase formation show that nucleation of c-Si in l-Si must be taken into account in simulations of recrystallization of a-Si induced by high energy density pulsed laser irradiation (>200 mJ/cm2). The presence of nanoclusters in the Si layer is possible if recrystallization starts from the strong supercooled l-Si, e.g., if a-Si is melted immediately.