Kinetics of cluster diffusion, evaporation, coalescence and coarsening

Several models for the diffusion, evaporation and coarsening kinetics of two-dimensional and three-dimensional islands and vacancy islands on surfaces are examined for a wide range of island sizes and temperatures. These kinetic processes are central in surface phenomena such as thin film coarsening, island aggregation and coalescence, which ultimately depend upon the mechanism by which single atoms evaporate diffuse and reattach to the islands. We find that the naive cluster scaling theories only very approximately describe the kinetics. In general, the diffusion and evaporation scaling exponents are both a function of the details of the system's energetics and the temperature. Using Monte Carlo models, we find that microscopic roughness, spatial and temporal correlations, lattice anisotropies and other kinetic factors account for this discrepancy. Furthermore, these thin films of adsorbates on solid surfaces often exhibit an irreversible clustering and island growth phenomena where the mean island size grows larger with a temporal power law dependence, accompanied by a scaling island size distribution function. We find that the traditional Ostwald ripening formulation is incomplete since it omits critical atomic level processes such as island mobility, spatial correlation between kinetic processes and surface roughening of the islands. Using Monte Carlo simulations for a simple broken bond model we have extracted rate constants for island evaporation-recondensation, island coalescence, and island diffusion. Using these rate equations, a quasi-chemical mean field approach is formulated as a high dimensional set of second order kinetics equations. The relative importance of Ostwald theory versus island coalescence is evaluated. Then we examine all realistic Monte Carlo/molecular dynamics model for Ag/Ag(100). We find that the role of exchange versus hopping diffusion and the detachment of adatoms from island edges to be crucial to both the island diffusion mechanism and the evolution of the coarsening adlayer. Finally, we examine the scaling kinetics of the post-deposition evolution of the morphology of a thin three-dimensional film of Ag on a mica substrate, using a combination of experimental and theoretical techniques. We find that the Ag film can be stabilized at room temperature by subjecting the film to a low temperature annealing process.