Coarsening mechanisms in surface morphological evolution

On the basis of a temperature-dependent, quantitative scanning tunnelling microscopy analysis of homoepitaxial growth and erosion of Pt(111), the atomic scale mechanisms responsible for coarsening of mounds and pits are identified. For an extended coarsening regime on Pt(111), step-edge diffusion including the thermal creation of species that are mobile along steps is found to be decisive. Only when step adatoms are thermally excited at kink sites does coarsening take place. It is argued that for many low-index surfaces this coarsening mechanism is likely to be operative. Instead of the surface diffusion current driven by changes in surface curvature frequently assumed in phenomenological theory as the origin of coarsening, it appears thus that even at high temperatures coarsening is driven only by differences in curvature along contours of constant height. At lower temperatures, when no mobile species are thermally created at steps on Pt(111), only an initial coarsening takes place, as long as the step-edge barrier to the descent of adatoms is small.