Coarsening dynamics of nanoscale Ti-silicide islands on Si surfaces

The late-stage coarsening dynamics of nanoscale Ti-silicide (TiSi2) islands on Si surfaces is explored in ultra high vacuum (UHV) by using ultraviolet photoelectron emission microscopy (UV-PEEM). The UV-PEEM is employed for real-time, in-situ monitoring of the nanostructure dynamics and evolution at high temperatures. Continuous annealing at similar to 1150 degrees C leads to an increase in the size of the initially nucleated silicide islands and to a reduction in the number density. By monitoring the relative position and the size of individual islands, we find that islands grow through Ostwald ripening and attractive migration and coalescence (AMC). The AMC is a new coarsening process where nearby islands are observed to migrate directly towards each other and subsequently coalesce. This process has been attributed to a growth-decay flow of the island edges driven by a non-uniform chemical potential around the islands. The non-uniform chemical potential results from a varying adatom surface concentration induced by local variations in size, number, and location of the neighboring islands. Significant shape distortions of the coarsening islands, migration of each island towards the center of mass of a group of islands, and screening of attractive migration of islands with a continuous Ti flux support our suggested model for the AMC mechanism. The mass exchange between the coarsening TiSi2 islands in the AMC process is explained in terms of a Ti adatom surface-diffusion-limited process.