DIFFUSION MECHANISMS RELEVANT TO METAL CRYSTAL-GROWTH - PT/PT(111)

We have studied theoretically diffusion processes relevant to metal crystal growth, focusing on the Pt(111) system. Using an EAM-type potential function to describe the atomic interactions, we have determined minimum energy paths and evaluated activation energy barriers for various adatom hop and exchange diffusion mechanism. We find a surprisingly wide range of activation energies for descent of adatoms from atop islands, with the energy barriers not scaling simply with the energy of the initial and final states. Short and irregular island edges can have an order of magnitude lower descent barriers than long, straight edges, primarily due to the presence of corner atoms. However, heptamers and smaller islands with only corner atoms at the island edge have very high barriers for descent. With this exception, our results support the hypothesis made earlier by Kunkel, Poelsema, Verheij and Comsa [Phys. Rev. Lett. 65 (1990) 733] that small and irregular islands provide lower barriers for adatom descent, which helps explain the observed reentrant layer-by-layer growth. We have also studied the approach of adatoms towards descending and ascending steps and find the adatom is in both cases attracted towards the step edge, resulting in trapping at low temperature.