An Investigation of the Rapid Cooperative Diffusion of Large Heteroepitaxial Islands

Monolayers of Ag on Cu(001) are well known to reconstruct forming a (10x2) reconstruction with Ag atoms packed hexagonally on the four-fold Cu(001) surface. We have investigated the surface diffusion of hexagonally-packed Ag monolayer islands of various sizes on Cu(001) using computer simulation techniques. Our examination reveals a novel cooperative diffusion mechanism consisting of core glide coupled with the migration of edge vacancies. In particular, analysis of several low-energy pathways reveals the two governing processes: a rapid, glide-centric process and a slower, vacancy-assisted one. It should be noted that this cooperative mechanism was observed for very large islands for which diffusion was orders of magnitude faster than for single Ag atoms at temperatures as low as 175K.

Our findings should provide insight to future experimental research on the size distribution and shapes observed during the growth of thin films in similar systems. Furthermore, these results should have implications for Monte Carlo models of film growth which generally allow only single atom diffusion, thus neglecting such complex, cooperative mechanisms.