American Vacuum Society - 51st International Symposium, Anaheim, CA, 11/14 - 11/19/04

How trace amounts of S enhances self-diffusion on Cu(111)

W.L. Ling, K.F. McCarty, J. de la Figuera (Sandia Nat'l Labs, CA), K. Pohl (UNH), and N.C. Bartelt (Sandia Nat'l Labs, CA)

We find that < 0.01 monolayer (ML) of S enhances surface diffusion on Cu(111) by several orders of magnitude. Using low-energy electron microscopy (LEEM) and scanning tunneling microscopy (STM), we probe the mechanisms of this enhancement by monitoring the decay rate of island stacks and the ripening of 2D island arrays as a function of S coverage and temperature. The decay rate of islands in stacks increases roughly as the S coverage cubed, consistent with the proposal that the enhanced surface diffusion is due to the formation of Cu3S3 clusters.1 However, we also find a dramatic change in the mechanism of island-stack decay and the ripening of 2D island arrays as a function of S coverage. While island decay on clean Cu is diffusion limited,2 this is not the case in the presence of a small amount of S. However, at higher S coverages, the kinetics revert to diffusion limited. Similarly, how island arrays evolve is strongly influenced by the S coverage. For clean Cu and at high S coverages, large islands grow at the expense of nearby small islands. In contrast, local ripening does not occur with smaller amounts of S - mass flows from islands to the steps bounding large terraces. We show that the strong dependence of the surface kinetics on S coverage can be quantitatively understood if the Cu diffusion at low S coverages is limited by the rate of reaction of S and Cu to form Cu3S3 clusters.
This work was supported by the Office of Basic Energy Sciences, Division of Materials Sciences of the U.S. DOE under Contract No. DE-AC04-94AL85000.

1P.J. Feibelman, Phys. Rev. Lett. 85, 606 (2000)
2M. Giesen & H. Ibach, Surf. Science 431, 109 (1999)