Surface diffusion of Pb on Cu(110) at low coverage: competition between exchange and jump

We have studied the surface diffusion of Pb on Cu(110) at low coverage by R utherford backscattering spectrometry ( RES) in the 500-800 K temperature r ange. The spread, after annealing, of a deposit achieved at room temperatur e is measured along the [110] and [001] directions. providing the correspon ding diffusion coefficients. The activation energy is found to be close to 0.6 eV along both directions: the ratio of the two diffusion coefficients D -[110]/D-[001] is nearly constant and equal to 2.4 in the temperature range studied. These results are interpreted in the frame of a model which takes into account the Fact that, at low coverages, Pb atoms form a disordered s urf,lcr alloy above room temperature. Thus, for a Pb atom, an elementary di ffusion event is a multi-process consisting successively in a de-insertion (exchange with a Cu adatom), some jumps along the 'open' [110] direction, a nd a reinsertion in the surface plane (exchange with a Cu surface plane ato m). The model predicts that, at low temperature, the diffusion becomes much more anisotropic as Pb adatoms undergo a great number of jumps before thei r insertion. This behaviour also accounts for the formation of two-dimensio nal Pb clusters which have been observed on various Cu surfaces by scanning tunneling microscopy experiments on low temperature deposits. (C) 1999 Pub lished by Elsevier Science B.V. All rights reserved.