The impingement and interdiffusion of adsorbed Pb and Bi layers spreading from separated 3D pure bulk sources on Cu(100) was studied, at 513K, by in situ scanning Auger microscopy. When the leading edges of the pure Pb and Bi diffusion profiles impinge, they both consisted of low-coverage lattice gas surface alloyed phases. In these low-coverage phases, Pb displaces surface alloyed Bi and the point of intersection of the profiles drifts towards the Bi source. These features led to the conclusion that Pb atoms were more strongly bound at surface alloyed sites in Cu(100) than Bi atoms. Once the total coverage (Pb+Bi) on the substrate reaches about one monolayer, Pb and Bi were de-alloyed from the substrate, and the interdiffusion profiles became essentially symmetric. Pb and Bi mix in all proportions, with an interdiffusion coefficient of about 10−13m2/s. This was considerably smaller than the self-diffusion coefficients previously observed for pure Pb and Bi in their respective high-coverage phases, indicating that the mechanism of interdiffusion was different from that of self-diffusion. As interdiffusion proceeds, the point of intersection of the Pb and Bi profiles reverses its drift direction leading to the conclusion that binding of Bi atoms to the Cu(100) substrate was stronger than that of Pb atoms in the highest-coverage surface de-alloyed layers.

Interdiffusion of Adsorbed Pb and Bi on Cu(100). J.P.Monchoux, D.Chatain, P.Wynblatt: Surface Science, 2007, 601[4], 1101-7