The diffusion from Pb deposits on Cu(100), at above-monolayer coverage, was studied under ultra-high vacuum conditions at 250 to 625K (figure 5). The growth mode was of Stranski-Krastanov type, and the deposits consisted of thick 3-dimensional Pb islands which formed above a dense Pb monolayer. The latter layer had a 2-domain structure which effectively melted at about 520K. The spread of deposits in the mm range was measured at 475 to 625K by using in situ Rutherford back-scattering analysis. In order to study diffusion at lower temperatures, and at a smaller scale, the Pb layer between the islands was removed by sputtering at about 150K. The kinetics of its re-formation were then studied at 250 to 380K by means of in situ Auger spectroscopy. The Pb diffusion was here monitored over distances which were of the order of the spacings between the Pb islands (about 1μm). The measurements demonstrated that the diffusing species was a Pb adatom which moved above the dense Pb layer and led to its spread, with a quasi-constant Pb concentration, with the Pb islands acting as an adatom source. The results were analyzed via the finite-difference numerical integration of a diffusion equation having a concentration-dependent diffusion coefficient. This showed that the activation energy which governed the process was the sum of the formation energy of Pb adatoms from islands and the activation energy for the motion of these adatoms on the Pb layer. The latter energy was markedly higher below 520K than above; thus indicating that, when the Pb layer was structured, diffusion was probably limited by the crossing of domain boundaries.

Macroscopic and Mesoscopic Surface Diffusion from a Deposit Formed by a Stranski-Krastanov Type of Growth - Pb on Cu(100) at above One Layer of Coverage. G.Prévot, C.Cohen, J.M.Guigner, D.Schmaus: Physical Review B, 2000, 61[15], 10393-403

Figure 5

Surface Diffusion of Pb on Cu