AVS 45th International Symposium
    Surface Science Division Thursday Sessions
       Session SS1+NS-ThM

Paper SS1+NS-ThM3
The Influence of Dislocations on the Intermixing Kinetics of Pd-Au Monolayer Films

Thursday, November 5, 1998, 9:00 am, Room 308

Session: Growth and Thin Films
Presenter: O. Schaff, Sandia National Laboratories
Authors: O. Schaff, Sandia National Laboratories
A.K. Schmid, Sandia National Laboratories
M.C. Bartelt, Sandia National Laboratories
R.Q. Hwang, Sandia National Laboratories
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The kinetics of surface alloying differ dramatically from the bulk counterparts. This is due to the fact that the dominant diffusion mechanisms are fundamentally different on surfaces. In this work, we used classical bulk studies as a guide to construct an effective experimental approach to directly determine such atomic mechanisms controlling intermixing kinetics in two-dimensional samples. On a clean Ru(0001) substrate, we prepared prototypical two-dimensional diffusion couples, consisting of monolayer regions of Au adjacent to monolayer regions of Pd. Between carefully controlled annealing steps, atomic resolution scanning-tunneling microscopy was used to image changes in the adlayer structure, and thereby identify and characterize the phenomena leading to the formation of a Pd-Au surface alloy. We report several striking observations: (I) The Pd-Au surface alloy forms only on the Au-rich side of the diffusion couple, reflecting strong asymmetry in the diffusion constants of the two metals across the Pd-Au seam. (II) Diffusion constants, and thus alloying, in this system are strongly anisotropic. Specifically, the rate of interdiffusion at a given Pd-Au boundary, measured from changes in the average position of the alloy interface, depends strongly on the orientation of the boundary with respect to the underlying Ru lattice. We relate these observations to the energetics of exchange of surface atoms with the "gas" of Au and Pd adatoms, as well as to the dynamical properties of the dislocation network present in the Au side of the diffusion couple.