Condensed Matter Physics Seminar, Oak Ridge National Laboratory, 05/15/06

Three-Dimensional Mapping of Atomic-Scale Mixing and the Origins of Heterogeneity in Ultra-Thin Films

Karsten Pohl

Department of Physics and Materials Science Program, University of New Hampshire, Durham, NH, U.S.A.

Controlling the local structure and composition of a surface alloy is of great importance in thin film technologies. However, measuring the alloy’s heterogeneity is very difficult, because existing experimental techniques either assume lateral homogeneity or have limited subsurface or chemical sensitivity. We have analyzed the electron diffraction intensity vs. incident energy curves of the (00) beam acquired from low-energy electron microscopy (LEEM) images. In contrast to conventional LEED-IV we are able to extract structural and local composition in the surface region with a lateral resolution of 8 nm.

We have investigated the local composition of the CuPd surface alloy phase grown on Cu(001) on the terrace, far from steps, and the step-induced inhomogeneous structure caused by the step-flow growth. We find that during growth the steps flow over a 2nd layer Pd alloy, bury it and convert it to a 3rd layer alloy. This 3rd layer Pd is immobile and its concentration increases exponentially toward the step edge along the upper terrace, in agreement with the step-flow-growth model.