ECOSS 23 - European Conference on Surface Science, Berlin, Germany, 9/4 - 9/9/05
James B. Hannon (IBM Research Div.), Jiebing Sun (UNH), Gary L. Kellogg (Sandia Nat. Labs), and Karsten Pohl (UNH)
Controlling the composition of thin-film alloys is critical in a wide range of technologies. However, measuring alloy compositions at surfaces is difficult. Quantitative information on surface alloy compositions can be obtained from analysis of low-energy electron diffraction intensity versus energy spectra (LEED-IV). However, so far the structure and composition had to be assumed as spatially uniform. In this presentation we will describe low-electron energy microscopy (LEEM) studies on the formation of the well-known CuPd surface alloy phase grown on Cu(001). We will show how the presence of steps in the growth process makes the alloy layer inherently inhomogeneous. These variations in the CuPd alloy composition introduce strong measurable changes in the electron reflectivity in the regions around steps. By analyzing spatially resolved IV curves taken every 10 nm along a scan line normal to a step, we have determined the local surface alloy composition in the first 3 layers by using the average t-matrix approximation on a c(2x2) grid. In the process we are simultaneously optimizing both structural and non-structural parameters, while special emphasis is given to characterize the individual error bars of the method.
We have investigated the composition on the terrace, far from steps, and the step-induced inhomogeneous structure caused by the step-flow growth. Depositing Pd at 500 K will cause an exponential increase of the Pd concentration with time and the formation of a c(2x2) Pd checkerboard structure in the 2nd Cu layer up to a 50% Pd concentration; far from the steps where the alloy is spatially uniform. However, during growth the steps flow over the 2nd layer Pd alloy, bury it and convert it to a 3rd layer alloy. The 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.