ECSCD-9 - 9th European Conference on Surface Crystallography and Dynamics, Vienna University of Technology, Austria, 9/2 - 9/5/07

Local structure and composition of heterogeneous surfaces and ultrathin films

Karsten Pohl

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

An important issue in the growth of ultrathin films is controlling compositional heterogeneity. It is well known that films can be inhomogeneous, but determining exactly how and why heterogeneity develops is extremely difficult. The great challenge in identifying these fundamental processes lies in the fact that an understanding of the time-evolution of the 3D composition profile of the film is required. Detailed information of this type has proven difficult to obtain because high spatial resolution must be combined with subsurface chemical sensitivity. Techniques that are capable of accurately determining subsurface structure and composition -- electron and x-ray diffraction -- implicitly assume a laterally homogeneous film. Techniques with high spatial resolution -- scanning probe microscopies -- typically have poor chemical and subsurface sensitivity. We have recently performed new spatially resolved electron diffraction measurements that overcome these limitations and allow us, in principle, to unambiguously determine how nanometer-scale compositional inhomogeneity develops during growth. By measuring the evolution of the 3D composition of a growing film in real time, we have shown that a simple step-overgrowth mechanism, potentially relevant in many systems, is responsible for the heterogeneity we measure [1].

We have measured the structure and chemical composition of ultrathin Pd films on Cu(001) using low energy electron microscopy (LEEM). We determine the local stoichiometry and structure, with 8.5 nm lateral spatial resolution, by quantitatively analyzing the scattered electron intensity, and comparing to dynamical scattering calculations, as in conventional LEED-IV analysis. The average t-matrix approximation (ATA) is used to calculate the total atomic scattering t-matrix for randomly substitutional alloys. As in traditional LEED analysis, the structural and compositional parameters are determined by comparing the computed diffraction intensity of a trial structure to that measured in experiment. Monte Carlo simulations show how the spatial and compositional inhomogeneity can be used to understand the energetics of Cu-Pd bonding [2].

[1] J.B. Hannon, J. Sun, K. Pohl, and G.L. Kellogg, Phys. Rev. Lett. 96, 246103 (2006)

[2] J. Sun, J.B. Hannon, G.L. Kellogg, and K. Pohl, (to be published)

Work at UNH supported by the U.S. National Science Foundation and Petroleum Research Fund. Work performed at Sandia supported by the U.S. DOE, Office of BES, Division of Materials Science and Engineering. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. DOE’s National Nuclear Security Administration under Contract No. DE-AC04-94AL85000