LEEM / PEEM Study of Anisotropic Diffusion Fields in the Ag/Si(001) System
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1088-W05-04
LEEM / PEEM Study of Anisotropic Diffusion Fields in the Ag/Si(001) System Dirk Wall1, Kelly Ryan Roos2, Michael Horn-von Hoegen1, and Frank-Joachim Meyer zu Heringdorf1 1 Department of Physics and Center for Nanointegration Duisburg-Essen (CeNIDE), Universität Duisburg-Essen, Lotharstrasse 1, Duisburg, 47057, Germany 2 Department of Physics, Bradley University, 1501 West Bradley Avenue, Peoria, IL, 61625 ABSTRACT We used direct imaging of diffusion fields in photoemission electron microscopy to study the diffusion (an)isotropy of Ag on flat and vicinal Si(001) surfaces with miscut angles between 0.2° and 4° in the [110] direction. While the diffusion field, represented by its iso-coverage zone, is isotropic on flat Si(001), it becomes elongated on the vicinal surface. The aspect ratio of the iso-coverage zone is used as a measure of the effective diffusion anisotropy on the surface. The aspect ratio of the iso-coverage zone increases continuously with the miscut angle, up to the maximum miscut that was studied. For the 4° miscut surface we find that the shape of the diffusion field is independent of the kink density of the substrate. From the temperature dependence of the aspect ratio, we determine an effective energy of ΔEani ~ 0.7 eV for the activation energy responsible for the anisotropy. INTRODUCTION The investigation of diffusion is of vital importance for the understanding of self organization processes at surfaces. In particular, systems that exhibit a direction-dependent surface diffusivity may tailor the self-assembly of anisotropic structures, like wires, and influence their arrangement. Indeed it was observed that anisotropic diffusion on surfaces leads to a slightly different arrangement of islands, compared to systems with isotropic diffusion [1]. Recently theoretical calculations have shown, however, that only enormously large diffusion anisotropies result in a measurable change of the island distribution [2]. While the anisotropy seems to only affect the distribution of islands in a minor way, it has been demonstrated that it can affect the shape of the islands dramatically and in fact trigger the formation of nanowires [3, 4]. Still, to quantify the parameters for diffusion, and in particular the diffusion anisotropy, remains a challenging task for both experiment and simulation [1, 2]. In our previous work [5] we have demonstrated how photoemission electron microscopy (PEEM) can be used to directly visualize surface diffusion in the Ag/Si(001) system on a micrometer scale during desorption, sampling at video rate while investigating a quasistatic equilibrium. Here, by using this technique, we investigate diffusion fields around Ag islands on well oriented as well as vicinal Si(001) surfaces, and compare the anisotropy of the diffusion field to the step structure of the Si substrate.
EXPERIMENTAL The experiments were performed in an ultra high vacuum (UHV) low energy electron microscope [6] in photoelectron emission microscopy (PEEM) mode, using a Hg discharge lamp for illumination o
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