Electron back-scattering contribution to the electron emission anisotropy by keV range electron beams

The dependence of the electron-excited Auger emission intensity on the inci dence direction for surfaces and ultra-thin overlayers has been investigate d. The interpretation of the intensity distribution curves is not straightf orward due to the superposition of true overlayer focusing-diffraction effe cts and back-scattering of the beam electrons by the substrate atoms. A mul ti-step mechanism that involves forward-focusing of the beam electrons in t he sub-surface region and their subsequent back-scattering to the surface r egion has been proposed to explain the non-zero anisotropy of the surface s ignal. In this work, we systematically test the different assumptions conta ined in the multi-step model both by direct measure of some of the relevant physical parameters such as the intensity and the anisotropy of the back-s cattered electrons by computer modelling of the experimental conditions. Mo nte Carlo simulations of the electron trajectories and single-scattering cl uster calculations helped to get information about the physical quantities which are not directly accessible by experiments. The relationship between the angular anisotropy of the surface signal and the surface structure itse lf is also discussed. (C) 2001 Elsevier Science B.V. All rights reserved.