Surface Analysis by Electron Spectroscopy

Abstract

In AES and XPS, electrons are emitted from the sample as a consequence of electron or X-ray irradiation respectively, and are subsequently energy-analysed and detected. For electrons in the energy range 100 to 1000 electron volts (eV), the distance that may be travelled before undergoing an inelastic collision, known as the inelastic mean free path, may be typically of the order of 2–3 nm. This distance corresponds to perhaps 10 atom layers in most materials, and it is this that gives the techniques their surface specificity. Experimentally, this inelastic mean free path is very difficult to measure and, in practice, a parameter known as the attenuation length, which also includes the effect of elastic scattering, is determined instead. Figure 2.1, from the work of Seah and Dench (1979), shows a compilation of measured attenuation length data for elements. These data are primarily derived from thin overlayer experiments in which the structure of the overlayers was usually not well characterised, with the result that the average of the compilation is systematically low. Nevertheless, a broad minimum in the energy range of interest is seen which rises at both high and low energies. More recent estimations of the attenuation lengths of Auger electrons and X-ray photoelectrons are discussed in Chapter 5.