Editing Auger electron spectroscopy (section)

===Instrumentation===

[[Image:AES Setup2-en.svg|thumb|340px|Figure 2. AES experimental setup using a cylindrical mirror analyzer (CMA). An electron beam is focused onto a specimen and emitted electrons are deflected around the electron gun and pass through an aperture towards the back of the CMA. These electrons are then directed into an electron multiplier for analysis. Varying voltage at the sweep supply allows derivative mode plotting of the Auger data. An optional ion gun can be integrated for depth profiling experiments.]]

Surface sensitivity in AES arises from the fact that emitted electrons usually have energies ranging from 50&nbsp;eV to 3&nbsp;keV and at these values, electrons have a short [[mean free path]] in a solid. The escape depth of electrons is therefore localized to within a few nanometers of the target surface, giving AES an extreme sensitivity to surface species.<ref name="oura2003"/> Because of the low energy of Auger electrons, most AES setups are run under [[ultra-high vacuum]] (UHV) conditions. Such measures prevent electron scattering off of residual gas atoms as well as the formation of a thin "gas (adsorbate) layer" on the surface of the specimen, which degrades analytical performance.<ref name="feldman_mayer"/><ref name="oura2003"/> A typical AES setup is shown schematically in figure 2. In this configuration, focused electrons are incident on a sample and emitted electrons are deflected into a cylindrical mirror analyzer (CMA). In the detection unit, Auger electrons are multiplied and the signal sent to data processing electronics. Collected Auger electrons are plotted as a function of energy against the broad secondary electron background spectrum.  The detection unit and data processing electronics  are collectively referred to as the electron energy analyzer.<ref>{{cite web |url=https://www.phi.com/surface-analysis-techniques/aes.html |title=Auger Electron Spectroscopy|publisher=Physical Electronics, Inc..(PHI)|date=2020 |website=Physical Electronics|access-date=January 8, 2020}}</ref>

Since the intensity of the Auger peaks may be small compared to the noise level of the background, AES is often run in a derivative mode that serves to highlight the peaks by modulating the electron collection current via a small applied AC voltage. Since this <math>\Delta V=k\sin(\omega t)</math>, the collection current becomes <math>I(V+k\sin(\omega t))</math>. [[Taylor series|Taylor expanding]] <!--or is that [[Taylor's theorem]]? I cannot tell--> gives:

:<math>I(V+k\sin(\omega t))\approx I_0+I'(V+k\sin(\omega t))+O(I'')</math>

Using the setup in figure 2, detecting the signal at frequency ω will give a value for <math>I'</math> or <math>\frac{dN}{dE}</math>.<ref name="feldman_mayer"/><ref name="oura2003"/> Plotting in derivative mode also emphasizes Auger fine structure, which appear as small secondary peaks surrounding the primary Auger peak. These secondary peaks, not to be confused with high energy satellites, which are discussed later, arise from the presence of the same element in multiple different chemical states on a surface (i.e. Adsorbate layers) or from relaxation transitions involving valence band electrons of the substrate. Figure 3 illustrates a derivative spectrum from a copper nitride film clearly showing the Auger peaks. The peak in derivative mode is not the true Auger peak, but rather the point of maximum slope of ''N(E)'', but this concern is usually ignored.<ref name="oura2003"/>
[[Image:Cu3NAES.JPG|thumb|340px|Figure 3. Auger spectrum of a copper nitride film in derivative mode plotted as a function of energy. Different peaks for Cu and N are apparent with the N KLL transition highlighted.]]