Electrons of energy 3-20keV are incident upon a conducting sample.
These electrons cause core electrons from atoms contained in the sample to be ejected
resulting in a photoelectron and an atom with a core hole. The atom then relaxes via
electrons with a lower binding energy dropping into the core hole. The energy thus
released can be converted into an Xray (see EDX) or emit an
electron. This electron is called an Auger electron after Pierre Auger who discovered this
relaxation process. After the emission of the Auger electron, the atom is left in a doubly
ionised state. The energy of the Auger electron is characteristic of the element that
emitted it, and can thus be used to identify the element. The short inelastic mean free
path (IMFP) of Auger electrons in solids ensures the surface sensitivity of AES.
AES is a popular technique for determining the composition of the top
few layers of a surface. It cannot detect hydrogen or helium, but is sensitive to all
other elements, being most sensitive to the low atomic number elements.
AES must be carried out in UHV conditions. A
popular method of looking at buried layers with AES is to use the technique in combination
with sputter cleaning. Normally, when a sample is brought into the UHV environment from
air, it will be coated with carbon and oxygen. This material has to be removed (usually by
sputtering) before the clean surface can be investigated. Sputtering involves directing a
beam of ions (usually Ar ions) at between 500eV and 5keV at the sample. This process
cleans the surface, but can also be used to erode away the sample to reveal structure
beneath the surface. Obviously this is a destructive technique.