Auger electron spectroscopy (AES)
About this technique
Auger electron spectroscopy (AES) is a surface analytical tool that uses secondary electrons as a probe of the elemental composition of the samples’ surface. It probes the same thickness (about 0.3 nm) as other techniques that employ secondary electrons (X-ray photoelectron spectroscopy) but has the advantage of a highly focused primary electron beam of typically 10–100 nm that enables small spot analysis and elemental mapping.
It derives it surface sensitivity from a small inelastic mean free path (IMFP), which is the distance a secondary electron will travel before suffering an inelastic collision (and therefore not contribute to the characteristic peak).
It is one of a few techniques that measures true surface composition (secondary ion mass spectroscopy and its variations being another). This is also a drawback since most clean surfaces exposed to air will oxidise and have a surface composition different to that of the bulk. The surface sensitivity also prevents coating insulators with a gold layer to prevent charging since the only Auger signal will be that from the gold layer.
The technique is therefore particularly suited for studies on conductors and semiconductors and samples that can be introduced and etched within an ultra-high-vacuum chamber. The etching is done by energetic argon ions and the surface layer atoms are sputtered to expose the underlying layers. After each etching cycle, the surface concentration is measured to construct a concentration profile versus time, which is then converted into a concentration-versus-depth profile for a given etch rate. These etch rates will depend on the chemical composition and structure and may have to be measured independently. Profiles up to a depth of several micrometres can be reconstructed with a depth resolution of a few nanometres. Preferential sputtering, radiation induced diffusion and damage may complicate data analysis.
The surface concentration is calculated from the total measured intensities, using elemental sensitivities, which in turn depend on the IMFP as stated above. These IMFP’s depend on atomic structure and the calculated atomic percentage may therefore include significant uncertainties. As the Auger process involves three electron levels, it is also not particularly suited to extract chemical bonding information. Provided these limitations are heeded, AES provides an easy-to-use technique that yields the surface elemental composition on a nanometre scale with little or no sample preparation.