Auger electron spectrometry is a surface sensitive analytical technique used mainly to determine the elemental composition of materials and, in certain cases to identify the chemical states of surface atoms. 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.
Auger electron spectrometry is named after Pierre Auger who investigated this process during 1923. The information depth for Auger analysis is the top 0.5…5 nm of the sample, and can be used in depth profiling applications in conjunction with ion beam sputtering.
Principle of AES
In this method the sample is irradiated by a primary electron beam, where in all elements with Z ≥ 3 (Li) emit Auger electrons by a process called Auger process. The energy distribution of the electrons emitted from the sample is analyzed in a spectrometer.
The Auger electrons, when analyzed as a function of energy, are used to identify the elements and at lower chemical states. The sensitivity for the elements varies over the periodic table; using Auger electrons the detection limit is about ≤ 1 %.
Electrons of energy 3-20keV are incident upon a conducting sample causing ejection of the core electrons from atoms contained in the sample 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 is converted into an X-ray or made to emit an electron called an Auger electron after which the atom is left in a doubly ionized state. The energy of the Auger electron is characteristic of the element that emitted it, and is used to identify the element. The short inelastic mean free path (IMFP) of Auger electrons in solids ensures the surface sensitivity of AES.
Conditions
Auger electron spectrometry is a destructive technique. AES technique is used in combination with sputter cleaning and is carried out in UHV conditions. Normally, when a sample is brought into the UHV environment from air, it will be coated with carbon and oxygen which is removed usually by sputtering process before the clean surface is investigated. Sputtering is carried out by directing a beam of 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.