Comparison between EDS and AES
- Practical Electron Microscopy and Database -
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An atom is ionized when an inner shell electron is removed by high-energy-electron radiation. In this step, vacancies in inner shell are formed. To return the excited atom to its ground state, the electron from an outer, higher energy shell fills the vacant inner shell, and then the atom releases an amount of energy equal to the potential energy difference between the two shells. This excess energy, which is unique for every atomic transition, will be emitted by the excited atom either as an X-ray photon (for EDS measurement: Energy Dispersive X-ray Spectrum) or will be self-absorbed and emitted as an Auger electron (for AES measurement: Auger Electron Spectrum). Auger electrons and X-rays are the only possible outcomes, and the sum of the two probabilities is unity.

Table 4660. Comparison between EDS and AES.

EDS
AES
Deep core relaxation
Dominates over Auger processes for deeper core relaxation Weaker information for deeper core relaxation
Characteristic of
excited atoms
Yes Yes
Signal Depth
The relatively large mean free paths of X-rays, particularly those of high-energy photons, allows signals to be collected from a large volume of the sample probed by the electron beam. The relatively short mean free path of the Auger electrons in bulk solids (on the order of few nanometers) means that only those produced near the surface layer can escape to the free space to be collected. This restricts the application of Auger spectroscopy to surface study.
Energy resolution
The typical energy resolution for EDS is of on the order of 150 eV. This is suffcient in most cases for resolving peaks of different elements, but is inadequate for detecting chemical shifts of the atoms which are of the order of a few electronvolts. EDS is thus mainly used for composition analysis.  
Yield at low accelerating voltages of incident electrons
Very few x-rays are emitted Auger electron emission dominates

Figure 4660a shows both generation processes of x-rays and Auger electron.

Generation processes of x-rays and Auger electron 

Figure 4660a. Generation processes of X-rays and Auger electron.

As an example, Figure 4660b shows the X-ray and Auger electron yields per K vacancy as a function of atomic number. For low-Z material Auger emission is dominant, while for high-Z material X-ray emission dominates. The two emission probabilities are approximately equal at Z ≈ 30 for K shell ionization.

X-ray and Auger electron yields per K vacancy as a function of atomic number

Figure 4660b. X-ray and Auger electron yields per K vacancy as a function of atomic number.

Figure 4660c shows the escape depths of various species generated by high-energy electrons penetrating into a solid. Auger electrons and "soft" X-rays are emitted in depth of <1 nm and in diameter of beam size, secondary electrons are emitted in depth of < 10 nm and in diameter of beam size, and the emission depths and diameters of backscattered electrons and "hard" X-rays depend on both the beam energy and beam size.

Escape depths of various species generated by high-energy electrons penetrating into a solid

Figure 4660c. Escape depths of various species generated by high-energy electrons penetrating into a solid.

In comparison between the emissions of Auger electrons and “soft” X-rays, for electron irradiation of high atomic-number (Z) elements, Auger electron emission predominates, while for irradiation of low Z elements, “soft” X-rays predominate.

 

 

 

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