AES

Auger Electron Spectroscopy

Reference: notes given in class

Auger Electron Spectroscopy, or AES, is the most common surface analysis technique. A high energy excitation source, typically 2-10 keV electrons excite an inner shell of an atom in the sample (just like in XPS). The atom with the inner shell hole is unstable. Repulsion between electrons cause one of them to fill in the hole, with the energy released given to another electron (the Auger electron) that is ejected. The Auger process thus competes with the radiative mode of decay of the atom (ejection of an X-ray). The fraction of decays that go into electrons (X-rays) is called the Auger yield (fluorescence yield).

Note that three electrons from the atom participate: the first electron removed from the inner shell (level A), the one that falls from level B to level A, and the emitted Auger electron (from level C). The Auger transition is labelled ABC. Levels B and C must be different from level A, since energy needs to be released, but levels B and C may be the same. Since the energy released when electron falls from B to A depends on the position of the energy levels, and thus on the element, the energy of the Auger electron is characteristic of the element and can thus be used to identify the constituents of the sample. Like in XPS, there is a chemical shift, but in this case is more complex because it involves three levels.

In comparison with XPS, analysis using AES:

	is much faster because the technology of electron sources allows more intense electron beams.
	electron beams can be focused to 50 nm or less in modern electron guns, thus allowing microscopy (scanning Auger microprobe).
	Auger lines are more complex (because different levels B and C can participate), which makes it more difficult to identify compounds and to do quantitative analysis.

Auger Energies

In X-ray emission, the photon energy is:

hn = E_A – E_B – relaxation

In Auger emission, the kinetic energy of the electron is:

KE = E_A – E_B – E_C – relaxation = E_A – E_B^' – E_C^'

where the levels B and C relax due to the presence of the core hole in A. The first order estimate of the Auger energy is to neglect relaxation:

KE = E_A – E_B – E_C

The second order is to consider that relaxation means that the electrons in levels B and C do not see the nuclear charge Z, as is the case of a neutral atom, but Z + 1, because of the hole in the A level produced by the initial inner-shell ionization. Thus, E_B and E_C can be taken to be those of the next element in the atomic table; e.g., E_B^'(Z) = E_B(Z+1). Thus,

KE = E_A(Z) – E_B(Z+1)– E_C(Z+1)

As an example, consider the Auger transition in vanadium with a hole in the L₂ shell (level A), and involving the M₂₃ and M₄₅ shells (levels B and C). The next element (Z+1) to vanadium is chromium. The binding energies needed are taken are values measured with XPS. That of V-L₂ is E_A = 520 eV. The binding energies of the other two levels are 38 eV and ť 2 eV for V, and 43 eV and ť 2 eV for Cr. Thus, the Auger energies are:

1^st order: KE = E_A – E_B – E_C= 520 – 38 – 2 = 480 eV

2^nd order: KE = E_A(Z) – E_B(Z+1)– E_C(Z+1) = 520 – 43 – 2 = 475 eV

The experimental value is 474 eV, in good agreement with the second estimate. In practice, calculations are rarely needed because one can rely on published tables. To understand chemical shifts, however, it is useful to consider the relaxation of electrons in the atom (intra-atomic) and around it (extra-atomic), in a different way. One can write:

KE = E_A – E_B^' – E_C^' = E_A – E_B – E_C – U

where U is the repulsion energy between the holes in the final state (atom with holes in levels B and C). The repulsion term is described in the literature to arise from the two contributions mentioned above:

U = H – P

where H is the internal term and P is the polarization energy in the environment around the atom.

Auger spectra and line shapes

Unlike XPS lines, Auger spectra are broad and more complex, because they involve more active electrons. The broadest spectra are those involving electrons in the valence band; e.g., the CVV spectra (C – core level, V – valence band) involves two valence band electrons. A core hole C can decay in transitions of the type CCC, CCV, or CVV, and the probability of each one is called the branching ratio. The order of likelihood is, typically, CVV > CCV > CCC.

In the prominent CVV Auger transitions, there are all possible combination of energies of the electrons within the valence band. The maximum Auger energy is obtained when the two V electrons come from the top of the valence band. In the case of metals, this means the Fermi level. In this case, and referencing the energies to the Fermi level, E_B = E_C = 0, so the maximum kinetic energy is E_A, the binding energy measured with XPS. (If the Auger energy is referred to the vacuum level of the spectrometer, then KE = E_A – f_s, where f_s is the work function of the spectrometer.) The minimum energy of the CVV transition is when both electrons come from the bottom of the valence band: E_B = E_C = E_F and is equal to E_A – 2E_F referred to the Fermi level. Therefore, the width of the CVV Auger transition is 2E_F, and that of a CCV is E_F. These values are very large compared to the small width of photoelectron lines in XPS (usually limited to ť 1 eV by instrumental resolution), and allows these Auger transitions to be easily distinguished from photoelectron peaks when doing XPS analysis.