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AUGER ELECTRON SPECTROSCOPY

Auger Electron Spectroscopy is a surface analysis technique based on the phenomenon discovered first by Lise Meitner, then independently by Pierre Auger in the early 1920's.  This process is called the Auger process and it will be described next.

AES can be separated in three main phases. In the first phase, a sample is irradiated with a high energy beam, usually an electron beam, these high energy electrons ionize some atoms at the surface of the sample (0.4 - 5 nm) by knocking out an electron from at least one of their core shells (K, L, or M). See Figure 1.

 
 
Figure 1: Ionization of an atom by a high energy x-ray or electron beam.


The second phase starts when an electron from a higher shell drops down to occupy the vacancy of the ejected electron; during this relaxation, energy is emitted in form of photons in the x-ray range, these photons in turn energize another electron from the same shell (can be from a different angular momentum quantum number 'l') and it is also ejected from the atom. This is called the Auger electron. See Figure 2.
Figure 2: Schematic of the Auger Effect.

The third phase is based on the measurement of the kinetic energy of the Auger electron using the equation:

                                                                      KE = Ecore - E1 - E2

where KE is the kinetic energy of the Auger electron, Ecore is the binding energy of the core electron, E1 is the binding energy of the electron that relaxed to the core level, and E2 is the binding energy of the Auger electron when it is still in its shell (with a slight, often ignorable, difference due to the ionization state). This data is then plotted in a number of electrons Vs. electron kinetic energy graph; this graph is then analyzed (enhanced, then plotting the derivative of the function) to determine the atoms from which the Auger electrons came from.

References.

  • Auger Electron Spectroscopy by Evans Analytical Group
    www.cea.com/techniques/analytical_techniques/aes.php
  • Surface Analytical Techniques - Auger Electron Spectroscopy
    www.chem.qmul.ac.uk/surfaces/scc/scat5_2.htm
  • Auger Electron Spectroscopy - Wikipedia
    en.wikipedia.org/wiki/Auger_electron_spectroscopy

EXAMPLE OF AES APPLICATION

The next example is completely based on the article:

  • A. B. M. O. Islam, Y. Nishiyama, T. Tambo, C. Tatsuyama, T. Ito, *{_}Characterization of GaS-deposited CVD diamond films by AES and XPS{_}*, Applied Surface Science, Volumes 159-160, June 2000, Pages 588-593.

In this study Islam et. al characterized GaS films deposited on diamond film on Si substrate using AES and XPS. In this example I'll focus on their AES analysis only.

Several samples were prepared for the experiment; the first and second samples were B-doped diamond films (as-grown) and B-doped diamond films annealed in oxygen at 500 oC (O-ann), both deposited on p^+^-type Si substrate via plasma-assisted chemical vapor deposition (CVD), this films had a thickness of about 5 µm.

For the third and fourth samples, a GaS film was deposited at 500 oC onto both as-grown and O-ann diamond films, with a GaS film thickness of 35 and 44 Å, respectively.

And lastly, for the fifth and sixth samples, the third and fourth samples were annealed at 900 oC and 970 oC, respectively.

The AES measurements were carried out in the ultrahigh vacuum chamber of a ULVAC PHI 548-SH, ESCA / AES system, using a base pressure of about 2x10^-9^ Torr. A double-pass CMA energy analyzer was used. First-derivative mode was used for AES measurements. The energy of the electron beam was 3 keV.

The AES spectra of both the as-grown and O-ann diamond films showed the Auger peak of C and the O-ann a smaller peak of O.

The spectra of the deposited GaS films samples (third and fourth) showed only the Auger peaks of Ga and S.

The spectra of the post-annealed samples (fifth and sixth) exhibited the Auger peaks of C and a small peak of S, neither showed the Ga or O peaks.

All these can be visualized in Figure 3.

 

Figure 3: AES spectra of each sample

 Based on the results from the whole analysis (AES, XPS, and SEE), Islam et. al concluded that a downward band bending was present due to oxygen treatment of the diamond films, then the GaS deposition caused an upward band bending. The annealing process caused an increase in the downward band bending. The as-grown films presented Negative Electron Affinity (NEA), and this disappeared when treated with oxygen. The NEA is also eliminated with the deposition of GaS. When the samples were annealed, a Positive Electron Affinity (PEA) surface was acquired due to the S present in the diamond surface.