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* Auger Electron Spectroscopy (AES)*

A technique used in material surface science is the Auger Electron Spectroscopy (AES) named after its discoverer Pierre Auger.  Auger discovered Auger electrons by a process whereby incident photons or electrons impinging upon an atom release core electrons from the atom.  A higher-level electron with less binding energy may transition to fill the resultant vacant space or hole.  The difference in energy between the electron vacancy and the down electron may be released as a photon known as X-ray fluorescence or transferred to another electron in the core levels that allows it to escape the atom with a kinetic energy exemplified by the following equation: 

Ekinetic = {EK- EL1} - {EL2, 3 - F}

     The EK quantity is the s level energy in the atom core, the EL's are the respective electron orbital energy levels, and the F is the beam work function.  These Auger electrons are sampled with an energy analyzer and counted to produce Auger electron peaks in the resultant spectra.  The Auger transitions are referred to as KLL, LMM, and MNN in X-ray notation depending on the energy levels of the electrons involved.  The energy analysis of Auger electrons is used to identify the particular elements in the surface [4]. 

     The spectral distribution is obtained by bombarding the sample with an electron beam in UHV.  The sample is positioned such that many Auger electrons can be captured by the energy analyzer so that by sweeping the analyzer settings, a graphical representation of electron signal versus kinetic energy can be obtained.  The energy sweep can be performed starting as low as 0 electron volts (eV) to a much higher energy that is arrived at from expected elemental composition or an arbitrary basis such as from 0-2000 eV.  The sample is bombarded with a beam voltage of 2.0 kV with the filament energized to a standard emission control setting of 0.6 milliamperes (mA) and controlled by the Perkin-Elmer F 11-045 A Electron Gun Control, Perkin-Elmer F 20-085 Power Supply, and Perkin-Elmer F 20-070 A Scanning System Control.  This provides a standard electron beam current of approximately 0.2 microamperes (µA). The energy analyzer, which on our system is a cylindrical mirror analyzer (CMA) Perkin-Elmer F 25-270 AR Energy Analyzer, then counts the number of auger electrons collected [N (E)] at their respective energies and renders an N (E) versus Energy graphical analysis of the material surface.  The graphical representation spectra can then be transformed mathematically to obtain further information about the sample.  The Software, AugerScan, uses the Savitzky-Golay algorithm to differentiate the data obtained from the Perkin-Elmer PHI 560 ESCA/SAM system. The spectra can be transformed by modifying the raw data by taking the moving average and then differentiating the result.  Since the auger peaks are generally small compared to the large secondary electron backgrounds, the technique of differentiating (plotting the dN (E)/dE curve) provides a better way of detecting the small peaks superimposed on a large background.  The change in gradient of the electron energy distribution can then be measured peak to peak and since the magnitude of the differentiated height is approximately proportional to the integrated area of the N (E) curve peak.  This information can be used to find the relative surface elemental concentrations on the sample.  The technique is sensitive enough to find monolayer and sub-monolayer amounts [4].

     Quantitative analysis is accomplished by comparison of known composition or pure elemental standards obtained from published results of Chart of Principal Auger Electron Energies, and the standard spectra.  Concentrating first on the major peaks of the obtained spectra, the possibilities are reduced to two or three elements.  After positive identification is made the dominant peaks are labeled.  Sensitivity of the system is determined from the collection efficiency of the analyzer, incident beam current and energy, and the probability of Auger transitions. To calculate the relative sensitivity, a comparison of a signal from a pure silver specimen and the one from the sample is used.  The formula employed is

Sx (Ep) =,

where A and B are the chemical formula indices of XA and YBelements, Ix and IAg are the magnitudes of the peak-to-peak amplitudes of the spectra, and Kx is the handbook scale factor of the element with KAg equal to one.  The formula

Cx =,

where the summation is made of one peak per element over the entire spectrum and dx is the scale factor, is used to find the relative elemental concentration of the sample surface. The scale factor more readily is applied as a magnification of some peaks with respect to others.  The experimental error arises from the difference in the system and the atom's work function, about 0.05%, which translates to about 1 eV.  The raw data can be shifted to accommodate a better read.  Listings of the sensitivity, scale factors, and Auger peaks can be found in the Handbook of Auger Electron Spectroscopy or relevant literature of the UHV system.  In this case, the Perkin-Elmer PHI 560 ESCA/SAM system was used [13].

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