Experiment Nr. 32
Auger Electron Spectroscopy (AES)
A. Introduction
Auger electron spectroscopy is a method for a quantitative and qualitative determination of
the composition of surfaces. The sensitivity, the surface-sensitivity, the fast data-recording and
the possibility for detection of all elements except hydrogen and helium are the basic advantages of this method. It’s found in large areas of physics, chemistries and material sciences,
manly applied in complementation with techniques for structural and morphological analysis
of solid-state surfaces like e.g. low energy electron diffraction (LEED) or scanning tunneling
microscopy (STM).
The basis of this method is the Auger-Meitner effect (after Lise Meitner 1922, Pierre Auger
1925), a relaxation process of excited atoms, competing with x-ray emission, where an ion goes
to an energetically favorable state by emitting an electron, going from a single ionized state
to a double ionized one. The emitted electrons are called Auger-electrons. Auger-spectra give
information about the distribution of chemical elements in the surface of a solid-state body,
about their binding among each other and to the surface, also a statement on the density of
states of the valence band is possible.
The Principle of the Auger-Meitner effect
a) Ionization of the atom via electron collision or the photo-effect
b) Non radiating decay of the hole-state by energy transfer to another hull-electron
Corresponding information about the surface are obtained from the energy loss of the primary electrons (electron energy loss spectroscopy, EELS), which is performable principally
with the same experimental setup. For exciting bound/localized/shell electrons a minimum
energy Emin is required. In the case of metals it corresponds with the binding energy referred
to the Fermi energy. This threshold energy is expressed trough a step in the energy distribution of the backscattered electrons (at Emin below the primary energy). Also good recognizable
in the energy loss spectrum are discrete excitations of collective electron oscillations (volume
and surface plasmons), which give information about the density of valence electrons. The
spectroscopy of interband transitions and phonons is possible too, although a monochromatic
electron source is needed due to the small loss energies (not realized in this experiment).
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B. Literature
The following is a selection of basic and extended literature for inquire purposes, it is not
expected that you read through all of the below but rather train extracting information of a
larger supply.
• introduction talk with comments at web page of the experiment
• literature map for the experiment (advanced courses library)
– G. Ertl. Küppers; Low Energy Electrons and Surface Chemistry, Page 1-52
– H. Viefhaus, in; Oberflächenanalytik in der Metallkunde, Page 7-58
– K. Müller; How much can Auger Electrons Tell Us About Solid Surfaces, Page 97-125
– F.P. Netzer, J.A.D. Matthew, E. Bertel; Electron-excited surface spectroscopy, in;
Spectroscopy of Surfaces, eds. R.J.H. Clark and R.E. Hester, Page 296-345
– M.L. Maede; Lock-in-amplifiers: principles and applications, Page 1-54
• L.E. Davis et al.; Handbook of Auger Electron Spectroscopy
Internet:
http://www.chem.qmw.ac.uk/surfaces/scc ºChapters 5.2, 7.2, 7.5
http://www.cem.msu.edu/%7Ecem924sg/LectureNotes.html ºChapter 10
http://en.wikipedia.org/wiki/Auger_electron_spectroscopy (not free of errors!)
C. Preparation
The following points are of importance for the understanding of the experiment. You should
cover them shortly in your written preparation.
• Experimental setup
Principle buildup of spectrometers for energy selective detection of electrons, measurement of differentiated spectra, as well as functionality of Lock-in amplifiers.
• Electron energy loss spectroscopy (EELS)
Energy loss mechanism and mean free path for electrons in solid state bodies, energy
of volume- and surface-plasmons, spectroscopic notation of bound/localized/shell
electrons.
• Auger effect
Principle of the Auger effect, nomenclature of the Auger electrons, line shape of the
Auger peaks, quantitative element analysis.
IMPORTANT
before attending the experiment, make yourself familiar with the experimental
setup (part D and "description of the devices"), as well as content and order of the
planed measurements (part E)
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D. Experimental Setup
The experiments are performed in a UHV-recipient, which is evacuated via an ion getter pump,
a turbomolecular pump and a rotary vane pump for the prevacuum on the outgoing side of
the turbomolecular pump.A Bayard-Alpert ionization gauge is used for pressure measurement
within the recipient. At a, from the outside movable and electric insulated, manipulator are
eight samples (A1 - A8, counted from top to bottom) located, and on the other side are additional six samples (B1 - B2) as well as a with luminescent material covered panel sheet (LS)
for making the electron beam visible. The vertical spacings between the samples are 6 mm,
Figure 1: experimental setup
Figure 2: sample holder
samples from front and backside are shifted with 3 mm against each other. The samples can be
cleaned from surface contaminations via ion bombardment from a plasma source. The samples
are irradiated with an electron beam from a electron gun with variable primary energy. The
backscattered respectively emitted electrons from the sample are detected energy selective with
a hemispherical analyzer and a channeltron, an electron multiplier, at its outgoing slit. The
outgoing signal is differentiated by adding a alternating voltage Umod to the voltage at the
analyzer and feeding the outgoing signal and the modulation signal into the Lock-in amplifier.
The pass energy of the hemispherical analyzer Epass = H · V can be selected independent to
the detection Energy E (constant analyzer energy, CAE modus) or proportional to the detection energy (constant retard ration, CRR modus). The retard ration is the proportion of
the detection energy to the pass energy e.g. CRR= 1 when detecting electrons with energy
of E = 800 eV the electrons passed the hemispherical analyzer with 800 eV too i.e. the pass
energy was Epass = 800 eV then, for CRR= 2 it would have been Epass = 400 eV.
The intrinsic energy resolution of the electron spectrometer depends linear from the pass
energy via ∆Esp = csp HV . Additionally the modulation amplitude Umod lowers the energy resolution via ∆Emod = cmod Umod . Under the assumption of statistically independent
and gaussian distributed broadening functions, the two contribution sum up quadratically:
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∆E 2 = ∆Esp
+ ∆Emod
. The time constant of the Lock-in amplifier τ in connection with the
scanning speed vsc = Ė would also lead to a lower energy resolution via ∆Esc = csc vsc τ , but
with decent measurement parameters this contribution is negligible.
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E. Experimental Procedure
IMPORTANT OPERATION NOTES
Despite all precaution measures, it is still possible to damage the experimental setup.
Please use with care and, when in doubt, ask the assistant before getting active by
yourself. Ion bombardment, if necessary, is only allowed after an introduction by the
assistant!
In the case that the pressure goes over p = 10−6 mbar, turn off the electron gun immediately
and inform the assistant. If the device is not showing pressure it’s ok though, in this case
electrons got into the ionization gauge, which is almost located opposite to the electron
gun.
The following parts of the setup must not be changed:
• Electric connections at the recipient, especially at the spectrometer.
• Electric connections at the backside of the supplies.
• Supplies of the turbomolecular and ion getter pump, which have to be always running.
• Valves on the prevacuum system
1. Getting to know the setup
a) Get familiar, where necessary by support from the assistant, with the functionality and
operation of the devices and their individual components and set up the hung out start
parameters.
b) Position the luminescence screen LS at the height of the spectrometer entrance, select a
primary energy of 800 eV and focus the electron beam onto the luminescence screen, ideally
slightly to the right of the center between the holding screws. By changing the energy of
the electron gun, it will be necessary to readjust the focus position from time to time.
c) Turn the sample holder 180° and lower it 3 mm, so that the electron beam hits the aluminum
sample A7 located at the rear side. The analog display of the Lock-in should now show a
signal at ca. 800 eV (negative at higher energy and positive at lower energy). Optimize the
signal of the primary peak by adjusting the beam position and focus as well as the angle of
the sample holder.
d) Optimize the relative phase position via the fine tuning at the Lock-in (note: with a phase
of 90° between signal and reference the Lock-in output should vanish!). Record the primary
peak with four different phase settings (the other parameters unchanged): 0°, +90°, 180°
and 270°.
e) Record the Primary peak with stepwise bigger time constants τ , at otherwise constant
parameters. Explain the observed differences.
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2. Recording of electron energy loss spectra
a) Record now the energy range between 600 − 820 eV with different sensitivity setting (each
with a factor 10 higher to the last one) and interpret the observed losses. Calculate the
energy of surface and volume plasmons of aluminum in your report. Assume 3 free valence
electrons per atom and a molar volume of 10.0 cm3 /mol. Compare the result with the
calculated values from theory.
b) Bring now the nickel sample A8 in front of the spectrometer and change into the CRR=1mode. Optimize beam position and focus with the characteristic MVV Auger transition
of Ni at 61 eV and select an adequate sensitivity factor maximizing the display in the
measurement program. Record a overview spectrum within the whole energy range of 0 −
820 eV.
Are there features present which indicate a contamination of the surface, can you identify
them via the outlaying standard spectra?
c) At which energy should the generation of holes in the M-shell set in? Are there more than
one kind of hole generation visible? Record another overview spectrum with slightly changed
primary energy (approx. 20 − 50 eV). Can you verify with your spectra that the features
near the primary peak are indeed energy loss peaks?
3. High resolution nickel spectra
a) Turn up the primary energy to 3 keV and re-optimize position and focus like before. With
this energy the electron beam should be visible at almost any material in the UHV-chamber.
Do not set the detection energy - the energy ramp - to higher energies than
1500 eV! (you dont need higher energies than 1 keV anyway). Record a overview spectrum
from 0 − 1000 eV and compare it to the standard spectrum, are there significant differences?
Identify the respective transition via the outlying table of the binding energies.
b) Switch to the CAE-mode and record the nickel peak group at high energies (approx.
600 − 900 eV) with stepwise better energy resolution: ca. 5 eV, 3 eV, and 2 eV. In order
to do so, vary modulation amplitude Umod , pass energy Epass , scanning speed vsc and time
constant τ in a adequate manner. The relevant constants are: csp = 7 · 10−3 , cmod = 7e and
csc = 1.3 (also see part D: experimental setup). You should observe a splitting of the peak
at ca. 848 eV, what could be the reason? What do you notice by observing the peak sizes
compared to each other, how does the energy resolution affect it?
4. Identification of Elements via their Auger spectra
a) Record overview spectra from two different single-element samples (A1-A6) in CRR=1mode within the energy range of 0 − 1000 eV. To find the most prominent peak from the
unknown sample, do a quick scan with vsc = 2500/10 eV/min and τ = 0.1 s. Then proceed
similar to the calibration at nickel, but make sure that you are not on the edge of the sample
with the electron beam, if necessary adjust the manipulator height.
Identify the samples by comparison to the standard spectra in the ’Auger-Handbuch’. What
are the determining transitions? Are there contaminations?
5. Quantitative analysis of the chemical composition of an alloy
a) Record an overview spectra from an alloy sample (B1-B5) in within the energy range of
0 − 1000 eV. Identify the occurring elements in the surface. Quantize the relative element
concentration, with the sensitivity factors from the ’Auger-Handbuch’. Discuss the accuracy
and error sources of your analysis, why can an error propagation be disregarded?
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