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An Introduction to Auger Electron Spectroscopy
Applications and Fundamental Studies on
Electronic Structure of Atoms Molecules and
Solids
DELTA Winter Semester 2004

• Abner de Siervo
• (16.11.2004)

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Outline

• Second Part Fundamental Studies
• Motivation
• Theoretical simulation for Auger process
• - coupling schemes and selection rules
• - multiplets calculation
• - transition probabilities (intensities)
• - examples of line shape calculation
• Different Mechanisms associated with
• Auger emission
• - satellites Coster-Kronig (C-K), Shake-up,
• plasmons
• Examples of opened possibilities with
  synchrotron radiation and XAES
• - shake-up versus C-K
• - Sudden and Adiabatic Approximation
• - Auger Cascade and Screening mechanisms

• First Part
• Historical Introduction
• Basics principles
• - Auger emission
• - Energies determination
• - Nomenclature
• - Analysis Volume
• - Advantages and Disadvantages
• Experimental Setups
• - RFA, CMA, HA
• - Simple methods for quantification
• Applications some examples.
• - chemical shift analysis
• - Auger Depth Profile
• - SAM Scanning Auger Microscopy

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Historical Introduction
AES means Auger Electron Spectroscopy This
spectroscopy technique uses Auger electrons as
probes for surface science analysis chemical and
elemental characterization.
TheAuger phenomenon is a not irradiative
de-excitation process for excited atoms. The
de-excitation occur by a Columbic interaction
where the atom loss energy by emission of one or
more electrons. This ejected electron to one
continuum state is named Auger electron. 1923 or
(1925) - This effect was discovered independently
by Lise Meitner (1923 -Journal Zeitschrift fur
Physik) and Pierre Auger (1925 -Radium ) 1953
- J. Lander uses electron to excited Auger
electrons to study surface
impurities. 1968 - L. Harris demonstrates
usefulness of technique when he
differentiates the energy distribution of Auger
electrons emitted from a bombarded
surface. About the same time, Weber and
Peria employ LEED optics as Auger
spectrometers. 1969 - Palmberg et. al invent the
cylindrical mirror analyzer (CMA),
greatly improving speed and sensitivity of the
technique. The mid-80s saw the implementation
of Schottky field emitters as electron
sources, allowing analysis of features 20 nm in
size. Improvements in analyzers and sources
have pushed this limit to the 10 nm regime.
Lise Meitner
Pierre Auger
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The Auger Process
or photons or
(PE)
Auger recombination and e- transport Final State
Ionization Initial State
Ground State

• IMPORTANT to Remember In the Auger process
  doesnt exist a REAL photon intermediating the
  transition.

Conservation Laws
-U
Observe Auger electron energy is independent of
the excitation energy !
U Electron-Electron interaction in the final
state Relaxation energies U is known as Auger
parameter
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Nomenclature for Auger Transitions
Spectroscopy Nomenclature (example
XPS) nlj ? 1s (2s, 2p1/2,2p3/2), 3s,
From the X-Ray techniques nlj ?K, (L1, L2,
L3), M1,
Conventionally is used the X-Ray type in the
nomenclature of Auger transition. In this
example KL1L23 . When the electronic levels
are energetically well distinguishable is common
to use more sub-indices, for example
L1,2,3M2,3M4,5. For a group of transition, the
sub-indices are in many times omitted (KLL, LMM,
MVV) and for transition involving level(s) in the
valence band is common to use V instead (L,M,N,O
..) Example M4,5VV.
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Auger Transitions lines
Intensity (a.u.)
LMM LMN
A. de Siervo (MSc. Thesis University of Campinas,
1988)
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Auger Transitions lines for different elements
Red dots are indicating the most intensity lines
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Analysis Volume

• Depending on the spot size of the e-gun is
  possible to have spatial resolution in the (nm)
  range.
• In the direction perpendicular to the surface
  the analysis volume depends on the electron mean
  free path.

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Advantages and Limitations

• Advantages
• Surface sensitive
• Elemental and chemical composition analysis by
  comparison with standard samples of known
  composition
• Detection of elements heavier than Li. Very good
  sensitivity for light elements.
• Depth profiling analysis quantitative
  compositional information as a function of depth
  below the surface (destroy the sample)
• Spatial distribution of the elements (SAM)
  Elemental or even chemical Auger maps analysis in
  lines, points and areas.
• Disadvantages / Limitations
• Samples must be compatible with UHV in most of
  cases.
• For samples not prepared in-situ is normally
  necessary cleaning procedures such as sputtering,
  heating or scraping of the surface (some times,
  it is not possible)
• Samples must be conductive. In some cases is
  possible to avoid charging effects also for
  non-conductive samples
• Possibility of beam damage of some surfaces, for
  example some organic samples and polymers
• Hydrogen and helium are not detectable (only by
  indirect ways when they are present in the
  compounds or physically adsorbed).
• Quantitative detection is dependent on the
  element light elements gt 0.1 heavier elements
  gt 1.
• Accuracy of quantitative analysis depending on
  the availability of adequate sensitivity factors
  (or standards). Typical accuracy 10.

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Seah and Briggs in Pratical Surface Analysis
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2) CMA Cylindrical Mirror Analyzer

• Important Characteristics
• Energy resolution scales with Ep.
• coaxial designing eliminates shadowing
• Better transmission than an Hemispherical
  Analyzers
• Relative Short work distance
• Normally uses the lock-in amplifier to get the
  differential distribution dN(E)/dE.

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3) HA Hemispherical Analyzer

• Important Characteristics
• Better Energy Resolution
• Long work distance possible
• -Angle-dependent measurements possible

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Quantification in AES
Quantification analysis using first principle is
possible but rarely done due the large
differences between coupling schemes that govern
the Auger transitions in a multi ionized atom.
The most common analysis use sensitive factors
derived from pure materials or standards. This
method also have a lot of imprecision and it
should be judiciously used.
Auger electron intensity
Simplified formula for Homogeneous materials
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Relative Sensitivity Factor for primary e 3KeV
PHI analyzers
The most important message is AES is very
useful, probably one of the best way to surface
analysis, but be careful when you start to write
for your sample !
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Examples for AES
P. Weightman, (review article)
1) Chemical Analysis

• AES is one of the best complementary technique
  for XPS in the chemical analysis. Depending on
  the kinetic energy of the Auger electrons, AES is
  much more sensitive to the surface that
  conventional XPS.
• Chemical shifts and Auger lineshape can be used
  to determine the chemical state for a given
  element in the sample, and in studies as charge
  transfer in alloys.

Differences in the line shape and peak Position
for the C Auger (KVV) in different CxHy
compounds
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Auger Depth Profiling

• Sources of artifacts
• sample charging
• topographical features resulting of non-uniform
  sputtering of the sample
• preferential sputtering
• beam effects
• Ion beam mixing

R.Nix, http//www.chem.qmw.ac.uk/surfaces/scc/
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SAM
Conventional SEM image
SAM
http//www.aquila.infn.it/infm/Casti/Tech/Sam/Exam
ples.html
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Second Part Fundamental Studies in AES

• Motivation
• Understanding the electronic structure
• -Chemical bonds, charge transfer, material
  properties,
• Possibilities to verify simple models
• -Atomic Theory, Complete Screening Model,
• - helpful in the development of other
  techniques example AED
• AES is a laboratory of excited states
• - theoretical determinations of branching
  ratios, fluorescent yields, ...

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Theoretical simulation of Auger process
electronic conf. of the atom .
Atomistic approach
Hamiltonian of the system
(Leighton,R.B. Principles of Modern
Physics) Robert D. Cowan, The theory of Atomic
Structure And Spectra
(more approximations Close shell approximation,
Central potential)
Average Energy

• Russell-Saunders ou LS Coulomb gtgt Spin-Orbit
  Astrophs. J. 61,38 (1925)
• jj Spin-Orbit gtgt Coulomb Condon and Shortley-
  The theory of Atomic Spectra
• Intermediate Coupling ( IC ) Coulomb ?
  Spin-Orbit Condon and Shortley ...

Coupling schemes

LS coupling (normally in the final State)
jj coupling (normally for the initial state)
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Spin Orbit
Coulomb Interaction
R.D. Cowan in The Theory of Structure and
Spectra

Transition Probabilities (Auger Intensities)
(Fermi Golden Rule)


For IC coupling
The complete equation, also including open shell
cases can be found in E.J.McGuire, Atomic
Inner-Shell Processes-I Ionization and
Transition Probabilites Chapter 7 (Academic
Press, NY, 1975)
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Practical Examples 1) Auger Lineshape
calculation
A. de Siervo, R. Landers, G.G. Kleiman, et al.
Phy. Rev. B 60 (1999)15790 A. de Siervo, R.
Landers, G.G.Kleiman, et al. J. Elec. Spec. Rel.
Phen. 103 (1999) 751 G.G.Kleiman,
R.Landers,S.G.C. de Castro, et al. Phy. Rev. B
58 (1998)16103 R. Landers, S.G.C. de Castro, A.
de Siervo, et al. J. Elec. Spec. Rel. Phen. 94
(1998) 253
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Open possibilities for XAES using synchrotron
radiation
Selecting channels for transition changing the
photon energies shake-up vs CK
J. Marais, A. de Siervo, R. Landers,et al.
Surface Science 435 (1999) 878
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J. Morais, A. de Siervo, R. Landers, et al. 103
(1999) 661 T.D.Thomas, PRL 52 (1984) 417
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More Complex Auger Transitions cascade Process

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MVV excited below and above L3 threshold
1. Enormous increase in normal MVV emission -
attributed to combination of normal MVV
fluorescence Auger cascade. Describe
observed intensities. 2. Pd seems to behave as
though it had a full d-band induced by core hole.
In Short, first observation of unambiguous
quasiatomic spectral structure produced purely by
screening mechanisms A. de Siervo, R.
Landers and G.G. Kleiman, PRL 86, (2001)
1362. A. de Siervo, R. Landers, M.F. Carazzolle,
et al. J.E.S.R.P 114 (2001) 679
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Vielen Danke Muito Obrigado -)