ISO/TR 18394:2016
(Main)Surface chemical analysis — Auger electron spectroscopy — Derivation of chemical information
Surface chemical analysis — Auger electron spectroscopy — Derivation of chemical information
ISO/TR 18394:2016 provides guidelines for identifying chemical effects in X-ray or electron-excited Auger-electron spectra and for using these effects in chemical characterization.
Analyse chimique des surfaces — Spectroscopie des électrons Auger — Déduction de l'information chimique
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TECHNICAL ISO/TR
REPORT 18394
Second edition
2016-05-01
Surface chemical analysis — Auger
electron spectroscopy — Derivation of
chemical information
Analyse chimique des surfaces — Spectroscopie des électrons Auger
— Déduction de l’information chimique
Reference number
ISO/TR 18394:2016(E)
©
ISO 2016
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ISO/TR 18394:2016(E)
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ISO/TR 18394:2016(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms . 1
5 Types of chemical and solid-state effects in Auger-electron spectra .1
6 Chemical effects arising from core-level Auger-electron transitions .3
6.1 General . 3
6.2 Chemical shifts of Auger-electron energies . 3
6.3 Chemical shifts of Auger parameters . 4
6.4 Chemical-state plots . 6
6.5 Databases of chemical shifts of Auger-electron energies and Auger parameters . 7
6.6 Chemical effects on Auger-electron satellite structures . 7
6.7 Chemical effects on the relative intensities and line shapes of CCC Auger-electron lines . 8
6.8 Chemical effects on the inelastic region of CCC Auger-electron spectra . 9
7 Chemical effects on Auger-electron transitions involving valence electrons .10
7.1 General .10
7.2 Chemical-state-dependent line shapes of CCV and CVV Auger-electron spectra .10
7.3 Information on local electronic structure from analysis of CCV and CVV Auger-
electron line shapes .15
7.4 Novel techniques for obtaining information on chemical bonding from Auger processes 16
Bibliography .21
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ISO/TR 18394:2016(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical
Barriers to Trade (TBT), see the following URL: Foreword — Supplementary information.
The committee responsible for this document is ISO/TC 201, Surface chemical analysis, Subcommittee
SC 7, Electron spectroscopies.
This second edition cancels and replaces the first edition (ISO/TR 18394:2006), which has been
technically revised.
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ISO/TR 18394:2016(E)
Introduction
This Technical Report provides guidelines for the identification of chemical effects on X-ray or electron-
excited Auger-electron spectra and for using these effects in chemical characterization.
Auger-electron spectra contain information on surface/interface elemental composition as well as
[1][2][3][4][5]
on the environment local to the atom with the initial core hole . Changes in Auger-electron
spectra due to alterations of the atomic environment are called chemical (or solid-state) effects.
Recognition of chemical effects is very important in proper quantitative applications of Auger-electron
spectroscopy and can be very helpful in identification of surface chemical species and of the chemical
state of constituent atoms in surface or interface layers.
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TECHNICAL REPORT ISO/TR 18394:2016(E)
Surface chemical analysis — Auger electron spectroscopy
— Derivation of chemical information
1 Scope
This Technical Report provides guidelines for identifying chemical effects in X-ray or electron-excited
Auger-electron spectra and for using these effects in chemical characterization.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 18115 (all parts), Surface chemical analysis — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18115 (all parts) apply.
4 Abbreviated terms
CCC core-core-core (Auger-electron transition)
CCV core-core-valence (Auger-electron transition)
CK Coster-Kronig
c-BN cubic boron nitride
CVV core-valence-valence (Auger-electron transition)
DEAR-APECS Dichroic Effect in Angle Resolved Auger-Photoelectron Coincidence Spectroscopy
h-BN hexagonal boron nitride
IAE Interatomic Auger Emission
ICD Interatomic Coulomb Decay
PAES Positron-Annihilation-induced Auger Electron Spectroscopy
REELS Reflection Electron Energy-Loss Spectroscopy
5 Types of chemical and solid-state effects in Auger-electron spectra
[1][2][3][4][5]
Many types of chemical or solid-state effects can be observed in Auger-electron spectra .
Changes in the atomic environment of an atom ionized in its inner shell can result in a shift of the kinetic
energy of the emitted Auger electron. In the case of X-ray-excited Auger-electron spectra, energy shifts of
Auger parameters (i.e. kinetic-energy differences between Auger-electron peaks and the photoelectron
peaks corresponding to the core levels involved in the Auger-electron process) can be detected as well.
Furthermore, the line shape, the relative intensity and the satellite structure (induced by the intrinsic
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ISO/TR 18394:2016(E)
excitation processes) of the Auger-electron lines can be considerably influenced by chemical effects,
as can the structure of the energy-loss region (induced by extrinsic, electron-scattering processes)
accompanying the intrinsic peaks. Strong chemical effects on the Auger-electron spectral shapes offer
ways of identification of chemical species using the “fingerprint” approach.
In the case of electron-excited Auger-electron spectra, the Auger peaks are generally weak features
superimposed on an intense background caused to a large extent by the primary electrons scattered
inelastically within the solid sample. As a consequence, the differential Auger-electron spectrum is
often recorded (or calculated from the measured spectrum) rather than the direct energy spectrum,
facilitating the observation and identification of the Auger-electron peaks and the measurement of the
respective Auger transition energies. Differentiation can, however, enhance the visibility of random
fluctuations in recorded intensities, as shown in Figure 1. If chemical-state information is needed from
a direct energy spectrum, then the relative energy resolution of the electron spectrometer should be
better than 0,15 % (e.g. 0,05 % or 0,02 %). A poorer energy resolution causes a significant broadening
of the Auger-electron peaks and prevents observation of small changes of spectral line shapes or peak
energies as chemical-state effects in the spectra. A great advantage of electron-excited Auger-electron
spectroscopy over X-ray excitation with laboratory X-ray source, however, is the possibility of using
high lateral resolution and obtaining chemical-state maps of surface nanostructures.
NOTE 1 Auger-electron spectra can be reported with the energy scale referenced either to the Fermi level or
to the vacuum level. Kinetic energies with the latter reference are typically 4,5 eV less than those referenced to
the Fermi level, but the difference in energies for these two references can vary from 4,0 eV to 5,0 eV since the
position of the vacuum level depends on the condition of the spectrometer and may, in practice, vary with respect
to the Fermi level. When energy shifts are determined from spectra recorded on different instruments, use of
different energy references should be taken into account.
NOTE 2 While the visibility of noise features in a differential spectrum can be reduced by use of a larger
number of channels in the calculation of the derivative, there may also be distortion of the resulting differential
spectrum and loss of fine details associated with chemical-state effects.
Key
X kinetic energy, eV
Y intensity
1 differential spectrum
2 direct spectrum
NOTE This figure is reproduced from Figure 2.8 of Reference [1].
Figure 1 — Comparison of direct and differentiated Auger-electron spectra for copper (Cu
LMM peaks)
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ISO/TR 18394:2016(E)
6 Chemical effects arising from core-level Auger-electron transitions
6.1 General
Core-level (or core-core-core, CCC) Auger-electron transitions occur when all of the levels involved in
the Auger transition belong to the atomic core for the atom of interest.
6.2 Chemical shifts of Auger-electron energies
The main effect of any change in the solid-state environment on Auger-electron spectra for Auger
transitions involving core levels is a shift of the Auger energies. This shift results from a change in the
core atomic potential due to the changed environment and from a contribution due to the response of
the local electronic structure to the appearance of core holes. Auger chemical shifts are generally larger
than the binding-energy shifts of the atomic levels involved in the Auger-electron process because the
two-hole final state of the process is more strongly influenced by relaxation effects. This phenomenon is
[6]
illustrated by the example of aluminium and its oxide in Figure 2 . Large chemical shifts in the energy
positions of the Auger-electron lines provide possibilities for chemical-state identification even in the
case of electron-excited Auger-electron spectroscopy with, in this case, moderate energy resolution. In
X-ray-excited Auger-electron spectra, the peak-to-background intensity ratios are usually larger than
those in electron-excited spectra, facilitating accurate determination of peak energies. Recommended
[7]
Auger electron energies are available for 42 elemental solids . Information on Auger chemical shifts of
[8][9][10][11] [12][13]
particular elements can be obtained from handbooks and online-accessible databases .
Key
X kinetic energy, eV
Y intensity, counts/s
Figure 2 — Photoelectron and Auger-electron spectra of an aluminium foil covered by a thin
overlayer of aluminium oxide: Excitation with Al and Mo X-rays
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With the advantage of high-energy-resolution analysers, small chemical shifts of Auger-electron lines
due to different type of dopants in semiconductors become discernible (for example, the kinetic-energy
[1]
difference between Si KLL peaks from n-type and p-type silicon is 0,6 eV ), allowing chemical-state
mapping in spite of the extremely low concentration (far below the detection limits of Auger electron
spectroscopy) of the dopants. Figure 3 shows a Si KLL Auger-electron map derived from a cross section
of a p-type silicon sample doped with phosphorus by implantation to obtain n-type Si at the sample
[1]
surface .
Key
1 vacuum
2 n-type Si (implanted with P)
3 p-type Si wafer
NOTE 1 A cross section of the sample is shown, and the Auger-electron spectra were excited with an
electron beam.
NOTE 2 This figure has been reproduced from Figure 5.30 of Reference [1].
Figure 3 — Silicon KLL Auger-electron map of a p-type silicon sample implanted with
phosphorus to produce n-type Si at its surface
6.3 Chemical shifts of Auger parameters
Auger parameters, obtained from X-ray-excited Auger-electron spectra, can also be strongly influenced
[2][14][15][16][17][18]
by the environment of the atom emitting photoelectrons and Auger electrons . The
Auger parameter, α, is given by Formula (1):
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ISO/TR 18394:2016(E)
α =−KE()jklKEi() (1)
where
KE( jkl) is the kinetic energy of an Auger transition involving core levels j, k and l of an atom;
KE(i) is the kinetic energy of a photoelectron from core level i (which may be the same as the
core level j).
[14][15][16]
In order to avoid negative values of the Auger parameter , the modified Auger parameter, α′, is
used in most practical cases. The modified Auger parameter is given by Formula (2):
′
αα=+ EK=+Ej()kl BE()i (2)
p
where
E is the exciting photon energy;
p
BE(i) is the binding energy of an electron in the core level i.
It is also preferable to use α′ rather than α because the value of α′ is independent of E .
p
The Auger parameters can be measured even in the case of static charging, since any charging shift is
[14][15]
cancelled as energy separations of peaks are determined. No energy-referencing problems occur
in the case of measuring Auger parameters; i.e. data obtained using the vacuum level as reference can be
compared directly to data obtained using the Fermi level as reference. Auger parameters can therefore
be very useful in the characterization of insulators and semiconductors, where the energy position of
the Fermi level of the sample is not well defined. A change in the atomic environment of a core-ionized
atom can result in a chemical shift of the corresponding Auger parameter. Auger-parameter shifts
depend on differences in the valence charge in the initial ground state and in the final state (intra-
atomic contribution), as well as on differences in the contribution to the relaxation process of all other
[16]
atoms in the system (extra-atomic contribution) .
When the intra-atomic contribution is dominant, a local screening mechanism of the core hole takes
place, while in the case when the extra-atomic contribution is dominant, the screening mechanism is
assumed to be non-local. In the latter case, simple electrostatic models can be used for estimating the
[5][16][17][18][19][20] [16]
electronic polarization energy . The model of Moretti describes the final-state
polarization process in which the sum of the electric fields (at the ligands) is generated by the central
positive charge and by induced dipoles on the ligands in the first coordination shell. This model can be
applied to estimate the extra-atomic polarization energy and the Auger-parameter shifts. Calculation
of Auger parameter shifts using the electrostatic model of Moretti is facilitated by the possibility of
applying the freely available Tinker molecular modelling and Molden computer graphic software
[18] [21][22]
packages . Weightman, et al. developed a different model, the “extended potential model”,
for estimating the Auger-parameter chemical shift; potential parameters were derived from atomic
calculations and the angular-momentum character of the electrons was taken into account. This model
gives a good approximation in the case of large charge transfer in the final state (conductors), where
the electrostatic model is not applicable, and describes well the local screening mechanism. In the case
[3]
of binary alloys, the magnitude of the transferred charge can be accurately derived . A review on first
principles calculations of Auger kinetic energy and Auger parameter shifts in metallic bulk solids from
[23]
the density functional theory are discussed in Reference .
In the case of nanoparticles, the Auger parameter can depend on the size of the particles. Figure 4
shows the dependence of the Auger parameter of Cu nanoclusters [deposited by evaporation onto highly
[24]
oriented pyrolitic graphite (HOPG)] on the nominal thickness of the Cu layer . It was found that the
[24]
Cu Auger parameter depends linearly on 1/d, where d is the average diameter of the Cu clusters .
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ISO/TR 18394:2016(E)
[24]
NOTE See Figure 2 of Reference .
Figure 4 — Dependence of the Cu Auger parameter on the nominal thickness of the Cu layer
consisting of Cu clusters deposited by evaporation onto HOPG surface
6.4 Chemical-state plots
For chemical-state identification and Auger-parameter analysis, the presentation of Auger parameters
[15]
in the form of a two-dimensional plot, as proposed by Wagner, proved to be very useful . The Auger-
electron kinetic energy is indicated on the ordinate of the plot and the corresponding photoelectron
binding energy is on the abscissa but oriented in the negative direction, as shown in Figure 5; constant
Auger-parameter values are represented on the plot by a straight line with a slope of –1 (note that the
abscissa axis in Figure 5 is increasing to the left). In the case of a negligible change in the intra-atomic
relaxation energy (due to the varying atomic environment), the change in the extra-atomic-relaxation
(final-state-effect) energy dominates, and components with higher extra-atomic relaxation energy
lie in the upper part of the chemical-state (or Wagner) plot. On the other hand, when the initial-state
effects (proportional to the sum of terms related to the ground-state valence charge and the Madelung
potential) dominate, the slope becomes –3 on the chemical-state plot; i.e. chemical states with similar
initial-state effects lie on straight lines with a slope of –3. This result illustrates that chemical-state
plots can be analysed to provide information on the nature of the changes in the environment of the
[19] [25]
core-ionized atom . Figure 5 shows a chemical-state plot for tin compounds . As can be seen, the
chemical-state plot can help to distinguish between chemical states not separable on the basis of core-
level binding energy shifts or Auger-electron-energy shifts alone.
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Key
X Sn 3d binding energy, eV
5/2
Y Sn MNN Auger kinetic energy, eV
1
Y Auger parameter + photon energy
2
[25]
NOTE Reprinted from Reference .
Figure 5 — Chemical-state plot for tin compounds
6.5 Databases of chemical shifts of Auger-electron energies and Auger parameters
The first comprehensive set of critically collected data on Auger parameters, Auger-electron kinetic
[15]
energies and photoelectron binding energies for several elements is found in Reference. A handbook
contains a collection of experimental photoelectron binding-energy and Auger-electron kinetic-energy
[8]
data for a large number of compounds and includes several chemical-state plots. The latest version
[12]
of the US National Institute of Science and Technology XPS Database provides online access to
over 33 000 records of photoelectron and Auger-electron data involving intense transitions for most
elements and many compounds. This database supplies Auger parameters, has the option of displaying
chemical-state plots, and is very useful for identification of chemical state as well as in studies of the
dependence of polarization energy on chemical state. Auger-parameter values for 42 elemental solids
[26]
are recommended in Reference .
6.6 Chemical effects on Auger-electron satellite structures
Auger-electron peaks can be accompanied by satellite lines due to intrinsic excitations. These
excitations are often of atomic origin. As a consequence of the creation of the core hole, electrons can
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be excited from occupied levels to unoccupied states (shake-up) or to the continuum (shake-off); the
Auger-electron process then takes place in an excited or multiply ionized atom. Note that not only the
shake-up but the shake-off process results in appearance of satellites in Auger-electron spectra, in
contrast to photoelectron lines where the shake-off process induces a continuous energy contribution
to the spectrum. The excited electron can be either a spectator or a participator in relation to the
Auger transition. In this latter case, the energy of the Auger satellite can be even higher than that of
the main line. Figure 6 shows an extraordinarily intense satellite occurring in the F KLL spectrum of
polycrystalline KF; this satellite is interpreted as a resonance between the ground state and the core-
[27]
ionized state . Coupling of unpaired spins in the ionized core level and in the outer shell leads to
multiple splitting of Auger lines; these splittings can be affected by changes in the atomic environment
as well.
Key
X kinetic energy, eV
Y intensity
Figure 6 — Satellite structure (designated by arrows) in fluorine KLL Auger-electron spectra
of fluorides
6.7 Chemical effects on the relative intensities and line shapes of CCC Auger-
electron lines
The relative intensities of CCC Auger-electron transitions may change as a result of different Auger-
[28][29]
transition probabilities in different atomic environments . Very fast Coster-Kronig (CK) processes
taking place prior to a particular Auger transition can convert the initial core hole into a vacancy in a
core level with a smaller binding energy, leading to a considerable change in the relative intensities of
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[30]
Auger transitions involving levels that participated in the CK process . The probability of these CK
[31]
processes can strongly depend on the chemical environment . In the case of some metals, CCC Auger
[32]
line shapes can be strongly asymmetric due to electron-hole pair excitation in the conduction band .
For adsorbates, a considerable broadening of the Auger-electron line can occur as a consequence of
[33]
phonon excitation by the appearance of the core hole .
6.8 Chemical effects on the inelastic region of CCC Auger-electron spectra
Strong variations in the energy-loss structure of Auger-electron spectra can be observed, following
changes of chemical state, e.g. in the case of some free-electron metals and their compounds. Figure 7
[34]
shows the difference between the KLL Auger spectra of metallic Al and Al O . Reflection Electron
2 3
Energy Loss Spectroscopy (REELS) can be used to confirm and separate chemical-state-dependent
energy-loss structures in Auger-electron spectra. On the other hand, the interpretation of REELS
spectra of multi-component systems can be confirmed by the surface chemical composition, obtained
[35]
from quantitative Auger analysis .
Key
X kinetic energy, eV
Y intensity
[34]
NOTE Reproduced from Figure 5 of Reference .
Figure 7 — Aluminium KLL Auger-electron spectra obtained by photo-excitation with Zr X-rays
from an oxidized sample (lower curve) and after various times of ion sputtering to remove the
oxide (upper curves)
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ISO/TR 18394:2016(E)
7 Chemical effects on Auger-electron transitions involving valence electrons
7.1 General
In the case of Auger transitions involving valence electrons, the Auger-electron line shapes are expected
to change due to changes in chemical state. The detection of these line shape changes may require high
(better than 0,5 %) relative energy resolution, although the changes are observable at moderate energy
resolution in many cases.
7.2 Chemical-state-dependent line shapes of CCV and CVV Auger-electron spectra
The line shapes of core-core-valence (CCV) and core-valence-valence (CVV) Auger-electron spectra can
depend strongly on the environment (i.e. the chemical state) of the atoms emitting the Auger electrons.
This effect can be utilized for fingerprinting, i.e. for identification of chemical state. Early measurements
of carbon CVV Auger-electron spectra showed obvious differences in line shapes for graphite, diamond,
[36][37]
metal carbides and carbon monoxide adsorbed on a metal surface . Figure 8 shows electron-
excited Si L VV Auger-electron spectra of solid silicon and of various silicon compounds that show
23
[38][39]
considerable differences in line shapes . Figure 9 shows Pd M N N Auger-electron spectra for
45 45 45
[40]
different Cu-Pd and Ag-Pd alloys .
Key
X kinetic energy, eV
Y intensity
[39]
NOTE Reproduced from Reference .
Figure 8 — Silicon L VV Auger-electron spectra in the differential mode for solid silicon and
23
various silicon compounds
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Key
X kinetic energy, eV
Y intensity
NOTE 1 Auger spectra were obtained with excitation by Mg X-rays.
[40]
NOTE 2 Reproduced from Reference .
Figure 9 — Palladium M N N Auger-electron spectra for the indicated Cu-Pd and Ag-Pd alloys
45 45 45
The stronger effect of the environment on the CVV peak compared with the CCV or CCC peaks is shown
[41]
clearly in the spectra of the metals and their oxides in the series Ti to Zn . These spectra show how
the environmental effect depends on the valence-level occupancy; this effect is a maximum in the
middle of the Ti-Zn series.
Direct-mode, high-energy-resolution, electron-excited Ti L M M Auger-electron spectra for TiN
3 23 45
...
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