Transcript An Introduction to Auger Electron Spectroscopy

DELTA – Winter Semester 2004
“An Introduction to Auger Electron
Spectroscopy : Applications and
Fundamental Studies on Electronic
Structure of Atoms Molecules and
Solids”
Abner de Siervo
(16.11.2004)
Outline
First Part:
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
•
•
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
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.
Lise Meitner
Pierre Auger
1969 - Palmberg et. al invent the cylindrical mirror analyzer (CMA),
greatly improving speed and sensitivity of the technique.
The mid-80’s 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.
The Auger Process
or photons or
…
Ground State
•
(PE)
Ionization: Initial State
Auger recombination
and e- transport : Final State
IMPORTANT to Remember: In the Auger process doesn’t exist a REAL photon intermediating the transition.
Conservation Laws
Z    Za  e
EB (a)   Nf 1   iN

Za  e   Zbc
 e  eA
EK A (abc)  EB (a)  EB (b)  EB (c) -U
Auger
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
Nomenclature for Auger Transitions
Spectroscopy Nomenclature
(example : XPS)
From the X-Ray techniques
nlj  1s; (2s, 2p1/2,2p3/2), 3s, …
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 subindices, 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.
Auger Transitions lines
45
In
45
LM M
45 45
XPS peaks
2
LM N
45
1
45
LM M
3
LM N
45 45
2
45
LM M
45
2
23
3
23
2
23
LM M
3
23
LM M
Intensidade
(u.a.)
Intensity (a.u.)
23
23
LM M
45
3
45
LM M
Excited with
Ti K=4511 eV
3s
3p
3d
LMM + LMN
2200
2400
2600
2800
3000
3200
3400
3600
3800
4000
Energia
(eV)
Kinetic Cinética
Energy (eV)
For a given element, several lines of Auger emissions can be observed.
A. de Siervo (MSc. Thesis University of Campinas, 1988)
Auger Transitions lines for different elements
Red dots are
indicating the
most intensity
lines
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.
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 > 0.1%; heavier elements > 1%.
Accuracy of quantitative analysis depending on the availability of adequate sensitivity factors (or
standards). Typical accuracy ± 10%.
Analyzer Setups
1) RFA in 4-grid LEED optics
Pre
Amplifier
Phase
shifter
2f
Lock in
Amplifier
f
signal
Frequency
doubler
computer
f
Signal
generator
Retarding H.V.
Supply
f
Isolated
transformer
Seah and Briggs in
“ Pratical Surface Analysis”
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.
3) HA – Hemispherical Analyzer
Important Characteristics :
-Better Energy Resolution
-Long work distance possible
-Angle-dependent
measurements possible
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:
I Ax ( XYZ )  I 0 Ax ( E p ) 1  rM ( EAx ,  )  T ( EAx ) D( EAx )  N A ( z) exp  z / M ( EAx ) cos  dz
0
Sensitive factor
Simplified formula for Homogeneous materials:
Ii / Si
Xi 
Ij / Sj
j
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 !
Examples for AES
1) Chemical Analysis
P. Weightman, (review article)
•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
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/
SAM
Conventional SEM image
SAM
http://www.aquila.infn.it/infm/Casti/Tech/Sam/Examples.html
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, ...
Theoretical simulation of Auger process
Atomistic approach:
(n1l 1 ) w 1 (n 2 l 2 ) w 2 . . . (n q l q )
wq
electronic conf. of the atom .
N
N 2Z
N i 1
N
2
  
2
H







(
r
Hamiltonian of the :
 i        i i )(li . si )  
i 1
i 1 ri
i 1 j 1 ri  r j
i 1
(Leighton,R.B. “Principles of Modern Physics”)
system
“Average Energy”:
Robert D. Cowan, “ The theory of Atomic Structure
And Spectra”
b
N

i 1
i2
N
2Z
   b'  bb' Ek
r
i 1 i
(more approximations:
Close shell approximation,
Central potential)
• Russell-Saunders ou LS: Coulomb >> Spin-Orbit [ Astrophs. J. 61,38 (1925)]
Coupling schemes:
•jj: Spin-Orbit >> Coulomb [ Condon and Shortley- “The theory of Atomic Spectra”]
•Intermediate Coupling ( IC ): Coulomb  Spin-Orbit [ Condon and Shortley ...]
jj coupling (normally for the initial state)
LS coupling (normally in the final State)
Spin Orbit
Coulomb Interaction
N i 1
2
b    b'   LSJM , LS' ' J ' M '  f k F k ( l1l2 )  gk G k ( l1l2 )
i 1 j 1 rij
k
2
l k l IF
l k l IR
l l LU
F
G
H0 0 0J
KG
H0 0 0 J
KS
Tl l k V
W
Fl k l I Rl l LU
2l  1G JS V
H0 0 0KTl l k W
f k  ( 1) L  2 li  1
i 1
gk  ( 1)
S
2

i 1
1
1
2
2
1 2
b
  
 i (ri )(li . si ) b   i di
L
M
N
i
 
1
3
di  li ji mi li . si li' ji'mi'   j , j ' m ,m'
ji ( ji  1)  li ( li  1) 
i i
i
i
2
4
2 1
2
1
2
1 2
i
1 2
R.D. Cowan in “ The Theory of Structure and Spectra”
Transition Probabilities (Auger Intensities)
Wifjj  LS
(Fermi Golden Rule)
2
e2
Wif 
i   f

r1  r2
2
2
1

JMj1 j2
l3l4 SLJM

2 j1  1  M
r12
a f
For IC coupling
4
2
1
Wifjj IC  (1   l3 ,l4 )(2 J  1) ( 2li  1) ( 2 x  1)  Ci ( LSJ )(1) L
2
i 1
x
LS
f   C f ( LSJ ) l3l4 SLJM
LS
l 2 L l1   1 1 

 S 
( 2l  1)(2 S  1)  1
  2 2  AS ( L, l 2 )
 2 j1 x   L x J 
2
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)
O
P
Q
Practical Examples : 1) Auger Lineshape calculation
Pd
L M M
3
23
45
Shake-Up
Nb
L M
jj-LS
2
L M
2
M
2,3
M
2,3
Ag
2,3
2,3
h=3190 eV
Auger
L =3174 eV
3
Plasmons
Mo
L M
2
L M
2
M
2,3
M
2,3
In
2,3
2,3
Plasmons
Rh
Sn
Plasmons
jj-IC
L M
2
M
2,3
2,3
L M M
2
23
23
Plasmon
Pd
L M
2
M
2,3
Plasmons+L2M23M23
Sb
2,3
Shirley Background
2180
2200
2220
2240
2260
2280
2300
2320
-80 -60 -40 -20
0
20
40
-80 -60 -40 -20 0
Energia Relativa à 1F (eV)
Kinetic Energy (eV)
3
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
20 40 60 80
Open possibilities for XAES using synchrotron radiation
Selecting channels for transition changing the photon energies: shake-up vs CK
Ag
L M M
3
45
45
D - acima do L =3808 eV (h=3829 eV)
1
C - acima do L =3525 eV (h=3597 eV)
2
B - acima do L =3352.6 eV (h=3520 eV)
3
A - threshold L =3352.6 eV (h=3359 eV)
3
D
C
Intensidade (u.a)
B
A
B-A (somente shake-up)
C-B ( Contribuição do CK L L X)
2 3
D-C (Contribuição do CK L L
X)
1 2,3
2550
J. Marais, A. de Siervo, R. Landers,et al.; Surface Science 435 (1999) 878
2560
2570
2580
2590
Energia Cinética (eV)
2600
From the Adiabatic to the Sudden Approximation
(B) Ag
L M M
3
45
Excitado longe do limiar de L
45
3
h   = L + 167.4 eV
Sudden Approx.
A proxim ação A brupta
3
Intensity (a.u.)
Sat.1
Sat.2
Ag L M M
(A)
3
45
Excitado próximo do limiar de L
45
3
h  = L + 6.4 eV
A proxim ação A diabática
3
Adiabatic Approx.
S hirley B ackground
1S
2555
0
Semsatellites
satélites
No
2560
2565
2570
2575
2580
2585
2590
2595
Energy (eV)
J. Morais, A. de Siervo, R. Landers, et al. 103 (1999) 661
T.D.Thomas, PRL 52 (1984) 417
More Complex Auger Transitions : cascade Process
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
Vielen Danke
Muito Obrigado :-)