“An Introduction to Auger Electron Spectroscopy : Applications and

Fundamental Studies on Electronic Structure of Atoms Molecules and

Solids”

Abner de Siervo(16.11.2004)

DELTA – Winter Semester 2004

Outline

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

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

AES means Auger Electron Spectroscopy – This spectroscopy technique uses Auger electrons as probes for surface science analysis: chemical and elemental characterization.

Historical Introduction

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-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 havepushed this limit to the 10 nm regime.

Lise Meitner Pierre Auger

The Auger Process

Ground State Ionization: Initial State Auger recombinationand e- transport : Final State

or photons or…

Z Z ea

(PE)

Z e Z e eaAuger

bc A ( ) ( ) ( ) ( )A B B BEK abc E a E b E c -U

Conservation Laws

• IMPORTANT to Remember: In the Auger process doesn’t exist a REAL photon intermediating the transition.

1( ) N NB f iE a

U= Electron-Electron interaction in the final state + Relaxation energiesU is known as Auger parameter

Observe: Auger electron energy is independent of the excitation energy !

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.

Auger Transitions lines

2200 2400 2600 2800 3000 3200 3400 3600 3800 4000

In

L 2M23

M23

3d

3p3sL 1M45

M45

L 2M45

N45L 3M

45N

45

L 2M45

M45

L 3M45

M45

L 2M23

M45

L 3M23

M45

L 3M23

M23

Inte

nsid

ade

(u.a

.)

Energia Cinética (eV)

Inte

nsi

ty (

a.u

.)

LMM + LMN

For a given element, several lines of Auger emissions can be observed.

Excited with Ti K=4511 eV

Kinetic Energy (eV)

XPS peaks

A. de Siervo (MSc. Thesis University of Campinas, 1988)

Auger Transitions lines for different elements

Red dots areindicating themost 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

Isolated transformer

Signal generator

Retarding H.V. Supplycomputer

Frequencydoubler

ff2f

f

Lock in Amplifier

signal

PreAmplifier

Phaseshifter

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.

Simplified formula for Homogeneous materials:

0

0

( ) ( ) 1 ( , ) ( ) ( ) ( ) exp / ( )cosx x x x x xA A p M A A A A M AI XYZ I E r E T E D E N z z E dz

Auger electron intensity:

/

/i i

ij j

j

I SX

I S

Sensitive factor

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

•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 peakPosition for the C Auger (KVV) in different CxHy compounds

P. Weightman, (review article)

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

(n l ) (n l ) . . . (n l )1 1w 1

2 2w 2

q qw q

Atomistic approach:

HZ

r r rr l si

i

N

ii

N

i jj

i

i

N

i ii

N

i i

2

1 1 1

1

1 1

2 2

( )( . )Hamiltonian of the :system (Leighton,R.B. “Principles of Modern Physics”)

Robert D. Cowan, “ The theory of Atomic StructureAnd Spectra”

electronic conf. of the atom .

“Average Energy”: b iii

N

bi

N

bb kZ

rE

2

11

2 ' '

(more approximations:Close shell approximation, Central potential)

• Russell-Saunders ou LS: Coulomb >> Spin-Orbit [ Astrophs. J. 61,38 (1925)]

•jj: Spin-Orbit >> 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)

bijj

i

i

N

b LSJM LS J M kk

kk

krf F l l g G l l

2

1

1

11 2 1 2

' ' ' ' ', ( ) ( )

R.D. Cowan in “ The Theory of Structure and Spectra”

f ll k l l k l l l L

l l kkL

ii

FHG

IKJFHG

IKJRST

UVW( )1 2 1

0 0 0 0 0 01

21 1 2 2 1

2

2

1

g ll k l l l L

l l kkS

ii

FHG

IKJRST

UVW( )1 2 1

0 0 01

21 2

21

1

2

2

b i ii

i i b i ir l s d ( )( . )

d l j m l s l j m j j l li i i i i i i i i j j m m i i i ii i i i

LNM

OQP

. ( ) ( )' ' '

, ,' ' 1

21 1

3

4

Coulomb Interaction Spin Orbit

W ie

r rfif

2 2

1 2

2

Transition Probabilities (Auger Intensities)

(Fermi Golden Rule) Wj

JMj jr

l l SLJMifjj LS

M

2

2 1

1

11 2

123 4

2

a f

f C LSJ l l SLJMfLS

( ) 3 4For IC coupling

2

4

12

1

1 2

, ),( 2

1

2

1

2

1

)12)(12()1)(()12()12()12)(

2

11(

43

i x LS

SL

iillICjj

if lLAJxL

S

xj

lLlSlLSJCxlJW

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)

Practical Examples : 1) Auger Lineshape calculation

2180 2200 2220 2240 2260 2280 2300 2320

S hirley B ackground

jj-IC

P d L3M

23M

45

K ine tic E nergy (eV )

L3=3174 eV

h=3190 eV

jj-LS L2M

2,3M

2,3

Nb

L2M

2,3M

2,3

Auger

Shake-Up Ag

Pd

L2M

2,3M

2,3

Mo

L2M

2,3M

2,3

Plasmons In

L2M

2,3M

2,3

L2M

2,3M

2,3

Rh

L2M

23M

23

Plasmons

PlasmonsSn

-80 -60 -40 -20 0 20 40 -80 -60 -40 -20 0 20 40 60 80

L2M

23M

23Plasmons+

Plasmon

Energia Relativa à 1F3 (eV)

Sb

A. de Siervo, R. Landers, G.G. Kleiman, et al. ; Phy. Rev. B 60 (1999)15790A. de Siervo, R. Landers, G.G.Kleiman, et al.; J. Elec. Spec. Rel. Phen. 103 (1999) 751G.G.Kleiman, R.Landers,S.G.C. de Castro, et al.; Phy. Rev. B 58 (1998)16103R. Landers, S.G.C. de Castro, A. de Siervo, et al.; J. Elec. Spec. Rel. Phen. 94 (1998) 253

Open possibilities for XAES using synchrotron radiation

Selecting channels for transition changing the photon energies: shake-up vs CK

2550 2560 2570 2580 2590 2600

D-C (Contribuição do CK L1L

2,3X )

B-A (somente shake-up)

Ag L3M

45M

45

C-B ( Contribuição do CK L2L

3X)

C

B

A

D - acima do L1=3808 eV (h=3829 eV)

C - acima do L2=3525 eV (h=3597 eV)

B - acima do L3=3352.6 eV (h=3520 eV)

A - threshold L3=3352.6 eV (h=3359 eV)

Inte

nsid

ade

(u.a

)

Energia Cinética (eV)

D

J. Marais, A. de Siervo, R. Landers,et al.; Surface Science 435 (1999) 878

From the Adiabatic to the Sudden Approximation

2555 2560 2565 2570 2575 2580 2585 2590 2595

(A)

h = L3 + 6.4 eVAproximação Adiabática

1S0

Sem satélites

Excitado próximo do limiar de L3

Inte

nsi

ty (

a.u

.)

Energy (eV)

Shirley Background

Ag L3M

45M

45

Sat.2Sat.1

(B)

Aproximação Abrupta h= L3 + 167.4 eV

Excitado longe do limiar de L3

Ag L3M

45M

45

Sudden Approx.

Adiabatic Approx.

No satellites

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 :-)