Experimental methods in physics

Surface analysis techniques

2. Photon and electron probes Elemental and electronic analysis

Jean-Marc BonardAcademic year 10-11

2

Chemical composition

• Electron transition between atomic energy levels

• Core electrons: binding energy depends on the element

• Excitation/ionisation by a probe particle (photon, electron, ion, …)

•Photoelectron

•After relaxation: X-ray, Auger electron, …

•Energy of interaction product is characteristic of the element

2

Chemical composition

• Electron transition between atomic energy levels

• Core electrons: binding energy depends on the element

• Excitation/ionisation by a probe particle (photon, electron, ion, …)

•Photoelectron

•After relaxation: X-ray, Auger electron, …

•Energy of interaction product is characteristic of the element

PhotonElectron…

Auger electronPhotoelectronUV Photon

X-ray

3

Chemical composition

• Experimental setup

• Source for probe particles

•UV photons to excite valence electrons

•X-Photons to excite core electrons

•Electrons

•Spectrometer for detection of interaction products

•Photoelectrons / Auger electrons

•X-rays

•Sample

•Sample holder, goniometer, sample movement x-y-z-rotation-tilt,…

•Vacuum system

2. Elemental analysis

2.1. Photoelectron spectroscopy

Photoelectron spectroscopy

•Principle•Excitation of core or valence electrons

•Energy-dispersive analysis of emitted photoelectrons

•Core levels

•Binding energy: 200 eV - a few keV

•Excitation: X-rays (or UV)

•Set of lines characteristic for each element

•Valence levels

•Binding energy: a few eV

•Excitation: UV light

•Reflects the electronic properties of the surface

Atom Bulk

2. Elemental analysis

2.1.1. ! Core-level photoelectron spectroscopy! ! ! (XPS, ESCA): ! ! ! chemical analysis

7

Photoelectron spectrum

• Analysis as a function of electron energy

•Kinetic energy Ekin allows to determine the binding energy EB

•Ekin = h! - EB - ϕs

•ϕs: work function

•In practice, spectrum calibrated with a well-known energy peak (C, O, ...)

Photoelectron spectrum

• Kinetic energy Ekin depends on binding energy EB

•Atom + h! " Atom+ + e–

•E(A) + h! = E(A+) + Ekin(e–)

•Ekin(e–) = h! - [E(A+) - E(A)]

•Ekin(e–): kinetic energy of photoelectron

•[E(A+) - E(A)]: binding energy of electron

•One-electron process (“photon in - electron out”)"

Identification of elements

• Energy of the peaks•Elements present in the sample

•Valence levels, core levels, Auger electrons, …

•Chemical environment

• Relative intensity of the peaks•Concentration of elements

• Angle-resolved analysis•Emission angle of photoelectrons

•Rough concentration profile as a function of depth (down to ~5 nm depth)

Chemical bonding configuration: “chemical shift”

• Shift of peak energy provoked by chemical environment and oxidation state

•Typ. 0.5-10 eV

• Allows to determine

•Oxidation state of metallic elements

•Nature of chemical bond - e.g. for

•C (C-H, C-O, C-F, …)

•O (O2-, OH, …)

•Si (metallic, SiO2, silicones)

Chemical bonding configuration: “chemical shift” for Carbon

• Carbon•Peak energy depends on nature of chemical bonding

•Shift of 1…10eV

•Makes possible to identify the configuration of the bond

•sp2/sp3 bond

•Hydrocarbon, fluorinated hydrocarbon, carbide, …

•Example: fluorinated polymer on AuCuCd

J.-M. Bonard, SSC’04

Chemical bonding configuration:

“chemical shift” for Carbon

! Carbon

" Peak energy depends on nature of chemical bonding

" Shift of 1…10eV

" Makes possible to identify the configuration of the bond

# sp2/sp3 bond

# Hydrocarbon, fluorinated hydrocarbon, carbide, …

" Example: fluorinated polymer on AuCuCd

J.-M. Bonard, SSC’04

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3

Survey spectrum! ! C1s peak

Depth profile

! XPS sensitive to surface (~5nm)

! Depth profile measurement by

sputtering the sample

" Cycles of ion beam etching (Ar+)

followed by measurement

! Max. depth: ~1 !m

" Degradation of sample due to ion

bombardment

" Atomic displacement

" Surface roughness

" Variable etching rates

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Depth profile

•XPS sensitive to surface (~5nm)

•Depth profile measurement by sputtering the sample

•Cycles of ion beam etching (Ar+) followed by measurement

•Max. depth: ~1 µm

•Degradation of sample due to ion bombardment

•Atomic displacement

•Surface roughness

•Variable etching rates

Spatial resolution and imaging

•Spatial (lateral) resolution depends on size of the surface area illuminated by the source and covered by the spectrometer•Spectrometer

•With transfer lens: ~15 µm resolution

•X-ray source

•Anode: scanning of electron beam on the anode allows to generate an X-ray beam of Ø 10 µm, and scanning over typ. 1mm2

•Synchrotron: 0.25 µm resolution has been demonstrated

Spatial resolution and imaging

! Spatial (lateral) resolution depends on size of the surface area illuminated by the source and covered by the spectrometer

" Spectrometer

#With transfer lens: ~15 !m resolution

" X-ray source

#Anode: scanning of electron beam on

the anode allows to generate an X-ray

beam of Ø 10 !m, and scanning over

typ. 1mm2

# Synchrotron: 0.25 !m resolution has

been demonstrated

13

Quartz crystal equivalent to 1:1 lens

Electron gun

Anode(Mg, Al, ...)

Sample

Spectrometer

Imaging

! Scanning the probed area

" Quantitative mapping of chemical composition

#One spectrum for each “pixel”

• One spectrum = several minutes

• Complete map = several hours

#Absolute intensity of signal depends

also on topography, not only

concentration

# In practice: qualitative mapping

(relative intensity)

• Ratio between element signal and background noise

14www.phi.com

C1s

Imaging

•Scanning the probed area•Quantitative mapping of chemical composition

•One spectrum for each “pixel”

•One spectrum = several minutes

•Complete map = several hours

•Absolute intensity of signal depends also on topography, not only concentration

•In practice: qualitative mapping (relative intensity)

•Ratio between element signal and background noise

www.phi.com

C1s

2. Elemental analysis

2.1.2. ! Valence band photoelectron ! ! ! spectroscopy (UPS): ! ! ! electronic analysis

4s

3d

Valence photoelectrons

• Transition between occupied and unoccupied states separated by h! = Ef-Ei

• Photoelectron current depends on initial and final density of states

•Final states: continuous distribution or free electrons (approximation)

• Informations•Measurement of occupied density of states near the Fermi level

•Interaction of the surface with adsorbed molecules

•Surface 2D/1D/0D states

•Angle-resolved analysis: band mapping

UPS: Fermi level

•Electron distribution around the Fermi level in Nb

•Measurements follow closely theoretical predictions for Fermi-Dirac distribution as a function of temperature

•Energy resolution of the analyser: some meV in this case

UPS: density of states in the valence band

• Sputtered Au on carbon nanotubes

•Nanotubes: wide band due to # electrons

•Bulk Au: 5d5/2 and 5d3/2 doublet, wide band due to 6s et 6p electrons

•Au on nanotubes

•Additional peaks due to Au

•Gradual transition to bulk Au behaviour as thickness increases

TEM

XPS

Bittencourt et al., Chem. Phys. 328, 385 (06)

on MWNTs

without Au

UPS: band structure mapping I

•Band structure analysis: determination of wave vector k

•Measuring the kinetic energy yields binding energy Ei, |k|

•Angle-resolved measurement

•Polar angle $ yields k// , k⊥

•Energy conservation: k// of photoelectron = k// of initial state

•k⊥ of photoelectron # k⊥ of initial state

•Angle %: crystallographic direction

•Band structure mapping as a function of k// and crystallographic direction

•2D band structure: surface states, molecules, “2D” crystals

•3D band structure: doable but more complicated

UPS: band structure mapping II

• Example:Pd(111) surface

•Measurement at fixed angle as a function of energy

•Transition from occupied to unoccupied states

•Derivation of band structure from measurements taken at different angles

•Corresponds well to theoretical predictions UPS as a function of polar

angle in &-M (<211>) direction0°: k// = 0

Band structure

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UPS: band structure mapping IIITheoretical band structure

• Example: mapping of graphite band structure (“quasi-2D” crystal)

•Synchrotron source

•hν variable, Ekin constant (130eV - continuum of non-occupied states)

•Position on image depends on k//

•! electron band

•0.5eV: K points

•3.5eV: M points

•6.5eV: Γ point

•σ electron bands

•14.5eV: K, M points

•21.5eV: Γ point

C. Heske et al., Phys. Rev. B 59, 4680 (1999)

Energy levels, ! electrons

Energy and angle-resolved UPS θ

φ

UPS: band structure mapping IIITheoretical band structure

• Example: mapping of graphite band structure (“quasi-2D” crystal)

•Synchrotron source

•hν variable, Ekin constant (130eV - continuum of non-occupied states)

•Position on image depends on k//

•$ electron band

•0.5eV: K points

•3.5eV: M points

•6.5eV: Γ point

•σ electron bands

•14.5eV: K, M points

•21.5eV: Γ point

C. Heske et al., Phys. Rev. B 59, 4680 (1999)

Energy levels, $ electrons

2. Elemental analysis

2.2.% Auger electron spectroscopy

Auger electron spectroscopy (AES)

• Excitation of core electrons with electrons or X-rays

• Relaxation of an electron

•X-ray emission

•Auger emission: energy transfer to a second electron

• Kinetic energy determined by

•Position of “initial” hole

•Position of two “final” holes

•Characteristic for each element

•Analysis of electron energy

X-Ray

Auger Emission

• Secondary process

• Competition between X-ray fluorescence and Auger emission

• Auger emission of shallow core levels favoured: light elements

• Auger electron XYZ

• Initial hole: X

• Relaxation of electron Y

• Energy transferred to another electron Z that is emitted into vacuum: Auger emission

• For example: (KL1L23)

• Energy Ekin(XYZ)

• Ekin(XYZ) = (EX-EY)-EZ-Einter

• Independent of probe particle energy

• Einter: interaction energy between holes L1 and L23; relaxation energy, … Einter << Ekin(XYZ) (typ. 1 eV)

Auger electron spectroscopy (AES)

! Excitation of core electrons with electrons or

X-rays

! Relaxation of an electron

" X-ray emission

" Auger emission: energy transfer to a

second electron

X-Ray

" Kinetic energy determined by

" Position of “initial” hole

" Position of two “final” holes

" Characteristic for each element

" Analysis of electron energy

Auger Emission

! Secondary process

" Competition between X-ray fluorescence and Auger emission

# Auger emission favoured of shallow core levels: light elements

" Auger electron XYZ

# Initial hole: X

# Relaxation of electron Y

# Energy transferred to another electron Z that is emitted by the solid: Auger emission

# For example: (KL1L23)

" Energy Ekin(XYZ)

# Ekin(XYZ) = (EX-EY)-EZ-Einter

• Independent of probe particle energy

• Einter: interaction energy between holes L1 and L23; relaxation energy, … Einter << Ekin(XYZ) (typ. 1 eV)

Auger spectrum

•Many transitions

•Strong background signal (low energy secondary electrons)

•Usually, 1st derivative of detected electron current for graphs/analysis

•High surface sensitivity

•Kinetic energy between 20…1000 eV (λ between 2…6 monolayers)

•Mapping by rastering an electron beam

•Resolution & 50 nm

•Depth concentration profiles by sputtering the surface

•Faster acquisition than XPS

•Quantitative analysis with composition standards

•Chemical effects difficult to interpret

AES: examples

• Chemical composition at the surface: FeCr steel in normal and brittle state

•Higher P, Cr and Ni concentration in brittle steel

• Depth profile: thin layers of Al2O3-Ti-TiN on Si

•After annealing: O diffuses in Ti layer, but not in TiN layer

•Ti+N: superposition of N and Ti peaks

•Layer thicknesses: 45nm of Ti, 55nm of Al2O3

brittle

normal

Spatial resolution and imaging

• Auger images: spectral analysis at every pixel

•Rastering of the probe electron beam, e.g. in a scanning electron microscope

• E.g., fracture surface of steel

•Sb/Cr particles at grain boundaries

•Resolution: '50 nm

SEM image!! Auger image: Fe, Sb, Cr

Spatial resolution and imaging

! Auger images: spectral analysis at every pixel

" Rastering of the probe electron beam, e.g. in a scanning electron microscope

! E.g., fracture surface of steel

" Sb/Cr particles at grain boundaries

" Resolution: !50 nm

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Auger Images – Fe (blue), Sb (red),

Cr (green)

AES identified the composition of grain

boundary particles to be Sb and Cr.

These phases resulted in the

embrittlement of an aged steel rotor.

Steel Fracture Surface

Secondary electron image, 10,000X

SEM image! ! Auger image: Fe, Sb, Cr

1 µm

2. Elemental analysis

2.3. X-ray fluorescence (XRF, EDX)

2. Elemental analysis

2.3. X-ray fluorescence (XRF, EDX)

X-ray spectroscopy

• Excitation of core electrons with electrons or X-rays

• Relaxation of an electron

• X-ray emission

• Auger emission: energy transfer to a second electron

• Kinetic energy determined by

•Position of “initial” hole

•Characteristic for each element

• Analysis of the energy of emitted X-rays

X-ray spectroscopy

• Excitation of core electrons with electrons or X-rays

• Relaxation of an electron

• X-ray emission

• Auger emission: energy transfer to a second electron

• Kinetic energy determined by

•Position of “initial” hole

•Characteristic for each element

• Analysis of the energy of emitted X-rays

X-Ray

X-Ray

X-ray spectroscopy: elements

Goldstein et al., “Scanning Electron Microscopy and X-Ray Microanalysis” (Plenum Press, 1992)

X-ray spectroscopy: analysis

• E.g. fluorescence (XRF)•X-ray excitation

•Widely used in industry

•Table-top analyser

•Quantitative analysis

•Chemical composition (alloys)

•Layer thicknesses

•Lateral resolution of 0.1mm

X-ray spectroscopy: analysis

• E.g. fluorescence (XRF)

•X-ray excitation

•Widely used in industry

•Table-top analyser

X-ray spectroscopy: quantitative analysis

• Identification of elements

• Quantification of composition from intensities

• Lateral and depth resolution: ~1 µm

•Not surface specific

• Commonly used incombination with electron microscopy

•EDX, excitationwith electron beam

Goldstein et al., “Scanning Electron Microscopy and X-Ray Microanalysis” (Plenum Press, 1992)

Infos XPS AES EDX / XRF

Quantification Yes Yeslight elements!

YesB and heavier

Chemical bond Yes Possible –

Electronicstructure

Yes – –

Mapping Possible (50 nm?) SEM (50 nm) SEM (1 !m)

Depth profiles Yes(Ar+ sputtering)

Yes(Ar+ sputtering)

Layer thickness

Surface sensitivity some nm some nm ~!m

Detection limit ~‰ ~% ~%

Vacuum UHV needed UHV needed XRF @ patm