Dr. Tim NunneyThermo Fisher Scientific, East Grinstead, UKDr. Nick BullossThermo Fisher Scientific, Madison, WI, USADr. Harry Meyer IIIOak Ridge National Laboratory, TN, USA
2
Introduction• New materials create new characterization challenges• Full characterization requires a variety of experimental techniques
3
Introduction
• Techniques need to be chosen that provide complimentary information• Elemental and chemical composition• Molecular “fingerprinting”• Composition with increasing depth• Surface structure• Morphology
XPS Surface Analysis Microanalysis
4
Introduction to X-ray Microanalysis
In an electron column, electrons are accelerated through an electric field, acquiring kinetic energy.
The energy is transferred to the sample
Yields a variety of signals for analysis
5
EDS Basic Overview
Energy Dispersive X-Ray Spectroscopy (EDX / EDS)
Analysis of elemental content of micro-volumes using non-destructive techniques.
Based on inner electron transitions between inner atomic shells.
Energetic electron from an electron column dislodges an orbital electron.
Electron from a higher energy shell fills the vacancy, losing energy in the process.
The lost energy appears as emitted radiation of energy (characteristic X-Ray).
6
Electron Beam & Sample Interaction
Interaction Volume
The volume in which the primary electrons interact with a sample
As electrons interact with the sample, they are scattered and spread.
7
Properties of X-Rays
The relationship between Energy, Frequency, and Wavelength may be expressed by one formula
E is the Energy of the radiation
F is the Frequency of the radiationc is the speed of light 3 x 1010 cm/second
is the Wavelength of the radiation
h is Planck’s constant 4.135 x 10-15 eV-sec = 6.625 x 10-27 erg-sec
8
Detectors
Energy-dispersive detectors (EDS)
• Collect the full range of x-ray energies simultaneously
Wavelength-dispersive detectors (WDS)
• Collect one element at a time via a diffracting crystal
9
Definition of a surface
Bulk
Surface (1 nm) 3 atomic layers
The modified layer is often far too thin to be characterized
with most techniques.
The extreme surface sensitivity of XPS ensures that only the top few nanometers of the
sample are analyzed.
Ultra-thin film (1 to 10 nm) 3 - 30 atomic layers
Thin Film (10 nm to 2µm) 30 - 600 atomic layers
Note: Approximate layer thickness only. Actual values depend upon materials
XPS surface analysis• What is a surface?
• XPS measures
Surfaces using XPS and angle resolved XPS
Ultra-thin films using XPS and angle resolved XPS (ARXPS)
Thin films using XPS in combination with sputter profiling
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XPS instrumentation
Hemispherical analyser
Detector
Ion gun
Flood gun
X-ray source
Monocrystal
Electron transfer lens
• UHV System• Ultra-high vacuum keeps surfaces clean• Allows longer photoelectron path length
• X-ray source• Typically Al Ka radiation• Monochromated using quartz crystal
• Low energy electron flood gun• Low energy e- (and possibly Ar+)• Analysis of insulating samples
• Ion gun• Typically noble gas ions• Sample cleaning• Depth profiling
11
• Surface is composed of atoms• Electrons surround the nucleus of an atom,
occupying orbitals at different energies• Surface is irradiated with X-rays from a photon
source• X-rays cause electrons to be ejected
(photoelectrons)• Kinetic energy, KE, of photoelectrons is
measured by an analyser• The binding energy, BE, of the electrons is
deduced from the kinetic energy and photon energy
• Binding energy depends upon• Element• Orbital from which electron was ejected• Chemical state of the element
Basic XPS theory
Photoelectron ejected
X-rays
BE = h
- KE
01002003004005006007008009001000110012001300Binding Energy (eV)Kinetic Energy (eV)
NaCl
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Surface analysis and XPS• Elemental identification
• Which elements are present?• Can detect all elements except for H
• Elemental quantification• How much of an element is present?
Detection limit >0.05% for most elements
Allows determination of stoichiometry
Cl S
Zn
P
020040060080010001200
Binding energy / eV
Elemental identification of tribology sample
Fe
F
O
Ca
C
Zn
Elemental identification
Element At% P 0.29 S 0.29 Cl 0.22 C 15.96 Ca 14.12 O 57.73 F 1.50 Fe 6.74 Zn 3.03 Mg 0.12
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• Elemental identification• Which elements are present?• Can detect all elements except for H
• Elemental quantification• How much of an element is present?
Detection limit >0.05% for most elements
Allows determination of stoichiometry• Chemical state identification and quantification
• Bonding states for each element• Chemical structure
Surface analysis and XPS
O
OO
O n
Poly(ethylene terephthalate), PET
280285290295
Binding energy / eV
Carbon
High energy resolution spectroscopy
CC CC
Chemical state identification
14
526531536541
Binding energy / eV
• Elemental identification• Which elements are present?• Can detect all elements except for H
• Elemental quantification• How much of an element is present?
Detection limit >0.05% for most elements
Allows determination of stoichiometry• Chemical state identification and quantification
• Bonding states for each element• Chemical structure
Surface analysis and XPS
O
OO
O n
Poly(ethylene terephthalate), PET
High energy resolution spectroscopy
Oxygen
CCCC
Chemical state identification
Case Study 1: Membrane electrode assembly
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MEA fuel cell Introduction
Schematic of polymer electrolyte membrane
MEA fuel cell
• Energy/environmental application• Fuel cells for clean conversion of hydrogen• 1Proton exchange membrane fuel cell (PEMFC)
• High conversion efficiency and low/zero pollution• Low operating temperature and relatively quick start-up
• Membrane Electrode Assembly (MEA) is important component of PEMFC
• Active layer of platinum/carbon black catalyst with polymeric adjacent layers
• Practical problem• Diffusion of Pt into adjacent polymer layers would reduce
Pt/C catalytic effect• Solution
• XPS imaging of sample
ULAM (ultra low angle microtomy) sample preparation to allow imaging of nanoscale layers
Quantitative elemental and chemical state mappingK-Alpha optical image of ULAM- prepared MEA fuel cell
1Korean J Chem Eng, 23(4), 555-559 (2006)
Pt/CPt/C
NafionNafion
17
MEA fuel cell Chemical analysis of Nafion
MEA fuel cell
• Chemical analysis of Nafion• Membrane Electrode Assembly (MEA) consists of layers
microns thick• Thin Pt/C layers around a thicker Nafion layer
• Nafion is an effective proton exchange membrane• Permits hydrogen ion transport while preventing
electron conduction• Requirement to analyze profile of Pt in Nafion layer
• Layers are too thick for XPS sputter profiling• Not enough data points per layer with standard
cross-sectioning• Use ultra-low angle microtomy to create cross-
section with layers on micron scale2
• Enables XPS mapping and imaging analysis
2Journal of Materials Science 40 (2005) 285– 293
Backscatter image of cross-section of MEA fuel cell component
Anode
Cathode
Membrane
Data points
Backscatter image of ULAM-prepared MEA fuel cell component
Anode
Cathode
Membrane
Data points
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MEA fuel cell
Pt/C
Pt/C
Nafion
ULAM-prepared MEA fuel cell
Epoxy
Nafion
Chemical analysis of Nafion
MEA fuel cell
• Chemical analysis of Nafion• XPS can distinguish carbon bonding states
• Epoxy (used in making the ULAM cross-section)• C-C, C-O and C=O bonding observed
• Nafion (part of the MEA)• CF2 bonding observed• Surface contamination of Nafion gives additional C-C
and C-O peaks
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Element At%
Pt 0.41
S 0.44
C 73.68
O 10.08
F 15.39
Atomic concentration of elements in Pt/C layer
MEA fuel cell Elemental quantification of MEA-ULAM sample
Pt/C Nafion
ULAM-prepared MEA fuel cell
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
Pt (x50) S (x50) C O F
At%
Element
Nafion
Pt/C
Epoxy
Elemental quantification of MEA-ULAM sample
MEA fuel cell
• Elemental quantification of MEA-ULAM sample• XPS can elementally quantify small areas on a sample
• Quantification of Pt/C, Nafion and Epoxy zones below
• Fluorine observed in Pt/C zones• Nafion intentionally mixed with Pt/C during manufacture
of catalyst layer• Sulfur detected in all regions, principally in Nafion
and Pt/C zones• XPS is able to detect elements at low concentration
(S<0.5at% in Pt/C zone)• Pt detected in catalytically active layer
20
MEA fuel cell Large area XPS mapping
MEA fuel cell
• Large area XPS mapping• Possible to quantify elements/chemical states over a wide
sample area• Map of Pt/C and Nafion zones shown below, overlaid on
optical image of MEA-ULAM sample• Linescan from map confirms no large scale diffusion of Pt
• 0% Pt in Nafion, in agreement with spectroscopic result on previous slide
Quantified Pt at% linescan taken from large area XPS map
Pt in catalytically active layers
No Pt in Nafion zone
Principal components phase map of MEA sample
Pt/C
Pt/C
Nafion
Large area XPS map of Pt / Nafion layers and interfaces in ULAM-MEA fuel cell sample
Pt/CPt/C Pt/CPt/CNafion
Linescan from mapping data
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Membrane Electrode Assembly
• Analyses performed at 10kV on a FESEM
Spectral Image net count maps
(background subtracted)
Nafion
Pt/C
22
Membrane Electrode Assembly
• Linescan analyses (50 points)
• No apparent migration from the Pt/C electrode into the Nafion
Si
C
PtF
Pt
Case Study 2: Thin film solar cell
24
Delaminated CIGS solar cell
3SEM of a Cu(In,Ga)Se2 solar cell (cross-section) and its mode of operation
• Energy/environmental application• Solar cells based on Cu(In, Ga)Se2 (CIGS)
• Thin-film stack on glass (or in this case steel)• Mo and Zn oxide layer form electrical contacts• p-type CIGS film (sunlight absorber) and n-type
CdS film form p-n junction• Excellent efficiency• Low cost compared to thicker silicon-based solar cells
• Practical problem• Delamination of device• Controlling film composition and interfacial chemistry
between layers (affects electrical properties)• Solution
• EDS imaging of good & delaminated areas
High resolution elemental information• XPS imaging & sputter depth profiling
Elemental and composition information as a function of depth
Identify delamination layer
3 D. Abou-Ras et al. Elemental distribution profiles across Cu(In,Ga)Se2 solar-cell absorbers acquired by various techniques. In: M. Luysberg, K. Tillmann, T. Weirich (Eds.), “EMC 2008”, Vol 1: Instrumentation and Methods, Proceedings of the 14th European Microscopy Congress 2008, Aachen, Germany, September 1-5, 2008 (Springer, 2008) p. 741.
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Delaminated CIGS solar cell
Delamination zone
Some of the CIGS material had delaminated from the substrate. Measurements were taken from the three distinct areas of the sample:
(1 & 2) the top layers of the sample(3) the metallic surface electrical contact material(4) the underlying metallic substrate.
EDS Point Analysis
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Delaminated CIGS solar cellEDS Point Analysis
27
Delaminated CIGS solar cell
Locations 1 and 2 on the film are the same material consisting of a majority of In and Se with a small amount of Cu and a small amount of Ga. Only small amounts of Zn, O, Cd and S are measured due to the thin nature ofthese layers.
Location 3 on the metallic contact layer is silver.
Location 4 where the film is removed shows the base Mo substrate that the layers are grown on.
EDS Point Analysis
28
Delaminated CIGS solar cell
1
34
XPS point analysis at the same locations as the EDS analysis
XPS Point Analysis
2
29
Delaminated CIGS solar cellXPS Elemental analysis
01002003004005006007008009001000110012001300
Cou
nts
/ s
Binding Energy (eV)Se
3d
Mo3
d
C1s
Ag3d
In3d
Sn3d
5O1s
Zn2p
3
Ga2
p3
Red spectrum is average of points 1 & 2
1 & 2
3
4
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Area 1
1.80E+04
2.00E+04
2.20E+04
2.40E+04
2.60E+04
2.80E+04
3.00E+04
3.20E+04
3.40E+04
480482484486488490492494496498500
Coun
ts / s
Binding Energy (eV)
Sn3d Scan
Elem
ent
SnO
0.00E+00
2.00E+04
4.00E+04
6.00E+04
8.00E+04
1.00E+05
1.20E+05
1.40E+05
1.60E+05
1.80E+05
440442444446448450452454456458460Co
unts
/ s
Binding Energy (eV)
In3d Scan
Elem
ent
InO
x
1.00E+04
2.00E+04
3.00E+04
4.00E+04
5.00E+04
6.00E+04
526528530532534536538540542544
Coun
ts / s
Binding Energy (eV)
O1s Scan
Met
al o
xide
Met
al C
O3
3000
4000
5000
6000
7000
280282284286288290292294296298
Coun
ts / s
Binding Energy (eV)
C1s Scan
C-C
or C
-H carb
ide
carb
onat
e
Cl2
s (M
etal
ClO
4) Chemical state analysis suggests that points 1 & 2 are indium-tin-oxide (ITO)
XPS Chemical analysis
31
Area 3XPS Chemical analysis
2300
2400
2500
2600
2700
2800
2900
3000
158160162164166168170172174
Coun
ts / s
Binding Energy (eV)
S2p Scan
Elem
ent
orga
nic
SO
2
Met
al SM
etal
SO
3
Met
al S
O4
2.00E+03
4.00E+03
6.00E+03
8.00E+03
1.00E+04
1.20E+04
1.40E+04
280282284286288290292294296298Co
unts
/ s
Binding Energy (eV)
C1s Scan
C-C
or C
-H
carb
ide
carb
onat
e
Cl2
s (M
etal
ClO
4)
0.00E+00
2.00E+04
4.00E+04
6.00E+04
8.00E+04
1.00E+05
1.20E+05
1.40E+05
1.60E+05
362364366368370372374376378380
Coun
ts / s
Binding Energy (eV)
Ag3d Scan
Ele
men
tA
g2O
AgO
2300
2400
2500
2600
2700
2800
2900
192194196198200202204206208210
Coun
ts / s
Binding Energy (eV)
Cl2p Scan
Met
al C
lO4
Met
al C
l
1.00E+04
1.10E+04
1.20E+04
1.30E+04
1.40E+04
1.50E+04
526528530532534536538540542544
Coun
ts / s
Binding Energy (eV)
O1s Scan
Met
al o
xide
Met
al C
O3
32
Delaminated CIGS solar cell
EDS
XPS
Point analysis comparison
Ga-K Zn-K O-K Cr-K In-L Cd-L Ag-L Mo-L Se-K Fe-K Cu-K
Area 1 2.90 2.51 17.38 0.82 24.32 7.42 2.31 22.93 2.41 12.39Area 2 3.01 2.88 17.60 0.69 24.99 7.77 2.09 21.63 2.49 12.31Area 3 100.00
Area 4 17.66 61.00 2.92 18.42
Ga2p3 Zn2p3 O1s Sn3d5 In3d Ag3d Mo3d Se3d C1s S2p Cl2p
Area 1/2 0.16 52.41 2.49 33.39 0.03 0.22 0.32 10.99
Area 3 12.38 0.23 3.13 28.40 52.18 2.01 1.67
Area 4 0.97 0.37 19.47 0.00 1.99 23.95 24.77 28.47
EDS shows the presence of the expected stack components.XPS shows the presence of small contaminants, and the nature of the outer layer.
33
Delaminated CIGS solar cellXPS Mapping
34
Delaminated CIGS solar cellPhase 1 Phase 2 Phase 3 Phase 4 Phase 5
Se3d 1.55 32.21 2.60 0.99 3.85S2p 0.38 0.75 3.84 0.39 0.47Mo3d 0.62 26.94 1.11 0.15 2.48C1s 16.54 6.27 52.72 21.67 17.32Ag3d 0.16 0.27 12.91 0.08 0.25Cd3d 0.11 0.02 0.05 0.17 0.24In3d 27.99 4.56 4.66 25.21 25.41Sn3d5 2.74 0.50 0.58 2.48 2.49O1s A 36.63 20.47 5.46 33.62 34.67O1s B 11.81 6.85 15.77 13.09 11.29Zn2p3 1.45 0.31 0.26 2.11 1.44
Ga2p3 0.02 0.84 0.02 0.04 0.09
XPS Phase Analysis
35
CIGS solar cell Cross-section of CIGS film stack
Low MagnificationA Spectral Imaging mapping analysis was performed at low magnification on the cross-section sample to understand all of the layers that constitute the material. The thickness of all of the layers was approximately 1/3 mm.
36
CIGS solar cell Cross-section of CIGS film stack
37
CIGS solar cell Cross-section of CIGS film stack
38
Delaminated CIGS solar cellXPS Depth Profile
3keV Ar+ ion beam60s etch timeCompucentric rotationDepth scale calibrated to EDS result
39
ZnO
CdS
CIGS Mo Cr SteelITO
0
10
20
30
40
50
60
70
80
90
100
0 1000 2000 3000 4000 5000 6000
Ato
mic
per
cent
(%)
Etch Depth (nm)
Se3dS2pMo3dC1sCd3d5In3d5Sn3d5O1s AO1s BCr2p3Fe2p3Cu2p3Zn2p3Ga2p3
Delaminated CIGS solar cellXPS Depth Profile
40
0
10
20
30
40
50
60
70
80
90
100
0 1000 2000 3000 4000 5000 6000
Ato
mic
per
cent
(%)
Etch Depth (nm)
Se3dS2pMo3dC1sCd3d5In3d5Sn3d5O1s AO1s BCr2p3Fe2p3Cu2p3Zn2p3Ga2p3
Delaminated CIGS solar cellXPS Depth Profile
0.00E+00
2.00E+03
4.00E+03
6.00E+03
8.00E+03
1.00E+04
1.20E+04
1.40E+04
438440442444446448450452454456
Coun
ts / s
Binding Energy (eV)
In3d
41
CIGS Mo Cr Steel
Delaminated CIGS solar cellXPS Depth Profile
0
10
20
30
40
50
60
70
80
90
100
0 1000 2000 3000
Ato
mic
per
cent
(%)
Etch Depth (nm)
Se3dS2pMo3dC1sIn3d5O1s AO1s BCr2p3Fe2p3
42
Delaminated CIGS solar cellXPS Depth Profile
ZnO
CdS
CIGS Mo Cr SteelITO
43
Delaminated CIGS solar cellXPS Depth Profile
CIGS Mo Cr Steel
44
Delaminated CIGS solar cellXPS Depth Profile
Ga/In gradient
Delamination zone
Ag contact
45
Acknowledgements
• Oak Ridge National Laboratory – (HTML)• RJ Lee Group• Application specialists in East Grinstead, UK & Madison, USA