MENA 9510 10th October 20161
X-ray photoelectron spectroscopy - An introduction
Spyros DiplasSINTEF Materials & Chemistry, Sector of Materials and Nanotechnology, Department
of Materials Physics-Oslo& Centre of Materials Science and Nanotechnology, Department of Chemistry, UiO
[email protected]@smn.uio.no
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Material Characterisation Methods
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hν
Photoelectron
2p1/2, 2p3/2
2s
1s
Ekin = hν – EB - ω
L23
L1
K
EKL2,3L2,3(Z) = EK(Z) – [EL2,3(Z) + EL2,3(Z + 1)]
Internal transition
(irradiative)
Auger electron
XPS-Basic Principle
valence bandFermi
Vacuum
De-excitationExcitation
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Auger electron vs x-ray emission yield
5
B Ne P Ca Mn Zn Br Zr
10 15 20 25 30 35 40 Atomic Number
Elemental Symbol
0
0.2
0.4
0.6
0.8
1.0
Pro
babi
lity
Auger Electron Emission
X-ray Photon Emission
It is easier to detect light elements with Auger than with EDS
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XPS spectrum ITOx 104
10
20
30
40
50
60
70
80
CPS
1200 1000 800 600 400 200 0Binding Energy (eV)
In 3dSn 3d
O 1s
In 3pSn 3p
In 3sIn 3s
In MNNSn MNN
O KLL
Auger peaks
Photoelectron peaks
In/Sn 4pIn/Sn 4s
C 1s
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Peak width (ΔE)ΔE = (ΔEn
2 + ΔEp2 + ΔEa
2)1/2
• Gaussian broadening:
-Instrumental:
There is no perfectly resolving spectrometer nor a perfectly monochromatic X-ray source.
-Sample
For semiconductor surfaces in particular, variations in the defect density across the surface will lead to variations in the band bending and, thus, the work function will vary from point to point. This variation in surface potential produces a broadening of the XPS
peaks.
-Excitation process such as the shake-up/shake-off processes or vibrational broadening.
• Lorentzian broadening.
The core-hole that the incident photon creates has a particular lifetime (τ) which is dependent on how quickly the hole is filled by an electron from another shell. From Heisenberg’s uncertainty principle, the finite lifetime will produce a broadening of the peak.
Γ=h/τ
Intrinsic width of the same energy level should increase with increasing atomic number
Natural widthX-ray source contribution
Analyser contribution
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Examples of XPS spectrometers
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Schematic of an XPS spectrometer
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Instrument: Kratos Axis UltraDLD at MiNaLab
Analyser
Monochromator
Sample
Detector
X-ray source
X-ray source
e-
e-
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Parallel angle resolved XPS instrument-Theta Probe
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Spectroscopy Source-defined small area XPS
15 µm to 400 µm
Snapshot spectrum acquisition Up to 112 channels Faster serial mapping Faster profiling
Unique parallel ARXPS with up to 96 channels Large samples (70 mm x 70 mm x 25 mm) Sputter profiles Mapping possible up to full size of sample
holder ISS included
Target applications• Thickness measurements• Surface modification, plasma & chemical• Self assembly• Nanotechnology• Ultra thin film technologies• Shallow interfaces
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Sample requirementsHas to withstand high vacuum (≤ 10-7 Torr).
Has to withstand irradiation by X-rays
Sample surface must be clean!
Reasonably sized.
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XPS Depth of Analysis
The probability that a photoelectron will escape from the sample without losing energy is regulated by the Beer-Lambert law:
Where λe is the photoelectron inelastic mean free path
Attenuation length (λ) ≈0.9 IMFPIMFP: The average distance an electron with a given energy travels between
successive inelastic collisions
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Features of the XPS spectrum Primary structure
- Core level photoelectron peaks (atom excitation)- Valence band spectra - CCC, CCV, CVV Auger peaks (atom de-excitation)
Secondary structure
- X-ray satellites and ghosts- Shake up and shake off satellites- Plasmon loss features- Background (slope)
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Quantification Unlike AES, SIMS, EDX, WDX there are little in the way of matrix effects to worry
about in XPS. We can use either theoretical or empirical cross sections, corrected for transmission function of the analyser. In principle the following equation can be used:
I = J ρ σ K λ I is the electron intensity J is the photon flux, ρ is the concentration of the atom or ion in the solid, σ s is the cross-section for photoelectron production (which depends on the element and
energy being considered), K is a term which covers instrumental factors, λ is the electron attenuation length.
In practice atomic sensitivity factors (F) are often used: [A] atomic % = {(IA/FA)/Σ(I/F)} Various compilations are available.
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Chemical shift
ΔE(i) = kΔq + ΔVM – ΔR
Initial state contribution
• Δq: changes in valence charge
• ΔVM : Coulomb interaction between the photoelectron (i) and the surrounding charged atoms.
.
final state contribution
• ΔR: relaxation energy change arising from the response of the atomic environment (local electronic structure) to the screening of the core hole
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Initial state: Before photoemission-ground stateFinal state: After photoemission
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Chemical shift - Growth of ITO on p c-SiIn
tens
ityar
bitra
ry u
nits
Binding Energy (eV)
SiOx
Si In oxide
In
Sn oxide
Sn
3/2 3/2 5/2 5/2
1.5 nm
0.5 nm
BHF 15 sec + 500oC 0.5 nm
1.5 nm
3.0 nm
0.5 nm
1.5 nm
3.0 nm
Si 2p In 3d Sn 3d
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Chemical shift
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Shake-up satellites in Cu 2p
Shake-up satellites
2p3/2
2p1/2
Cu
CuO
CuSO4
970 960 950 940 930
Binding energy (eV)
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Plasmons
Pure elements
Mo-Si-AlCompound
They describe the interaction (inelastic scattering) of the PE with the plasma oscillation of the outer shell (valence band) electrons
Plasmons in their quantum mechanical description are pseudoparticles with energy Ep=hω
ω = (ne2/ε0m)1/2/2π n =valence electron density, e, m electron charge and massε0=dielectric constant of vacuum
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Peak asymmetry
Peak asymmetry in metals caused by small energy electron-hole excitations near EF of metal
Arb
itrar
y U
nits
16 14 12 10 8 6 4 2 0 -2Binding Energy (eV)
Arb
itrar
y U
nits
1055 1050 1045 1040 1035 1030 1025 1020 1015 1010Binding Energy (eV)
Zn
ZnO
ZnZnO
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Depth profile with ion sputtering
Use of an ion gun to erode the sample surface and re-analyse Enables layered structures to be investigated Investigations of interfaces Depth resolution improved by:
Low beam energiesSmall ion beam sizesSample rotation
SnO2
Sn
Depth500 496 492 488 484 480
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Probing different depths by choosing different areas in the XPS spectrum
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Binding Energy (eV)
Ge3d
Ge(IV)
Ge(0)
4.0 eV
1220
Binding Energy (eV)
Ge2p3/2Ge(IV)
Ge(0)
4.0 eV
1140 1150 1160 1170 1180
Kinetic Energy (eV)
GeLMM
Ge(IV)
Ge(IV)
Ge(0)
Ge(0)8.0 eV
8.2 eV
Ge3d: EK = 1453 eV
λ = 2.8 nm
Ge2p: EK = 264eV
λ =0.8 nm
GeLMM: EK = 1140 -1180 eV
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Angle Resolved XPS (ARXPS)for non-destructive depth profile
Substrate
I(d) = Io*exp(-d/λcos θ)θ
Film
I (d) = Io* exp(-d/λ)
λ=attenuation length (λ ≈0.9 IMFP)
λ=538αA/EA2 +0.41αA(αA EA)0.5
(αA3 volume of atom, EA electron energy)
Arb
itra
ry U
nits
536 534 532 530 528 526 524Binding Energy (eV)
OH oxide
bulk
AR
surface
RT
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XPS-Check list Depth of analysis ~ 5nm All elements except H and He Readily quantified (limit ca. 0.1 at%) All materials (vacuum compatible) Chemical/electronic state information
-Identification of chemical states-Reflection of electronic changes to the atomic potential
Compositional depth profiling by -ARXPS (ultra thin film <10 nm), -change of the excitation energy -choose of different spectral areas-sputtering
Ultra thin film thickness measurement Analysis area mm2 to 10 micrometres
XPS gives information of what elements exist on the sample surface, how much of each element, at what chemical state, how much of each chemical state.
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Some application examples
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Interfacial studies of Al2O3 deposited on 4H-SiC(0001)Avice, Diplas, Thøgersen, Christensen, Grossner, Svensson, Nilsen, Fjellvåg, WattsAppl. Physics Letters, 2007;91, 52907, Surface & Interface Analysis,2008;40,822
• d=λSi cosθ ln(1+R/R∞)
• d: SiOx film thickness
• λSi:inelastic mean free path for Si,
• Θ: the angle of emission,
• R: the Si 2p intensity ratios ISiox/ISiC,
• R∞ the Si 2p intensity ratios I∞Siox/I∞SiC where I∞ is the intensity from an infinitely thick substrate.
• R∞=(σSi,SiO2 . λSi,SiO2) / (σSi,Si . λSi,Si)
• where σSi,SiO2 and λSi,SiO2 are the number of Si atoms per SiO2 unit volume and the inelastic mean free path respectively
• The σSi,SiO2 / σSi,Si ratio is given by
• σSi,SiO2 / (σSi,Si = (DSiO2 . FSi) / DSi . FSiO2
• where D is the density of the material and F the formula weight.
• For the calculations we also assumed that the Si 2p photoelectrons from both SiC and Si oxide film will be attenuated by the same amount as they travel through the Al2O3 film therefore, their intensity ratio will reflect the attenuation of the Si 2p electrons coming from the SiCthrough the Si oxide film.
From XPS
d= 1nm at RT, d=3nm at 1273 K
Si0
Si4+Si3+
SiC/Si2+ Al 2p plasmon contribution
Si+
SiC
Al2O3
SiOx
as-grown
N2/H2 873 K 15 mins
N2/H2 873 K 30 mins
Ar 1273 K 15 mins,
Ar 1273 K 30 mins
Ar 1273 K 60 mins
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XPS on ITO e-beam deposited prior and after annealing (SINTEF internal)
Sn 3d In 3d
O 1s Valence band
e-beam deposited
e-beam deposited
Air annealed 300 C
Air annealed 300 C
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Nitrogen in ZnO
Arb
itra
ry U
nits
410 408 406 404 402 400 398 396 394 392 390 388Binding Energy (eV)
AG
900 oC
200 sec sputtering
200 sec sputtering
outermost
outermost
Arb
itra
ry U
nit
s
414 412 410 408 406 404 402 400 398 396 394Binding Energy (eV)
N
N-C=Oor
N-CNO2
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Band bending in ZnOR. Schifano, E. V. Monakhov, B. G. Svensson, and S. Diplas, 2009, Appl. Phys. Lett. 94, 132101
Arb
itrar
y U
nits
1050 1045 1035 1030 1025 1020 1015Binding Energy (eV)
Arb
itrar
y U
nits
980 982 984 986 988 990 992 994 996 998 1000Kinetic Energy (eV)
Zn 2p - O 1s energy difference
490
490,2
490,4
490,6
490,8
491
491,2
491,4
491,6
0 20 40 60 80 100
Etching time (sec)
Zn 2
p - O
1s
ener
gy d
iffer
ence
(eV)
LR4 Zn2p-O1sLR3 Zn2p-O1s
Zn 2p-LM45M45 Auger parameter comparison
2008,5
2009
2009,5
2010
2010,5
2011
0 20 40 60 80 100
Etching time (sec)
Zn 2
p-LM
45M
45 A
uger
par
amet
er (e
V)
LR4 2p-LM45M45LR3 2p-LM45M45
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3SEM of a Cu(In,Ga)Se2 solar cell (cross-section) and its mode of operation
CIGS solar cell• Energy/environmental application
• Solar cells based on Cu(In, Ga)Se2 (CIGS) • Thin-film stack on glass• 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• Controlling film composition and interfacial chemistry
between layers (affects electrical properties)• XPS solution
• XPS sputter depth profiling Elemental and composition information as a function of depth Identify chemical gradients within layers Investigate chemistry at layer interfaces
Acknowledgement: Thermo Electron Corporation
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Depth profile of CIFS film stack
CIGS solar cell• Depth profile of CIGS film stack
• Demonstrates standardless quantification of XPS• Excellent quantification agreement between XPS and
Rutherford BackScattering (RBS)• Both techniques show cross-over of In and Ga close to
1.6µm depth• XPS tool is able to analyze product solar cell device
Zn
O
Se
Cu
Ga
In
Mo
CdS
C
Sputter depth profile of CIGS film stack Rutherford BackScatter profile of CIGS film stack
Acknowledgement: Thermo Electron Corporation
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Interfaces in Solar cells
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Interface between p-Si/ZnO: Si HF with and without Ar etched (SINTEF SEP 09)
SiO2
Si Zn mixed oxide
Si
RT depos + 300 C annealingRT deposDepos at 500 C Depos at 500 C + Ar etching Depos at RT + Ar etching
No Ar etchingAr etching
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Elemental distribution and oxygen deficiency of magnetron sputtered ITO filmsA. Thøgersen, M.Rein, E. Monakhov, J. Mayandi, S. Diplas
JOURNAL OF APPLIED PHYSICS 109, 113532 (2011)
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XPS depth profiling reveals metallic In and Sn at the interface
High resolution TEM showing metallic In at the interface
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Initial stages of ITO/Si interface formation studied with XPS and DFTO.M. Løvvik, S. Diplas, A Romanyuk, A. Ulyashin, JOURNAL OF APPLIED PHYSICS 115, 083705 (2014)
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• Presence of pure In and Sn, as well as Si bonded to oxygen at the ITO/Si interface wereobserved. • The experimental observations were compared with several atomistic models of ITO/SiInterfaces. • These results support and provide an explanation for the creation of metallic In and Sn alongwith the growth of SiOx at the ITO/Si interface.
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Theoretical efficiency of Cu2O/ZnO based solar cell:[1-3]
~10-20 %
Highest experimental efficiency reported: ~ 2 % without buffer layer [2]
~ 1-4% with buffer layer [4,5]
[1] S. Jeong et al. , Electrochim. Acta (2008), 53, 2226 .[2] A. Mittiga et al. Appl. Phys. Lett. (2006), 88, 163502 .[3] T. Gershon , et al., Sol. Energy Mater. Sol., C. (2012), 96,48[4] Z. Duan et al. Solar Energy Materials & Solar Cells 96 (2012)[5]T. Minami et al., Appl. Phys. Exp.4, 062301(2011)
Cu2O as a Potential Solar Cell MaterialNFR project "Heterosolar" (SINTEF-UiO)
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Samples Investigated with XPS
Top film nominal thicknesses: 1, 3, 5, 10 nm
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AZO/Cu2O AZO/CuO
XPS on ultrathin AZO deposited on Cu2O. The satellite indicates formation of CuO at the interface in agreement with TEM (see next slide). Variations in the satellite intensity as the interface is approached indicate that ZnO influence the electronic structure of the interfacial CuO
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STEM-HAADF imaging of Cu2O/ZnO cross-section interface. (A) The overview STEM HAADF image of the Cu2O film grown on O-polar ZnO substrate. (B) The HRSTEM HAADF image of the boxed region indicated
in (A). The different layers in the cross-section are marked by false coloring. (C) and (D) The FFT patterns from the top and the interfacial
layer in (B) respectively were identified as Cu2O and CuO phases.
STEM HHADF and FFT
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