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Ming H. Cheng, Allie Margarella, Yu LiuJohn C. Hemminger group
January 11, 2011
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Surface structure of Pyrite thin films on Si Raw Fitting Elemental sulfurBulk sulfide
S2p KE=200eVBulk sulfide
(Fully coordinated, 1S, 3Fe) Surface sulfide
(Lower coordinated 1S, 2Fe)
Monosulfide
S
159 160 161 162 163 164 165 166 167 168
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159 160 161 162 163 164 165 166 167 168B.E. (eV)
Advanced X‐Ray Analysis Methods
XPS (Photoemission) Binding Energy(beamiline 11.0.2, beamline 9.3.2)XAS (Absorption) Unoccupied Density of state (HOMO)XAS (Absorption) Unoccupied Density of state (HOMO)(beamiline 11.0.1, beamiline 10.3.2)XES (Emission) Occupied Density of state (LOMO)XES (Emission) Occupied Density of state (LOMO)(beamiline 8.0.1)
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All in one?
XPS, XAS, XES all in Beamline 11.0.2?
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Purpose•XPS & Electron Yield XAS
Binding Energy, Density of State of Conduction Band F i S f D t i ti V l B d S t k BE l t Fermi Surface Determination: Valence Band Spectrum, or know BE element Band GapTesting experiment on Si
• Electron Yield XES & XAS• Electron Yield XES & XAS Density of State of Valence and Conduction Band Band Gap and Core Hole Effect
5Chem. Mater. 2009, 21, 2568–2570
XPSOne Photon process: Photon in Electron outElectron out
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XPS Cartoon Mechanism
E kin
Free electron level
Conduction bandVacuum level
Fermi level
Φs Incident X‐rayh
Valence band
EB
hν
S
2P1/2
2P3/2
1S
2S
XPS: EB= hν‐E kin ‐Φs For example, EB for S2p3/27
Surface Sensitivity•Electron Inelastic Mean Free Path
Detector
X‐ray (constant hν )
Photoelectron effect
e‐X ray (constant hν )
E kin = hν‐EB‐Φs
8J. Phys. Chem. Ref. Data, Vol. 28, No. 1, 1999
Depth profile experiment and inelastic mean free path (IMFP)mean free path (IMFP)
Depth profile experiment
DetectorSynchrotron Light (various hν)
Depth profile experiment
e‐e‐
•Continuous•Changeable
ZChangeable
Photoelectrons with different kinetic energies come from
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gdifferent depth of the sample.
XES and XASTwo Photons process: Photon in Photon outPhoton out
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XES Cartoon Mechanism
E kin
Free electron level
Conduction bandVacuum level
Fermi level
ΦsIncident X‐rayh
Valence band
EB
hν
S
2P1/2
2P3/2 Fluorescent Photon
1S
2S
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XES:(Photon in Photon out) Fluorescent photon created by electron decay from valence band to core level
XAS Cartoon Mechanism
V l l
E kin
Free electron level
Conduction band
l b d
Vacuum level
Fermi level
ΦsIncident X‐rayhν
Valence band
EB
P
2S
2P1/2
2P3/2
1S
XAS: Electrons from core level to unoccupied
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S ect o s o co e eve to u occup edconduction band, For example, A1 for S_L edgeEA= hν‐ hνT For example
Limitation
•Traditional Measurement (Transmission) Signal‐to‐background ratios limited by thickness (~500Å) Radiation damage Reflection geometry experiment Surface Sensitive?!
•We can only collect electron not photon in beamline 11.0.2We can only collect electron not photon in beamline 11.0.2
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Electron Yield
•Electron Yield or Secondary Electron A l & Fl Ph Auger electron & Fluorescent Photon
Photoelectron
Ev
Auger electron
Higher shell
Incident X‐rayhν Fl thν Fluorescent
Photon
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Electron Yield
•Auger electron yield dominateFor K shell excitation of low‐Z atoms For L shell excitation of all Z <90C, N O, S, Si
15J. Phys. Chem. Ref. Data 8, 307 (1979)
Electron Yield
•Detection Mode Auger Electron Yield (AEY) Auger Electron Yield (AEY) Partial Electron Yield (PEY) Total Electron Yield (TEY)
16Stöhr, Joachim, NEXAFS Spectroscopy, Springer‐Verlag 1996
Reviews:l lXPS & Electron Yield XAS
Band Gap DeterminationBand Gap Determination
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Fermi Surface Determination( )
Conduction bandVacuum levelΦs band gap
•Testing Experiment on Si (band gap 1.11eV)
Valence bandFermi level
2P3/2
band gap
•Binding Energy calibration Au 4f7/2=84.00 eV
2P3/2
hv=293eV
•Valence Band Spectrum
7/2 Ag 3d5/2=368.27 eV Cu 2p3/2=932.67 eV
M P Seah Surf Interface Anal 14 488 (1989)
Valence Band spectrum
hv=293eV
M. P. Seah, Surf. Interface Anal. 14, 488 (1989)
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-10 -5 0 5 10 15B.E. (eV)
Take‐home messagesa e o e essages
• Band Gap Determination by XPS+XAS Fermi Surface Determination by Valence Band Spectrum Fermi Surface Determination by Binding Energy Calibrationy g gyTesting Experiment on Si
• Band Gap Determination by Electron Yield XES+XASp yDensity of State Information of Valence band and Conduction bandCore Hole Effect Analysis
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