(Electron-)Spectroscopy - uni-stuttgart.de · Microscopy273 273 1s 2s 2p K L1 L 2,3 M Valenzband...
Transcript of (Electron-)Spectroscopy - uni-stuttgart.de · Microscopy273 273 1s 2s 2p K L1 L 2,3 M Valenzband...
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Microscopy 271 271Microscopy 271
(Electron-)Spectroscopy
1s
2s
2p
K
L1
L2,3
M
Valence bandFermi-level
Vacuum-level
Orbitals of free atoms form crystal bands à Often target of spectroscopy
[bands are often drawn in the reduced band schemata (projected into first Brioullin-zone)]
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Microscopy 272 272Microscopy 272
Excitation of electrons in the vacuum
1s
2s
2p
K
L1
L2,3
M
Valence bandFermi-level
Vacuum level
X-Ray induced Photoelectron Spectroscopy (XPS)
hn
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Microscopy 273 273Microscopy 273
1s
2s
2p
K
L1
L2,3
M
ValenzbandFermi-Niveau
Vakuum-Niveau
Auger Electron Spectroscopy (AES)
hn oder e-
Excitation of electrons in the vacuum
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Microscopy 274 274Microscopy 274
MPI für Metallforschung ZWE Dünnschicht labor
Group1 - Survey
x 104
0
2
4
6
8
10
12
CPS
800 600 400 200 0Binding Energy (eV)
2s
2p
3s 3p
LMM
EF
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Microscopy 275 275
Fingerprint analysis
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Microscopy 276 276
Instrumentation for XPS:
X-Ray Source Monochromator
Anodenmaterialien für Röntgenquellen:
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Microscopy 277 277
Electron detector:
Electrostatic hemispherical analyzer (HAS)
Double pass cylindrical mirror analyzer (DPCMA)
Energy resolution HAS:
0
20
2rrw
EE a+=
D
w: aperture width, a: e-beam convergence of angle
Elektronenvervielfacher in Verwendung
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Microscopy 278 278Microscopy 278
• Binding energy, Influence of chemical bonding- Initial and final energy state of atom
EB = Ef(n-1) - Ei(n)
EB: Binding energy; Ei: Initial atom energy; Ef: final atom energy; n: number of electrons in atom
- Without relaxation of electrons: EB = - ekek: Orbitalenergie
- With relaxations due electron loss:
EB = - ek + Er(k)
Er(k): Relaxationsenergie
• Effect of initial state:- Change of EB due to chemical binding
• e.g. Oxidization leads tl increase of EB by DEB
DEB = - Dek
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Microscopy 279 279Microscopy 279
MPI für Metallforschung ZWE Dünnschicht labor
Group1 - Fe 2p
x 103
15
20
25
30
35
40
45
CPS
740 735 730 725 720 715 710 705 700Binding Energy (eV)
2p1/2720.1 eV
2p3/2707.0 eV
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Microscopy 280 280
4504554604654701500
2000
2500
3000
3500
4000
4500
Binding Energy (eV)
c/s
Ti4+
Ti2p1/2Ti2p3/2
Ti3+ + Ti2+
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Microscopy 281 281Microscopy 281
(a) Verschiebung des S1s Peaks als Funktion des Oxidations-zustandes für verschiedene Verbindungen
(b) S2p Bindungsenergie für verschiedene Schwefelverbindungenals Funktion der berechneten Ladung
Korrelation zwischen Ladung des Atoms undder Bindungsenergie
Chemical shift:
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Microscopy 282 282
• Effects from final state - Reorganization of the electrons reduces EB
• often no clear correlation between EB and oxidization state
• Reference level for bining energy: Fermi energy - Electric conducting specimen and detector in contact:
F = Ef - EvacF: Work function; Ef: Fermi energy; Evac : Energy needed for complete removal of electron from specimen
EBf = hn - Ekin - Fsp
Fsp: Work function of detector
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Microscopy 283 283Microscopy 283
• Aufspaltung der Peaks - Spin-Bahn-Kopplung:
- Plasmonenanregungen:
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Microscopy 284 284Max-Planck-Institut für Metallforschung; ZWE Dünnschichtlabor
Pr:Einbau - Au_4f
Arbitrary U
nits
100 98 96 94 92 90 88 86 84 82 80Binding Energy (eV)
DE= 1.0 eV
Mg ka XPSSi 2p
Au 4f7/2Au 4f5/2
as derived
650�C
700�C
750�C
800�C
• Au and Si form eutectica (chemical shift)• evaporation at T > 750�CÞ Nano structuring of surface
DCA-MBE: Au/SiOx/Si Surfaces
Beri Mnbekum, Department Spatz
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Microscopy 285 285Microscopy 285
Auger Electron Spectroscopy (AES)
General remarks:• AES based on detection of Auger electrons• one of the most common used method for the analysis of the chemical composition of surfaces• Excitiation by e-beam or by photons• Energy of primary electrons 3...30 keV• Information depth ~10 monolayers (ML)
• typical energy of Auger electrons: up to 3 keV
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Microscopy 286 286
Basics:Energy defined by quantum numbers
Kinetic energy of Auger electrons:- 3 electrons participate in the process:
e.g.. EWXY = EK - EL - EV - FA
EWXY: kinetic energyof Auger-electrons; EK, EL : energy of orbitalsFA: work function of detector
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Microscopy 287 287
Übersicht AES-Prozess
Auger-Prozess: EF ist die Fermienergie, Fe und FA sind die Austrittsarbeitender Probe und des Analysators
EWXY = EW(Z)- EX (Z +D) - EY (Z + D) - FA
- Korrektur D für fehlende Elektronen: Erhöhung der Bindungsenergie nachIonisation D : 0...1
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Microscopy 288 288Microscopy 288
Auger excitations as function of kinetic energyfor elements Z > 2
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Microscopy 289 289Microscopy 289
• cross section sW for ionization for AES- Depending on several processes (removal of one electron,transfer of one electron from higher to lower orbital, excitation of Auger electron): quanten mechanica calculations needed (e.g. Bethe)
sW = C ln(cEP/Ew)/(EPEW)
EP: Energie der Primärelektronen; EW : Energie der SchaleC: Konstante
Experimentelle und berechneteWerte für sW
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Microscopy 290 290Microscopy 290
• Photon/Electron emission:- Energy difference DE = EW - EX could be used for AES or X-ray emission- Emission probabilities for both processes:
•Back scattering of electrons- Back scattered electrons can excite Auger electrons:
Itotal = I0 + IM = I0 (1 + rM )rM: back scatter factor is function of Z (Atom number)
- Approximation rM
1 + rM = 1 + 2,8 [1 - 0,9 (Ew/Ep)]h(Z)h(Z) = -0,0254 + 0,16 Z - 0,00186 Z2 + 8,3 × 10-7 Z3
Auger-Elektron (A)Photon (X)
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Microscopy 291 291Microscopy 291
Elektronenrückstreufaktor alsFunktion der kinetischen Energie(Ep = 5 keV, q = 30�)
Auger-Übergänge und relative Intensitätsfaktoren
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Microscopy 292 292Microscopy 292
•Emission depth L- Electron mean free path lel
L = lel cos q
Approximation: lel = 0,41 a1,5 Ekin0,5
a: thickness of monolayer (nm)
Ekin (eV), l (nm)
• Influence of binding on peak positions (Chemical Shift)- Shift due to changing chemical binding (Änderung der elektronischen Struktur des Materials)- Additional peaks might appear
320 340 360 380 400 420 440 460 480
Ti O
TiO
Ti(LMV)
Ti(LMM)
Ekin (eV)
2
2 3
Ti
d/dE
[EN(
E)]
0.2 nm Ti
0.1 nm Ti
0 nm Ti
E (eV)
Ti(MVV)
Al(LVV)
Al(LVV)
Al(L2,3)O(L2,3)O(L2,3)`
20 30 40 50 60 70
d/dE
{EN
(E)}
Ekin (eV)
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Microscopy 293 293Microscopy 293
Experimental set up- e-gun- Electrostatic energy analyzer- UHV chamger - Ion gun(for depth profiling)- Detector for surfcae imaging
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Microscopy 294 294Microscopy 294
•Elektronenquellen- W-Filament Strahlfleck 3...5 µm- LaB6 Kristall < 20 nm- Feldemissionskathode < 20 nm
- Strahlenschädigung bei Stromdichten über 1 mA/cm2 (1 nA/10 µm2)
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Microscopy 295 295Microscopy 295
• recording of spectra- Point analysis- Line profile or Mapping- Depth profiling
• normal spectra of differntiated spectra
Prinzip der Ermittlung der chemischenKonzentration einer Schicht in der Tiefe:(a) für Schichten mit Dicken unter 2...3 nm;(b) Schichtdicke < 200...1000 nm;(c) Schichtdicke < 20 µm
Auger-Map einer AlSiMg Probe,die mit Sekundärelektronen auf-gezeichnet wurde(a) REM Bild(b) Al(c) S(d) Si
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Microscopy 296 296Microscopy 296
Änderung der Austrittstiefe derElektronen mit dem Winkel
Stahlprobe, die mit TiN Schichtbedeckt ist
Linienprofil über die Vertiefung
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Microscopy 297 297Microscopy 297
•Auflösungsgrenzen- Konzentration 0,1...1% einer Monolage- Masse 10-16...10-15 g (1µm × 1µm × 1 nm)- Atome 1012...1013 Atome/cm2
- Feldemissionskathode < 20 nm
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Microscopy 298 298Microscopy 298
Auger-Übergänge als Funktion derkinetischen Energie der Elektronen für Elemente Z > 2
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Microscopy 299 299Microscopy 299
• Aufspaltung der Peaks - Spin-Bahn-Kopplung:
- Plasmonenanregungen:
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Microscopy 300 300Microscopy 300
Anregung von Elektronen in das Vakuum
1s
2s
2p
K
L1
L2,3
M
ValenzbandFermi-Niveau
Vakuum-Niveau
3.1 Röntgen induzierte Photoelektronenspektroskopie X-Ray Photoelectron Spectroscopy (XPS)
hn