(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)]
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
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
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
Microscopy 275 275
Fingerprint analysis
Microscopy 276 276
Instrumentation for XPS:
X-Ray Source Monochromator
Anodenmaterialien für Röntgenquellen:
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
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
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
Microscopy 280 280
4504554604654701500
2000
2500
3000
3500
4000
4500
Binding Energy (eV)
c/s
Ti4+
Ti2p1/2Ti2p3/2
Ti3+ + Ti2+
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:
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
Microscopy 283 283Microscopy 283
• Aufspaltung der Peaks - Spin-Bahn-Kopplung:
- Plasmonenanregungen:
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
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
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
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
Microscopy 288 288Microscopy 288
Auger excitations as function of kinetic energyfor elements Z > 2
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
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)
Microscopy 291 291Microscopy 291
Elektronenrückstreufaktor alsFunktion der kinetischen Energie(Ep = 5 keV, q = 30�)
Auger-Übergänge und relative Intensitätsfaktoren
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)
Microscopy 293 293Microscopy 293
Experimental set up- e-gun- Electrostatic energy analyzer- UHV chamger - Ion gun(for depth profiling)- Detector for surfcae imaging
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)
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
Microscopy 296 296Microscopy 296
Änderung der Austrittstiefe derElektronen mit dem Winkel
Stahlprobe, die mit TiN Schichtbedeckt ist
Linienprofil über die Vertiefung
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
Microscopy 298 298Microscopy 298
Auger-Übergänge als Funktion derkinetischen Energie der Elektronen für Elemente Z > 2
Microscopy 299 299Microscopy 299
• Aufspaltung der Peaks - Spin-Bahn-Kopplung:
- Plasmonenanregungen:
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