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)

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