Spectroscopy of Hybrid Inorganic/Organic Interfaces

Electron Spectroscopy

Dietrich RT Zahn

Photoemission Spectroscopy: UPS and XPS

X-Ray Source (Mg KX-Ray Source (Mg K/ Zr M/ Zr M))

UV Lamp (He I/ He II) UV Lamp (He I/ He II)

Lens System: 5 operation modesLens System: 5 operation modes

Angular Resolved Energy AnalyserAngular Resolved Energy Analyser

Detector (Channeltron)Detector (Channeltron)

Data acquisition system Data acquisition system

VGX900IC data system

Ah

,

Ratemeter and

Channeltron supply

Lense Supply

SPECTROMETER

CONTROL UNIT

Lens

System

Retard

ChanneltronElectrons

path

Pre Amp

Lens Voltages

Fermi analyserHVKE = +R +WF

OMBD SystemOMBD Systemand Electrical Measurementsand Electrical Measurements

O M BD &M etalliza tion

C ham ber

In s itu IV /C V

K nudsence lls

P lasm aC ell

LE E D

X P S U P S

K e lv inP r o b e

R A SS tage

II-V I M B EC ham ber

R H E E DG un

AnalysisC ham ber

(AR U PS10)

E lectronA nalyzer

S pectroscopicE llipsom etry

in situIV / CV

X5

EF

cutoffE

h

HOMO

VBM

IES-GaAs

S -GaAs

CBM

LUMO

IEPTCDA

PTCDA 1PTCDA 0 nm

×S -GaAs(10 ) : (20 1)

h

K inetic energy Binding Energy

(Vacuum Level shift)

Interface dipole

1PTCDA 0 nm×S -GaAs(10 ) : (20 1)

Determination of Determination of Energy Diagram using Photoemission Spectroscopyusing Photoemission Spectroscopy

E = - h E - K IN B

,g tE

gE

Reduction of Reduction of IInhomogeneous nhomogeneous FFermi ermi LLevel evel PPinning by inning by PTCDAPTCDA DDepositioneposition

S.Park, D.R.T. Zahn et al., APL 76 (22) (2000) 3200.

• PTCDA/Se-GaAs(100)

• Lineshape remains unchanged Negligible interaction between PTCDA and Se-GaAs(100).

• Gaussian broadening of Se3d core level is reduced:0.87 0.78 eV by 0.09 eV. Reduction of inhomogeneous Fermi level pinning by preferential adsorption of PTCDA on defect sites.

Valence Band Offset at thePTCDA/S-GaAs Interface

• Valence band – HOMO offset : (1.1eV0.1)eV

• No change in band bending of the substrate upon PTCDA deposition.

10 8 6 4 2 0 -2

1.97eV

S-GaAs(100) PTCDA(6nm)

0.89eV

Inte

nsi

ty /

a.u.

Binding energy / eV

• IE spans from 5.18 to 6.4eV. • A wide range of IE of wet S treated surfaces (5.53~5.91eV).Due to the degree of the surface dipole formation.

• Similar IE for GaAs(100)-c(44) and H-plasma treated GaAs(100).

Ionization Energy of Differently Ionization Energy of Differently Treated Treated GaAs(100) SurfacesGaAs(100) Surfaces

Valence Band Spectra of Valence Band Spectra of PTCDAPTCDA//SS-GaAs(100)-GaAs(100)

• Assignment- A: MO in perylene- B,C,D: MO in perylene and C=O- E: mixture of and states

• No change in energy position of A – E upon PTCDA deposition.

• Shift of Ecutoff towards higher binding energy.

Valence Band Spectra of Valence Band Spectra of PTCDAPTCDA//GaAs(100)-c(4GaAs(100)-c(44)4)

Change in direction of interface dipole is observed.

Band Diagram of Band Diagram of PTCDAPTCDA on on Differently treated Differently treated GaAs(100)GaAs(100)

IEGaAs=6.40eVSe-GaAs PTCDA

IEGaAs=5.75eVS-GaAs PTCDAGaAs PTCDA

IEGaAs=5.23eV

• Possible LUMO position:(Eg,o=2.2eV)–(Eg,t=2.8eV from Kahn et al.)• Correlation between interface dipole and relative energy position of ELUMO to ECBM. EA difference is the driving force for the formation of the interface dipole.

Interface Dipole vs. Electron Interface Dipole vs. Electron Affinity of Affinity of GaAs(100)GaAs(100)

3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2

-0.8

-0.6

-0.4

-0.2

0.0

0.2

PTCDA

=4.12eV

Interface dipole Linear fit

Inte

rfac

e d

ipol

e / e

V

Electron affinity of GaAs / eV

Se-GaAs-(21)

S-GaAs-(21)

GaAs-c(44)

• Linear relation of interface dipole to GaAs.

• At interface dipole=0, GaAs=(4.120.1)eV=PTCDA

• Eg,t(PTCDA)=2.44–2.55eVPTCDA=4.12eV

VB Spectra of Ag on VB Spectra of Ag on PTCDAPTCDA

• At low Ag thickness, features from PTCDA are still seen without energy shifts.

• Very weak charge transfer between Ag atoms and PTCDA molecules.

VB Spectra of Ag on VB Spectra of Ag on DiMe-DiMe-PTCDIPTCDI

• Very weak charge transfer between Ag atoms and DiMe-PTCDI molecules.

• Slightly different Ag4d band lineshape.

DiMethyl-3,4,9,10-Perylenetetracarboxylic diimide

Influence of Influence of Organic SubstrateOrganic Substrate on Metal Workfunctionon Metal Workfunction

IH P R esea rch Tra in ing N e tw orkAg(111), Ag,poly :Dweydari et al., Phys. Stat. Soli. A 17 (1973) 247

• Ag film on PTCDA: closer to Ag(111), stronger (111) diffraction peak

• Crystalline structure of underlying organic film strongly influence the crystalline structure and of metal film.

interface dipole =-0.68 eV

strong surface dipole good interface properties

Energy band alignment DiMePTCDI / S-GaAs(21)

EFS

1.18eV2.04eV

6.28eV

6.46eV

=-0.68eV

EVBM

EHOMO

Gianina Gavrila 26.06.03

Gianina Gavrila 26.06.03

Density of states for a neutral molecule of DiMePTCDI

valence band states corresponding

to bonding combinations of C2s, C 2p,

N2s, N2p or O2s, O2p

Gianina Gavrila 26.06.03

Molecular orientation of DiMePTCDI on S-GaAs(100)

56°

deviation from the predicted value by ~ 15 ° better estimation of V0

Photon energy dependence spectra

16 15 14 13 12 11 10 9 8 7 6 5 4

90 eV 85 eV 80 eV 75 eV 70 eV 65 eV 60 eV 55 eV 50 eV 45 eV 42 eV 40 eV 37 eV 35 eV

Inte

ns

ity

/a

.u.

EB-E

VAC /eV

HOMO

Gianina Gavrila 26.06.03

Intermolecular energy band dispersion

2

01/ 2

0*

/ 2 , ( / )2 ( - - ) /

/ 1B

E k m V k n ak m h E V

m m

the final continuum state is a parabolic free-electron-like band in a

constant inner potential V0.*

0( ) 2 cos( * )B BE k E t a k

Parameters:Parameters: V0=5.8 eV, t=0.04eV, a= 4.1 Å

tilt 42°

* D. Yoschimura at al, PRB, 60, 12, 9046-9060, 1999

The Transport Gap from Combined PES and IPES Measurements

HOMO

LUMO

EF

EVAC

IE EA