Dietrich R. T. ZahnInstitut für Physik, Technische Universität Chemnitz, Germany

Raman Spectroscopy as a Tool to StudyInterfaces, Thin Films,

and Low Dimensional Structures

SemiconductorSemiconductor PhysicsPhysics ––ActivitiesActivities in Chemnitzin Chemnitz

hω

ee

SemiconductorInterface

hω

Electrical Measurements:Current-Voltage (IV)Capacitance-Voltage (CV)(Deep Level) Transient Spectroscopy

Surface Science:Photoemission Spectroscopy(UPS and XPS)X-ray Absorption Fine Structure(NEXAFS)Auger Electron Spectroscopy(AES)Low Energy Electron Diffraction(LEED)Inverse PhotoemissionKelvin Probe (CPD)

Growth:(Organic) Molecular Beam Depositionin Ultra-High Vacuum(Metal-Organic) Vapour Phase Deposition

Optical Spectroscopy:Raman Spectroscopy (RS) PhotoluminescenceSpectroscopic Ellipsometry (SE) UV-visInfrared Spectroscopy (IR)Reflection Anisotropy Spectroscopy (RAS)

The Chemnitz Semiconductor Physics and Organic Semiconductors Groups

Photons in – photons out

Raman Spectroscopy

Optical Spectroscopy

Raman spectroscopy

• Different principles. Based on scattering of (usually) visible monochromatic light by molecules of a gas, liquid or solid

• Two kinds of scattering encountered:– Rayleigh (1 in every 10,000) same frequency– Raman (1 in every 10,000,000) different frequencies

99.99%MONOCHROMATIC

RADIATION

TRANSPARENT DUST-FREE SOLID, LIQUID or GAS

Information Depth

Dielectric Function

describes light – matter interaction

Light – Matter Interaction

incident

reflected

transmitted or absorbed

( ) ( ) κωεω inn +==~

( )xIxI α−= exp)( 0

cωκα 2

=

( ) ( ) ( )ωεωεωε ir i+=

Refractive index:with n real part of refractive index (refraction !) and κ the so-called extinction coefficient (absorption).

Absorption coefficient:Light intensity as function of distance x travelled in a medium:

GaAs

Energy E / eV

1 eV = 1,602×10-19 J

1 nm = 10-9 m = 10 Å

410 495 620 700

Wavelength λ / nm

560

3,0 2,5 2,2 2,05 1,7

UV IR

Energy units for optical spectroscopy

hω =hck = 2πhcλ

k ≡ wave vector; 1/λ = wavenumber

1/λ ∝ energy

1 eV = 8067.5 cm-1

300 cm-1 = 37.2 meV

Historical Introduction

• Studying scattering of light by transparent media

• Serendipitous discovery in 1921

• Phenomenon named for Raman in 1928

• His illuminating source was sunlight!!

Published in Nature in 1928

Origin of Rayleigh and Raman scattering

44 or 1 ν

λ∝I

Raman spectrum for CCl4 excited by laser radiation of λ0 = 488 nm and ν0 = 20, 492 cm-1. The number above the peaks is the Raman shift in cm-1.

Example:

a) At what wavelengths in nm would the stokes and anti-stokes Raman lines for carbon tetrachloride (Δν = 218, 459 and 769 cm-1 ) appear if the source wasa helium/neon laser (632.8nm)?

(an argon ion laser (488.0 nm)?)

a helium/neon laser (632.8nm):

Δν = 218, 459 and 769 cm-1

Raman signal:

Wavenumber: 15802.8 cm-1

Stokes line: ν =(15802.8 – 218) cm-1 =15584.8 cm-1

λ = 641.65 nm

anti-Stokes line: ν =(15802.8 + 218) cm-1 =16020.8 cm-1

λ = 624.2 nm

Infrared absorption and Raman scattering

-300 -200 -100 0 100 200 300Raman shift (cm-1)

0 50 100 150 200 250 300Frequency (cm -1)

Comparison of Raman and infrared spectra

Vibrational Raman

• Symmetric stretching vibration of CO2

• Polarisability changes– therefore Raman band at 1,340 cm-1

• Dipole moment does not– no absorption at 1,340 cm-1 in IR

Vibrational Raman

• Asymmetric stretching vibration of CO2

• Polarisability does not change during vibration– No Raman band near 2,350 cm-1

• Dipole moment does change– CO2 absorbs at 2,349 cm-1 in the IR

Raman SpectroscopyRaman SpectroscopyR - Rayleigh Scattering

S - Stokes Raman Scattering

ωi- ω(q)AS - Anti-Stokes

Raman Scatteringωi+ ω(q)

ωi

v=0v=1

ω(q)ω(q)

Virtual levels

qkk

qEP

i

i

S

S

rh

rh

rh

rhhh

rrrr

±=

±=

=

)(0

ωωωχε

ωi ωiωi+ ω(q)ωi- ω(q)

Inelastic scatteringInelastic scattering of the light mediated by the polarisabilitypolarisability of the medium.

ω

I

Reflected light

Incident light

Scattered light

Theory of Raman Scattering

• Scattering is based on the fact that incident radiation induces an oscillating dipole moment:

)2cos(0 tEM πναrtr

⋅=

)2cos( 110 tπνααα ttt +=• If the atoms execute a periodic motion:

)})(2cos())(2{cos(21

1101 ttE ννπννπα ++−⋅+rt

)2cos(00 tEM πναrtr

⋅=

Depolarization resulting from Raman scattering

Conservation rules for a Raman process

hωL = hωS + hΩphononhKL = hKS +hqphonon

⎧

⎨ ⎪

⎩ ⎪

BACKSCATTERING

qphonon

KS KL

Raman

Brillouin

4π500 nm

π0.5 nm

Ω (cm-1)

300

qphononω L − ωS[ ] q( ) = ω KL( )− ω q − KL( )= cKL − c q − KL( ) = 2cKL − cq

The ‘phonon’ spectrum

IR & Raman Active

Raman Intensity:

polarizability; intensity of the source; the concentration of the active group;

Raman signal intensity increases with the fourth powerof the frequency of the source;

directly proportional to the concentration of the active species

Principle of Raman Scattering

Raman Instrumentation

three major components:

- laser source - sample illumination system and - a suitable spectrophotometer

1,5 2,0 2,5 3,0 3,51

10

100

1000

laser lines

Info

rmat

ion

dept

h / n

m

Photon energy / eV

Information depth for GaAs= ½ of light penetration depth

Typical geometries for Raman scattering

90o scattering

180o scattering

Two sample excitation systems

(a) Schematic of a system for obtaining Raman spectra with a fiber-optic probe; (b) end view of the probe; (c) end view of the collection fibers at the entrance slit of the monochromator. The blackened circle represents the input fiber, and the hatched circles the collection fibers.

Schematic of Raman Spectrometer

Spectrographs for Raman

Spex 1877 triple monochromator

Spex 1403/4 double monochromator

Single Monochromator

Multichannel dispersive Raman spectrometer with a charge-coupled device (CCD). BP is an interference band-pass filter; BR is a Rayleigh band-rejection filter.

Photo-Detectors

Photodiode array detector

Charge coupled device (CCD)

Have a good spectrograph!

Optical diagram of an FT-Raman spectrometer

Spectra of anthracene. A: Conventional instrument, 514.5 nmexcitation; B: FT instrument, 1.064μm excitation.

Raman Spectroscopy

hωs=hωi+hΩ

200 250 300 350

ZnSe LO

Intensity / ctsmW-1s-1

GaAs LO

Raman Shift / cm-1

Raman Spectroscopy

hωs=hωi+hΩ

200 250 300 350

ZnSe LO

Intensity / ctsmW-1s-1

GaAs LO

Raman Shift / cm-1

1,5 2,0 2,5 3,0 3,51

10

100

1000

laser lines

Info

rmat

ion

dept

h / n

m

Photon energy / eV

RamanRaman SpectroscopySpectroscopy

phis Ω±= hhh ωω

buried layers

surface

small focus

intensity∝ω4

high Eg materials

Inten

sity

/ ctsm

W -1

s-1

Raman Shift / cm-1

Inten

sity

/ ctsm

W -1

s-1

Raman Shift / cm-1

Frequency Position and Lineshape

frequency shift by

temperature ≈-2cm-1/100°Cpressure ≈1cm-1/1kbar

lineshape:

asymmetric broadening and shiftoccurs as a result of latticedisturbance

0 100 200 300 400284

286

288

290

292

+/- 10°C

+/- 0.2 cm-1

Peak

Pos

ition

in /

cm -1

Temperature / °C

Determination of Surface Temperature

Using temperature induced shift of substrate phonon peak:

cm-1/100°CInSb: 2.1InP: 2.0GaAs: 1.8Si: 2.2ZnSe: 2.4

Resonance Raman scattering

0ij0

I ∝0 Light j j phononi i Light 0

hωL −hωphonon − Ej⎛

⎝

⎜ ⎜

⎞

⎠

⎟ ⎟ hωL − Ei

⎛

⎝ ⎜

⎞

⎠ ⎟ ij

∑

2

LightLight Phonon

Resonance Raman excitation profiles

100 150 200 250 300Raman shift (cm-1)

100 150 200 250 300Raman shift (cm-1)

100 150 200 250 300Raman shift (cm-1)

100 150 200 250 300Raman shift (cm-1)

100 150 200 250 300Raman shift (cm-1)

1.65 1.70 1.75 1.80 1.85 1.90 1.95

Inte

nsity

(arb

. uni

ts)

Laser Photon Energy (eV)

hωL

100 200 300 400 500 600 70050

100150

2002500.1

0.2

0.3

0.4 LOZnS LOZnSe+LOZnS

2 LOZnSeLOZnSe

Intensity / counts mW -1s -1

Temperature / °CRaman Shift / cm-1

with increasingtemperaturethe bandgapof ZnS0.05Se0.95approaches thephoton energyof 2.66 eV

typical gain oftwo orders ofmagnitude

Resonance enhancement

Sub-Monolayer Sensitivityvia Resonance Enhancement

Growth Chamberultra-high vacuum: base pressure<1⋅10-10mbar

up to 3 Knudsen cells

rf plasma source foratomic nitrogen

LEED/Auger

0.45 0.50 0.55 0.60 0.650

1

2

3

4

visi

ble

ligh

tred

blue

(620 nm)

(414 nm)

InSb

CdTeInPSiGaAs

ZnSe CdS

ZnS

GaN

Ener

gy b

andg

ap /

eV

Lattice constant / nm

Eg vs Lattice Constant

CdS Growth on InP(100)

substrate: ammonium sulfidepassivated InP

wafers annealed in UHV to 330°C for 10 min; TS=200°C compound source for CdS at 620°C

laser excitation:2.34 eV

CdS Growth on InP(100)

0 50 100 150 2000.0

0.1

0.2 calculation experiment

Inte

nsity

LO

CdS

/ co

unts

s-1

mW

-1

CdS Layer Thickness / nm

Determination of CdS Layer Thickness

Fabry-Perotinterferencescause intensitymodulation of Ramansignals

200 300 400

Δd=4nm

Scat

terin

g In

tens

ity

Raman Shift / cm-1

Initial Phase of CdS Depositionon InP(100) at 200°C

broad shoulderon low frequencyside of CdS LO phonon peakindicates an interfacialreaction leadingto an In-S richlayer

CdTe Growth on InSb

substrate: cleaved n-type InSb(110) surface

CdTe deposition from single Knudsencell kept at 550°C

laser excitation: 2.41 eV

CdTe Deposition at 300°C

no CdTe growth

strong interfacereaction

100 150 200 250

In2Te

3

A1g

(Sb)

D

C

B

A

Experiment Fit

Sca

tterin

g In

tens

ity

100 150 200 250

77K

D

C

B

AIn

2Te

3

Sca

tterin

g In

tens

ity

Raman Shift / cm-1

Interfacial Reaction Products

Reaction of Te with InSb leading to the formation of In2Te3 and liberatedSb confirmed.

CdTe Deposition at RT

no interfacereaction

Fabry-Perotmodulation

change in InSbLO/TO ratio

ZnSSe Growth on GaAs(100)

substrate:As capped MBE grownGaAs layer

compound sources for ZnSe and ZnS

atomic nitrogen provided by rf plasma sourcelaser excitation: 2.54 eV for doping at

TS=260°C2.66 eV for ZnSSe at

TS=250°C

MolecularMolecular BeamBeam EpitaxialEpitaxial Growth of Growth of ZnSeZnSe: : EffectEffect of of NitrogenNitrogen DopingDoping

Modulation due to Fabry-Perotinterference: Determination of growth rate and layer thickness

Identical experimental conditions, except: undoped doped

Incorporation of nitrogen causesbroadening of electronicresonance; plus compressivestrain in substrate

0 50 100 150 200 250 300284.7

285.0

285.3

285.6

285.9

286.2

286.5

286.8

ZnSe:N ZnSe undoped

Ram

an S

hift

/ cm

-1

Thickness / nm

Dependence of GaAsLO Frequency on ZnSe Doping

Nitrogeninducescompressivestrain in GaAs

125 150 175 200 225 250 275 300 325 350 375

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

5.7 cm-1

5.7 cm-1

20.2 cm-1

13.7 cm-1

ZnSe LO

GaAs LO

ZnSe:N

ZnSeundoped

TM =260°C

Eex

= 2.54 eV (488 nm)d = 200 nm

Ram

an In

tens

ity /

coun

ts m

W-1

s-1

Raman Shift / cm-1

ZnSe with and without Nitrogen

broadeningof ZnSe LO phonon mode indicateslatticedisturbancebynitrogenincorporation

Raman Monitoring of ZnSSe Growth

ZnS- and ZnSe-like LO phononscatteringobservableup to up to third order

0.0 0.2 0.4 0.6 0.8 1.040

60

80

100

120

140

Theory after Hayashi et al. measured peakdifference

at nominal x

LOZn

S-LO

ZnSe

/ cm

-1

sulphur content x

Determination of S Content in ZnSxSe1-x

dependence of the relative frequency shiftof ZnS- and ZnSe-like LO modes onsulphur contentK.Hayashi et al. ,Jpn.J.Appl.Phys. 30, 501(1991)

200 220 240 260 280 300 320 340

LO1+LO

2

LO2

Sca

tterin

g In

tens

ity

Raman Shift /cm-1

460 480 500 520 540 560 580

xnom

= 0.05

LO1: ZnSe-like

LO2: ZnS-like

LO2-LO

1

2LO1

LO1

Composition of Ternary Compounds

increasing frequencysplitting of ZnS- and ZnSe-like LO modescan be seen in LO and 2LO features

GaN Growth on GaAs(100)substrate:As capped MBE grown

GaAs layer

atomic nitrogen provided by rf plasma source

Ga from Knudsen cell at 870°C

laser excitation: 3.05 eV

Raman Monitoring of GaN Growth on GaAs(100) at 600°C

resonanceenhancement of scattering in thecubic modification:

Eex=3.05eV≈Eg,cub

at 600°C

200 400 600 800 1000

T=600°C

E2

GaAs LO

GaN

E2

A1+LO

dGaN

=

230nm

30nm

clean GaAs

Sca

tterin

g In

tens

ity

Raman Shift / cm-1

GaN Growth on GaAs(100)

high sensitivityachieved for GaNdetection at elevatedtemperatures

Substrate strain and GaN crystalquality

0 50 100 150 200 250

34

36

38

40

42 A1+LO GaN

FWH

M /

cm-1

GaN layer thickness / nm

281

282

283

284

LO GaAsPosi

tion

/ cm

-1

shift of GaAs LO phonon again revealsthe evolution of compressive strain in the substrate

evolution of FWHM is related to thecompetitive growth of cubic and hexagonal GaN

RamanRaman SpectroscopySpectroscopy::

DesorptionDesorption of a of a Se Se CappingCapping LayerLayerCrystallisation during annealing

Temperature induced shift

Background due to roughness

Simple molecules<1nm

IBM PowerPC 750TM

Microprocessor7.56mm×8.799mm

6.35×106 transistors

semiconductor nanocrystal (CdSe)5nm

10-10 10-510-9 10-7 10-610-8 10-4 10-3 10-2

m

Circuit designCopper wiringwidth 0.2μm

red blood cell~5 μm (SEM)DNA

proteins nm

bacteria1 μm

Nanometer memory element(Lieber)1012 bits/cm2 (1Tbit/cm2)

SOI transistorwidth 0.12μm

control biological machines

diatom30 μm

Nanoparticles: What are they?Nanoparticles are small elemental ensembles (typically on the nanometer scale).

They often have unique properties due to the fact that they are do not exhibit the characteristics of the bulk or individual particles.

solid state molecular

Low-dimensional semiconductor structures

3D: 2D: 1D: OD: Crystals Quantum Quantum Quantum

wells (SLs) wires dots

E

N(E

)

E

N(E

)

E

N(E

)

E

N(E

)

SubstrateSubstrate

WL

QD

÷8

÷8

N = 4096n = 1352

N = 4096n = 3584

N = 4096n = 2368

N = total atoms; n = surface atoms

Surface Area

One intrinsic benefit is the increased surface area available in nanoparticles.

Structural characterisation: HRTEM

Ge quantum dots:

Dot base size: ~15 nm,

Height: ~ 1,5 nm

Period of structure is 10

100Å

Structural characterisation of samples: STM

0 20 40 60 80Distance, nm

2.0

1.0

0.0

Height,

nm

Ge quantum dots:

Dot base size: ~15 nm,

Height: ~ 1,5 nm

Density of the dots:

~ 3.1011cm-2

Dot uniformity: ~ 20%

Structural Characterisation: STM

Ge quantum dots:

Dot base size: 10x10 nm,

Height: ~ 1,5 nm

Experimental Set-up

Basic principles of Raman spectroscopy in crystals

1. Energy conservation:

2. Quasi-momentum (wavevector) conservation:

Ω±= hhh si ωω

qkk ±= si

ki

ks qki

ks q 0≈

kiks

q 2k≈ i

λi ~ 5000 Å, a0 ~ 5 Å ⇒ |q| << π/a0 ⇒ only close to BZ center phonons are seen in the 1st order Raman spectra of bulk crystals

i

nqλπ40 ≤≤⇒

sii kk ≈⇒Ω⟩⟩hhω

Raman tensor. Symmetry selection rules.

Scattering intensity: Is ~ |es·χ(q)·ei|2

χ(q) – dielectric susceptibility tensor, modulated by phonons:χ(q) = χ0 + (∂χ/∂q)0q + ... ⇒ 1st order Raman intensity is

Is ~ |es· (∂χ/∂q)0·q·ei|2

R = (∂χ/∂q)0·q — Raman tensor Is ~ |es·R·ei|2

For zinc-blende-type crystals, (001) surface:

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛=

0000000

)( L

L

LO dd

zR⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛=

00000

00)(

T

T

TO

d

dxR

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛=

0000

000)(

T

TTO

ddyR

Reduction ofsymmetry:

Td → D2d

• Optical phonons: confinement

• Acoustic phonons: folding

General Properties of Phonons in Superlattices

m = 1, 2, 3... d = (n1+n2)a0

0

Averagedispersion

GaAs AlAs (GaAs) (AlAs)8 8

00.5 0.51 1

0 0.5 1

100

200

300

400

3

2

1

2

1

3Al Ga

As

q (units /a )π 0

Ω(cm )-1

4 nπ λ/ 4 nπ λ/

q (units /d)π Atomic displacements

01 )( anmδ

π+

GaAs AlAs

Selection Rules forGe/Si Superlattices

LA LO

LO TOy‘(x‘,z)-y‘TO

z(x,y)-z,

z(x‘,y‘)-z,

y‘(x‘,x‘)-y‘

LO

z(x,x)-z,z(x‘,x‘)-z

LA

geometryphonon

Typical Raman Spectrum of Ge Dot Superlattice

200 300 400 500

m=±4

m=±3

m=±2

m=±1

Ge-Ge

Ge-Si

localSi-Si

Si

Ram

an in

tens

ity/ a

.u.

Raman shift/ cm-1

40 60 80 100

z(xx)-z geometry

• Ge-Ge phonons,

• Ge/Si phonons,

• folded acoustic phonons

Influence of Strain, Confinement, Intermixing

280 300 320 400 420

Ge-Ge

Ge-Si

Ram

an in

tens

ity/ a

.u.

Raman shift/ cm-1

Strain-induced shift

ω=304 cm-1, εxx=εyy=0.04,

εzz =2C12/C11.εxx

frequency shift due to atomic intermixing

Confinement-induced shift

117)]([21 −=++=Δ cmqp yyxxzz εεεω

ω

Ge-Ge Optical Phonons: Information about Strain

z(xy)-z geometry

wetting layer (no QDs): strained structure; confinement influence

QDs separated by thin Si layers (≤25Å): a partial strain relaxation

(strain is 2.8%)

QDs separated by thick Si layers (≥45Å): strained QDs ( 4%)

280 300 320 340

11/100

14/125

14/45

14/25

14/15

6/100

Ram

an in

tens

ity, a

.u.

Raman shift/ cm-1

Bulk Ge

Ideal strained

Ge-Si Optical Phonons: Information about Atomic Intermixing

z(xy)-z geometry

wetting layer (no QDs): almost abrupt interface

QDs separated by thin Si layer (≤25Å): atomic intermixing x≈0.09

QDs separated by thick Si layer (≥45Å): atomic intermixing x≈0.04

Model of atomic arrangement:380 400 420 440

11/100

14/125

14/45

14/25

14/15

6/100

Ram

an in

tens

ity, a

.u.

Raman shift/ cm-1

Si Si Ge GeGexSi1-x Ge1-xSix

Abrupt interface

Confinement of Optical Phonons

Ge/Si structure with the Ge thickness of 5ML

Ge1 Si

2

0 π/L π/aWavenumber, cm-1

ω1

3

4

5

2

Ge

qm Lm=π

Folded acoustical phonons

Dispersion of folded acoustic phonons (S.M.Rytov, Akoust. Zh.2, 71 (1956))

0 π/d π/aWave vector, cm-1

ω Si

Ge

)sin()sin(2

1)cos()cos()cos(2

2

1

12

2

2

1

1

υω

υω

υω

υω dd

kkddqd +

−=

k =υ ρ

υ ρ1 1

2 2d=d1+d2; d1 and d2, ρ1 and ρ2, υ1 and υ2 are the thickness, density and sound velocity in Ge and Si layers

qs

Folded Acoustical Phonons in Ge Dot SLs

Nominal and calculatedstructure periods:

214 Å 238 Å211 Å 229 ÅElastic continuum theory is applicable for periodical structures with QDs !!!

1,0

0,8

0,6

0,4

0,2

0,0

wav

evec

tor,

π/d

20 40 60 80 100

x10

wavenumber/cm-1

Ram

an in

tens

ity/a

.u.

20 40 60 80

x10

wavenumber/cm-1

14/200 11/200

Experimental Raman Spectra of Ge QD Superlattices

50 100 300 350 400

x3

1.0

0.8

0.6

0.4

0.2

0.0 w

aven

umbe

r /

π/d

LTLOTO

y'(x'x')-y'

y'(x'z)-y'

z(yx)-z

z(x'x')-z

z(xx)-z

y'(zz)-y'

y'(zx')-y'

Ram

an in

tens

ity/ a

.u.

Raman shift/ cm-1

Observation of „prohibited“ phonons:

Deviation fromselection rulesfor ideal Ge/Si superlattices

Resonance Profile of Ge OpticalPhonons

• maximum of Raman intensitycorresponds to resonance with E1exciton in Ge layers

• decreasingfrequency positionof Ge phononsmanifests size-confinement in Ge quantum dots

1,8 2,0 2,2 2,4 2,6Energy/ eV

Ram

an in

tens

ity/ a

.u.

303

306

309

312

315

Ge

phon

on fr

eque

ncy/

cm

-1

Comparison of Strained and Relaxed Ge QDs

• size distribution

• strain in QDs of particular size

• QD shape

1.8 2.0 2.2 2.4 2.6Energy/ eV

Ram

an in

tens

ity/ a

.u.

275

280

285

290

295

300310

315

Ge

phon

on fr

eque

ncy/

cm

-1

SiSi

Ge SiOx

strained relaxedQDs

Raman Spectroscopy and OMBD

Dilor XY 800 SpectrometerMonochromatic light source: Ar+ Laser (2.54eV), Detector: CCD • resonance condition with the absorption band of the organic material.• resolution: ~ 3.5 cm-1.

1.5 2.0 2.5 3.0 3.5 4.0

0

2

4

6

Abs

orbt

ion

coef

ficie

nt *

105

S0-S2 transition

S0-S1 transition

DiMe-PTCDI

PTCDA

Energy / eV

800 700 600 500 400

0

2

4

Wavelength / nm

Ar+ line

PTCDA DiMe-PTCDI

Symmetry D2hRaman active: 19Ag+18B1g+10B2g+7B3g

IR active: +10B1u+18B2u+18B3u

Silent: + 8Au108 internal vibrations

Molecular Vibrational Properties

CC2424HH88OO66

• DiMe-PTCDI: Cambridge Structural Database.

• PTCDA: α- and β-phases: S. R. Forrest, Chem. Rev. 97 (1997), 1793.

Monoclinic crystallographic system in thin films:

CC2626HH1414OO44NN22

C2h44Ag+22Bg

+23Au+43Bu

+ 8Au132 internal vibrations

200 400 600 1200 1350 1500 1650

Inte

nsity

/ ar

b. u

nits

Raman shift / cm-1

Raman Spectra of a PTCDA Crystal

• assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP, 3-21G).

x0.1

200 400 600 12

Inte

nsity

/ ar

b. u

nits

Raman sh

Raman Spectra of a Raman Spectra of a PTCDAPTCDA CrystalCrystal

• assignment of modes and their relative atomic contribution using Gaussian `98 (B3LYP:3-21G).

Raman shift /cm-1

and a and a DiMeDiMe--PTCDIPTCDI

DiMe-PTCDI PTCDA

PTCDA DiMe-PTCDI

DiMe-PTCDI

PTCDA experimental

ω m= =0.97ω m

ω 221= =0.95ω 233

⎛ ⎞⎜ ⎟⎝ ⎠

• Molecules remaining at the surface:NPTCDAPTCDA(0.04nm) ~ 1013 cm-2

NddSiSi ~ 1012 cm-2

Strong interaction between PTCDAPTCDA molecules and defectsdefects mainlymainly due to SiSi at the GaAsGaAs surface.

Interaction of Interaction of PTCDAPTCDA with the with the SS--GaAs(100):2x1 GaAs(100):2x1 SurfaceSurface

Annealing of a 14 nm thick film at 623 K for 30 min:

1300 1400 1500 1600

Inte

nsity

/ ct

s m

W-1 s

-1

Raman shift / cm-1

0.00

2

40 nmx 0.01

0.45 nm(x 0.6)

0.18 nm

ann.x 4.4

300 600 9000

10

20

30

1200 1400 16000

500

1000

1500

Inte

nsity

/ A

4 am

u-1

Raman shift / cm-1

Calculated Vibrational Properties:PTCDA

1340 1350

2.7 cm-1

• calculations with Gaussian `98 (B3LYP:3-21G).

Raman Monitoring ofRaman Monitoring of PTCDAPTCDA Growth on Growth on SS--GaAs(100):2x1GaAs(100):2x1

• Thin PTCDAPTCDA film: “first layer” SERS effect: molecules in contact with AgAg

• 15 nm PTCDAPTCDA film: mainly long range SERS:no AgAg diffusion into PTCDAPTCDA

S-GaAs(100)

AgAg//PTCDA:PTCDA: Evidence for Abrupt InterfaceEvidence for Abrupt InterfaceSimilar interface formation for AgAg//DiMeDiMe--PTCDIPTCDI

1350 1500 1650

Inte

nsity

/ ct

s m

W-1s-1

0.03

PTCDA(0.4 nm)

Raman shift / cm-11200 1350 1500

PTCDA(15 nm)

0.001

S-GaAs(100)

Ag:1.6 nm/minAg:5.5 nm/min

2.2 nm Ag

11 nm Ag

/ 30

/ 5

Indium/PTCDA: Evidence for Strong Indiffusion

1200 1350 1500 1650

Inte

nsity

/ ct

s m

W-1s-1 0.03

PTCDA

PTCDA(15 nm)

Raman shift / cm-11350 1500 1650

PTCDA(0.4 nm)

0.001

x 0.017+ Inx0.045

In: 0 →100 nm

In: 1 nm/min

PTCDA~0.4 nm(~1 ML) S-GaAs(100)

~15 nm(~50ML)

PTCDA

S-GaAs(100)

PTCDAPTCDA / / SS--GaAs(100)GaAs(100)

Morphology of Organic Thin FilmsMorphology of Organic Thin Films

15 nm thick films (nominal coverage) preferential orientation of DiMeDiMe--PTCDI PTCDI islands with their

long axis parallel to the [011] GaAs[011] GaAs substrate axis.

200nm200nm200nm200nm

DiMeDiMe--PTCDIPTCDI / / SS--GaAs(100)GaAs(100)

Determination of Molecular Orientation:Determination of Molecular Orientation:DiMeDiMe--PTCDIPTCDI

Azimuthal rotation of a 120 nm thick film; normal incidence.Periodic variation of signal in crossed and parallel polarization.

M. Friedrich, G. Salvan, D. Zahn et al., J. Phys.: Cond. Matter, 15 (2003) S2699 .

γ=0°: x II [011]GaAs

γ=90°:x II [0-11]

γ

phononsphonons phononsphonons

STM tip-enhanced Raman spectroscopyA new approach, tip-enhanced Raman spectroscopy (TERS), is explored that combines Raman spectroscopy at smooth surfaces with a local electromagneticfield enhancement provided by an optically active Ag STM or AFM tip. This optical activity is achieved by exciting local surface plasmon modes by focussing the laser light through a thin metal film onon a glass slide onto the tip apex. The local enhancement of the Raman scattering cross section in the vicinity of the tip opens promising avenues towards single molecule Raman spectroscopy.

Raman Spectroscopy