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

Optical Spectroscopies of Thin Films and Interfaces

1. Introduction

2. Vibrational Spectroscopy, i.e. Raman

3. Spectroscopic Ellipsometry

4. Reflectance Anisotropy Spectroscopy

Principle of Raman Scattering

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

Raman Spectroscopy

hωs=hωi+hΩ

200 250 300 350

ZnSe LO

Intensity / ctsmW-1s-1

GaAs LO

Raman Shift / cm-1

Raman Spectroscopy

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

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

Resonance Raman scattering

0ij0

I∝0 Light j j phononi i Light 0

hωL −hωphonon−Ej

hωL−Ei

ij

∑

2

LightLight Phonon

Sub-Monolayer Sensitivityvia Resonance Enhancement

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

up to 3 Knudsen cells

LEED/Auger

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

Pea

k Po

sitio

n 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

0.45 0.50 0.55 0.60 0.650

1

2

3

4

visi

ble

ligh

t

red

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

∆d=4nm

Sca

tterin

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

Scat

terin

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

100 200 300 400 500 600 050

100150

200

0.02

0.04

0.06

0.08

0.10

Intensity / counts mW -1s -1

Thickness / nmRaman Shift / cm-1

Raman Monitoring of ZnSe Growth

100 200 300 400 500 600 050

100150

200250

0.02

0.04

0.06

0.08

0.10

Intensity / counts mW -1s -1

Thickness / nmRaman Shift / cm -1

Raman Monitoring of ZnSe Growth: Nitrogen Doping

weak ZnSe2LO scatteringrevealschange in resonancecondition as a result of nitrogendoping

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

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

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 GaAsPos

ition

/ 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

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

Abso

rbtio

n co

effic

ient

*10

5

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

2-fold

DavydovSplitting

internal molecular modes: external molecular modes (phonons):

200 300 400 500 600 700

1200 1300 1400 1500 1600 1700

Inte

nsity

/ a.

u.

x2

Raman shift / cm-1

CC--OOBBgg

CC--HH CC--CC

CC--CC

SymmetrySymmetry: : DD2h2h CC2h2h (monoclinic)(monoclinic)

25 50 75 100 125 Raman shift / cm-1

Inte

nsity

/ a.

u. 6 rotationalvibrations:3Ag+3Bg

19Ag+18B1g+10B2g+7B3g

BBgg

AAgg

AAgg

BBgg

AAgg

RamanRaman--active vibrations of active vibrations of PTCDA PTCDA ((CC2424HH88OO66))::Effect of crystal formation Effect of crystal formation

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

external molecular modes (phonons): 6 rotational vibrations: 3Ag+3Bg

SymmetrySymmetry: : CC2h2h (monoclinic)(monoclinic)

25 50 75 100 125 Raman shift / cm-1

Inte

nsity

/ a.

u.Phonons in PTCDA:

BBgg

AAgg

BBgg

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

200 250 300 350 400

LO Ω−

Nd = 2.7 *1018 cm-3

Ram

an in

teni

sty

/ a. u

.

Raman shift / cm-1

0 2 4 60.00.20.40.60.81.01.21.4

Raman PES

S-GaA

s

Ban

d B

endi

ng /

eV

Film Thickness / nm

PTCDA/S-GaAs

Electronic Properties at Electronic Properties at PTCDAPTCDA//SS--GaAsGaAs

• Relative intensities of GaAs LO and PLP (Ω-) bands:

Band bending within the substrate: minor changes upon PTCDA adsorption.

Good agreement with photoemission (PES) studies: S. Park, D.R.T. Zahn, et al. Appl. Phys. Lett. 76 (2000) 3200.

J. Geurts, Surf. Sci.

Rep. 18 (1993), 1.

4882

( 0)

GaAsn

nmdLO

n s

I eI

V z

δ

δ

−Ω

∝

∝ =

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. Mater. submitted.

γ=0°: x II [011]GaAs

γ=90°:x II [0-11]

γ

phononsphonons phononsphonons

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

yx

xx

IDep =

I

56 4 ;,

θψ ϕ

= ° ± °

( ) ( )θ ψ ϕ θ ψγ ϕ γ⋅ ⋅g

-1g

m= R , ,A ,A, R , , Good agreement with IR and NEXAFS results

( )s igAI = e e⋅ ⋅r r

0 60 120 180 240 300 3600.0

0.5

1.0

1.5

2.0

2.5

Dep

olar

izat

ion

Rat

io/ a

.u.

Experimental angle (γ)/°

BreathingBreathing mode at 221 cmmode at 221 cm--11

200 400 600 1200 1350 1500 1650

Inte

nsity

/ ar

b. u

nits

Raman shift / cm-1

x0.1

Ag Raman Modes of PTCDAwith In

200 400 600 1200 1350 1500 1650

Inte

nsity

/ ar

b. u

nits

Raman shift / cm-1

x0.1

Ag Raman Modes of In4PTCDA

In Situ Raman: Monitoring of IndiumDeposition onto PTCDA (15 nm)

1200 1400 1600

0.05

Raman shift / cm-1200 400 600

Inte

nsity

/ ct

s m

W-1s-1

0.005

43/5

In thickness / nm

00.4/0.71.1/1.52.8/135.0/288.0/3315.0/5826.0/10

Influence of Indium on VibrationalSpectra of PTCDA

1200 1400 1600

0.0025

+ InB3g

B1u

Ag

Ag

B3g

B2u

Ag

B3g

(B3g)B1u

B3gAg

Raman shift / cm-1

PTCDA

200 400 600

B2uAg

B3g

Ag

Ag

In15 nm

Inte

nsity

/cts

mW

-1s-1

0.0025Ag

B2g

GaAs

• 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)

200 300 400 500 600 1300 1400 1500 1600

Inte

nsity

Raman shift / cm-1

5x10- 2

cts.mW- 1S - 1

5x10-3

cts mW-1s-1

+ Mg

DiMe-PTCDI

+ In

+ Ag

Comparison of In, Ag and Mg deposition on DiMePTCDI

Raman Spectroscopy

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.