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

Optical Spectroscopies of Thin Films and Interfaces

1. Introduction

2. Vibrational Spectroscopies (Raman)

3. Spectroscopic Ellipsometry

4. Reflectance Anisotropy Spectroscopy

Spectroscopic Ellipsometry(SE)

Reflectance AnisotropySpectroscopy (RAS)

variable angle of incidence

)iexp(tanrr

ρs

p ∆Ψ==

+ΦΦ+Φ=

ρ1ρ1ε 0

20

20

2 -tansinsin~

near normal incidenceRAS signal:effective dielectric function:

with: Φ0 - angle of incidence,rp, rs – Fresnel coefficients

and

βα

βα

rr

rr

+−

=∆

rr

2

where α and β correspond to e.g. the [110] and [-110] directions in the surface plane of the (001) oriented substrate

RAS

SE Φ0

Substrate

Experimental in situ Techniques

Schematic Presentation of Ellipsometry

Measuring Principle

Widely used because :

• Very sensitive

• Very acurate

• Non-destructive

• Easy and quick measurement

The Fresnel equations

{ }ppip

opp ir

nnnn

EE

r δϕϕϕϕ exp

cos~cos~cos~cos~

1001

1001 ⋅=+−

==

{ }ssis

oss ir

nnnn

EEr δ

ϕϕϕϕ exp

cos~cos~cos~cos~

1100

1100 ⋅=+−

==

iKnn −=~Complex refractive index

K: extinction coefficient

Eip Eos

Eop

Eis

Plane of incidence

Incominglight wave

Outcominglight wave

Sample

2

02

02

02

11tansinsin

+−

ΦΦ+Φ=ρρε

( )∆== irr

s

p exptanψρ

Scheme of Experimental Setup

UV-VIS-NIR ellipsometer

GaAs

Ellipsometry– Ellipsometry can measure the oxide depth.– Intensity doesn’t vary much with film depth but∆ does.

Reflectometry

S-GaAs(001) Substrate

1 2 3 4 50

4

8

12

16

20

Experiment Fit

Im<ε

>

Energy /eV1 2 3 4 5

-10

-5

0

5

10

15

20

Experiment Fit

Re<

ε>Energy /eV

GaAs substrate – sulfur treated

• The topmost layer was modeled by a Cauchy layer with Urbachabsorption: 2 4

n n nn( ) a b cλ λ λ= + + k kk

1 1k( ) a exp bc

λλ

= −

,

• Woollam VASE ellipsometer- spectral range: 0.8 - 5 eV; step 0.02 eV

• Determination of ellipsometric parameters ψ, ∆:

Variable Angle Variable Angle SpectroscopicSpectroscopic Ellipsometry (VASE)Ellipsometry (VASE)

ES

EP

substrate (ns,ks)film (nf,kf,df)

EL Eel

φ0

%%

%

%%

%

r e f l e c t e dp

P i n c i d e n tp

r e f l e c t e ds

S i n c i d e n ts

Er =

E

Er =E

( )tan expP

S

r ir

ψρ = = ∆%%

%

(n, k, t)(n, k, t)

ModelModel the optical response!

Models for data evaluationModels for data evaluation::- Cauchy (when k=0) ⇒ (n,t)

- Point-by-point fit ⇒ (n,k) at each λ

- Oscillator model

2λλ)( nn ban +=MultiMulti--

sample sample analysisanalysis

• 0.1 - 5 Torr• Vapor Phase• External Sources• MFC Source Control• Directed Carrier Flow • Hot chamber walls

• Large Substrates, Webs• Efficient Materials Use• Low Source Cross-Contamination• High Control of Doping • Higher Throughput

Organic Vapor Phase Deposition:The Concept

with courtesy of UDC

OVPD: Produktionskonzept

Großflächige Beschichtung!

Copyright © 2001-04 AIXTRON AGhttp://www.aixtron.com/

Institut für HochfrequenztechnikTU Braunschweig

AIXTRON´s OVPD-Module at University Braunschweig

HalbleiterphysikTU Chemnitz

Copyright © 2001-04 AIXTRON AGhttp://www.aixtron.com/

Institut für HochfrequenztechnikTU Braunschweig

Thickness uniformity of OVPD-Alq3 on 8“ Si-wafer

STDEV=0,7%

HalbleiterphysikTU Chemnitz

Homogeneity of Homogeneity of AlqAlq33/Si/Si sample derived from sample derived from ellipsometryellipsometry

Overlapping of the ellipsometric spectra measured on 12 different points!!!

Alq3/ SiO2 /Si

Thickness profile along 8“ wafer diameterThickness profile along 8“ wafer diameter

edge effects edge effects ––originating from originating from

distortion of the flow distortion of the flow profile outside the 6” profile outside the 6”

mother glass area.mother glass area.

6“ mother glass area

Alq3

α-NPD

Tris (8- hydroxiquinoline) aluminumN,N’- diphenyl- N,N’- bis(1- naphtyl)- 1,1’-

biphenyl- 4,4’- diamine

electrontransport material

holetransport material

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

0

20

40

60

80

100

65°

65°

75°

75°

sample a2 sample a1

Energy / eV

Alq3

Ψ /

deg

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0-150

-100

-50

0

50

100

150

200

250

300

65°

65°

75°

75°

sample a2

sample a1

Energy / eV

Alq3

∆ / d

eg

Ellipsometric spectra of Alq3 films deposited on Si/SiOSi/SiO22

SymbolsSymbols:experiment LinesLines: fits

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

1.4

1.6

1.8

2.0

2.2

2.4

0.0

0.2

0.4

0.6

0.8

1.0

Cauchy n k

Alq3

n

Energy / eV

k

Optical constants of Alq3

BESSY VUVBESSY VUV--XUV XUV EllipsometerEllipsometer setupsetup

• in situ Vacuum Ultraviolet spectroscopic Ellipsometry (VUV-SE) in the range 4-10eV• ex situ Variable Angle Spectroscopic Ellipsometry (VASE) in the range 0.8-5eV.

The dielectric function of The dielectric function of bulk Alqbulk Alq33

2

02

02

02

11tansinsin

ρ+ρ−

ΦΦ+Φ=ε

4 5 6 7 8 9

1

2

3

4

5

6

ε 1

Photon energy/eV

experimental <ε1> ε1 from model fit

4 5 6 7 8 90

1

2

3

4

ε 2

Photon energy/eV

experimental <ε2> ε2 from model fit

Effective dielectric function

ε=ε The light does not see the substrate

Comparison between the Comparison between the εε22 the bulk the bulk AlqAlq33and and εε22 of a monolayer of of a monolayer of AlqAlq33

4 5 6 7 8 90

5

10

15

20

25

30

35

40

45

0 1 2 3 4 5 6 70

1

2

3

4

5

6

7

<ε2>

Photon energy /eV

H-Si 0.6 nm 1.8 nm 3 nm 5.6 nm

Thic

knes

s fr

om fr

eque

ncy

shift

/nm

Thickness from ellipsometry /nm

4 5 6 7 8 90

2

4

ε 2

Photon energy /eV

bulk Alq3 submonolayer of Alq3

model fit

0

2

4

ε 2

submonolayer of Alq3 calculated [7]

The sharpening and the shift of the absorption peaks whencompared to the bulk can be explained by a change in either electronic coupling or due to a molecular ordering of the Alq3molecules in the bulk.

4 5 6 7 8 90

2

4

ε 2

Photon energy /eV

bulk Alq3 submonolayer of Alq3

model fit

Comparison between the Comparison between the εε22 the bulk the bulk AlqAlq33and and εε22 of a monolayer of of a monolayer of AlqAlq33

where εs is the dielectric function of the substrate, εL is the dielectric function of the layer and d is the layer thickness (valid if d<1 nm)

( )( )( ) )7(

11sinid4

sL

LLss2ss −εε

−εε−εεΦ−ε

λπ

+ε=ε

[7] D. Aspnes – Spectroscopic Ellipsometry of Solids, Chap 15, Optical Properties of Solids-New Developments, ed B.Seraphin, North Holland 1976

1 2 3 4 5

-20

-10

0

10

20

30

1.8 2.0 2.2 2.4 2.6

0.2

0.4

0.6

0.8

Energy /eV

[110]

DiMe-PTCDI

DiMe-PTCDI Films Grown on S-GaAs(001):

Ellipsometry Spectra

1 2 3 4 50

20

40

60

80

100

2.0 2.2 2.4 2.60

3

6

9

Im<ε

>

Energy /eV

[-110] DiMe-PTCDI

• very pronounced anisotropy compared with the PTCDA films

DiMe-PTCDI Film on S-GaAs(001)Optical Constants

• very strong in plane anisotropy