Dietrich R. T. Zahn Institut für Physik, Technische Universität Chemnitz, Germany

Dr. Ovidiu D. Gordan & Dr. Raul D. Rodriguez

Surfaces, Interfaces, and Thin Films: Characterisation at the Nanometre Scale

1. Photons vs. electrons: Basic considerations for

achieving surface/interface sensitivity

2. Extending the photon energies towards the vacuum

ultraviolet region: Ellipsometry using synchrotron

radiation

3. Special ways of achieving surface/interface sensitivity

in Raman and reflectance anisotropy spectroscopy

4. High resolution photoemission and X-ray absorption

fine structure measurements

Semiconductor Physics – Activities in Chemnitz

ω e e

Semiconductor Interface

ω

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 Photoemission Kelvin Probe (CPD)

Growth: (Organic) Molecular Beam Deposition in Ultra-High Vacuum (Metal-Organic) Vapour Phase Deposition Spray coating

Optical Spectroscopy: Raman Spectroscopy (RS) Spectroscopic Ellipsometry (SE) Infrared Spectroscopy (IR) Reflection Anisotropy Spectroscopy (RAS) Photoluminescence and UV-vis

Nanoscale Characterization: •  Micro-Raman spectroscopy and imaging •  Atomic force microscopy •  Kelvin probe force microscopy, current sensing

AFM Tip- and surface-enhanced Raman spectroscopy (TERS and SERS)

•  ANSYS: finite element simulation package Matlab

Organic semiconductors

Displays (Kodak) Organic field-effect transistors

GaAs(100)

Organic Interlayer

V

I Metal Organic/Inorganic

Microwave Diodes

Electrically driven organic lasers

Organic-modified Schottky Diodes Plastic solar cells

First large area OVPD-OLED displaying the Logo of TU Braunschweig processed on a substrate size of 35 x 50 mm².

silver

U

magnesiumAlq3

α-NPDITO

glass substrate

silver

U

magnesiumAlq3

α-NPDITO

glass substrate

Structure of the large area OVPD-OLED device

First OVPD-OLED

PVD TiN 450 Å

HfO2 21 Å Interfacial layer 12 Å

Silicon substrate

Avinash Agarwal et al., Alternatives to SiO2 as Gate Dielectrics for Future Si-Based Microelectronics, 2001 MRS Workshop Series (2001)

Scanning Tunneling Microscopy

Moving atoms one at a time…

Monatomic Regular Steps on Si(111)-3º

9.0nm5.7nm

0 5 10 15 200

0.20.40.60.8

11.21.4

X[nm]

Z[nm

]

5.7nm

Monatomic layer on each step (0.3 nm/layer)

Equidistant terraces

Atomically straight edges

Experimental slope ~ Nominal value of wafer

~2.8°

After 30sec. x 2times of flash

Photodetector

Mirror

Laser

Sample

Cantilever →

Piezo

Atomic force microscopy (AFM)

AlAs QDs in InAs matrix. Ion milling with cooling.

We  can  see  the  stripes!  

Lattice period (50±4) nm

Optical Spectroscopy

Dielectric Function

describes light – matter interaction

Light – Matter Interaction

incident

reflected

transmitted or absorbed

( ) ( ) κωεω inn +==~

( )xIxI α−= exp)( 0

cωκ

α2

=

( ) ( ) ( )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:

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

Penetra'on  depth  

zk

eIzI λπ2

0)(−

=

-  extinction coefficient

zk

kp πλ

δ4

= -­‐  Informa'on  depth  

400 500 600 7000

250

500

750

1000

Pen

etra

tion

dept

h / n

m

Wavelength / nm

Si GaAs InAs AlAs

-  depth

¨  Informa'on  about  atomic  arrangement  ¡  Chemical/molecular  analysis  ¡  Crystallinity  ¡  Polymorphism  ¡  Phase  

Phonon   frequency   depends   on  elas1c  constant  of  chemical  bond  and  atom  mass  

Photon  

•  Applied  to  any  op'cally  accessible  sample  

•  Solid,  liquid,  gas,  transparent,  non-­‐transparent,  bio-­‐applica'ons  

•  Microscopic  spa'al  resolu'on  •  Confocal  analysis  

¨  Non-­‐destruc've  

Raman Spectroscopy

17

Iden'fica'on  and  use  with:  • Narco'cs  &  illicit  street  drugs;  • Explosives  &  chemical  warfare  agents;  • Hazardous  materials  &  chemical  spills;  • Pharmaceu'cal  IQ/OQ/PQ  quality;  • Gemstone  authen'ca'on,  etc.  

18

Medical applications

20

1 µm

0 1 Intensity of the G+ band / a.u.

21

1200 1300 1400 1500 1600 1700

1553.81569.1

1561.5 1601.21569.3

1540.71559.8

1572.0

1556.81565.9

1571.1

1591.4

1590.9

1590.2

1591.4

1288.5

1340.7

1341.2

Ram

an in

tens

ity

Raman shift / cm-1

G peak region 829 nm

632.8 nm

532 nm

514.7 nm

1319.0

cm-1 Tentative type

Diameter / nm

829 nm

1569.1 S 1.5 1553.8 S 1.1

632.8 nm 1572.0 S 1.6 1559.8 M 1.6 1540.7 M 1.3

532 nm 1569.3 S 1.5 1561.5 S 1.3

514.7 nm 1571.1 S 1.6 1565.9 S 1.4 1556.8 S 1.2

Diameter distribution 1.1 – 1.6 nm

Raman spectroscopy: Key technique for the analysis of CNT

Laser  spot    ̴  2000  nm   Nanopar'cle    ̴  40  nm  

hν0  h  (ν0  -­‐  νph)  

1)  Illuminate  sample  with  laser  

2)  Measure  intensity  of  sca=ered  light  at  each  νph  

Raman Spectroscopy

22

Metallic  Tip  

Still we have contributions from a sample region comparable in size to the laser spot but…

Laser  spot    ̴  2000  nm  

Nanopar'cle    ̴  40  nm  23

Sample signal comparable in size to the tip!

This is TERS

TERS:  Tip-­‐enhanced  Raman  Spectroscopy  

The  key:  Excita'on  of  localized  surface  

plasmons  at  the  'p  apex  confines    and  

amplifies  the  electromagne'c  field    

24

500 510 520 530 5400

50

100

150

200

250

Inte

nsity

/ co

unts

Raman shift / cm-1

Tip Approached Tip retracted

1200 1400 1600 2400 27000

30

60

90

120

Inte

nsity

/ ct

s

Raman shift / cm-1

Tip Approached Tip retracted

2D

G+

G-

D

Tip-enhanced Raman Spectroscopy TERS

Transistors

60 nm"

Technology generation: L → L/√2

“Transistorized” PBS, Nov. 8, 1999 www.pbs.org/transistor/

Bell Labs "1947"

TI 2001"

“Moore’s Law”

21st Century Electronics: Transistors at the nano/molecular scale

Gate"

Drain"Source"

~100 nm"

Texas Instruments"~2000 "

~10 nm" ?""

~2015 "

electron flow

The Scale of Things

•  1 meter "(1m)"

•  1 mm "(10-3 m)"

•  1 µm "(10-6 m)"

•  1 nm "(10-9 m)"

•  1 pm "(10-12 m)" Silicon atom (0.118 nm)"

human hair (100 µm)"

biomolecules (10’s nm)"

transistor (100 nm -2000)"248 nm -DUV lithography"

wavelength of light (< 1µm)"

Putting it in Scale

÷8

÷8

N = 4096 n = 1352

N = 4096 n = 3584

N = 4096 n = 2368

N = total atoms; n = surface atoms

Surface Area

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

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-9-8-7-6-5-4-3-2-10

Ag/GaAs

Ag/PTCDA/GaAs

Log(

Cur

rent

den

sity

/ A

/cm

2 )

Voltage / V

Δ=-0.45 eV

EHOMO

1.18 eV

6.58 eV

EVBM

EVL

ECBMELUMO

2.0 eV

6.95 eV

EF

S-GaAs / PTCDA

Δ=-0.45 eV

EHOMO

1.18 eV

6.58 eV

EVBM

EVL

ECBMELUMO

2.0 eV

6.95 eV

Δ=-0.45 eV

EHOMO

1.18 eV

6.58 eV

EVBM

EVL

ECBMELUMO

2.0 eV

6.95 eV

EF

S-GaAs / PTCDA

Ag/S-GaAs

Ag/PTCDA/S-GaAs

Δ= 0.14 eV

EHOMO

0.6 eV

5.75 eV

EVBM

EVL

ECBM ELUMO

2.0 eV

6.58 eV

EF

GaAs / PTCDA

Δ= 0.14 eV

EHOMO

0.6 eV

5.75 eV

EVBM

EVL

ECBM ELUMO

2.0 eV

6.58 eV

Δ= 0.14 eV

EHOMO

0.6 eV

5.75 eV

EVBM

EVL

ECBM ELUMO

2.0 eV

6.58 eV

EF

GaAs / PTCDA

J-V characteristics of organic modified Schottky diodes

Si GaAs

Crystal structure

Diamond & Zincblende lattices – two interpenetrating fcc sublattices one displaced from the other by ¼ of the distance along the diagonal of the cell (a√3/4)

a=5.43 A a=5.63 A Semiconductor Devices, 2/E by S. M. Sze Copyright © 2002 John Wiley & Sons. Inc. All rights reserved.