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.