Transcript Spectroscopy of Hybrid / Interfaces

Spectroscopy of
Hybrid Inorganic/Organic Interfaces
Introduction
Dietrich RT Zahn
Semiconductor Physics –
Activities in Chemnitz
Surface Science:
Photoemission Spectroscopy
(UPS and XPS)
X-ray Absorption Fine Structure
(NEXAFS)
Auger Electron Spectroscopy
(AES)
Low Energy Electron Diffraction
hw
(LEED)
e
Inverse Photoemission
Kelvin Probe (CPD)
Growth:
(Organic) Molecular Beam Deposition
in Ultra-High Vacuum
(Metal-Organic) Vapour Phase Deposition
Semiconductor
Interface
e
Electrical Measurements:
Current-Voltage (IV)
Capacitance-Voltage (CV)
(Deep Level) Transient Spectroscopy
hw
Optical Spectroscopy:
Raman Spectroscopy (RS)
Photoluminescence
Spectroscopic Ellipsometry (SE)
UV-vis
Infrared Spectroscopy (IR)
Reflection Anisotropy Spectroscopy (RAS)
My Team
Major Research Areas
I. Inorganic/Organic Hybrid Structures
II. Low Dimensional Semiconductor Structures,
in particular Quantum Dot Superlattices;
Cooperation with the Institute of
Semiconductor Physics in Novosibirsk
III. Wafer Bonding; Arrays of Micromechanical
Sensors and Actuators, SFB 379
Designing Inorganic/Organic DEvices
EU Funded Human Potential Research Training Network
Contract No. HPRN-CT-1999-00164, www.tu-chemnitz.de/diode
DIODE Project
(v) The Overall Device Performance
(iv) The Interface between the
Organic Molecules and the Metal
(iii) The Organic
Molecular Film
Metal
I
V
Organic Interlayer
GaAs(100)
(ii) The Interface between
GaAs Substrate and
Organic Molecules
(i) GaAs Substrate Surface
I
H
P
R
e
s
e
a
r
c
h
T
r
a
i
n
i
n
g
N
e
t
w
o
r
k
D. R. T. Zahn,
TU Chemnitz
Molecular Control of III-V Diodes
Metal/III-V Semiconductor contacts
High frequency
application
e.g. Mixers,
Modulators
Challenge:
lowering operating voltage.
Organic thin interlayer
• Metal/organic/InP
A. Böhler et al. Mater. Sci. and Eng. B
51 (1998) 58
: Rectifying behavior
: Superior to commercial diode
: High frequency limit: 42 GHz
I
H
P
R
e
s
e
a
r
c
h
T
r
a
i
n
i
n
g
N
e
t
w
o
r
k
Dietrich RT Zahn,
TU Chemnitz
Technology at TU Braunschweig
OMBD
Ag
PTCDA
Lift-off
OMBD
- PTCDA(10nm)
- Ag(200nm)
Lift-off in 20%NaOH
------------------------------------------------------

Ag contact
Active area
SiO2 layer
Au/Ni/Ge
n-- GaAs/ n+- GaAs substrate
100 m
PTCDA modified Ag/GaAs(100)
Schottky Contacts
-3
Ag/PT CD A/ G aAs
Ag/PTCDA/GaAs
Ag/ G aAs
Ag/GaAs
log(Current / A)
-4
• 30 nm PTCDA interlayer.
• Reverse bias/Low forward bias
Increase in current ( 1000)
Decrease in barrier height ( -150 meV)
-5
-6
-7
• High forward bias
Deviation from thermionic emission.
-8
-9
-10
-11
-12
-1,0
-0,5
0,0
V oltage / V
0,5
3,4,9,10-Perylenetetracarboxylic dianhydride ( C24H8O6)
I
H
P
R
e
s
e
a
r
c
h
T
r
a
i
n
i
n
g
N
e
t
w
o
r
k
Dietrich RT Zahn,
TU Chemnitz
Organic Semiconductors
• Primary interest: Organic LEDs and displays.
• Low cost plastic electronics.
• Modification of semiconductor devices.
Polymers
Long molecular chains, „Spin-Coating“.
Monomers
Extended and conjugated -electron system.
Phthalocyanines, perylene derivatives.
Organic Molecular Beam Deposition (OMBD).
(S. R. Forrest, Chem. Rev. 97 (1997) 1793)
Organic
field-effect transistors
Displays (Kodak)
Electrically driven
organic lasers
Organic semiconductors
Organic/Inorganic
Microwave Diodes I
V
Plastic solar cells
Metal
Organic Interlayer
GaAs(100)
Organic-modified Schottky Diodes
I
H
P
R
e
s
e
a
r
c
h
T
r
a
i
n
i
n
g
N
e
t
w
o
r
k
G. Salvan,
TU Chemnitz
Multilayer Device Structure
-
+
Electron Injection Electrode
Electron Transporting Layer
Light Emitting Layer
Hole Transporting Layer
Substrate
Light
Hole Injection Electrode
Bilayer Structure
Vacuum level
Ea
eFa
Oe
+ +
+ +
eFc
fe
Ip
Cathode
fh
- - -
Anode
P-I
Oh
P-2
Advantages
- Optimize injection of each carrier type separately : Balance charge injection
- Optimize mobility of each carrier type separately
- Control transport by energy level offsets at the heterojunction interface : confinement of charges ;
optimize charge recombination
- Keeps exciton decay away from electrodes : minimize electrode quenching
Organic Semiconductors
Molecular / Polymeric Materials
Van-der-Waals Bonds
 no dangling bonds
Conjugated Materials (extended (delocalized) -electrons)
‘Bandgap’ of 1.5 to 3 eV
Delocalised -Electron System
Benzene
Energy E / eV
3,0
2,5
2,2
2,05
1,7
UV
410
IR
495
560
Wavelength  / nm
1 eV = 1,60210-19 J
1 kT = 0,025 eV (T = 300 K)
1 nm = 10-9 m = 10 Å
620
700
• Inorganic semiconductor:
wide bands and delocalized states
-
CB
small exciton
binding energy
Transport gap ~ optical gap
VB
+
+
Energy levels and transport: Bloch states and single-electron approximation
• Organic molecular solid:
small transfer integral between molecules; charge
carrier = molecular ions; electronic polarization + molecular relaxation
-
LUMO
Single-particle
or
Transport gap

+
HOMO
Ground
state
of neutral
molecule
Molecular ions
Optical gap
+
On-molecule
neutral excitation
 strong e-h coulomb
interaction
Transport gap – optical gap = exciton binding energy
Excitons
Organic Semiconductors
Semiconducting properties:
!
Inorganic Semiconductor
Organic Molecules
Conduction Band
Valence Band
Lowest Unoccupied Molecular Orbital
Highest Occupied Molecular Orbital
Band gap
Eg = Ecb - Evb
(ELUMO – EHOMO)Transport > (ELUMO – EHOMO)Optical
PTCDA: Eg,Transprt = 2.8 eV, Eg,Optic = 2.2 eV
p- and n-type doping
Large anisotropy of carrier mobility
Doping: alkali metals or other molecules
I
H
P
R
e
s
e
a
r
c
h
T
r
a
i
n
i
n
g
N
e
t
w
o
r
k
„small molecules“
Polymers
e.g. PTCDA: 38 atoms !
Compare:
Inorganic semiconductors: typically 2 atoms/unit cell
e.g. Silicon (Si) or Galliumarsenide (GaAs)
Some Semiconductors
a-quartertiophene
a-sexithiophene
a -6T
a -4T
S
S
S
S
Tetracene
S
S
S
S
S
S
Pentacene
-conjugated Molecules (oligomers)
Transport  -* overlap
C60
Perylene derivatives
PTCDA: 3,4,9,10- Perylenetetracarboxylic dianhydride
DiMe-PTCDI: 3,4,9,10- Perylenetetracarboxylic diImide
C24H8O6
C26H14O4N2
+ Phthalocyanines at UWA
I
H
P
R
e
s
e
a
r
c
h
T
r
a
i
n
i
n
g
N
e
t
w
o
r
k
DRTZ
TUC
Absorption Spectra
Wavelength / nm
450
300
Absorbtion coefficient *10
5
750 600
6
DiMe-PTCDI
PTCDA
4
2
0
2
3
4
5
6
Energy / eV
Dietrich RT Zahn, TU Chemnitz
PTCDA Crystal
Experimentally derived
geometry via X-ray analysis
3.21 Å
c
b
 two molecules per unit cell
 a - and  - phases
b
c
a - PTCDA
 - PTCDA
PTCDA crystal structure
3,4,9,10,-Perylene TetraCarboxylic DiAnhydride (C24H8O6)
 Single crystals:
 double sublimation of PTCDA
powder  needle - like crystallites
(50  200  2000 m3)
 a - phase
c’
b
 Thin films:
 40 nm PTCDA / H-Si(001) grown
at Tsub = 410 K  polycrystalline
structure with the domain size
below 500 nm
 dominantly a - phase
 monoclinic structure (C2h):
2 molecules per unit cell
d=3.21 Å
PTCDA/GaAs(100)-S:2x1 Topography
Tsubstrate =360 K
Atomic
Force
Microscopy
Tsubstrate =295 K
Preferred orientation
of crystallites – one
edge to GaAs(110).
Tsubstrate =410 K
Crystal Structure - Anisotropy
Herringbone-Structure
van-der-Waals (attraction)
Pauli-Principle (repulsion)
Layered Semiconductor
Anisotropy
spar / sper
Thiophenes
spar1 /
spar2
~70
~1.5
Acenes
~3
~1.5
Tetracene
Herringbone pattern
PTCDA
Alq3
3,4,9,10- Perylenetetracarboxylic
Tris(8-hydroxyquinoline)Aluminum
dianhydride (C24H8O6)
Al(C9H6NO)3
z
x
y
2.21eV
Optical absorption(exp)
2.7eV
D2h
Symmetry
C3v (Facial-isomer )
108 internal vibrational modes:
150 internal vibrational modes:
Raman active: 19Ag+18B1g+10B2g+7B3g
Raman and IR active
Thin films:
mixture of two crystalline phases: monoclinic space group
Dietrich RT Zahn, TU Chemnitz
Copper Phtalocyanine (CuPc)
Organic Vapor Phase Deposition:
The Concept
•
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
with courtesy of UDC
OVPD: R&D Deposition System
4
Design
Carrier Gas source
barrels
Inlets
Standard Glass
fittings chamber
Clean Deposition
Chamber
4-zone heater
Temperature
probes
To Pump
Mechanical
Shutter
Thickness
monitor
Rotating
cooled holder
Isothermally heated source
 Efficient & Controlled Evaporation:
0.3 grams of material last > 150 films
with courtesy of UDC
OVPD vs. Vacuum
Deposition
Technology Comparison
Task
LP-OVPD
Vacuum
Process
Multi-Layer
Single-Layer
Interface
Control
Reproducible
No Crosstalk
Multi-Sources
No Bowing
No Control
(Mask on Substrate)
(Mask below Substrate)
Precursor
Shadow-Mask
Crosstalk
Limited- Sources
Bowing
OVPD: The Production Concept
OVPD: System Overview
Computer Control
GMS
(Gas Mixing System)
Reactor
Ink jet printing
INK
With a modern ink jet printer it is possible to make
printed circuits with a dissolved polymer up to 2400 dpi
Paper based electronic
The dream: Organic circuits printed on paper by
offset printing, ’soft’ lithography
• Paper is a cheap and friendly to the environment
• (Theoretically) very high speed of production
• Cheap electronics:
• Sensors in food containers
• Electronic magazines
• Billboards, simple animations
• Wireless communication, prize tags