Plenarvorträge - DPG-Tagungen

Symposium Organic and Hybrid Systems for Future Electronics Donnerstag
devices. Our work will present dielectric spectra of organic heterojunctions
formed by di-(pyridyl)-perylenetetracarboxylic diimide (Py-PTC)
and copper phthalocyanine (CuPc). This organic junction exhibits a photovoltaic
effect with effective photogeneration of charge carriers occuring
near the Py-PTC/CuPc interface. The main processes determining
the heterojunction performance will be discussed. Acknowledgement: The
work was supported by Marie-Curie Host Fellowships: Project MODERN
No.HPMD-CT-2001-00083 and by KBN: Project No.4T11B 057 22.
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An impedance spectroscopy study of Ag/PTCDA/n-GaAs(100)
heterostructures — •A. Bekkali, I. Thurzo, T. U. Kampen und
D. R. T. Zahn — Insitut für Physik, TU Chemnitz, D-09107 Chemnitz
In this work impedance spectroscopy (IS) was applied to investigate
Ag/PTCDA/n-GaAs(100) heterostructures. Describing the samples
using an equivalent circuit consisting of resistors and capacitors, the potential
drop across each relevant area of the sample, that is, the GaAs and
PTCDA bulk as well as the Ag/PTCDA and PTCDA/GaAs interface,
can be determined. The IS characteristics can be simulated by modeling
the Ag/PTCDA interface using a parallel RoCo circuit connected in series
with a bulk resistance Rb for the PTCDA layer. All the elements are
found to be nonlinear with respect to the applied bias and frequency independent.
The depletion layer in GaAs is correctly characterized by its
parallel resistance Rd and capacitance Cd. The most important feature
of the Ag/PTCDA interface is the capacitance Co, which consists of a
geometric capacitance and an excess capacitance δC, the latter originating
from recharging a Gaussian density of states (DOS). This Gaussian
DOS is attributed to surface states Nss(E), their respective capacitance
δCss(E) = q 2 Nss(E) peaking for an applied bias of 0.2 V. This value
agrees with the thermal activation energy for carrier transport in the
PTCDA layer.
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Characterization of polymeric metal-insulator-semiconductor
diodes — •Silviu Grecu, Markus Bronner, Andreas Opitz, and
Wolfgang Brütting — Experimentalphysik IV, Universität Augsburg,
86135 Augsburg, Germany
Metal-insulator-semiconductor (MIS) diodes are the two-terminal pendants
of thin film transistors sharing the same basic layer structure. However,
instead of the current-voltage characteristics one has to study the
capacitance-frequency and capacitance-voltage behavior, which can give
information about mobile charges and trapping processes in these devices.
We have investigated MIS structures based on poly(alkyl-thiophene) as
semiconductor which were fabricated on glass substrates with polymeric
insulator layers and compared their response to devices fabricated on
Si/SiO2 substrates. From capacitance-voltage measurements the acceptor
dopant concentration is determined for different preparation conditions.
Typically these measurements show hysteresis between forward and reverse
bias sweeps. We have investigated this behavior as a function of
external parameters like sweep speed and temperature and discuss their
possible origin. The analysis of the frequency response using appropriate
equivalent circuits allows the extraction of material parameters, like
conductivity and charge carrier mobility, which are compared to data
obtained on thin film transistor structures.
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Charge injection into thin fullerene films — •Elizabeth von
Hauff, Vladimir Dyakonov, Jürgen Parisi, and Zivayi Chiguvare
— Energy and Semiconductor Research Laboratory, Institute of
Physics, University of Oldenburg, D-26129, Oldenburg, Germany
In this study, charge injection currents into thin film fullerene diodes
composed of a thin film of PCBM sandwiched between a metal and an
ITO (indium tin oxide) electrode. Decreasing the thickness of the semiconductor
layer in the diodes results in a transition from bulk limited to
injection limited currents. From the injection limited currents through the
thin film fullerene diodes, parameters describing the energy barrier between
metal and semiconductor, the transport level within the bulk, and
the activation energy could be calculated according to an already existing
hopping model. The model describes charge carriers being injected from
the metal into energy sites in the semiconductor medium distributed in a
Gaussian density of states. The barrier height experienced by the charge
carriers described by the above model was compared to a value calculated
earlier from the Fowler-Nordheim model for quantum mechanical
tunneling. In an attempt to shed more light on the metal/semiconductor
interface, the fullerene diodes were futher investigated via capacitance-
voltage measurements.
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Quantum transport through carbon-based nanojunctions —
•Miriam del Valle 1,2 , Carlos Tejedor 1 , and Gianaurelio Cuniberti
2 — 1 Dpto. Física Teórica de la Materia Condensada, UAM,
Spain — 2 Institut für Theoretische Physik, Universität Regensburg
We present a quantum transport study of two- and three-terminal carbon
nanotube (CNT) based junctions. We concentrate our analysis on
functionalized and recapped zig-zag CNTs exhibiting peculiar surface
electronic states localized at the zig-zag cut and at the cap defect atoms
respectively. We calculate the linear conductance by means of the Green
function technique, within a π-orbital description of the carbon network,
and show the interplay between these peculiar localized states and the
open transport channels through the device. In correspondence of the
energies of the localized states, the conductance behavior shows a Fanolike
resonance. Possible experimental conditions and setup in which such
effect could be detected are also discussed.
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Electron Transport in Carbon Nanotube-Metal Systems: Contact
Effects — •Rafael Gutierrez 1 , Nitesh Ranjan 2 , Stefan
Krompiewski 3 , and Gianaurelio Cuniberti 1 — 1 Institute for Theoretical
Physics, University of Regensburg, D-93040 Regensburg, Germany
— 2 Institute for Physical-Chemistry and Electrochemistry,Technical University
Dresden, 01062 Dresden, Germany — 3 Institute of Molecular
Physics, Polish Academy of Sciences,PL-60179 Poznan, Poland
Carbon nanotubes (CNT) posses a very large application potential in
the rapid developing field of molecular electronics. Infinite single-wall
metallic CNTs have theoretically a conductance of 2×G0, (G0 = 2e2)
be- h
cause of the two electronic bands crossing the Fermi level. For finite size
CNTs experiments have shown that other values are also possible, indicating
a very strong influence of the contacts. We study electron transport
in single- and double-wall CNTs contacted to metallic electrodes with a
fcc(111) surface within the Landauer transport formalism combined with
Green function techniques. The electronic structure of the CNTs is described
with a simple π-orbital model.We show that the symmetry of the
contact region may lead in some cases to blocking of one of the transport
channels reducing the conductance to 1×G0 for single-wall CNTs. In the
case of double-wall systems with both inner and outer shells being metallic,
non-diagonal selfenergy contributions from the electrodes may induce
mixing of the CNT’s channels, again reducing the conductance from the
theoretically expected value (4×G0). Finally, we discuss the influence of
inter-wall interactions on the conductance.
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Nonequilibrium-Transport through Metallic Point Contacts
and Single-Molecule Junctions — •Fabian Pauly 1 , J. Heurich 1 ,
J.C. Cuevas 1 , W. Wenzel 2 , and Gerd Schön 1,2 — 1 Institut für
Theoretische Festkörperphysik, Universität Karlsruhe, 76128 Karlsruhe,
Germany — 2 Institut für Nanotechnologie, Forschungszentrum
Karlsruhe, 76021 Karlsruhe, Germany
The future of molecular electronics depends crucially on our understanding
of the transport mechanism in single-molecule junctions. From
the theoretical point of view an important problem, not yet fully solved,
is the way of dealing with the high voltage applied in the experiments.
Building upon traditional quantum-chemistry density functional calculations,
we extend our method [1,2] to compute ab initio current-voltage
characteristics of molecular devices. We show our first results on the
bias-dependent conductance of metallic point contacts, illustrating the
predictive power of the developed method.
[1] J. Heurich, J.C. Cuevas, W. Wenzel, G. Schön, Phys. Rev. Lett. 88,
256803 (2002)
[2] J.C. Cuevas, J. Heurich, F. Pauly, W. Wenzel, G. Schön, Nanotechnology
14 (2003)
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Transport properties of [2,2]-paracyclophane thin films
— •Bruno Gompf 1 , Wenping Hu 1 , Martin Dressel 1 , Jens
Pflaum 2 , and Dieter Schweitzer 2 — 1 1. Physikalisches Institut,
Universität Stuttgart — 2 3. Physikalisches Institut, Universität
Stuttgart
The transport properties of [2,2]-paracyclophane thin films prepared by
molecular beam epitaxy were studied by space charged limited current
measurements. This organic semiconductor, which has a three dimen-