Plenarvorträge - DPG-Tagungen

Symposium Organic and Hybrid Systems for Future Electronics Donnerstag
sional π-system comparable to that of C60 is an interesting new material
for molecular electronics applications due to its remarkably high mobility
and its insensitivity against photo-oxidation. The I-V characteristics
were recorded using a coplanar electrode geometry. This geometry has
the advantage, that after deposition no further processing of the organic
semiconducting layer is necessary. In polycrystalline films hole mobilities
up to 10 −2 cm 2 V −1 s −1 were observed at room temperature.
SYOH 5.61 Do 18:00 B
Single crystalline organic field effect transistors — M. Fischer
1 , •J. Niemax 2 , B. Gompf 1 , M. Dressel 1 , and J. Pflaum 2
— 1 1.Physikalisches Institut, Universität Stuttgart — 2 3.Physikalisches
Institut, Universität Stuttgart
Organic field effect transistors are receiving increased attention during
the last few years due to their potential application in low cost electronics.
It has been proven that grain boundaries significantly influence
charge transport in these devices. In order to evaluate the maximum
performance that can be achieved for a certain type of organic molecule,
it is thus necessary to investigate the charge carrier transport in single
crystalline devices.
We have studied the field effect in organic single crystals. The crystals
were grown by a sublimation technique in a stream of vapor. We have
investigated the influence of different organic materials (Tetracene, Anthracene),
different gate insulators (Mylar, PPX) and different electrode
materials (silver paste, colloidal graphite) on the field effect.
SYOH 5.62 Do 18:00 B
Downscaling of thiophene based organic field-effect transistors
to the nanometer regime — •M. Leufgen 1 , U. Bass 1 , T.
Borzenko 1 , G. Schmidt 1 , J. Geurts 1 , T. Muck 2 , and V. Wagner 2
— 1 Physikalisches Institut der Universität Würzburg, EP III, Am Hubland,
D-97074 Würzburg — 2 International University Bremen, School of
Engineering and Science, Campus Ring 8, D-28759 Bremen
For future electronics high-performing organic field-effect transistors
(OFETs) are required. Downscaling to channel lengths L in the nanometer
regime means higher accessible frequency, integration and current of
transistors. To vary L from 2 µm down to 50 nm, we use an assembly
of n-type silicon wafer as common gate with thermally grown oxide
as insulator layer and Au/Ti source and drain interdigitated electrodes,
defined by electron beam lithography. In order to keep good characteristics
downscaling the channel requires a thinner insulator layer because
of the ratio of horizontal and transversal electrical field in the device.
The thickness of the SiO2 investigated ranges from 200 nm down to 30
nm. The active material is the soluble hexyl substituted quaterthiophene
(DH4T). For the 30 nm oxide layers our results show high mobility of
about 2 × 10 −2 cm 2 /Vs for a 200 nm channel DH4T-device with ratio
W/L of 800. Moreover, the linear and saturation regime are proven and
on/off-ratios for devices with L down to 200nm are as high as 10 5 . Below
200nm field-effect is still observed, but the transistor performance
decreases.
SYOH 5.63 Do 18:00 B
Organic phototransistors based on intramolecular charge transfer
— •Tobat Saragi, Robert Pudzich, Thomas Fuhrmann, and
Josef Salbeck — Makromolekulare Chemie und Molekulare Materialien,
Fachbereich Naturwissenschaften und Center for Interdisciplinary
Nanostructure Science and Technology, Universität Kassel
The conductivity of organic field effect transistors is based on the
generation of radical ions as additional charge carriers by the field effect.
Similarly, charge carriers can be produced by intramolecular charge
transfer reactions in an organic photoconductor. Using an asymmetric
spiro compound, 2,7-bis-(N,N’-diphenylamino)-2’,7’-bis(biphenyl-4- yl)-
9,9’-spirobifluorene (Spiro-DPSP), which consists of a hole transport moiety
in one molecular half and an electron-accepting chromophore in the
other, we demonstrate an one-component organic phototransistor. The
sensitivity of the photosensing device is better than 1 A/W for UV light.
SYOH 5.64 Do 18:00 B
Influence of contact metal on the serial resistance in organic
field effect transistors — •U. Bass 1 , M. Leufgen 1 , T. Muck 2 , J.
Geurts 1 , and V. Wagner 2 — 1 Physikalisches Institut der Universität
Würzburg, EP III, Am Hubland, D-97074 Würzburg — 2 International
University Bremen, School of Engineering and Science, Campus Ring 8,
D-28759 Bremen
The performance of organic field effect transistors (OFETs) depends
essentially on the channel resistance of the organic material and the resistance
between the organic material and the metal contacts. With improving
film morphology and reducing channel lengths the relative influence
of the contact resistance increases and becomes the dominating factor
of the OFETs performance. We present a systematic study of various
noble metals as source and drain contact materials for OFETs, based on
DH4T. We applied gold and platinum, which both required a titanium
adhesion layer, and palladium, which we could deposit directly on the
SiO2 substrate. The smallest contact resistance was obtained when using
Pd. However, these contacts turned out to be rather vulnerable during
the chemical processing prior to the deposition of the organic active layer.
From the contacts with Ti adhesion layers, the Au results were superior
to those of Pt. For a more detailed study of the Au/Ti system, we varied
the Ti adhesion layer thickness from 10 nm down to 1 nm, yielding an
increasing performance with decreasing Ti thickness. Finally, a reduction
of the channel resistance of the DH4T film was achieved by employing a
pretreatment of the SiO2 substrate with octadecyltrichlorosilane (OTS)
instead of hexamethyldisilane (HMDS).
SYOH 5.65 Do 18:00 B
In situ electrical characterization of DH4T transistors — •T.
Muck 1 , V. Wagner 1 , M. Leufgen 2 , J. Geurts 2 , E. Bentes 3 , and
H. L. Gomes 3 — 1 International University Bremen, School of Engineering
and Science, Campus Ring 8, D-28759 Bremen — 2 Experimentelle
Physik II, Universität Würzburg, Am Hubland, D-97074 Würzburg —
3 University of the Algarve, Campus de Gambelas, FCT, 8000, Portugal
The performance of organic field effect transistors (OFETs) was
improved over the last years enormously, e.g., dihexyl-quaterthiohene
(DH4T) has proven high mobility values and is compatible with cheap
solution processing, which is of high interest due to low cost production.
The transport properties of OFETs are influenced by different parameters,
e.g., temperature, film morphology, and the ambient atmosphere.
For a systematic analysis we performed in situ electrical measurements on
DH4T thin film transistors during the deposition of the active layer onto
prepatterned templates by organic molecular beam deposition (OMBD).
Here we get information about the charge transport in the first monolayers
and the dependence of the mobility values on film thickness. Results
for different substrate temperatures will be discussed, including the phase
transition of the active layer at elevated temperatures.
Applying a gate voltage leads to filling of traps (bias stress). This effect is
influenced by the ambient atmosphere. Performing these measurements
in situ as well as in air enables us to systematically analyze these stress
effects.
SYOH 5.66 Do 18:00 B
Transparent organic field effect transistors based on rf magnetron
sputtered alumina films — •Michael Voigt and Moritz
Sokolowski — Institut für Physikalische und Theoretische Chemie,
Universität Bonn, Wegelerstrasse 12, 53115 Bonn
Organic field effect transistors (OFETs) were fabricated by evaporation
of pentacene (Pc) onto alumina insulator films (d = 160–320 nm).
The alumina films were prepared on ITO covered glass (gate–electrode)
by rf magnetron sputtering and show under optimized sputter conditions
breakdown fields of 1.2 MV/cm and a dielectric constant of ∼7. AFM
investigations reveal that their surface is rather rough (rms = 3.6 nm).
Source and drain contacts (Au; d = 50 nm) were prepared by shadow
mask technique either on top of the Pc films or on top of the alumina.
The channel length L was 50 µm. In the first case, a charge mobility of
0.01 cm 2 /Vs was observed, a value of the same order as we obtained for
comparable OFETs on SiO2. In the second case, no field effect mobility
was observed, which we relate to a detrimental influence of Au clusters on
the alumina surface on the Pc film formation. Since the OFET–channel
is transparent, we can control growth of the Pc films by polarization microscopy.
Supported through the DFG-priority program ”Organic field
effect transistors”.
SYOH 5.67 Do 18:00 B
Field Effect Transistor and Space Charge Limited Current Measurements
on Tetracene and Perylene Single Crystals — •G.
Ulbricht 1 , J. H. Smet 1 , J. Niemax 2 , Ch. Herb 2 , and K. von Klitzing
1 — 1 MPI-FKF, Stuttgart — 2 3. Phys. Inst. Uni Stuttgart
The study of charge carrier transport in organic single crystals is an
experimental challenge. Among other things, the intrinsic carrier concentration
is very low and doping quite difficult. Increasing the amount of