45 research outputs found
Organic small molecule field-effect transistors with Cytop(TM) gate dielectric: eliminating gate bias stress effects
We report on organic field-effect transistors with unprecedented resistance
against gate bias stress. The single crystal and thin-film transistors employ
the organic gate dielectric Cytop(TM). This fluoropolymer is highly water
repellent and shows a remarkable electrical breakdown strength. The single
crystal transistors are consistently of very high electrical quality: near zero
onset, very steep subthreshold swing (average: 1.3 nF V/(dec cm2)) and
negligible current hysteresis. Furthermore, extended gate bias stress only
leads to marginal changes in the transfer characteristics. It appears that
there is no conceptual limitation for the stability of organic semiconductors
in contrast to hydrogenated amorphous silicon.Comment: 4 pages, 3 figures, to be published in Appl. Phys. Let
Intrinsic Josephson junctions in the iron-based multi-band superconductor (V2Sr4O6)Fe2As2
In layered superconductors, Josephson junctions may be formed within the unit
cell due to sufficiently low interlayer coupling. These intrinsic Josephson
junction (iJJ) systems have attracted considerable interest for their
application potential in quantum computing as well as efficient sources of THz
radiation, closing the famous "THz gap". So far, iJJ have been demonstrated in
single-band, copper-based high-Tc superconductors, mainly in Ba-Sr-Ca-Cu-O.
Here we report clear experimental evidence for iJJ behavior in the iron-based
superconductor (V2Sr4O6)Fe2As2. The intrinsic junctions are identified by
periodic oscillations of the flux flow voltage upon increasing a well aligned
in-plane magnetic field. The periodicity is well explained by commensurability
effects between the Josephson vortex lattice and the crystal structure, which
is a hallmark signature of Josephson vortices confined into iJJ stacks. This
finding adds (V2Sr4O6)Fe2As2 as the first iron-based, multi-band superconductor
to the copper-based iJJ materials of interest for Josephson junction
applications, and in particular novel devices based on multi-band Josephson
coupling may be realized.Comment: Accepted in Nature Physic
Oxygen-related traps in pentacene thin films: Energetic position and implications for transistor performance
We studied the influence of oxygen on the electronic trap states in a
pentacene thin film. This was done by carrying out gated four-terminal
measurements on thin-film transistors as a function of temperature and without
ever exposing the samples to ambient air. Photooxidation of pentacene is shown
to lead to a peak of trap states centered at 0.28 eV from the mobility edge,
with trap densities of the order of 10(18) cm(-3). These trap states need to be
occupied at first and cause a reduction in the number of free carriers, i.e. a
consistent shift of the density of free holes as a function of gate voltage.
Moreover, the exposure to oxygen reduces the mobility of the charge carriers
above the mobility edge. We correlate the change of these transport parameters
with the change of the essential device parameters, i.e. subthreshold
performance and effective field-effect mobility. This study supports the
assumption of a mobility edge for charge transport, and contributes to a
detailed understanding of an important degradation mechanism of organic
field-effect transistors. Deep traps in an organic field-effect transistor
reduce the effective field-effect mobility by reducing the number of free
carriers and their mobility above the mobility edge.Comment: 13 pages, 14 figures, to be published in Phys. Rev.
Defect healing at room temperature in pentacene thin films and improved transistor performance
We report on a healing of defects at room temperature in the organic
semiconductor pentacene. This peculiar effect is a direct consequence of the
weak intermolecular interaction which is characteristic of organic
semiconductors. Pentacene thin-film transistors were fabricated and
characterized by in situ gated four-terminal measurements. Under high vacuum
conditions (base pressure of order 10E-8 mbar), the device performance is found
to improve with time. The effective field-effect mobility increases by as much
as a factor of two and mobilities up to 0.45 cm2/Vs were achieved. In addition,
the contact resistance decreases by more than an order of magnitude and there
is a significant reduction in current hysteresis. Oxygen/nitrogen exposure and
annealing experiments show the improvement of the electronic parameters to be
driven by a thermally promoted process and not by chemical doping. In order to
extract the spectral density of trap states from the transistor
characteristics, we have implemented a powerful scheme which allows for a
calculation of the trap densities with high accuracy in a straightforward
fashion. We show the performance improvement to be due to a reduction in the
density of shallow traps <0.15 eV from the valence band edge, while the
energetically deeper traps are essentially unaffected. This work contributes to
an understanding of the shallow traps in organic semiconductors and identifies
structural point defects within the grains of the polycrystalline thin films as
a major cause.Comment: 13 pages, 13 figures, to be published in Phys. Rev.