6 research outputs found
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.
Direct observation of phase coherence in 3-<b>k</b> magnetic configurations
International audienceWe report the observation by neutron diffraction of phase coherent Bragg reflections in a multi-k magnetic configuration with a spatial periodicity outside the conventional scattering cross-section. The peaks, which exist in the 3-k state of UAs0.8Se0.2, display long-range order with a wavevector dependence characteristic of a magnetic interaction. The results confirm the long-range order and temperature dependence reported in an earlier study of similar peaks in this material using x-ray resonant scattering by (a) the non-trivial extension to the technique of neutron diffraction, and (b) the observation of similar 3-k phase-coherent reflections in other samples by x-ray resonant scattering. The importance of the neutron diffraction results lies primarily in the fact that magnetic neutron diffraction is well established as a weak probe operating on thermodynamic time scales. This alleviates concern that the rapid (10-15 - 10-14 s), strong interaction, characteristic of the resonant x-ray technique, is imaging a transient or non-equilibrium configuration. Likewise, the extension of the x-ray resonant scattering results to other samples establishes the generality of this effect. The enigma of how to understand the observed diffraction, which appears to lie strictly outside both the conventional neutron and x-ray scattering cross sections, remains