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. Photo-oxidation 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 . As the gate voltage is ramped up, these trap states are occupied at first and cause a reduction in the number of free carriers at a given 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.

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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 10 −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 cm 2 /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.

The temperature dependence of the magnetization, the electrical resistivity, and the Hall effect has been measured for 11 different uranium concentrations in single crystals of U x La 1Ϫx S and is compared with corresponding data for U 0.2 ͑La 0.15 Y 0.85 ͒ 0.8 Te and UTe. While the electrical transport data for diluted UTe show maxima related to crystal-field splittings, such structures are absent in all sulfide compounds. The magnetic susceptibility of the sulfides, measured up to 1200 K, is fitted by the sum of a Curie-Weiss and a Pauli paramagnetism term in second-order approximation. In a quasi-free-electron model the variation with the uranium concentration of the Fermi energy, the carrier concentration and the effective mass is derived. It is found that the degree of localization varies nonmonotonically and, in a certain range, contrary to the U-U separation. It is concluded that hybridization and magnetic exchange play dominant roles. ͓S0163-1829͑96͒00921-6͔

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