No abstract

We have fabricated organic thin-film transistors and integrated circuits using pentacene as the active material. Devices were fabricated on glass substrates using low-temperature ionbeam sputtered silicon dioxide as the gate dielectric and a doublelayer photoresist process to isolate devices. These transistors have carrier mobility near 0.5 cm 2 /V-s and on/off current ratio larger than 10 7. Using a level-shifting design that allows circuits to operate over a wide range of threshold voltages, we have fabricated ring oscillators with propagation delay below 75 s per stage, limited by the level-shifting circuitry. When driven directly, inverters without level shifting show submicrosecond rise and fall time constants.

The charge-carrier transport mechanism in the organic semiconductor pentacene is explored using thin-film transistor structures. The variation of the field-effect mobility with temperature differs from sample to sample, ranging from thermally activated to temperature-independent behavior. This result excludes thermally activated hopping as the fundamental transport mechanism in pentacene thin films, and suggests that traps and/or contact effects may strongly influence the observed characteristics. These results also indicate that field-effect transistors may not be appropriate vehicles for illuminating basic transport mechanisms in organic materials.

We report a solvent-induced phase transition in pentacene thin films, from a “thin film” phase to a bulk-like phase. X-ray diffraction indicates that as-deposited thermally evaporated pentacene films consist mainly of (001)-oriented pentacene with an elongated (001) plane spacing of 15.5±0.1 Å, and a minor amount with a (001) plane spacing of 14.5±0.1 Å. When such films are exposed to solvents such as acetone, isopropanol, or ethanol, the plane spacing of the entire layer shifts abruptly from the elongated (001) plane spacing to the bulk value and this shift is accompanied by a macroscopic change in film morphology. While molecular ordering is maintained as indicated by x-ray diffraction, thin film transistor performance is severely degraded, most likely as a result of the morphological changes in the film.

We report stable, high performance zinc oxide thin film transistors grown by an atmospheric pressure atomic layer deposition system. With all deposition and processing steps kept at or below 200°C, the alumina gate dielectric shows low leakage (below 10−8A∕cm2) and high breakdown fields. Zinc oxide thin film transistors in a bottom gate geometry yield on/off ratios above 108, near zero turn-on voltage, little or no hysteresis, and mobility greater than 10cm2∕Vs. With alumina passivation, shifts in threshold voltage under gate bias stress compare favorably to those reported in the literature.