Kiyomi Tsukagoshi

Contact-metal dependent current injection in pentacene thin-film transistors

Contact-metal dependent current injection in top-contact pentacene thin-film transistors is analyzed, and the local mobility in the contact region was found to follow the Meyer-Neldel rule. An exponential trap distribution, rather than the metal/organic hole injection barrier, is proposed to be the dominant factor of the contact resistance in pentacene thin-film transistors. The variable temperature measurements revealed a much narrower trap distribution in the copper contact compared with the corresponding gold contact, and this is the origin of the smaller contact resistance for copper despite a lower work function.

Bias stress instability in pentacene thin film transistors: Contact resistance change and channel threshold voltage shift

Bias stress instability in top-contact pentacene thin film transistors was observed to be correlated not only to the channel but also to the metal/organic contact. The drain current decay under bias stress results from the combination of the contact resistance change and the threshold voltage shift in the channel. The contact resistance change is contact-metal dependent, though the corresponding channel threshold voltage shifts are similar. The results suggest that the time-dependent charge trapping into the deep trap states in both the contact and channel regions is responsible for the bias stress effect in organic thin film transistors.

Correlation between grain size and device parameters in pentacene thin film transistors

We develop a general approach to precisely extract the device parameters in top-contact pentacene thin film transistors. The charge trap sites are clarified by analyzing the grain size dependence of the device parameters. The channel mobility and threshold voltage are limited by the charge traps in the channel region, most of which are located not at the grain boundaries but at the organic/insulating-layer interface. The contact resistance decreases by increasing the grain size and is controlled by the charge traps in the contact region, which are suggested to be concentrated at the grain boundaries and at the metal/organic interface.

Charge trapping induced current instability in pentacene thin film transistors : Trapping barrier and effect of surface treatment

The current instability of pentacene thin film transistors is described by the energetic distribution of the barrier height for the trapping of mobile charges at the organic/insulator interface. The trapping energy was quantitatively analyzed by measuring the temperature dependence of current decay, which follows a stretched exponential function. The distribution of the barrier becomes higher and narrower by the use of a self assembled monolayer (SAM) on the insulator surface, whereas the pentacene film morphology has little influence on the trapping barriers. The increase in the barrier height in the SAM-treated device suppresses charge trapping, resulting in stable device operation.

Current transport in short channel top-contact pentacene field-effect transistors investigated with the selective molecular doping technique

The contact doping profile is controlled in the top-contact configuration to clarify a transistor operation based on a current injection process from the metal contact to the organic channel in a submicron channel pentacene field-effect transistor. The molecular doping in the pentacene film underneath the metal contact, in which a thin layer of iron (III) chloride was introduced, drastically changes transistor characteristics. The doping profile control directly revealed the resistive part for current injection. A model to explain the saturation behavior of the top-contact short channel organic transistor is presented.

Anisotropic transport in graphene on SiC substrate with periodic nanofacets

Anisotropic transport in graphene field-effect transistors fabricated on a vicinal SiC substrate with a self-organized periodic nanofacet structure is investigated. Graphene thermally grown on a vicinal substrate contains two following regions: atomically flat terraces and nanofacets (atomically stepped slopes). The graphene film at a nanofacet is continuously connected between two neighboring terrace films. Anisotropic transport properties are clearly observed, indicating a difference in the graphene properties of the two regions. The observed anisotropic properties are discussed in terms of the effects of nanofacet structures on conductivity and electron mobility.

Transition-Voltage Method for Estimating Contact Resistance in Organic Thin-Film Transistors

We report a simple method for evaluating the contact resistance (R C) in organic thin-film transistors, utilizing the measurement of the transition voltage between the linear and saturation regimes in the output characteristics. This method does not need the complex electrode patterning, and thus, it could be useful for the practical R C test. The R C extracted from the transition-voltage method shows a decrease with increasing gate voltage, and the results are similar with those extracted from the transfer-line method, indicating the preciseness of the proposed transition-voltage method.

Contact resistance instability in pentacene thin film transistors induced by ambient gases

Top-contact pentacene thin film transistors show the strong susceptibility of the contact resistance to ambient gases. Moisture and oxygen lead to the increase and decrease in the contact resistance, respectively. The phenomenon is interpreted by the local conductivity change in the contact region induced by the adsorption/desorption of ambient gases. The present works suggest that the current injection in the pentacene thin film transistors is strongly correlated with the charge transport property in the contact region, where the contact resistance is mainly controlled by the charge trap states.

Dynamic bias stress current instability caused by charge trapping and detrapping in pentacene thin film transistors

The current instability in pentacene organic thin film transistors (OTFTs) under dynamic bias stress was investigated. Current instability is strongly influenced by pulse duty ratio and pulse on-period and off-period voltages, but is independent of pulse frequency. A stable on-current without current instability was achieved by controlling the pulse duty ratio and pulse voltage. On the basis of the experimental results, a reversible transition model of dynamic charge trapping and detrapping in pentacene OTFTs is proposed, and trapping and detrapping time constants were estimated.

Preparation of Polyimide-Polyamide Random Copolymer Thin Film by Sequential Vapor Deposition Polymerization

A new method for preparing aromatic polyamide-polyimide random copolymer films has been developed. Oxydianiline, pyromellitic dianhydride, and terephthaloyl chloride were evaporated separately from thermostatically controlled pulse molecular evaporation sources, which were actuated by solenoid valves, producing a polyimide-polyamide film after curing treatment. By selecting the appropriate combination of monomers, other copolymer films could be obtained through the sequential vacuum deposition polymerization process.