K. Tsukagoshi

High-performance transparent flexible transistors using carbon nanotube films

Transparent flexible thin-film transistors (tf-TFTs) are an important focus of research since present silicon-based electronics cannot realize such devices. Here, we demonstrate a single-walled carbon nanotube (SWNT) tf-TFTs based on the solution process using transparent electrodes. SWNT tf-TFTs typically exhibit a mobility of 0.5cm2∕Vs and an on/off current ratio of ∼104. More importantly, these transistors are highly flexible and can be bent to a radius of 7.5mm without a significant loss in performance. This study therefore represents a major step towards “SWNT transparent plastic electronics.”

Modification of the electric conduction at the pentacene∕SiO2 interface by surface termination of SiO2

A surface treatment method has been developed for the SiO2∕Si substrate to control the electrical properties of pentacene thin-film transistors (TFTs). The surface treatment is performed by spin-coating 1,1,1,3,3,3-hexamethyldisilazane liquid, resulting in a drastic improvement of the off current although the surface treatment never shows a pronounced morphological change in the pentacene channel in comparison with the one on the nontreated substrate. The off current improvement directly enhances the transistor performance especially in the TFTs with a few monolayers channel thickness. The off current suppression could be caused by the reduction of the interfacial floating charge trapped at the pentacene∕SiO2 interface.

Origin of the relatively low transport mobility of graphene grown through chemical vapor deposition

The reasons for the relatively low transport mobility of graphene grown through chemical vapor deposition (CVD-G), which include point defect, surface contamination, and line defect, were analyzed in the current study. A series of control experiments demonstrated that the determinant factor for the low transport mobility of CVD-G did not arise from point defects or surface contaminations, but stemmed from line defects induced by grain boundaries. Electron microscopies characterized the presence of grain boundaries and indicated the polycrystalline nature of the CVD-G. Field-effect transistors based on CVD-G without the grain boundary obtained a transport mobility comparative to that of Kish graphene, which directly indicated the detrimental effect of grain boundaries. The effect of grain boundary on transport mobility was qualitatively explained using a potential barrier model. Furthermore, the conduction mechanism of CVD-G was also investigated using the temperature dependence measurements. This study can help understand the intrinsic transport features of CVD-G.

Hall Effect of Quasi-Hole Gas in Organic Single-Crystal Transistors

Hall effect is detected in organic field-effect transistors, using appropriately shaped rubrene (C42H28) single crystals. It turned out that inverse Hall coefficient, having a positive sign, is close to the amount of electric-field induced charge upon the hole accumulation. The presence of the normal Hall effect means that the electromagnetic character of the surface charge is not of hopping carriers but resembles that of a two-dimensional hole-gas system.

High-density electrostatic carrier doping in organic single-crystal transistors with polymer gel electrolyte

High-density carrier accumulation in organic semiconductors is demonstrated in Au∕polymer gel electrolyte/rubrene crystal∕SiO2∕doped Si dual-gate transistors, forming electric double layers in the polymer gel. Application of only 1.2V across the polymer gel electrolyte drastically enhances the conductance of the rubrene single crystal with the field-induced carrier density up to ∼5×1013cm−2. Directly comparing the transfer characteristics of the same device channel in the dual-gate transistors revealed that the achieved doping level is beyond the maximum of the SiO2-based transistor on the opposite side of the organic crystal.

Influence of Disorder on Conductance in Bilayer Graphene under Perpendicular Electric Field

Electron transport in bilayer graphene placed under a perpendicular electric field is revealed experimentally. Steep increase of the resistance is observed under high electric field; however, the resistance does not diverge even at low temperatures. The observed temperature dependence of the conductance consists of two contributions: the thermally activated (TA) conduction and the variable range hopping (VRH) conduction. We find that for the measured electric field range (0-1.3 V/nm) the mobility gap extracted from the TA behavior agrees well with the theoretical prediction for the band gap opening in bilayer graphene, although the VRH conduction deteriorates the insulating state more seriously in bilayer graphene with smaller mobility. These results show that the improvement of the mobility is crucial for the successful operation of the bilayer graphene field effect transistor.

Inter-Layer Screening Length to Electric Field in Thin Graphite Film

Electric conduction in thin graphite film was tuned by two gate electrodes to clarify how the gate electric field induces electric carriers in thin graphite. The graphite was sandwiched between two gate electrodes arranged in a top and bottom gate configuration. A scan of the top gate voltage generates a resistance peak in ambiploar response. The ambipolar peak is shifted by the bottom gate voltage, where the shift rate depends on the graphite thickness. The thickness-dependent peak shift was clarified in terms of the inter-layer screening length to the electric field in the double-gated graphite film. The screening length of 1.2 nm was experimentally obtained.

Spin-dependent boundary resistance in the lateral spin-valve structure

We report the detection of clear spin-valve signal without any spurious magnetoresistive signal in a lateral spin-valve structure consisting of Cu∕Ni−Fe ohmic junction using local current injection. The obtained spin-valve signal is much larger than that of the nonlocal spin-valve configuration because of the efficient spin accumulation. The local current injection experiments with different probe configurations proved that the spin-valve signal is caused by the spin-dependent boundary resistance at the interface between the ferromagnetic voltage probe and the spin-polarized nonmagnetic wire.

Gate control of spin transport in multilayer graphene

We experimentally studied the gate voltage dependence of spin transport in multilayer graphene (MLG) using the nonlocal spin detection technique. We found that the spin signal is a monotonically decreasing linear function of the resistance of MLG, which is characteristic of the intermediate interfacial transparency between the MLG and the ferromagnetic electrodes (Co). The linear relation indicates a large spin relaxation length significantly exceeding 8μm. This shows the superiority of MLG for the utilization of the graphite-based spintronic devices.

Pentacene transistor encapsulated by poly-para-xylylene behaving as gate dielectric insulator and passivation film

We present pentacene thin-film transistors with poly-para-xylylene gate dielectric and passivation films for plastic electronics. Both the poly-para-xylylene films are formed by dry chemical vapor deposition at room temperature. An organic pentacene channel is fully encapsulated by poly-para-xylylene films, except in the area of electrode connection. Passivation induces little degradation of the organic transistor properties. In addition, a small amount of charge transfer molecules are introduced between the pentacene channel and the metal electrodes to improve device performance. Contact resistance is sufficiently reduced by the employment of the charge transfer molecules, which is analyzed using a transmission-line model.