工藤 一浩

Organic light emitting transistors which are vertically combined with the organic static induction transistor and organic light emitting diode are fabricated and the device characteristics depending on the structure of gate electrode are investigated. By optimizing the layer thickness and the size of slit-type Al gate electrode, high luminance modulation by low gate voltage as high as 1 V are obtained. The organic light emitting transistor described here is a suitable element for flexible sheet displays. (c) 2004 Published by Elsevier B.V.

Grain morphology, crystal structure and transistor characteristics of pentacene films have been studied using molecular beam deposition under various conditions: growth temperature ranging from - 5 to 80 degreesC and growth rate from 0.15 to 2 nm/min. Grain morphologies observed with atomic force microscopy are categorized into five groups, which are summarized in a phase diagram. Above the growth temperature of 40 degreesC, many 'recessed regions' which extend independently of molecular steps are observed on the film surfaces. Carrier mobilities of the films estimated from transistor measurements are distributed from 1 x 10(-5) to 0.20 cm(2)/Vs. A comparative study between the structural and electrical results indicates that the mobility does not explicitly depend either on grain size or on specific grain morphology. Lamellar grains associated with flat-lying molecules among differently oriented grains, gaps between grains and the recessed regions are considered to restrict the overall carrier mobility. (C) 2004 Elsevier B.V. All rights reserved.

Ultraviolet photoelectron spectroscopy (UPS) of C-60 thin-film field-effect transistors was measured with biasing gate voltages. A time-dependent change in the electronic structure of the C-60 film was, observed during the UPS measurement, which has never been observed in a C-60 film grown on a conductive substrate. The change was attributed to the accelerated polymerization of C-60 by comparing the UPS with that of the photopolymerized C-60. The polymerization Was associated with the increase in the field-effect electron mobility. This result indicates that mobile carriers produce reactive radicals in organic semiconductors. (C) 2004 American Institute of Physics.

The field-effect transistor (FET) characteristics of isolated copper phthalocyanine (CuPc) nano-crystals have been studied using AFM conductive probes as source electrodes. To extract the intrinsic carrier mobility of a single grain, the characteristics obtained were simulated by assuming a sub-exponential diode connected in series. The proposed model successfully reproduced most of the FET characteristics and the mobility was estimated to be 10(-3)-10(-2) cm(2)/V s. On the other hand, characteristics with high and low on/off ratio were sequentially obtained using the same combination of CuPc grain and AFM probe. The characteristics of high on/off ratio disagreed with the model near its off-state. The high on/off ratio is explained by the dependence of the carrier injection rate on the gate voltage, which suppresses the off current. (C) 2003 Elsevier Science B.V. All rights reserved.

Charge-Transfer (CT) complexes are stabilized by partial transfer of electric charges from donor molecules to acceptor molecules. The conductivity of the film of CT-complexes can be controlled if the degree of charge transfer is varied by applying an external field, for example an electric field. In this study, we fabricated field-effect transistors (FETs) using CT complex Langmuir-Blodgett (LB) films (TMPD-C-n TCNQ), and have attempted to control the conductivity of the CT-complex layers by gate voltages. The temperature dependence of conductivity shows that semiconducting and metallic parts coexist in parallel between the electrodes. (C) 2003 Elsevier B.V. All rights reserved.

Thermally stimulated current (TSC) and current-voltage measurements were carried out for Schottky diodes using pentacene thin films in order to investigate the relation between carrier traps and electrical conduction. Two type of TSC peaks originated from the traps at the metal-pentacene interface and in the depletion layer were observed. The current-voltage characteristics of the diodes were governed by the space charge limited conduction depending on these traps.

A novel technique, AFM potentiometry (AFMP), for nanoscale potential mapping has been developed by combining an intermittent-contact-mode AFM with a conductive cantilever which is connected to an electrometer. Potential resolution and limitations of the technique are discussed. TTF-TCNQ wire-like crystals connecting two electrodes were measured with the AFMP under application of a voltage to the electrodes. The potential image indicated that the resistance localized at the grain boundaries was much larger than that distributed within the grains.

The tetramethyltetraselenafulvalene-7,7,8,8-tetracyanoquinodimethane (TMTSF-TCNQ) charge transfer complex shows metallic conductivity and has a large dipole moment due to the charge transfer between donor and acceptor molecules. We have investigated the characteristics of TMTSF-TCNQ field effect transistors (FETs) by controlling the growth conditions during the deposition. The FET characteristics strongly depend on the growth temperature, growth rate, and applied gate voltage. The results indicate that p- and n-type conduction and enhancement and depletion mode can he controlled by choosing appropriate growth conditions of the electric-field-assisted deposition.