Field Effect of Fullerene Thin Film Studied by Displacement Current Measurements and Infrared Absorption Spectroscopy in
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Field Effect of Fullerene Thin Film Studied by Displacement Current Measurements and Infrared Absorption Spectroscopy in The Multiple Reflection Geometry S. Ogawa, Y. Kimura, H. Ishii, and M. Niwano Research Institute of Electrical Communication, Tohoku University 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, JAPAN Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Corporation (JST), 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, JAPAN ABSTRACT Au/fullerene(C60)/SiO2/Si field effect transistor (FET) has been investigated to examine the behavior of carrier injection by using displacement current measurement (DCM) and infrared absorption spectroscopy in the multiple internal reflection (MIR-IRAS). At the first scan of DCM, an increasing displacement current due to electron injection was clearly observed above a threshold voltage in forward scan, while no flow-back of the injected electrons was observed, indicating that almost all electrons injected in the scan were trapped. The capacitance obtained from the increasing current suggests that the electrons injected from a Au electrode spread laterally along C60/SiO2 to some extent. At the further scans of DCM, the threshold voltage was shifted to higher voltage, and the lateral spread of the electron was quite suppressed due to the space charge of the filled traps. These results clearly demonstrate that DCM is a useful method to examine the behavior of carriers in field effect transistor (FET) devices. The spectral change in MIR-IRAS observed under bias voltage for electron injection is also reported. INTRODUCTION Recently, organic field effect transistors (OFETs) have attracted much attention [1-7]. The control of carriers by external field, so-called field effect doping, is a key issue for understanding and improving OFETs. However, the mechanism about the field-induced carriers is not well understood. Especially, the origin of the carriers accumulated in the channel is not clear. For example, OFETs are believed to perform with the carrier generations in not inversion mode but only accumulation and depletion modes. The validity of such mechanism of carrier generation is under discussion: Most organic semiconductors applied to OFETs are not intentionally doped, and the total number of carriers in the organic layer may be often insufficient to enrich the carrier concentration in the channels. Instead, the carrier-injection can supply enough carriers in the channel. These points suggest that we have to take care of not only the carriers due to atmosphere
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but also those due to injection from the electrodes. In this study Au/fullerene(C60)/SiO2/n-Si FET device has been investigated to examine the behavior of carrier injection by using displacement current measurement (DCM) and infrare
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