Effects of Polarized Organosilane Self-Assembled Monolayers on Organic Single-Crystal Field-Effect Transistors
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Effects of polarized organosilane self-assembled monolayers on organic single-crystal field-effect transistors J. Takeya1, T. Nishikawa2,3,4, T. Takenobu2,3, H. Shimotani2,3, S. Kobayashi2,3, T. Mitani4, and Y. Iwasa2,3 1
Materials Science Research Laboratory, CRIEPI, Komae, Japan. Institute for Material Reasearch, Tohoku University, Sendai, Japan. 3 CREST, Japan. 4 JAIST, Tatsunokuchi, Japan. 2
ABSTRACT Organosilane self-assembled monolayers, embedded in organic single-crystal field-effect transistors, significantly affect subthreshold properties in the transfer characteristics. The monolayer of either polarized or nearly unpolarized molecules is deposited on a SiO2/doped Si substrate before softly attaching a rubrene thin single crystal to form the “laminated crystal” transistors. As a result of effective passivation of the SiO2 surface, the device has achieved a subthreshold swing as low as 0.11 V/decade. It is also demonstrated that threshold gate voltage is shifted by polarization of the monolayers, indicating that threshold of the device is adjustable by the choice of silane materials. Both of the functions are closely associated with low-power applications such as logic-circuit components.
INTRODUCTION Significant efforts are being made to bring organic field-effect transistors (OFETs) into practical use, taking advantages of their potential of low-cost production, mechanical flexibility and optical as well as chemical sensitivities. So far, rather “high-power” applications such as pixel control devices for active matrix flexible displays are mostly argued as promising items; however, it is not yet clear whether or not the OFETs are capable of being used for low-power devices such as logic circuit components, which impose stricter requirements especially in their subthreshold regime [1]. In order to approach the maximum performance of the OFETs, we developed a method to use organic single crystals for the active semiconductor material instead of polycrystalline thin films; the crystals were laminated onto SiO2/doped Si substrates by natural electrostatic force [2]. Employing the four-probe measurements, genuine transfer characteristics of the OFETs are obtained avoiding influence of grain boundaries and parasitic contacts [2]. The laminated single-crystal devices are also useful to examine whether technologies achieved for polycrystalline thin-film OFETs are intrinsic to the semiconductor-SiO2 interface or are an artifact due to grain-boundaries, because the process of single-crystal device fabrication is almost identical to the commonly studied bottom-electrode thin-film OFETs. As organosilane self-assembled monolayers (SAMs) are often argued as a useful tool to modify the surfaces of the SiO2 dielectrics by directly acting on the subthreshold property of the OFETs, we have incorporated them in the “laminated crystal” FETs. In this bulletin, we report the intrinsic
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effects of the SAMs based on our measurements on the rubrene crystal devices.
EXPERIMENTAL DETAILS Molecularly flat surface
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