Anisotropic and Enhanced FET Performance in Rubbing-Aligned CuPc Thin Film Transistor with Top Gate Structure
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Anisotropic and Enhanced FET Performance in Rubbing-Aligned CuPc Thin Film Transistor with Top Gate Structure Masanao Goto, Yohei Watakabe, Hideo Takezoe, and Ken Ishikawa Dept. of Org. and Polym. Mater., Tokyo Institute of Technology, S8-42, 2-12-1, O-Okayama, Meguro, Tokyo, 152-8552, Japan ABSTRACT We successfully fabricated the top-gate OFETs utilizing an aligned CuPc thin film. We achieved enhanced performance with aligned CuPc thin film compared with non-aligned one. The mobility reached to 0.07 cm2/Vs along the preferable axis, which was 10 times faster than that of the non-aligned one. The mobility perpendicular to the preferable axis was also enhanced. INTRODUCTION Because the electronic properties of organic crystals are usually highly anisotropic, structure-controllable materials are desired for understanding the device physics in OFETs. Hence the control of the crystal orientation is one of the most important issues. However it is not easy in most thin film organic semiconductors. Moreover we cannot examine the exact morphological structure of bottom gate OFETs where the current flows. Therefore we need to study the relationship between the electrical properties and the semiconductor morphology of top gate FET structures using morphology controllable active layers. For this purpose, copper phthalocyanine (CuPc), which is one of the most classical organic semiconductors, was chosen as an active material. In the former studies [1-3] it was shown that the crystal orientation and the crystal size at the surface of CuPc thin films could be independently controlled by rubbing treatment and film thickness, respectively. Therefore CuPc was expected to be a good candidate for this study. EXPERIMENT The device was constructed on an indium-tin-oxide (ITO) coated glass substrate. The ITO electrode was photolithography patterned and served as source and drain electrodes. Prior to rubbing the substrate was sequentially sonicated by acetone, isopropyl alcohol and distilled water. Then the substrate was cleaned by UV/O3 treatment. The rubbing treatment was carried out by using a rubbing machine for liquid crystal alignment. A rotating roller covered with cotton velvet was pressed to sliding substrate with extremely high pressure compared to the usual rubbing process for fabricating liquid crystalline cells. The rubbing strength was determined by the spacing between the roller surface and the substrate surface. After the rubbing process, CuPc was vacuum deposited as an active layer at a rate of 0.05 nm/s. The temperature of the substrate was 150 oC during the deposition. The base pressure was under 1.0 x 10-3 Pa. The layer thickness was 50 nm. Subsequently the amorphous fluoropolymer CYTOP (CTX-809S) was spin-coated from a 6 wt% solution as the gate dielectric. This process
normally gives a 600-nm-thick layer at 2500 rpm. Then aluminum was evaporated as a gate electrode. The base pressure was under 1.0 x 10-3 Pa. Evaporation rate was crucial for the leakage current and kept as slow as possible. The molecul
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