Monolithically Integrated p- & n- Channel Thin Film Transistors of Nanocrystalline Silicon on Plastic Substrates
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Monolithically Integrated p- & n- Channel Thin Film Transistors of Nanocrystalline Silicon on Plastic Substrates I-Chun Cheng and Sigurd Wagner Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, U.S.A. ABSTRACT Inverters made of monolithically integrated p- and n-channel thin film transistors of nanocrystalline silicon were demonstrated on both Corning 1737 glass and Kapton E polyimide substrates. The TFT’s geometry is staggered top-gate, bottom-source/drain. A nc-Si:H seed layer promotes the structural evolution of the nc-Si:H channel. Electron field-effect mobilities of 15 - 30 cm2V-1s-1 and hole mobilities of 0.15 - 0.35 cm2V-1s-1 were obtained. Slightly lower carrier mobilities were observed in the TFTs made on polyimide than on glass substrates. High gate leakage currents and offsets between the supply HIGH voltages and the output voltages in the inverters indicate that the low-temperature gate dielectric needs improvement. INTRODUCTION Electronics on organic polymer (plastic) foil are appealing because they possess the desired features, lightweight and ruggedness, for many new generation large-area electronic products. The basic prerequisite for these large flexible or conformable electronic systems is a versatile transistor back plane. Among the various active materials available for back plane transistors, hydrogenated amorphous silicon (a-Si:H) is the current industrial standard. a-Si:H thin film transistors (TFTs) on plastic substrates have been demonstrated [1,2]. However, complementary circuits are out of reach for a-Si:H, because its hole mobility is insufficient for p channel operation. Monolithically integrating circuits with plastic substrates calls for radical innovation in transistor technology and fabrication. This is the driver behind present programs on transistors of excimer laser annealed polycrystalline silicon [3-5], organics [6-8], p-channel organic/n-channel a-Si:H hybrids [9], fluidic self-assembled silicon nanoblocks [10], physical transfer of separately fabricated devices [11] and nanocrystalline silicon (nc-Si:H). nc-Si:H is pursued because it is obtained by direct deposition, which is compatible with existing a-Si:H technology, provides better uniformity than excimer laser annealing, and has higher potential for integration into a roll-to-roll process [12-14]. EXPERIMENTS A. Substrate preparation We work with both glass (1.1-mm thick Corning 1737 glass) and plastic (50-µm thick DuPontTM Kapton® E polyimide) substrates. At present glass is the standard substrate for large area electronics. The Kapton E polyimide is chosen because of its high thermal stability, with a glass transition temperature of ~ 354°C, a low coefficient of thermal expansion of 12x10-6/°C, and good TFT film adhesion. On glass substrate we fabricate devices directly, while the Kapton substrate are first passivated on both faces with ~ 500-nm thick PECVD SiNx:H.
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B. Deposition of nanocrystalline thin films High ON and low OFF currents are the two principal goals for TFTs
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