Solution-Processed ZnO Nanowire Network Thin Film Transistors for Transparent Electronics
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Solution-Processed ZnO Nanowire Network Thin Film Transistors for Transparent Electronics Teymur Bakhishev 1, Steven K. Volkman, and Vivek Subramanian Department of Electrical Engineering and Computer Sciences, University of California, Berkeley 1 144MB Cory Hall, #1770, Berkeley, CA 94720-1770, (510) 643-4232, ABSTRACT We describe a process to grow ZnO nanowires in solution to obtain a network of nanowires 0.5-1.5μm long. The growth was initiated on ZnO nanoparticles spun-cast onto thermal SiO2. We demonstrate bottom-gate FET structures formed using these nanowire networks as channel layers with gold (Au) and aluminum (Al) source/drain pads. Resulting transistors are well-behaved with on-off ratios >10 4 and mobility >10-2 cm 2/V-s, calculated without accounting for actual surface coverage of nanowires. Actual nanowire mobility is therefore substantially higher, attesting to the potential for this technique as a method for realizing low-cost, high-brightness displays. INTRODUCTION ZnO has garnered substantial attention in recent years as a candidate material for use in transparent electronics. ZnO is a transparent n-type semiconductor that has been used in accumulation-mode NMOS thin film transistors (TFTs) [1]. Since these TFTs are transparent, they are extremely attractive for use as pixel transistors in active matrix flat panel displays. Due to their transparency, they may be sized to cover substantial fractions of the pixel without reducing aperture ratio. Consequently, they have reduced mobility requirements for given pixel current specifications compared to conventional amorphous Si or polysilicon-based TFTs. In recent years, several groups have demonstrated sputtered [2] or laser-deposited ZnO TFTs [3]. While these are attractive due to their transparency and performance, they still require the use of conventional vacuum-based processing techniques. Solution-processing techniques are substantially more attractive for low-cost displays, since they may be used to deposit films on a wide variety of low-cost, large area substrates. Recently, we demonstrated ZnO TFTs using sintered ZnO nanoparticles [4]. Here, we use solution-deposited ZnO nanowires to realize the first ZnO nanowire network TFTs, offering transparency, air stability, and NMOS operation, all of which are desirable for active matrix displays. EXPERIMENTAL DETAILS ZnO nanoparticles ~3nm in size were prepared as previously reported [4]. They were dissolved in chloroform and spun onto a standard substrate-gated FET test vehicle with N+ Si gates and 100nm thermal SiO2 gate dielectric. The dilution was adjusted to get discrete nanoparticles on the substrate, rather than a film. The substrate was annealed at 190°C to unencapsulate the nanoparticles. The nanowires were grown in 1mM aqueous solution of Zinc Nitrate and Hexamine [5] at 65°C for 5 hours, resulting in nanowires 0.5-1.5μm long (Fig 1). The samples were then annealed in forming-gas for 1 hour at 300°C. Gold source/drain pads were evaluated in bottom contact configuration where p
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