Nanoimprinted Glass Substrates for Nanoscale Growth Control of Transparent Conducting Oxide Films
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Nanoimprinted Glass Substrates for Nanoscale Growth Control of Transparent Conducting Oxide Films Yuki Sugimoto1, Yasuyuki Akita1, Yuta Nakasone1, Masahiro Mita2, Hideo Oi2, and Mamoru Yoshimoto1, 3 1 Department of Innovative & Engineered Materials, Tokyo Institute of Technology, 4259-J2-46 Nagatsuta, Midori, Yokohama 226-8503, Japan 2 Kyodo International Inc., 8-5-1 Chiyogaoka, Asao, Kawasaki 215-0005, Japan 3 Patent Attorney, Tokyo Institute of Technology ABSTRACT The crystal growth of indium tin oxide (ITO) thin films on nanoimprinted glass substrates was examined by applying pulsed laser deposition. The nanoimprinted glass was fabricated by thermal nanoimprint using a nanostriped NiO thin film mold. The nanopatterned glass had a straight nanowire array with intervals of about 180 nm, and wire height of about 30 nm. The surface morphology of the ITO thin film grown on the nanoimprinted glass accurately reflected the morphology of the glass surface nanopattern. The ITO thin film on the imprinted glass exhibited preferentially (111)-oriented polycrystalline growth, and had 35% lower resistivity in the direction perpendicular to the nanowire array than that of the film grown on the nonpatterned commercial glass. INTRODUCTION Nanoscale patterning technologies of glass surfaces are of great importance for fabricating functional devices such as optical circuits, optical memory, displays, and solar cells. Formation of nanoscale patterns has been usually conducted by electron-beam lithography and dry etching. Nanoimprint lithography has attracted much interest as a nanoscale surface modification technique with simple, low-cost, and high-throughput features [1-7]. The procedure of thermal nanoimprinting replication is to press the nanopatterned mold onto the surface of glassy materials such as polymers and oxide glasses, at a temperature above their glass transition temperatures (Tg), and then to cool down the sample and demold [8-11]. Previously, we succeeded in fabricating nanoimprinted glass with straight nanowire arrays by glass nanoimprint [12, 13]. The thin film mold used in glass nanoimprint could be obtained by annealing epitaxial Li-doped NiO (NiO:Li) thin films deposited on a sapphire (αAl2O3 single crystal) substrate with straight steps and terraces [14, 15]. For NiO film deposition, we employed a pulsed laser deposition (PLD) process in which the composition of the deposited film is expected to be close to that of the target for laser ablation. The nanopatterned NiO thin film mold had a straight nanogrooved array. Indium tin oxide (ITO) has been used as the transparent conducting electrode in optoelectronic devices, because the film has good conductivity and transparency in the visible region. It is commonly known that the electrical and optical properties of ITO films depend strongly on the deposition conditions [16, 17]. Several groups reported that the nanoscale pattern on the substrate affects the size of the crystal grains and position of the grain boundaries [18]. The use of these nanoimpri
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