Heavy Doping of Li + -ion into NiO Epitaxial Thin Films via Unequilibrium Room-temperature Processing for New Functional
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Heavy Doping of Li+-ion into NiO Epitaxial Thin Films via Unequilibrium Roomtemperature Processing for New Functionalization Naoki Shiraishi1, Yushi Kato1, Hideki Arai1, Nobuo Tsuchimine2, Susumu Kobayashi2, Masahiko Mitsuhashi3, Masayasu Soga3 and Mamoru Yoshimoto1,4 1 Department of Innovative & Engineered Materials, Tokyo Institute of Technology 4259-J2-46, Nagatsuta, Midori, Yokohama, 226-8503, Japan 2 TOSHIMA Manufacturing Company Limited Japan 1414, Shimonomoto, Higashimatsuyama, Saitama, 355-0036, Japan 3 Kanagawa Industrial Technology Research Institute 705-1, Shimoimaizumi, Ebina, Kanagawa 243-0435, Japan 4 Patent Attorney, Tokyo Institute of Technology ABSTRACT Heavily Li-doped (up to 50 mol%) NiO thin films were deposited on an ultrasmooth sapphire (0001) substrate at room -temperature (RT) by applying the pulsed laser deposition (PLD) process. From in situ reflection high energy electron diffraction (RHEED) and ex situ Xray diffraction (XRD), it was found that 50 mol% Li-doped NiO ((Li0.5Ni0.5)O thin film deposited at RT ) could be epitaxially grown with (111) orientation; however the film became polycrystalline when deposited at 515 °C under the same atmosphere. These results indicate the possible exploration of novel growth of the oxide films through unequilibrium low temperature processes, such as PLD film growth at RT. Increasing the doped Li content decreased the electrical resistivities at RT. of Li-doped NiO epitaxial films (Li-content of 0～50 mol%) from 600 to 0.05 Ωcm. INTRODUCTION Low temperature processes, such as room -temperature (RT) film growth demonstrate several characteristics. One is that thermal diffusion or stress can be avoided at the boundary, resulting in a sharp interface and very flat surface. In addition, the process is expected to create novel materials, different from those obtained from a thermal equilibrium phase. Pulsed laser deposition (PLD), which is based on laser ablation of a solid target, has the advantage of depositing high quality thin films at relatively low substrate temperatures because photochemical ablation generates highly excited film precursors [1]. Previously, we demonstrated RT epitaxial growth of various oxide thin films, such as CeO2 [2], α-Al2O3 [3], ZnO [4], and Sn-doped In2O3 [5], with the PLD process. We also reported that NiO thin films were epitaxially grown on a sapphire substrate at RT by PLD [6]. Annealing the RT -grown NiO epitaxial thin film at 700 °C reconstructed the film surface to form a straight striped nanogroove pattern with a depth of about 3.0 nm over the entire film area. This nanopattern formation on the film might be related to the RT growth on the atomically stepped substrate [7]. By doping Li+-ion into NiO thin films, we could obtain a deeper nanogroove array about 20 nm in depth. These Li-doped NiO thin films with a striped nanogroove pattern had anisotropic electric conduction, depending on whether the electric path was parallel or perpendicular to the straight nanogrooves. Thus, film deposition via an RT
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