Quality control of epitaxial LiCoO 2 thin films grown by pulsed laser deposition
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Quality control of epitaxial LiCoO2 thin films grown by pulsed laser deposition T. Ohnishia) National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan; and Japan Science and Technology Agency, Core Research for Evolutional Science and Technology (JST-CREST), Saitama 332-0012, Japan
B.T. Hang and X. Xu National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan
M. Osada and K. Takada National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan; and Japan Science and Technology Agency, Core Research for Evolutional Science and Technology (JST-CREST), Saitama 332-0012, Japan (Received 13 February 2010; accepted 17 May 2010)
Thin films of c-axis-oriented LiCoO2 were epitaxially grown by pulsed laser deposition (PLD). The ablation laser conditions greatly affect the crystal quality of the epitaxial LiCoO2 thin films. In addition, high-quality LiCoO2 thin films were found to grow without any impurity phases under relatively low oxygen partial pressure, although high pressure had been often selected to suppress the formation of Co3O4 with a lower valence state as an impurity. This result clearly indicates that the ablation laser conditions are an essential growth parameter, and that composition control is indispensable to grow high-quality complex compound thin films by PLD. Progress in thin-film technologies has enabled not only electronic materials but also ionic ones to be made into thin films. One aim of thin-film ionic materials, including electroactive materials and solid electrolytes, is to develop thin-film batteries. For example, lithiumion batteries are now widely used in portable electronic devices and their use is spreading. There are also increasing demands for large batteries for electric vehicles and load leveling equipment, and for miniature batteries for contactless smart cards, electric tags, and so forth. Thinfilm batteries are a typical example of the latter, and a number of related papers have been published since 1983.1 Most of the cathodes used in these works were LiCoO2 thin films,2,3 because LiCoO2 is one of the major cathode materials used in rechargeable Li-ion batteries. However, little information has been reported on the influences of deposition conditions on film characteristics. Thin films often show somewhat different properties from those of bulk single crystals. In fact, in a study on thin films of multication complex oxides, the author has demonstrated that the material properties of thin films easily change; for example, film performance is usually a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0250
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J. Mater. Res., Vol. 25, No. 10, Oct 2010
degraded by the inclusion of lattice defects caused by the composition deviating from stoichiometry in the homoepitaxial growth of SrTiO3 thin films using pulsed laser deposition (PLD).4,5 The study revealed that the ablation laser conditions affected the composition of deposited thin films in the PLD process, even if a stoichiometric single
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