Titanium disulfide thin film prepared by plasma CVD a)
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M. Koizumi Faculty of Science and Technology, Ryukoku University, Ohtsu, Shiga 520-21, Japan (Received 20 September 1989; accepted 16 August 1990)
Titanium disulfide films were prepared by plasma CVD. Crystalline orientation of layered TiS2 was investigated in relation to deposition rate, film thickness, and kinds of substrate. The preferred orientation of TiS2 basal plane perpendicular to substrates was obtained on the films with their thickness of more than ca. 10 jum at the deposition rate of ca. 4 x 10"3 g/cm2 • h on all kinds of substrate. This orientation resulted in a large discharge capacity in a lithium battery cathode application.
I. INTRODUCTION
Application of ceramic thin film attracts increasing attention. It is very important to orient crystals in these films such that the characteristics of the crystals are emphasized. Layered crystals, e.g., MoS2 and h-BN, have lubricity along their basal plane. There have been several investigations of MoS2 films prepared by sputter deposition.1"8 The film texture has been investigated with different deposition conditions; several kinds of surface texture were reported, but the mechanism has not yet been fully explained. Titanium disulfide has CdI2-type layered crystal structure similar to MoS2, and is one of the most promising candidate materials for lithium secondary battery cathodes. Sputter deposition is not suitable for TiS2 due to its wide nonstoichiometric compositional range. We have reported a preparation of its very fine powder by plasma CVD to obtain a high current density in batteries.910 Its Li intercalation occurs along the layer, and it is difficult to intercalate Li from a direction perpendicular to the host layer. The battery charge and discharge occur with lithium intercalation and deintercalation. It seems very important that the very fine TiS2 powder orients its layer plane perpendicular to the lithium anode surface in order to enhance current density. In a discharging process, most of the lithium ions are electrostatically absorbed on the TiS2 film surface if TiS2 crystals orient their layer planes parallel to a substrate, as schematically shown in Fig. 1(A). If the crystals stand perpendicularly, as shown in Fig. 1(B), the Li+ is easily intercalated to the TiS2 interlayer space in an applied electric field. The perpendicularly oriented TiS2 thin film had been apa)
Paper presented, in part, at the Fall Meeting of the Materials Research Society, on "Plasma Assisted Deposition of New Materials" (Symp. N), Boston, Massachusetts, 1987. b) Address correspondence to this author. 2894
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 12, Dec 1990
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plied to make a solid state battery using Li3.6Sio.6Po.4O4 solid electrolyte.11 However, there has been no report on an orientation control of TiS2 crystals in its thin film. We obtained preferred orientations of TiS2 fine powder both parallel and perpendicular to the substrate. In the present investigation, crystalline orientations in TiS2 film were studied in relation
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