Preparation of MoS 2 thin films by chemical vapor deposition
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The chemical vapor deposition (CVD) of MoS 2 by reaction of H 2 S with molybdenum halides was determined to be thermodynamically favored over a wide range of temperature, pressure, and precursor concentration conditions as long as excess H 2 S was available. The thermochemical stability of H 2 S, MoF 6 , and M0CI5 was also assessed to address their suitability as precursors for the CVD of MoS 2 . The results from the thermodynamic analysis were used as guidance in the deposition of MoS 2 thin films from MoF6 and H 2 S. The (002) basal planes of MoS 2 films deposited above 700 K were preferentially oriented perpendicular to the substrate surface.
I. INTRODUCTION Lamellar solid lubricants such as MoS 2 have been developed for tribological applications in high temperature and vacuum environments where the use of traditional liquid lubricants becomes ineffective or cannot be tolerated.1 The lattice structure of MoS 2 consists of a hexagonal sheet of Mo atoms, which lies between two hexagonal sheets of S atoms. The Mo and S sheets are held together strongly by covalent bonding whereas the interaction between the MoS 2 layers (i.e., between adjacent S sheets) is through weak van der Waals forces. This layered crystallographic arrangement allows the MoS 2 layers to easily shear between basal planes and is responsible for its excellent lubricity. Also, MoS 2 has been considered as a cathode material for solidstate secondary lithium batteries.2 In this application, the ability to intercalate the MoS 2 layers with lithium metal ions is being utilized. MoS 2 has been prepared as a thin film by a variety of methods including sputter deposition, 34 pulsed laser evaporation,5 sulfurization of electroplated Mo,6-7 drying or heating of chemical solutions containing liquid Mo and S precursors,8"10 chemical vapor deposition (CVD),2 and metal-organic CVD (MOCVD). 1112 Among these techniques, MoS 2 prepared by sputter deposition has been most frequently evaluated for tribological purposes. The processing-microstructureproperty relationships of sputter-deposited MoS 2 are relatively well-documented.3'4'13"16 Interestingly, the preparation of MoS 2 by CVD has not received much attention, although it seems that the technique offers several advantages over sputter deposition. First, the non-line-of-sight nature of CVD should provide superior film uniformity and conformal coverage on substrates having intricate shapes, internal passages, or large dimensions. Second, CVD offers the possibility of incorporating MoS 2 into hard nitride and carbide CVD film matrices as finely dispersed, discrete 1474 http://journals.cambridge.org
grains via codeposition. This type of self-lubricating composite film, if appropriately engineered, might provide the improved wear protection required for some of the high temperature automotive and aircraft applications. It is noted that the versatility of CVD in creating ceramic composite films by codeposition and in controlling their microstructure has been previously demonstrated for many binary combinations in
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