Crystal orientation and near-interface structure of chemically vapor deposited MoS 2 films
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Crystalline MoS 2 films were deposited on Si and graphite substrates using MoF 6 and H 2 S as precursors. The crystal orientation and near-interface structure of the MoS 2 films were studied using transmission electron microscopy. In general, the preferred orientation of the (002) basal planes of the MoS 2 films with respect to the substrate surface changed from parallel to perpendicular with increased deposition temperature from 320 to 430 °C. At 430 °C, the basal planes were primarily oriented perpendicular to the Si substrate, except for the presence of a ~ 5 nm interface region in which the basal planes were oriented in the parallel direction. The formation of this transitional region was also observed on the graphite substrate. I. INTRODUCTION Because of its lamellar lattice structure, MoS 2 is an excellent solid lubricant for use in high temperature and vacuum environments.1 MoS 2 is currently prepared in the form of thin films by physical vapor deposition (PVD) techniques such as sputter deposition. The tribological performance of the MoS 2 film mainly depends on film adhesion and the orientation of (002) basal planes. 2 ^ While orienting the basal planes parallel to the substrate surface might be desirable from a lubrication point of view, some investigators observed that film adhesion was superior with perpendicular orientation.3 Recently Moser and Levy used transmission electron microscopy (TEM) to study the crystal structure and orientation of sputter deposited MoS 2 films and their wear behavior. 45 Crystalline MoS 2 was deposited on Si at 300 °C. The orientation of the (002) basal planes could be either parallel or perpendicular, depending on the Ar pressure in the sputtering chamber. Interestingly, in the film identified to have perpendicular orientation by x-ray diffraction (XRD), the presence of a very thin transitional layer ( 3 - 6 nm) was observed at the filmsubstrate interface. In this thin transitional layer, the basal planes of MoS 2 crystallites were primarily oriented parallel to the Si substrate. Between the transitional layer and the Si surface, there was a 5 nm native oxide layer. After the transitional layer, the basal planes became oriented perpendicular to the substrate surface. The parallel-to-perpendicular transition in the MoS 2 film was explained in terms of local branching during nucleation and growth. In ball-on-disk wear tests, the film with parallel orientation was shown to have a much longer life than that having perpendicular orientation. As surveyed in an earlier paper,6 the preparation of MoS 2 by chemical vapor deposition (CVD) has not received much attention, although the non-line-of-sight nature of the CVD technique may provide the film uniJ. Mater. Res., Vol. 10, No. 1, Jan 1995
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formity and conformal coverage required for processing substrates with intricate shapes and internal passages. The deposition of MoS 2 by reaction of H2S with MoF 6 (or MoCl4) was determined to be thermodynamically favorable over a wide ra
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