Orientation of rf-sputter-deposited MoS 2 films
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2H—MoS 2
I. INTRODUCTION The adhesion and orientation of rf-sputter-deposited films of MoS2 are critical to their application as solid lubricants on metal and ceramic surfaces. It has been suggested that adhesion of the film to the substrate determines wear life,1 that films fail when they are pushed out of the wear track. It has also been suggested that if the films are oriented with the crystallite basal planes parallel to the substrate, the friction will be lower than in the case of perpendicular orientation.2 Unfortunately, it is not clear how to produce films with the proper orientation, although the amount of water in the deposition chamber has been identified as a factor by one worker.3 The crystal structure of MoS2 is illustrated in Fig. 1. Each S-Mo-S sandwich is tightly bound internally and interacts with neighboring sandwiches only through van der Waals forces. The plane between sandwiches, then, is a plane of easy shear. Since there are no vacant orbitals or dangling bonds on the basal plane, there is no possibility of strong chemical bonding between the basal plane of a crystallite and another surface. Thus a crystallite with parallel orientation (as it will be called here) will not adhere well to a substrate. At the edges of a crystallite perpendicular to the basal plane, of course, there will be unsatisfied bonds: S atoms, for instance, that are missing their neighboring Mo. These bonds may be satisfied by oxygen or other impurities or by the substrate. If the bonds are satisfied by the substrate, the crystallite will be forced to align with its basal plane perpendicular to the substrate because of the directionality of the S dangling bonds: they point to where the missing Mo should be. This investigation is concerned with the orientation of MoS2 films. A simple theory of film growth is tested. Active sites on the substrate react with S atoms from the plasma in the rf sputter deposition chamber. This forces edge-on (perpendicular) orientation of the crystallites because only edge sulfurs may form bonds to other atoms due to the crystal geometry and electronic structure previously discussed. If the active sites can be removed, the 180
http://journals.cambridge.org
J. Mater. Res., Vol. 4, No. 1, Jan/Feb 1989
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Mo
Q) s
crystallites will form with their lowest energy surfaces against the substrate, that is, with the basal plane parallel, in order to minimize the interfacial energy. What are the active sites? Probably H2O and -OH groups on the surface, associated with high energy locations such as ledges, kinks, and grain boundaries. The exact identification of these sites is not the subject of this paper, although we report the manipulation of the sites in ways that suggest their identity. II. EXPERIMENTAL Single crystal Si was chosen as a model substrate for the important engineering ceramics SiC and Si3N4. The specimens were prepared in a variety of ways to manipulate the active sites on the surface. Some specimens were 1989 Materials Research Society
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