Studies of reactions between gaseous organo-silicon compounds and metal surfaces
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I. INTRODUCTION A recent article in Science1 has highlighted a new method of making metallic silicides from reactions of gaseous silicon-containing molecules with metal surfaces. This gas-solid reaction method offers many advantages over the usual method of making silicides by solid-solid reaction. Conventional semiconductor silicide technology 23 starts from crystalline silicon; the metal of interest (e.g., Ni, Pt, Fe, etc.) is then evaporated onto the surface. The solid sandwich so formed is heated in vacuum to elevated temperatures to produce the silicide phases. It is known that the reaction temperatures do not have to be very high. For example,3 Au-Si is formed at 100 °C, Ni2Si at 200-350 °C, Pd2Si at 100-300 °C, and Pt2Si at 200-500 °C. Even refractory metals have been reported to form silicide phases at temperatures not much higher than the above, e.g., MoSi2 at 525 °C, and VSi2 at 600 °C. Dubois and Nuzzo and co-workers have reacted organometallic compounds of Group III A or IV A elements with metal surfaces, both in the form of finely divided nickel catalysts,1'4 and as metal foils.5 In addia>
Now at Department of Mechanics and Materials Science, Rutgers University, Piscataway, New Jersey 08854.
714
J. Mater. Res. 3 (4), Jul/Aug 1988
http://journals.cambridge.org
tion, they have studied the reaction of silane with oxidized copper, silver, and gold surfaces,6 and the reaction of silane and germane with single-crystal nickel.7 The catalytic and surface properties of intermetallics have been studied only since the mid-1970s and have been reviewed by Wallace.8 Studies have been reported on low-surface-area (0.5-1.7 mVg) silicides of Fe, Co, and Ni that exhibit methanation activity.9-10 Such low surface areas were obtained by pulverization (in a mortar and pestle) of bulk alloys prepared by high-temperature fusion, and hence, as mentioned elsewhere,4 have limited practical utility. Chemisorption properties of metal silicides9"11 show some of the characteristics of supported catalysts in the strong metal-support interaction state.12 However, all of the samples were made either by grinding the bulk silicides,9>1° or by reactive growth on Si( 111) single-crystal wafers.11 No detailed chemisorption properties have been reported for high-surface-area metal silicides, either supported or unsupported. As there are many problems involved with treating surfaces with silanes, we have studied the reactions of a series of catalytically active metals with hexamethyldisilazane (HMDS), which has the formula [(CH 3 ) 6 Si 2 NH].
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Yates, Behal, and Kear: Organo-silicon compounds and metal surfaces
TABLE I. Experimental conditions for runs.
II. EXPERIMENTAL PROCEDURE The apparatus used for sample treatment was essentially the same as that employed earlier in a study of filamentous carbon growth on nickel and iron surfaces.13'14 It consists of a silica reactor tube with silica specimen holders, Fig. 1, an
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