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Abstract
The kinetics of the direct reaction of solid Si and gaseous CH$\sb3$Cl was studied on high-purity, low surface area Cu$\sb3$Si and CaSi$\sb2$ samples in an ultrahigh vacuum (UHV) surface analysis/atmospheric-pressure reaction apparatus. Samples were cleaned by Ar$\sp+$ sputtering and annealing in UHV and characterized by Auger electron spectroscopy (AES) both before and after reaction.
The effects of trace amounts of Zn and Sn ($<$0.1 atom%) on the Cu-catalyzed direct reaction were studied on Cu$\sb3$Si surfaces at 550-600 K. Fresh, unpromoted Cu$\sb3$Si surfaces are highly selective for $\rm (CH\sb3)\sb2SiCl\sb2.$ Tin, when present as the only promoter, inhibits $\rm (CH\sb3)\sb2SiCl\sb2$ formation and promotes $\rm CH\sb3SiCl\sb3$ and nonsilane formation. Tin segregates dramatically to Cu$\sb3$Si surfaces. Zinc, and Zn plus Sn promote $\rm (CH\sb3)\sb2SiCl\sb2$ formation, apparently by enhancing Cu-Si intermixing, Si diffusion, or participating in surface reaction steps. Concentration depth profiles of reacted surfaces showed that although a thick sub-surface region of Cu$\sb3$Si and Cu$\sb3$Si-Sn samples were depleted of Si by reaction, no depleted region formed on Zn-promoted Cu$\sb3$Si surfaces. Dimethyldichlorosilane and $\rm CH\sb3SiCl\sb3$ form by significantly different mechanisms on Cu$\sb3$Si surfaces, which become less active for $\rm (CH\sb3)\sb2SiCl\sb2$ formation at long times.
The direct reaction of high-purity CaSi$\sb2$ with CH$\sb3$Cl and HCl/CH$\sb3$Cl gas mixtures was studied at 525-700 K. Reaction of CaSi$\sb2$ with CH$\sb3$Cl is similar to the uncatalyzed reaction because Si-Si bonding in CaSi$\sb2$ and elemental Si are similar, although Ca catalyzes SiCl$\sb4$ formation. Above 600 K, methyl groups tend to decompose to form surface C- and H-species or react to form nonsilanes.
In 25% HCl/CH$\sb3$Cl, CaSi$\sb2$ reacts immediately at 575 K to form primarily HSiCl$\sb3,$ plus SiCl$\sb4$ and $\rm (CH\sb3)\sb3SiCl.$ When CaSi$\sb2$ is activated in 35% HCl/CH$\sb3$Cl and then reacted in 100% CH$\sb3$Cl at 575 K, $\rm (CH\sb3)\sb3SiCl$ forms selectively. Selectivity for $\rm (CH\sb3)\sb3SiCl$ on the HCl-activated surface decreases with time in 100% CH$\sb3$Cl because SiCl$\sb4,$ HSiCl$\sb3,$ and CH$\sb3$SiCl$\sb3$ formation rates increase; HCl is necessary to generate sites for $\rm (CH\sb3)\sb3SiCl$ formation. Hydrogen chloride may form Si-Cl active sites at low temperatures ($<$700 K). Selective $\rm (CH\sb3)\sb3SiCl$ formation is discussed in terms of ionic and silylene-insertion reaction mechanisms.
