Kinetics of the Silicon-Silicontetrachloride Reaction in a Flow System

The kinetics of the heterogeneous reaction Si + SIC14 = 2SiCt2 have been studied in a flow system with helium as the carrier gas. For this reaction the postulated rate expression d (0 00o) ( 0o00) S i--1.0 X 106 exp P41/2 + 9 exp P2 dt RT RT g Si cm-2 sec-1 was shown to describe the experimental results. The flow system data has been analyzed in terms of quantitatively measurable system parameters. Con-sequently true rather than apparent constants are found. The reduction of silicon and germanium tetrahalides in a hydrogen atmosphere is widely used as a method for preparing epitaxial films of these materials. The type reaction is written usually as MX4 + 2II2 = M-5 4HX [I] where M is the group-IV element and X is the halogen. This form is not even an accurate indication of the material balance. For example, a variety of other products occurs in the silicon-chlorine process includ-ing SiHCI3 and miscellaneous polymeric forms. In spite of the large amount of experimental work in this field, a high level of uncertainty remains as to the mechanism by which hydrogen is involved in the over-all process. However, there is a general consensus in the literature [silicon (I-5) and germanium (6-7)] that the type disproportionation iV [ ~- 1V;.X 4--- ~ 2MX2 [2] is one of the reactions in the over-all process. The equil ibrium for the Si-SiCI ~ reaction following Eq. [2] has been studied quantitatively by Sch~ifer and Nickl (8-9). They concluded that no other molecular species were present o any significant degree. Bylander (2) and 1V[onchamp, McAleer, and Pollack (5) have studied the disproportionation reaction in a flow sys-tem, but no quantitative kinetic data were obtained in either case. Monchamp et al. then used a low-pressure closed system, somewhat similar to that used by Heinecke and Ing (10) in germanium studies, for the investigation of the kinetics of the disproportionation reaction. They found tha