Sequential clustering reactions of Si sup + with SiD sub 4: Identification of a bottleneck preventing rapid growth of hydrogenated silicon particles.

Gas phase nucleation of hydrogenated silicon dust is a poorly understood problem in silane CVD of silicon films. We have used Fourier transform mass spectrometry to study growth of subcritical size Si sub x D sub y sup (x) nuclei initiated by clustering of atomic silicon cations with SiD sub 4 at room temperature. The clustering reactions proceed in a highly specific fashion. Si sup + grows initially by sequential addition of three -SiD sub 2 units. This cluster growth mechanism then encounters a critical bottleneck and ceases. Further aggregation occurs only by slow bimolecular attachment of SiD sub 4. The fundamental mechanisms and energetics for the individual reaction pathways have been calculated by Raghavachari using ab initio electronic structure theory. The clustering mechanism involves insertion into a SiD bond of SiD sub 4 followed by elimination of D sub 2. Addition of -SiD sub 2 serves to increasingly saturate the bonds of all of the silicon centers which leads to the observed growth limitations at Si sub 4 D sub 6 sup (+).