Catalytic Effect of SiO on Thermomigration of Impurities in SiO sub 2.

We have found that encapsulated thin Si films, adjacent to SiO sub 2, are dissolved at elevated temperatures by a reaction: Si + SiO sub 2 -> 2SiO, with SiO diffusing through the silica network. The presence of SiO in SiO sub 2 in a concentration =1 at. % has a profound effect on thermomigration of dopant impurities in the oxide. We propose a model for transport of As-, Sb-, P-, and Ge-rich precipitates, 50-1000angstrom in diameter, based on a reaction between SiO sub 2 and the precipitates that is mediated by a catalytic influence of SiO molecules. Only in the presence of SiO are the bonds in the SiO sub 2 network broken ahead of the drifting clusters and regrown behind them. The model predicts that the migration of precipitates is controlled by diffusivity of SiO in silica, consistent with the narrow range of drift velocities for clusters of different chemical compositions. The data also suggest that SiO sub 2 dissociation at the Si/SiO sub 2 interface is diffusion limited, which explains why decomposition of thin SiO sub 2 in vacuum is spatially inhomogeneous, while tick films used in our experiments react uniformly.