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A model for As clustering in single crystal silicon is presented that allows for the participation of arbitrary numbers of As ions, electrons, and arbitrarily charged vacancies. From this model the saturation behavior of the concentration of unclustered, electrically active As impurities is derived. It is shown that saturation (or electrical solubility) demands the participation of at least one negative charge in the formation of positively charged clusters, be it electron or negative vacancy. By analysis of our own experimental results and those by other authors, this general cluster model indicates that the participation of exactly one negative charge is the most likely case. In the course of this discussion the equilibrium saturation concentration of electrically active As is deduced from experiment as a function of temperature.

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