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Diffusion of tin in n-type GaAs

A series of experiments is described in which radioactive tin was diffused into GaAs for a wide range of conditions. Radio-tracer profiles were plotted and diffusion coefficients were calculated. It was found that at the lowest temperatures used ( approximately 850 degrees C), the measured diffusion coefficients depended on the doping of the original GaAs slice used in the experiment. At higher temperatures ( approximately 1100 degrees C), however, the diffusion coefficient was independent of the substrate material. Electrical measurements were also carried out on the diffused specimens so that direct comparison could be made between tin concentration and free carrier density. It was found that at high doping concentrations there were many more tin atoms than electrons in the samples. The results are interpreted in terms of a model in which the tin diffuses by way of charged gallium vacancies.