Radiotracer study of cobalt diffusion and solubility in electronic-grade germanium wafers

The diffusion rate of Co in Ge is found to be as fast as 2×10-6cm2s-1 at 900 °C, whereas the Co solubility comes close to 1×1016cm-3 at the same temperature. Based on these properties and its acceptor activity, Co may cause serious contamination problems during the fabrication of Ge-based electronic devices. In contrast to an early radiotracer study [L.Y. Wei, J. Phys. Chem. Solids 18 (1961) 162], we observe common diffusion profiles of complementary error function type, which are indicative of a constant diffusivity depending only on temperature. A preliminary analysis of the data points to the dissociative diffusion mechanism involving interstitial-substitutional exchange via vacancies. However, in contrast to Cu, which migrates via the vacancy-controlled mode of the dissociative mechanism, the diffusion of Co may be rate-limited by the transport properties of its interstitial modification (Coi).