Boron diffusion across silicon-silicon germanium boundaries

Most baron diffusion studies in Si-Ge have been made in regions of uniform germanium content. In this paper diffusion is observed from a baron-doped epitaxial silicon layer across surrounding Si-Ge layers. Pileup of baron in the Si-Ge layers shows that the activity coefficient for baron in Si-Ge is lower than that for pure silicon. A simple pairing model for Si-B interaction fitted the pileup quite well, with the same equilibrium constant applying to both Si0.9Ge0.1 and Si0.97Ge0.03 layers. The effect of this was simply to immobilize a significant fraction of the boron while retaining its acceptor qualities, the ratio of immobile baron to normal substitutional baron being proportional to the germanium content. Quasielectric field effects at the Si-SiGe interface have a strong effect on the results obtained. (C) 1998 American Institute of Physics.