The diffusion of B in samples with, or without, a 20nm-thick molecular beam allotaxial CoSi2 top-layer was investigated during annealing and oxidation by using doping superlattices. The latter were grown by means of molecular beam epitaxy, and consisted of 6 spikes with peak concentrations of 1018/cm3; with peak centres that were spaced 100nm apart. The shallowest spike was capped with 100nm of Si, followed by 20nm of CoSi2. Annealing in pure N2 and oxidation in pure O2 was carried out at 800 to 1200C. The concentration depth-profiles were measured by means of secondary ion mass spectrometry. The results showed that dopant diffusion was markedly different with, as compared to without, the CoSi2 layer. In the latter specimens, oxidation-enhanced diffusion of B was observed. However, the effect of CoSi2 layers was to retard B diffusion strongly (table 21). The B diffusivity was retarded by a factor of between 2 and 10, as compared to the thermal diffusivity, and by a factor of 20 to 100 when compared with the corresponding diffusivity for oxidized Si without a CoSi2 layer.

Effect of an Epitaxial CoSi2 Layer on Diffusion of B and Sb in Si during Annealing and Oxidation A.K.Tyagi, L.Kappius, U.Breuer, H.L.Bay, J.S.Becker, S.Mantl, H.J.Dietze: Journal of Applied Physics, 1999, 85[11], 7639-45. See also: Fresenius’ Journal of Analytical Chemistry, 1999, 365[1-3], 282-5