Effect of the impurity content and of the surface on the electrical activation of low energy implanted boron in crystalline Si

The electrical activation of B in Si after low and ultra-low energy ion implantation is investigated. It is found that it critically depends on the purity of the substrate as well as the distance from the surface. In particular, while in very pure epitaxial Si layers typically the total B content is electrically active, in Czochralski Si, containing high C and O concentrations, the active fraction is very low. For shallow B implants (<10 keV) the electrical activation in Czochralski Si further decreases to a few percent of the total amount. These results are interpreted in terms of the formation of boron-impurity complexes deactivating the dopant. Close to the surface enhanced vacancy annihilation, concomitant with a supersaturation of interstitials, produces the observed dramatic effects on the electrically active profiles. Finally, at very low energies (<3 keV) also in epitaxial Si layers part of the dopant can be electrically inactive due to B clustering.