B couples formation and dissolution in ion implanted Si.

The off-lattice displacement of electrically active, substitutional B in presence of Si interstitials generated by light ion irradiation has been studied by channeling along the $<$100$>$ and $<$110$>$ axes. The channeling yield $\chi $ of B increases with the ion fluence until it saturates at $\chi \approx $ 0.5 suggesting a non-random B displacement. At the saturation B is not electrically active and accurate angular scans indicates the formation of B-B couples aligned along the $<$100$>$ direction in agreement with first principle calculations. The same kind of defect is formed upon B implantation at room temperature as demonstrated also by angular scans with $\chi _{B}\approx $0.5. A peculiar behavior is observed upon annealing: at 800 \r{ }C a significant increase of randomly located B occurs and $\chi _{B}\approx $1, at higher temperatures B recovers progressively into substitional site. The $\chi _{B}$ reaches 0.1 at 950 \r{ }C and the carrier concentration coincides with the amount of substitutional B. The increase of $\chi _{B}$ at 800\r{ }C can be due to the dissolution of B couples and to an intermediate off lattice location of B before to occupy a substitutional site.