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 χ of B increases with the ion fluence until it saturates at χ 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 χ_B 0.5. A peculiar behavior is observed upon annealing: at 800 C a significant increase of randomly located B occurs and χ_B 1, at higher temperatures B recovers progressively into substitional site. The χ_B reaches 0.1 at 950 C and the carrier concentration coincides with the amount of substitutional B. The increase of χ_B at 800 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.