Role of implantation-induced defects in surface-oriented diffusion of fluorine in silicon

Open-volume defects introduced in Si(100) crystals during fluorine implantation were investigated by variable-energy positron beam depth profiling. The behavior of the implantation-induced lattice defects upon high temperature annealing and their role in the surface-oriented diffusion of F impurities were examined. The defects become mobile and undergo recovery at temperatures below 550 °C, i.e., well before the onset of fluorine diffusion as seen by secondary ion mass spectroscopy (SIMS) profiling. This behavior suggests that after irradiation and annealing the fluorine occupies substitutional sites to which positrons are insensitive. The anomalous F diffusion seen in SIMS has been explained through a two-step diffusion mechanism, in which the diffusion kinetics is determined by dissociation of the substitutional F into an interstitial F and a vacancy, followed by a rapid diffusion of the interstitial F and the vacancy through the crystal to the surface.