Fluorine in Si: native-defects complexes and the suppression of impurity diffusion

The transient enhanced diffusion of acceptor impurities severely affects the realization of ultrahigh doping regions in miniaturized Si-based devices. Fluorine codoping has been found to suppress this transient diffusion, but the mechanism underlying this effect is not understood. It has been proposed that fluorine-impurity or fluorine-native-defect interactions may be responsible. Here we clarify this mechanism combining firstprinciples theoretical studies of fluorine in Si and purposely designed experiments on Si structures containing boron and fluorine. The central interaction mechanism is the preferential binding of fluorine to Si-vacancy dangling bonds and the consequent formation of vacancy-fluorine complexes. The latter effectively act as traps for the excess self-interstitials that would normally cause boron transient enhanced diffusion. Instead, fluorineboron interactions are marginal and do not play any significant role. Our results are also consistent with other observations such as native-defect trapping and bubble formation.