The formation of highly activated ultrashallow junctions is one of the main challenges for the forthcoming generation of complementary metal oxide semiconductor (CMOS) devices. Co-implantation of impurities such as carbon (C) or fluorine (F) is an attractive technique. However, junction optimization can only be achieved with a complete understanding of the underlying physical mechanisms. In this paper, the effect of C/F co-implant on boron (B)-doped preamorphized silicon during the soak annealing is extensively studied. C/F atoms are located in the middle range between the B/BF2 concentration profiles and the end-of-range (EOR) defect band, with the aim of reducing the interactions of dopants with the interstitials released from EOR region. Isochronal annealing study is performed to investigate the impact of C/F codoping on the dopant de/reactivation behavior. It is shown that transient enhanced diffusion can be reduced by both co-implant schemes. The B-doped junction formed with the C co-implant is relatively stable and dopant deactivation is inhibited, while it is presumed that F atoms form B-F complexes, which reduces the B activation level. A physical insight on the dopant-defect interactions associated with C/F co-implant is established through the combination of diffusion and activation studies during soak annealing. c 2007 The Electrochemical Society.