High dose implantations of boron molecular ions such as BF+2and BCl+2ions in Si have been investigated in an attempt to form shallow P+layers in the dose range 1×1015–1×1016ions/cm2, in comparison with those of boron. For BF+2implants it was found that both the effective surface carrier densityNsand junction depthXjin Si after 1000°C anneal tend to completely saturate at doses above 5×1015ions/cm2, while they tend to increase for B+implants. From profile measurements of total and electrically active boron it was clarified that the additional implantation of BF+2ions in excess of 5×1015ions/cm2results in the accumulation of inactive boron atoms particularly around the surface region. Junction depths in Si resulting from BF+2implants are generally shallower than those resulting from B+implants. For BCl+2implants the values ofNsare generally much lower than those for BF+2and B+implants.