We have studied boron profiles by using the ion beam recoil implantation. A boron layer was first deposited onto Si, followed by irradiation with Si ions at various energies to knock the boron. Conventional belief is that the higher the implantation energy, the deeper the recoil profiles. While this is true for low-energy incident ions, we show here that the situation is reversed for incident Si ions of higher energy due to the fact that recoil probability at a given angle is a strong function of the energy of the primary projectile. Our experiments show that 500-keV high-energy recoil implantation produces a shallower B profile than lower-energy implantation such as 10 keV and 50 keV. The secondary-ion-massspectrometry (SIMS) analysis shows that the distribution of recoiled B atoms scattered by the energetic Si ions agrees with our calculation results. Sub-100 nm p+/n junctions have been realized with a 500-keV Si ion beam.