We optimized Ge+and Si+preimplantation to eliminate the channeling tail and prevent the rapid diffusion of boron and the formation of serious defects. We examined the dependence of the microchanneling of BF+2implantation or the lattice disorder of preimplanted silicon using secondary‐ion mass spectroscopy and grazing exit Rutherford backscattering spectroscopy. The optimum doses are about 25% those for full amorphization, i.e., preamorphization. The channeling tail is eliminated by disordered layers containing about 60% silicon atoms on irregular sites.