A high concentration of hole is generated without annealing, by high dose ion-implantation of boron into silicon substrate. A model has been proposed that B₁₂ clusters are generated in crystalline Si and act as double acceptors.Recently, we investigated the atomic and electronic structures for B₁₂ clusters by the ab initio calculation. It was found that the clusters capture the valence electrons, which results in the generation of unoccupied level in valence band of crystalline Si and the generation of holes. It was also found thatthe icosahedral B₁₂ in Si is more stable than the cubo-octahedral B₁₂.