The precise control of dopant atom is one of the most important challenges to fabricate ultra-shallow and highly doped junctions. In the present study, the activation process of B atom in Si crystal was investigated at low temperature of 500 degrees C by using our tight-binding quantum chemical molecular dynamics method, which is over 5000 times faster than the conventional first-principles molecular dynamics method. The simulation results indicate that the B atom diffuses through the interstitial sites in the Si crystal even at low temperature of 500 degrees C. Moreover, we found that the boron atom tends to migrate into the lattice vacancy and however the diffusion of the B atom is very hard after the boron atom is trapped in the single lattice vacancy. On the other hand, when there are two adjacent lattice vacancies in the Si crystal, the B atom migrates frequently between two adjacent vacancies back and forth. This result predicts that two adjacent lattice vacancies impede the B activation in the Si crystal. Finally, we confirmed that our tight-binding quantum chemical molecular dynamics program is very effective to elucidate the boron activation process in the Si crystal, considering the electronic states and electron transfer dynamics. (c) 2004 Elsevier B.V. All rights reserved.