Effect of Nitrogen Doping on Vacancy State in Silicon Crystals Observed by Low-Temperature Ultrasonic Measurements

For the B-doped silicon crystals grown with and without N-doping, we measured the temperature dependence of the elastic constant in low-temperature region, to examine whether the N-doping annihilates the elastic softening caused by the gap-states of the isolated single vacancy. We have found that the elastic softening clearly observed for the N-free crystals is not observed for the N-doped ones, suggesting that the gap-states of the vacancies causing the elastic softening are destroyed by the N-doping. This is consistent with the model of Abe [T. Abe, J. Crystal Growth, 327 (2011) 1] in which the nitrogen molecule (N-N pair) occupies the vacancy to destroy its original gap-states. We have further observed that the N-doped silicon, which exhibits no softening in its as-grown state, exhibits the softening after the short-time annealing. This suggests that during the annealing the N-N pair is thermally activated to jump off the lattice site leaving the vacancy.