Stability of native defects in hexagonal and cubic boron nitride

We investigate, through first-principles calculations, the stability and electronic structure of self-interstitials and vacancies in both hexagonal (graphite-like) and cubic boron nitride. We find that the self-interstitials N_i and B_i in hexagonal boron nitride (h-BN) have low formation energies, comparable to those of the vacancies V_N and V_B. For instance, we find that N_i is the most stable defect in h-BN under N-rich and p-type conditions followed by the nitrogen vacancy. This is consistent with experimental findings of large concentrations of nitrogen interstitials and vacancies, and of the trapping of nitrogen in the hexagonal phase of BN thin films grown by ion-bombardment assisted deposition techniques. In contrast, in cubic boron nitride (c-BN) the self-interstitials have high formation energies as compared to those of the vacancies. As a consequence, the formation of vacancy-interstitial pairs in kickout processes would typically require much more energy in c-BN than in h-BN. This suggests that a possible role of the ion bombardment in favoring the growth of c-BN films is to generate a much larger amount of defects in the hexagonal phase than in the cubic phase.