The pseudopotential and super-cell methods were used to study the optical absorption of graphite-like hexagonal boron nitride involving deep levels of N vacancies and their clusters. The impurity-induced absorption was shown to be mainly related to electron transitions between the states that were anti-symmetrical with respect to the horizontal plane. Therefore, this absorption was highly anisotropic and was maximum for light waves polarized normally to the hexagonal axis. The optical absorption and photoconductivity spectra before and after neutron irradiation and thermal treatment were interpreted, and the activation energies for the thermoluminescence and conductivity of N-depleted boron nitride before and after fast-neutron irradiation and vacuum annealing were found.

Optical Absorption of Hexagonal Boron Nitride Involving Nitrogen Vacancies and Their Complexes. S.N.Grinyaev, F.V.Konusov, V.V.Lopatin, L.N.Shiyan: Physics of the Solid State, 2004, 46[3], 435-41