CHEMICAL-BOND AND ELECTRONIC-STRUCTURE ANISOTROPIES IN THE GRAPHITIC AND RHOMBOHEDRAL MODIFICATIONS OF BORON-NITRIDE

The pseudopotential method is used to calculate the band spectra, vale nce charge densities, and electroneutrality levels in the graphitic (h -BN) and rhombohedral (r-BN) modifications of boron nitride. It is sho wn that these compounds are indirect-band insulators with the minimum energies of the indirect (4.65 and 4.8 eV) and direct (5.27 and 5.5 eV ) forbidden zones in h- and r-BN, respectively. The band energies of h -BN correlate well with the experimental data. Both crystals are chara cterized by ionic-covalent chemical bonds. In the plane of hexagonal l ayers, the bonds are more covalent in h-BN. In h- and r-BN, the electr oneutrality levels correspond to the energy of about 1.9 eV over the t op of the valence band which is responsible for the hole conductivity of the substances. In superlattices based on these compounds, the rhom bohedral phase is a quantum well for holes and a barrier for electrons .