CALCULATION OF GROUND-STATE AND OPTICAL-PROPERTIES OF BORON NITRIDES IN THE HEXAGONAL, CUBIC, AND WURTZITE STRUCTURES

The electronic structures, the charge-density distribution, and the total energies of boron nitrides (BN) in the hexagonal, cubic, and wurtzite structures are studied by first-principles self-consistent local-density calculations. For the ground-state properties, the band structures, the equilibrium lattice constants, the bulk modulus and their derivatives, and the cohesive energy are in good agreement with other recent calculations and with experimental data. The relative stabilities and possible phase transitions among these three phases are discussed. The linear optical properties of these three crystals are also calculated and compared with the available measurements. For hexagonal BN, all the structures in the electron-energy-loss function as measured by inelastic electron scattering have been reproduced by the calculation. For cubic BN, the calculated dielectric functions is also in good agreement with the reflectance data. For wurtzite-structure BN, no optical data are available for comparison. These results are discussed in the context of crystal structure and bonding in these three crystals. Based on the analysis of the calculated and measured optical data on cubic and hexagonal BN, it is argued that the assessment of the accuracy of the conduction-band states should rely mainly on the reproduction of major structures in the optical-absorption curves rather than on the size of the band gap. The accuracy of the higher conduction-band states as calculated by the local-density theory is strongly energy and momentum dependent. Furthermore, a determination of the optical gap is complicated by the different roles of the direct and indirect transitions, and by the difficult task of extrapolating data to the low-frequency region.