Optical properties of hexagonal boron nitride

Optical absorption, reflectivity, and photoconductivity in the near-uv range (1950-3200) of a thin film of hexagonal boron nitride were measured. The main absorption peak was observed at 6. 2 eV. A sharp fall at about 5. 8 eV was attributed to the direct band gap. The temperature dependence of the band gap was found to be less than 4 10^-5 eV/^. Self-consistent tight-binding band-structure calculations were performed on a two-dimensional hexagonal crystal model, using Hamiltonian matrix elements calculated by semiempirical LCAO (linear combination of atomic orbitals) methods. The calculated value for the band gap of hexagonal BN was in reasonably good agreement with the experimental value obtained in the present work, as well as with values reported earlier from electron-energy-loss and photoelectron-emission measurements. The calculations also predicted a very small change in the band gap with temperature, in agreement with the experimental observations.