Optical properties of pyrolytic boron nitride in the energy range 0.05—10 eV

Optical reflectance studies of the c face of pyrolytic boron nitride were carried out at room temperature in the photon energy range 0.45---10 eV. A Kramers-Kronig analysis was performed to determine the in-plane real (ε₁) and imaginary (ε₂) dielectric functions. Infrared-active transverseoptic phonons were observed at 770 and 1383 cm^-1. Strong impurity- and defect-associated absorption was observed throughout the visible and near-ultraviolet region (1---5 eV) which masks, to some extent, the threshold for absorption across the electronic energy band gap. From ε₂(ω) and the absorption coefficient we estimate the direct band gap between π bands to be 5.2±{}0.2 eV. Interband transitions associated with high joint density of states were observed at 6.10 and 6.85 eV. The structure in ε₂(ω) at energies ω>5 eV is identified with specific transitions predicted by previous energy-band calculations.