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

Abstract

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 (${\ensuremath{\epsilon}}_{1}$) and imaginary (${\ensuremath{\epsilon}}_{2}$) dielectric functions. Infrared-active transverseoptic phonons were observed at 770 and 1383 ${\mathrm{cm}}^{\ensuremath{-}1}$. 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 ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ and the absorption coefficient we estimate the direct band gap between $\ensuremath{\pi}$ bands to be 5.2\ifmmode\pm\else\textpm\fi{}0.2 eV. Interband transitions associated with high joint density of states were observed at 6.10 and 6.85 eV. The structure in ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ at energies $\ensuremath{\hbar}\ensuremath{\omega}>5$ eV is identified with specific transitions predicted by previous energy-band calculations.