Ellipsometry Study of Hexagonal Boron Nitride Using Synchrotron Radiation: Transparency Window in the Far‐UVC

Abstract

The linear optical response of the ultrawide bandgap h-BN is investigated by spectroscopic ellipsometry. The ordinary dielectric function of h-BN is determined up to 25?eV on a high-quality single-crystal platelet. The direct bandgap and the high-energy transitions are characterized with the aid of ab initio self-consistent GW calculations and the optical properties are calculated using the Bethe?Salpeter equation. The dispersion of the ordinary refractive index in the visible and UV part of the spectrum below the bandgap is in excellent agreement with previous transmission experiments. A sharp excitonic peak at 6.1?eV associated with the direct bandgap dominates the absorption spectrum, and a second peak is observed at 6.8?eV. At higher energies, a strong absorption peak emerges at 15?eV associated with higher σ???σ* transitions. As a consequence of the unique electronic band structure of h-BN, a transparency window is observed in the far-UV region between 7 and 13?eV, where the light penetration depth reaches a value of 38?nm, as opposed to the value of 0.8?nm at the absorption peak of the fundamental bandgap.