Artús, L., Feneberg, M., Attaccalite, C., Edgar, J.H., Li, J., Goldhahn, R. and Cuscó, R. (2021), Ellipsometry Study of Hexagonal Boron Nitride using Synchrotron Radiation: Transparency Window in the Far-UVC. Adv. Photonics Res.. Accepted Author Manuscript 2000101.


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 have been characterized with the aid of ab initio self-consistent GW calculations and the optical properties have been calculated using the Bethe-Salpeter equation. The dispersion of the ordinary refractive index in the visible and ultraviolet 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 ?? ? ??* transitions. As a consequence of the unique electronic bandstructure of h-BN, a transparency window is observed in the far ultraviolet 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.

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