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.

Tuning the Direct and Indirect Excitonic Transitions of h-BN by Hydrostatic Pressure

“…Figure 6 b shows the absorption spectrum at ambient pressure of a 50 nm thick h -BN sample in the spectral region of the direct excitonic transition. The absorption peak is observed around ∼6.12 eV, in reasonable agreement with the values observed in Figure 1 a,b and coincident with previous ellipsometric measurements, 3 thus validating the approach used to extract excitonic transition energies from the reflectance data. Since the calculated electronic transitions are affected by the underestimation of the quasi-particle bandgap and the neglect of vibrational renormalization, 33 a small correction of ∼196 meV independent of pressure has been applied to the theoretical results to match the calculations with the experimental absorption peak at ambient pressure [magenta circle in Figure 6 a].…”

“…As opposed to the large reduction in E gd with pressure, the direct exciton transition is essentially unaffected by hydrostatic pressure. This is due to the fact that the direct exciton is formed by transitions around the K and H points of the Brillouin zone, 3 which are only slightly affected by the external pressure. 28 This different sensitivity to pressure implies that the exciton binding energy decreases significantly with pressure, as shown in Figure 5 b.…”

“… 26 The strong excitonic effects in the optical response were taken into account using the Bethe–Salpeter equation. 27 The calculation parameters are the same as those used in ref ( 3 ). A rigid shift of 196 meV independent of pressure has been applied to the calculated conduction band structure to bring the calculated absorption peak in full accord with the absorption peak determined by synchrotron radiation experiments.…”

“…A rigid shift of 196 meV independent of pressure has been applied to the calculated conduction band structure to bring the calculated absorption peak in full accord with the absorption peak determined by synchrotron radiation experiments. 3 …”

Excitons under strain: light absorption and emission in strained hexagonal boron nitride

“…[35]. The spectra has been shifted to match the position of the iX1 exciton at equilibrium, and compensate the difference between one-shot and self-consistent GW [36]. Dashed lines are just for the eyes.…”