Determination of the optical bandgap of the Bernal and rhombohedral boron nitride polymorphs

We report a study of polymorphic boron nitride (BN) samples. We interpret the photoluminescence (PL) line at 6.032 +/- 0.005 eV that can be recorded at 8 K in sp(2)-bonded BN as being the signature of the excitonic fundamental bandgap of the Bernal BN (bBN) [or graphitic BN (gBN)] polymorph. This is determined by advanced PL measurements combined with x-ray characterizations on pure hexagonal BN (hBN) and on polymorphic crystal samples, later compared with the theoretical predictions of Sponza et al., [Phys. Rev. B 98. 125206 (2018)]. The overall picture is consistent with a direct excitonic fundamental bandgap of the bBN (or gBN) polymorph. This value dX(b) = 6.032 +/- 0.005 eV is higher than the indirect bandgap of hBN (iX(h) = 5.955 +/- 0.005 eV).

Automated summary

The study reports on the properties of different polymorphic boron nitride samples based on photoluminescence measurements. It shows that the Bernal BN or graphitic BN polymorph has a higher excitonic fundamental bandgap compared to hexagonal BN.