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±0.0050.16em0exeV that can be recorded at 8 K in sp²-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 dXb=6.032±0.0050.16em0exeV is higher than the indirect bandgap of hBN (iXₕ=5.955±0.0050.16em0exeV).