Determination of the optical bandgap of the Bernal and rhombohedral boron nitride polymorphs
Adrien Rousseau, M. Moret, Pierre Valvin, W. Desrat, Jiahan Li, Eli Janzen, Lianjie Xue, James H. Edgar, Guillaume Cassabois, Bernard Gil
Abstract
We report a study of polymorphic boron nitride (BN) samples. We interpret the photoluminescence (PL) line at $6.032\ifmmode\pm\else\textpm\fi{}0.005\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ that can be recorded at 8 K in $s{p}^{2}$-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 $d{X}_{b}=6.032\ifmmode\pm\else\textpm\fi{}0.005\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ is higher than the indirect bandgap of hBN $(i{X}_{h}=5.955\ifmmode\pm\else\textpm\fi{}0.005\phantom{\rule{0.16em}{0ex}}\mathrm{eV})$.