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Luminescence efficiency of hexagonal boron nitride

hBN has recently become a strategic material for the fabrication of 2D heterostructures, where it is stacked with graphene or transition metal dichalcogenides. Its optical properties though remain unusual among semiconductors and have been debated for 15 years. hBN indeed exhibits a high luminescence efficiency in the deep ultra-violet despite an indirect electronic structure. In this work [1], we quantitatively determined the luminescence efficiency of high quality hBN crystals, thanks to the calibration of a cathodoluminescence set-up. The internal quantum yield of exciton recombinations in hBN is found as high as 50% at 10K, close to the values observed for direct bandgap semiconductors. Contrary to diamond, the luminescence remains stable up to room temperature in hBN, indicating a higher stability of excitons. Ab-initio calculations under the Bethe Salpeter approach confirms the lowest-energy exciton in hBN is indirect, with a high stability characterized by a 300 meV binding energy. Moreover, the absorption maximum measured at 6.03 eV is attributed to a direct exciton, located only slightly above the indirect one at 5.96 eV, solving the Stoke shift issue in hBN.