Hexagonal boron nitride (h-BN) is one of the most promising candidates for light emitting devices in the far UV region, presenting a single strong excitonic emission at 5.8 eV. However, a single line appears only in extremely pure monocrystals that can hardly be obtained only though complex synthesis processes. Common h-BN samples present more complex emission spectra that have been generally attributed to the presence of structural defects. Despite a large number of experimental studies up to now it was not possible to attribute specific emission features to well identify defective structures. In this work we address this fundamental questions by adopting a theoretical and experimental approach combining few nanometer resolved cathodoluminescence techniques with high resolution transmission electron microscopy images and state of the art quantum mechanical simulations.
We show that emission spectra are strongly inhomogeneus within individual flakes with emission peaks close to the free exciton appearing at lines crossing the flakes. Complementary investigations through high resolution transmission electron microscopy allow to associate these emission lines with extended crystal deformation such as stacking faults and folds of the planes. Finally, by means of ab-initio calculations in the framework of Many Body Perturbation Theory (GW approximation and Bethe-Salpeter equation) we provide an in-depth description of the electronic structure and spectroscopic response of bulk hexagonal boron nitride in the presence of these extended morphological modifications. In particular we demonstrate that, in a good agreement with the experimental results, additional excitons can be lighten up at crystal stacking faults.