Effects of isotopic purification and stacking modification on optical properties of sp² boron nitride

Juliette PLO

Hexagonal boron nitride (hBN) is a wide indirect bandgap semiconductor, around 6 eV, exhibiting emission properties in the deep-UV range that could be useful for developping efficient UV-LEDs, for which III-nitrides semiconductors are ones of the most promising candidates. On the other hand, hBN is also part of the two-dimensional materials family, such as grahene or transition metal dichalognneides. (TMDs). Due to its lamellar structure, several stackings of hBN exist, called sp²- polytypes : for instance AA’, AB, AA or ABC stackings. In the early 2000s, the first high-quality AA’-hBN crystals have been obtained by Taniguchi & Watanabe, leading to a large number of fundamental studies to understand its intrinsic properties, in particular the demonstration of its indirect band gap, resulting in surprisingly efficient phonon-assisted light emission. Then, studying the impact of stacking on the intrinsic properties of hBN proved to be also a topic of interest for the BN community: this results in a band gap shift with stacking of a few meV, i.e., a relative difference compared to AA’-hBN of the order of %. Nevertheless, ABC stacking, also known as the rhombohedral phase (rBN), had been poorly studied until then due to a lack of crystals of sufficient quality and size for optical spectroscopy. Thanks to the obtention of rBN crystallites grown at the L2C laboratory in Montpellier, I realized a relatively complete study focused on the crystallographic, vibrational, and optoelectronic properties of rBN, by performing Raman and PL- UV spectroscopies. On the other hand, the extrinsic emission of hBN related to point defects, is also one of the key topics in the BN community, as a large number of fine lines are observed between 1 eV and 4 eV in hBN crystals, the origins of which are poorly understood. So another chapter of my thesis focused on the study of the defect emitting around 4 eV: thanks to isotopic purification, we were able to demonstrate that the emission around 4 eV comes from a carbon dimer, i.e., two carbon atoms substituting a neighboring boron atom and a nitrogen atom.