We study the deep-ultraviolet emission in Bernal boron nitride as a function of temperature. The quasidegeneracy of indirect and direct excitons in Bernal boron nitride leads to their simultaneous recombination, allowing a comparison of phonon-assisted broadening in the two cases. Temperature-dependent measurements reveal that below 200 K, the efficiency of phonon-assisted broadening is one order of magnitude lower for the direct transition at 6.035 eV than for the phonon replicas of the indirect exciton. This striking effect results from the inhibition of quasielastic acoustic phonon scattering in the strong-coupling regime of the light-matter interaction, where the density of final states is reduced by the curvature of the polaritonic dispersion.

The sp2-bonded layered compound boron nitride (BN) exists in more than a handful of different polytypes (i.e., different layer stacking sequences) with similar formation energies, which makes obtaining a pure monotype of single crystals extremely tricky. The co-existence of polytypes in a similar crystal leads to the formation of many interfaces and structural defects having a deleterious influence on the internal quantum efficiency of the light emission and on charge carrier mobility. However, despite this, lasing operation was reported at 215 nm, which has shifted interest in sp2- bonded BN from basic science laboratories to optoelectronic and electrical device applications. Here, we describe some of the known physical properties of a variety of BN polytypes and their performances for deep ultraviolet emission in the specific case of second harmonic generation of light.

The presence of metastable Bernal stacking boron nitride is verified by combining second harmonic generation (SHG) and photoluminescence (PL) spectroscopy. The scanning confocal cryomicroscope, operating in the deep-ultraviolet range, shows a one-to-one correlation between inversion symmetry breaking probed by SHG and the detection of an intense PL line at ∼6.035 eV, the specific signature of the noncentrosymmetric Bernal stacking. The coherent character of the Bernal phase in boron nitride crystals is demonstrated by two-photon excitation spectroscopy. Direct and indirect excitons are simultaneously detected in the emission spectrum; they are quasi-degenerate, in agreement with theoretical predictions for Bernal boron nitride. The transition from AA′ to AB stacking is characterized by an intense emission from stacking faults at the grain boundaries of hexagonal and Bernal boron nitride crystals.

Boron Nitride (BN) layers with sp2 bonding have been grown by Metal Organic Chemical Vapor Deposition (MOCVD) on AlN underlayers themselves deposited on c-plane sapphire substrates. Two different boron precursors were employed: trimethylboron (TMB) and triethylboron (TEB) while ammonia was used as the nitrogen precursor. The BN obtained epitaxial BN films contain ordered rhombohedral (rBN) and partially ordered turbostratic (tBN) stackings as evidenced by Xray Diffraction analysis. We discriminatively identify the PL signatures of the rBN and tBN from those typical of the hexagonal (hBN) and Bernal stackings (bBN). The optical signature of tBN appears at 5.45eV and it intercalates between the two recombination bands typical of rBN at 5.35 eV ( strong intensity) and 5.55 eV(weaker intensity) . The analogs of the high intensity band at 5.35 eV in rBN sit at 5.47 eV for hBN and at 5.54 eV for bBN.

The nature of point defects in hexagonal boron nitride (hBN) is of current interest for the potential to alter its optical and electrical properties. The strong interaction between neutrons and the boron-10 isotope makes neutron irradiation a controllable way to introduce point defects in hBN. In this study, we perform Raman spectroscopy, photoluminescence, electron paramagnetic resonance (EPR), and optically detected magnetic resonance (ODMR) characterization of neutron-irradiated monoisotopic (hBN with a single boron isotope) 10 B-and 11 B-enriched hBN crystalline flakes and a pyrolytic BN (pBN) reference sample. In h 10 BN and pBN, neutron irradiation produced two new Raman bands at 450 and 1335 cm −1 , which could be associated with B-related vacancies or defects. The near-bandedge optical emission was also significantly impacted by the neutron irradiation. EPR measurements clarified the origin of a high-spin defect center due to negatively charged boron vacancies, which was recently reported for similar neutron-irradiated hBN crystals. The ODMR experiments further confirmed this assignment. High-temperature annealing partially recovered some of the hBN vibrational and optical properties. Our results are helpful to identify the nature of defects in hBN and enable defect-engineered applications such as quantum information and sensing.