Bernal Boron Nitride Crystals Identified by Deep-Ultraviolet Cryomicroscopy.

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.