Hexagonal BN (h-BN) is attracting a lot of attention for two-dimensional electronics and as a host for single-photon emitters. We study the properties of native defects and impurities in h-BN using density functional theory with a hybrid functional. Native vacancy and antisite defects have high formation energies, and are unlikely to form under thermodynamic equilibrium for typical growth conditions. Self-interstitials can have low formation energies when the Fermi level is near the band edges, and may form as charge compensating centers; however, their low migration barriers render them highly mobile, and they are unlikely to be present as isolated defects. The defect chemistry of h-BN is most likely dominated by defects involving carbon, oxygen, and hydrogen impurities. Substitutional carbon and oxygen, as well as interstitial hydrogen and boron vacancy-hydrogen complexes, are low-energy defects in h-BN. Based on our results, we can rule out several proposed sources for defect-related luminescence in h-BN. In particular, we find that the frequently observed 4.1 eV emission cannot be associated with recombination at CN, as has been commonly assumed. We suggest alternative assignments for the origins of this emission, with CB as a candidate. We also discuss possible defect origins for the recently observed single-photon emission in h-BN, identifying interstitials or their complexes as plausible centers.

Fluorescent defects recently observed under ambient conditions in hexagonal boron nitride (h-BN) promise to open novel opportunities for the implementation of on-chip photonic devices that rely on identical photons from single emitters. Here we report on the room-temperature photoluminescence dynamics of individual emitters in multilayer h-BN flakes exposed to blue laser light. Comparison of optical spectra recorded at successive times reveals considerable spectral diffusion, possibly the result of slowly fluctuating, trapped-carrier-induced Stark shifts. Large spectral jumpsreaching up to 100 nmfollowed by bleaching are observed in most cases upon prolonged exposure to blue light, an indication of one-directional photochemical changes possibly taking place on the flake surface. Remarkably, only a fraction of the observed emitters also fluoresce on green illumination, suggesting a more complex optical excitation dynamics than previously anticipated and raising questions on the physical nature of the crystal defect at play.

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Interaction of a trapped charge with a point defectLet us consider a sheet of h-BN of thickness ݀ (see Fig. S1a); h-BN is a strongly anisotropic material with static dielectric constants ߝ // = 5.0 and ߝ ୄ = 7.0, respectively parallel and perpendicular to the sheet plane. For the sake of simplicity, let us take the value averaged over the three spatial directions: ߝ = 5.7. Let us place a point charge ‫ݍ‬ in the middle of the plane and let us calculate the electric field in the same mid-plane a distance ‫ݎ‬ away from the point charge. Due to symmetry, the electric field is radially symmetric. It can be found using the method of images, and it is given by (in atomic units) ‫ܨ‬ሺ‫ݎ‬ሻ = ‫ݍ‬ ‫ݎߝ‬ ଶ ൝1 + 2 ߛ ሾ1 + ሺ݀ ‫ݎ‬