Defect Engineering in Hexagonal Boron Nitride: Optical Properties of Stable Defect Complexes Arising from Boron Interstitials

Hexagonal boron nitride (hBN) is a wide-band-gap semiconductor that is promising as a host material for solid-state quantum technologies through defect engineering. It has been shown that boron atoms can be removed from the lattice upon irradiation by electrons or light ions, creating boron vacancies and boron interstitials. While the optical properties of boron-vacancy-derived defects have been studied extensively, little is known about the optical properties of boron-interstitial-derived defects. In this work, we use state-of-the-art first-principles calculations to predict the electronic and optical properties of boron interstitials (B_int) and defect complexes comprising B_int and substitutional carbon impurities at boron and nitrogen sites (C_B and C_N). These carbon impurities can be present in as-grown hBN and can also be introduced intentionally. We demonstrate that these complexes are expected to be stable at room temperature. Our GW-Bethe-Salpeter equation (BSE) calculations show that B_int-C_B and B_int-C_N have low-energy optical transitions that are isolated in energy, making them suitable as single-photon emitters. Together with constrained density functional theory calculations to capture the red shift due to emission, we predict that B_int-C_B and B_int-C_N have zero phonon lines at ∼2.0 eV and ∼2.6 eV, respectively. Defects involving B_int are likely to be the source of blue emitters recently observed in regions several microns away from ion-irradiated parts of hBN. Our work sheds light on these recent experiments and introduces a fresh perspective to the field of quantum emitters in hBN─we show that defects related to B_int are potential single-photon emitters that can be intentionally created in hBN.