Atomic structure of carbon centres in hBN: towards engineering of single photon sources.

Revealing atomic and electronic structure of quantum defects in carbon doped hexagonal boron nitride (hBN:C) is crucial for the development of future technology to integrate them within solid state devices. Here, we investigate atomically thin hBN:C films deposited on graphite substrates via scanning tunnelling microscopy and spectroscopy. We observe positively charged defect centres which lower the onset of differential conductance below the conduction band of the host material and induce narrow resonances in the mid-gap regime of the tunnelling spectra. We corroborate atomically-resolved lattice structure of defect sites and their mid-gap states observed in tunnelling dI/dV spectra with the predictions of the atomic and electronic structure of carbon impurities in hexagonal boron nitride by density functional theory. Our findings demonstrate that carbon substitution for boron constitutes a stable and energetically favourable impurity. Its unique properties, when analysed comparatively with other plausible defect centres, provide a good agreement with our experimental observations.