Direct Observation of Optically Active Mid-Gap Electronic States in Hexagonal Boron Nitride by Electron Spectroscopy

Optically active point defects in wide-bandgap semiconductors have been demonstrated to be attractive for a variety of quantum and nanoscale applications. In particular, color centers in hexagonal boron nitride (hBN) have recently gained substantial attention owing to their spectral tunability, brightness, stability, and room-temperature operation. Despite all of the recent studies, precise detection of the defect-induced mid-gap electronic states (MESs) and their simultaneous correlations with the observed emission in hBN remain elusive. Directly probing these MESs provides a powerful approach toward atomic identification and optical control of the defect centers underlying the sub-bandgap emission in hBN. Combining optical and electron spectroscopy, the existence of mid-gap absorptive features is revealed at the emissive sites in hBN, along with an atom-by-atom identification of the underlying defect configuration. The atomically resolved defect structure, primarily constituted by vacancies and carbon/oxygen substitutions, is further studied via first-principles calculations, which support the correlation with the observed MESs through the electronic density of states. This work provides a direct relationship between the observed visible emission in hBN, the underlying defect structure, and its absorptive MESs, opening venues for atomic-scale and optical control in hBN for quantum technology.