Substitutional Oxygen as the Origin of the 3.5 eV Luminescence in Hexagonal Boron Nitride

Although point defects in hexagonal boron nitride (hBN) exhibiting single-photon emission attract considerable interest, a broader understanding of defect physics and chemistry in hBN remains limited, potentially hindering further development. Oxygen is among the most common impurities in hBN, and numerous studies have reported a pronounced photoluminescence (PL) band centered near 3.5 eV following oxygen incorporation, yet its microscopic origin has remained unresolved. Here, we demonstrate that this emission originates from hole capture by neutral oxygen substituting for nitrogen (ON), which acts as a recombination center under optical excitation. The transition mechanism is nontrivial, involving not only a change in charge state but also a substantial structural reconfiguration: while the positively charged defect adopts a high-symmetry in-plane geometry, the neutral state stabilizes in a low-symmetry configuration characterized by out-of-plane displacements of the oxygen atom and its neighboring boron atoms. Strong lattice relaxation accompanying hole capture lowers the emission energy well below the zero-phonon line energy of 5.09 eV. The calculated luminescence spectrum forms a broad emission band with a maximum at 3.66 eV and a full width at half-maximum (FWHM) of 0.69 eV, in excellent agreement with the experimentally observed peak position and line shape in oxygen-doped hBN.