Carbon dimers as the source of 4.1 eV luminescence in hexagonal boron nitride

Hexagonal boron nitride (h-BN) is an exciting material for electronics and optoelectronics, as well as for quantum information applications. h-BN samples typically display bright luminescence with a zero-phonon line (ZPL) at 4.1 eV, and single-photon emission associated with this line has been observed. The source of the luminescence has been intensely debated, though there seems to be broad agreement that carbon is involved. We propose that the carbon dimer, C_B–C_N, gives rise to this ubiquitous narrow luminescence band. Such carbon dimers have actually been observed in transmission electron microscopy. Our first-principles calculations, based on hybrid density functional theory, show that the neutral state of the dimer is stable over a wide range of electron chemical potentials. The calculated ZPL energy, Huang-Rhys factor, and radiative lifetime are all close to the experimental values. The optical transition occurs between two localized π-type defect states, with the lower-lying state localized on C_N, while the higher-lying state is localized on C_B. We find the transition to be dipole allowed with polarization in the h-BN plane.