We propose that the carbon dimer defect in hexagonal boron nitride gives rise to the ubiquitous narrow luminescence band with a zero-phonon line of 4.08 eV (usually labeled the 4.1 eV band). Our first-principles calculations are based on hybrid density functionals that provide a reliable description of wide band-gap materials. The calculated zero-phonon line energy of 4.3 eV is close to the experimental value, and the deduced Huang-Rhys factor of S≈2.0, indicating modest electron-phonon coupling, falls within the experimental range. The optical transition occurs between two localized π-type defects states, with a very short radiative lifetime of 1.2 nanoseconds, in very good accord with experiments.
@article{arxiv.1907.02303,
title = {Carbon dimer defect as a source of the 4.1 eV luminescence in hexagonal boron nitride},
author = {Mazena Mackoit-Sinkeviciene and Marek Maciaszek and Chris G. Van de Walle and Audrius Alkauskas},
journal= {arXiv preprint arXiv:1907.02303},
year = {2019}
}