Electronic and optical properties of a hexagonal boron nitride monolayer in its pristine form and with point defects from first principles

While the optical band gap of pristine hexagonal boron nitride (hBN) is about 6 eV, emissions from defects in the visible regime with single-photon characteristics have been observed in experiment for a long time. To tackle the question which kind of defects are responsible for these emissions, many ab initio studies have been conducted. Most of them are on the level of density-functional theory (DFT). While DFT provides an efficient method to obtain band dispersion and optimal structure, it lacks the ability to calculate the quasiparticle energy levels correctly. In this work, we employ an efficient approximation of the $GW$ theory to calculate quasiparticle energy levels of small atomic defects: Two carbon substitutions ${\mathrm{C}}_{\mathrm{N}}$ and ${\mathrm{C}}_{\mathrm{B}}$, the nitrogen vacancy ${\mathrm{V}}_{\mathrm{N}}$, and the divacancy ${\mathrm{V}}_{\text{NB}}$. Optical spectra are calculated by solving the Bethe-Salpeter equation. The defect systems which are examined in this work allow for transitions between intrinsic states of hBN and deep defect states which lie inside the band gap, resulting in bright excitons at $\ensuremath{\sim}2\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$.