Phonon-assisted light absorption and emission in cubic boron nitride

Abstract

Cubic boron nitride (cBN) is a wide-bandgap polymorph of boron nitride whose optical response remains only partially understood due to the coexistence of indirect electronic transitions and strong exciton–phonon coupling. Using first-principles Many-Body Perturbation Theory, we investigate the optical properties of cBN by combining GW quasiparticle corrections with Bethe–Salpeter equation calculations of excitonic effects. Phonon-assisted absorption and emission processes are explicitly included through the exciton–phonon coupling formalism. We find that phonon-mediated optical transitions provide a dominant contribution to both absorption and luminescence spectra, partially reconciling the discrepancy between the theoretical optical gap (≈11 eV) and experimental emission around 6–7 eV. Our results demonstrate the importance of including exciton–phonon interactions for the correct interpretation of experimental spectra, offering new insights into light emission in wide-bandgap materials.