Phonon assisted light absorption and emission in cubic-Boron Nitride

TL;DR

This study uses advanced first-principles calculations to reveal that phonon-assisted processes dominate optical absorption and emission in cubic boron nitride, clarifying experimental observations.

Contribution

It introduces a comprehensive first-principles approach combining GW, Bethe-Salpeter, and exciton-phonon coupling to explain optical spectra in cBN.

Findings

Phonon-mediated transitions dominate optical spectra.

Reconciliation of theoretical gap (~11 eV) with experimental emission (6-7 eV).

Highlights importance of exciton-phonon interactions in wide-bandgap materials.

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 ($\simeq$ 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.