Phonon assisted light absorption and emission in cubic-Boron Nitride

TL;DR

This study integrates GW quasiparticle corrections, Bethe–Salpeter equation excitons, and explicit exciton–phonon coupling to model phonon-assisted absorption and luminescence in cubic boron nitride (cBN). The results show that phonon-mediated transitions dominate both absorption and emission, shifting the observable absorption onset from the direct-gap estimate near $11$ eV down toward $9.8$–$10.0$ eV, and predicting intrinsic luminescence near $5.57$–$5.60$ eV. Although exciton–phonon interactions are crucial, they do not fully bridge the gap between theory and experiment, suggesting that defect states or phase inhomogeneities (e.g., hBN inclusions) contribute to experimentally observed features around $6$ eV. Overall, the work provides a unified, first-principles framework for interpreting wide-bandgap materials where strong exciton–phonon coupling shapes optical spectra, with implications for BN-based optoelectronics and spectroscopic characterization.

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.

Figures (5)

• Figure 1: In the top panel, we show the electronic band structure calculated at the Kohn-Sham (KS) (dashed red line) and G$_0$W$_0$ (solid blue line) approximation. In the same panel, we also show the lowest phonon-assisted transitions, with the green (phonon-emission) and orange (photon-emission) arrows. In the bottom panel, we show the phonon bandstructure, and we indicate in red the phonons at ${\bf q}=X$ that are responsible for the lowest phonon-assisted transitions between valence and conduction bands.
• Figure 2: Optical absorption of cBN calculated at the Bethe-Salpeter equation (BSE, solid blue line) and independent particle approximation (IPA, dot-dashed orange line) levels. In the figure, we have indicated the first two peaks of the spectrum with A and B. Results are compared with experimental measurements from Ref. pouch1990synthesis. In the same figure, we also report the direct KS (black dashed line) and the direct and indirect G$_0$W$_0$ gaps (red dash-doted lines and dotted lines).
• Figure 3: Excitonic dispersion of cBN for the first lowest 20 excitons along the high-symmetry points of the Brillouin Zone. Excitonic bands are interpolated using a smooth Fourier transform.interpolation We report also the position of the 3 times degenerate exciton states responsible for the first absorption peak $A$ at the $\Gamma$ point (see Fig. \ref{['bse_dbgrid']}) and the the direct KS (black dashed line) and the direct and indirect G$_0$W$_0$ gaps (red dash-doted lines and dotted lines).
• Figure 4: Total phonon-assisted luminescence of cBN (continuous line) and the contribution of the different phonon modes at $q=X$: LO branch (red loosely dashed line), LA branch (green dotted line), TO branch (orange dot-dashed line), TA branch (blue dashed line).
• Figure 5: Low energy tail of the optical absorption of cBN at different levels of approximation: independent particle approximation(IPA), Bethe-Salpeter (BSE), and Bethe-Salpeter plus exciton--phonon coupling (BSE-EXCPH). Results are compared with experimental measurements from Ref. pouch1990synthesis.