Direct and indirect excitons in boron nitride polymorphs: a story of atomic configuration and electronic correlation

TL;DR

This study investigates how atomic stacking and electron-hole interactions influence the electronic and excitonic properties of hexagonal boron nitride in various configurations using advanced ab initio methods.

Contribution

It provides a detailed analysis of excitonic dispersion and the impact of stacking sequences on the electronic gap in hBN, highlighting the importance of electron-hole interactions.

Findings

Transition from direct to indirect band gap from monolayer to bulk

Electron-hole interaction flattens exciton dispersion

Lowest exciton in AB stacking remains direct despite indirect band gap

Abstract

We compute and discuss the electronic band structure and excitonic dispersion of hexagonal boron nitride (hBN) in the single layer configuration and in three bulk polymorphs (usual AA' stacking, Bernal AB, and rhombohedral ABC). We focus on the changes in the electronic band structure and the exciton dispersion induced by the atomic configuration and the electron-hole interaction. Calculations are carried out on the level of \textit{ab initio} many-body perturbation theory (GW and Bethe Salpeter equation) and by means of an appropriate tight-binding model. We confirm the change from direct to indirect electronic gap when going from single layer to bulk systems and we give a detailed account of its origin by comparing the effect of different stacking sequences. We emphasize that the inclusion of the electron-hole interaction is crucial for the correct description of the momentum-dependent dispersion of the excitations. It flattens the exciton dispersion with respect to the one obtained from the dispersion of excitations in the independent-particle picture. In the AB stacking this effect is particularly important as the lowest-lying exciton is predicted to be direct despite the indirect electronic band gap.