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

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 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. arXiv:1806.06201v2 [cond-mat.mtrl-sci]

Exciton-Phonon Interaction and Relaxation Times from First Principles

“…Figure 2(a) shows the exciton band structure, defined here as the exciton energy versus momentum dispersion curves, along a high-symmetry line for the lowest four exciton bands (numbered in order of increasing energy). Overlaid to the exciton band structures are the ex-ph relaxation times at 77 K. Our exciton band structure agrees well with previous results [65,66], apart from a small rigid energy shift; the global minimum is located close to a point called here Q, the halfway point between Γ and K, which corresponds to the excitation across the indirect electronic band gap of h-BN. Note also that in our calculation the degeneracy between the third and fourth exciton bands at Γ is lifted due to the inclusion, different from Refs.…”

Boron nitride for excitonics, nano photonics, and quantum technologies

“…This led to several theoretical calculations for hBN monolayers and bulk crystals. They have been of increasing complexities with time passing [49][50][51][52][53]. Intentional substitutional doping of hBN with foreign atoms is very much challenging [54], incorporation of C, Si, and Mg have been attempted with moderate fortunes [55].…”

“…The determination of this AA′ stacking was far from trivial as there exists a lot of very close hexagonal polytypes (see Figure 3) that correspond to very similar values of the in-plane lattice parameter a and slightly different values of c [65][66][67]. These polytypes will have different band structures for electrons [65] and phonons and different excitonic energies [53]. We will restrict here our article to the AA′ stacking or to monolayers, the cubic wurtzitic and rhomboedral phases will not be discussed either.…”

“…These bands were not clearly seen in ref. [108] as they are not so robust with T compared to the other ones and it has been argued that they are probably associated to lattice vacancies or incorporation of atoms in antisite positions [53,112].…”

“…The issue is the doping which still remains routinely unsolved. One important result is the residual p-type doping of hBN [122], most probably due to native defects [53,60].…”