Coupling of excitons and defect states in boron-nitride nanostructures

TL;DR

This study uses many-body perturbation theory to analyze how defects in hexagonal boron nitride influence optical spectra, revealing defect-related excitons and their impact on luminescence features.

Contribution

It provides a detailed ab initio analysis of defect states and excitons in BN nanostructures, explaining experimental luminescence observations.

Findings

Defect states produce characteristic emission bands around 4 eV.

Bound and free excitons coexist at high defect concentrations.

Defect-related excitons explain luminescence sub-structures.

Abstract

The signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many body perturbation theory. A single BN-sheet serves as a model for different layered BN- nanostructures and crystals. In the sheet we embed prototypical defects such as a substitutional impurity, isolated Boron and Nitrogen vacancies, and the di-vacancy. Transitions between the deep defect levels and extended states produce characteristic excitation bands that should be responsible for the emission band around 4 eV, observed in luminescence experiments. In addition, defect bound excitons occur that are consistently treated in our ab initio approach along with the "free" exciton. For defects in strong concentration, the co-existence of both bound and free excitons adds sub-structure to the main exciton peak and provides an explanation for the corresponding feature in cathodo and photo-luminescence spectra.