Bright luminescence from indirect and strongly bound excitons in hBN

TL;DR

This study demonstrates that bulk hexagonal boron nitride exhibits bright, stable luminescence due to strongly bound indirect excitons with high quantum yield, confirmed by experimental and theoretical analysis, making it promising for optoelectronic applications.

Contribution

It provides the first quantitative analysis of excitonic luminescence efficiency in hBN, revealing the indirect exciton nature and high quantum yield comparable to direct bandgap semiconductors.

Findings

Quantum yield of ~50% at 10 K

Luminescence remains stable up to room temperature

Exciton binding energy of 300 meV

Abstract

A quantitative analysis of the excitonic luminescence efficiency in hexagonal boron nitride (hBN) is carried out by cathodoluminescence in the ultraviolet range and compared with zinc oxide and diamond single crystals. A high quantum yield value of ~50% is found for hBN at 10 K comparable to that of direct bandgap semiconductors. This bright luminescence at 215 nm remains stable up to room temperature, evidencing the strongly bound character of excitons in bulk hBN. Ab initio calculations of the exciton dispersion confirm the indirect nature of the lowest-energy exciton whose binding energy is found equal to 300 meV, in agreement with the thermal stability observed in luminescence. The direct exciton is found at a higher energy but very close to the indirect one, which solves the long debated Stokes shift in bulk hBN.