Radiative lifetime of free excitons in hexagonal boron nitride

TL;DR

This study measures the radiative lifetime of free excitons in hexagonal boron nitride using a novel time-resolved cathodoluminescence system, revealing a surprisingly short lifetime and high luminescence efficiency.

Contribution

First direct estimate of free exciton radiative lifetime in hBN at room temperature using a new experimental setup.

Findings

Radiative lifetime of indirect excitons in hBN is 27 ns.

hBN exhibits high luminescence efficiency despite being an indirect bandgap material.

Shorter lifetime explained by electron-hole proximity and small energy gap.

Abstract

Using a new time-resolved cathodoluminescence system dedicated to the UV spectral range, we present a first estimate of the radiative lifetime of free excitons in hBN at room temperature. This is carried out from a single experiment giving both the absolute luminescence intensity under continuous excitation and the decay time of free excitons in the time domain. The radiative lifetime of indirect excitons in hBN is equal to 27 ns, which is much shorter than in other indirect bandgap semiconductors. This is explained by the close proximity of the electron and the hole in the exciton complex, and also by the small energy difference between indirect and direct excitons. The unusually high luminescence efficiency of hBN for an indirect bandgap is therefore semi-quantitatively understood.