Radiative lifetime of free excitons in hexagonal boron nitride

Using a time-resolved cathodoluminescence system dedicated to the UV spectral range, we present an estimate of the radiative lifetime of free excitons in hexagonal boron nitride (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 band gap 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 band gap is therefore semiquantitatively understood.