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.

Figure 1

(a) cw-CL spectra at room temperature of hBN crystals grown from three different methods HPHT, APHT, and PDC. The spectra are dominated by the radiative recombination of free excitons at 215 nm in hBN. (b) Temporal decay of the free exciton luminescence after interruption of the incident electron beam at initial time. The CL intensity under continuous excitation is measured simultaneously before the beam interruption. More information on the samples is given in Table 1.

Figure 2

Free exciton lifetime ($\tau$) as a function of the internal quantum yield for the intrinsic luminescence in hBN. The radiative lifetime of hBN indirect excitons is estimated at 27 ns from the linear fit (red curve).

Figure 3

Radiative lifetimes of free excitons in silicon [43], diamond [42], and hexagonal boron nitride (this work) as a function of the average distance between electron and hole in the indirect exciton.