Time-resolved photoluminescence in band-edge region of hexagonal boron nitride single crystals

We studied time-resolved photoluminescence spectra of hexagonal boron nitride single crystals grown using a temperature gradient method under high pressure and high temperature. Decay curves for each luminescence band near the bandgap energy were observed separately by using a prism double monochromator as a variable bandpass filter. The most luminous band of 215 nm, which can be assigned as free exciton luminescence, shows a very short decay time of about 70 ps. However, the 227-nm band, which is assigned as bound exciton luminescence because of stacking disorders, shows a relatively slow decay of a few ns. The short lifetimes, ranging from ns to ps, and the high luminous intensities of these bands are specific properties for direct exciton luminescence rather than ones for large momentum phonon assisted luminescence. (C) 2008 Elsevier B.V. All rights reserved.