Near band-gap photoluminescence properties of hexagonal boron nitride

Near band-gap luminescence (hν≥5 eV) of hexagonal boron nitride has been studied by means of the time- and energy-resolved photoluminescence spectroscopy method. Two emissions have been observed at 5.5 and 5.3 eV. The high-energy emission at 5.5 eV is composed of fixed subbands assigned to bound excitons at 5.47, 5.56, and 5.61 eV. The nonstructured low-energy emission at 5.3 eV undergoes a large blueshift (up to 120 meV) with a linear slope ΔE_lum/ΔE_exc≪1 with increasing excitation energy E_exc. At E_exc≥5.7 eV, the band position is fixed and marks the transition from the Raman to the photoluminescence regime. We assign the 5.3 eV band to quasi-donor-acceptor pair (q-DAP) states due to electrostatic band fluctuations induced by charged defects. The shift is explained by photoinduced neutralization of charged defect states. The absence of contribution to the q-DAP luminescence from exciton suggests the existence of a large exciton binding energy, which is qualitatively consistent with theoretical predictions.