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Temperature effects in luminescence of associated oxygen-carbon pairs in hexagonal boron nitride under direct optical excitation within 7-1100 K range

We have studied the temperature dependencies of the photoluminescence (PL) intensity of 4.1 eV in microcrystalline powder of hexagonal boron nitride in the range of 7-1100 K. The results obtained have been analyzed within the band model of energy levels of associated donor-acceptor pairs based on impurity (ONCN) complexes. Luminescence enhancement processes at T<200 K and two independent channels of external thermal activation quenching are typical of the observable luminescence mechanisms under direct (4.26 eV) excitations of the samples. It has been shown that, at T>220 K, when directly excited, the samples diminish the PL intensity because of the processes of thermal ionization of the donor level of the ON-center (122 meV) and the deep acceptor level of the CN-center (1420 meV) as parts of the (ONCN)-complex. The temperature enhancement region with an activation energy of 15 meV is due to the decay of a bound Wannier-Mott exciton followed by transfer of excitation to the associated donor-acceptor pair.