Point defects in hexagonal boron nitride. I. EPR, thermoluminescence, and thermally-stimulated-current measurements

When hexagonal boron nitride is exposed to ionizing radiation two types of paramagnetic centers appear: three-boron centers and one-boron centers. Results of electron paramagnetic resonance, thermoluminescence, and thermally-stimulated-current measurements associated with these centers are described, and a model is proposed to explain these results. The model is consistent with views of others investigators on the role of carbon impurities in hexagonal boron nitride, and with the hypothesis that the three-boron centers are $F$ centers. The three-boron centers and one-boron centers were found to introduce trapping levels at 1.0 and 0.7 eV, respectively, below the conduction band. It is suggested that carbon impurities produce luminescence centers, with an energy level at about 4.1 eV below the conduction band. Ionizing radiation frees electrons from these levels into the conduction band. Some of the electrons may then be trapped either in three-boron centers or in one-boron centers. Some may fall back to the centers, thus emitting blue photoluminescence. The unpaired trapped electrons give rise to the EPR signals. When the samples are heated, the electrons escape from the traps (first from the one-boron centers and then from three-boron centers and give rise to the glow curves and at the same time also cause a decrease in the EPR signal. This model was supported by quantum-mechanical defect-model calculations, given in part II of this study (following paper).