Defect pairing, concept

Several investigators (28, 29, 45, 94) criticized as energetically unfavorable the Bjerrum concept of a valence defect pair formed by rotation of a rigid molecule in an otherwise undisturbed lattice. Alternative mechanisms were proposed. 

The concept of resonance is necessary because the LE model postulates that electrons are localized between a given pair of atoms. However, nature doesn t really operate this way. Electrons are actually delocalized—they can move around the entire molecule. The valence electrons in the N03 molecule distribute themselves to provide equivalent N—O bonds. Resonance is necessary to compensate for this defective assumption of the LE model. However, because this model is so useful, we retain the concept of localized electrons and add resonance to accommodate species like NO,-. 

Based on the concepts of localized recombination of close DAPs with random distribution of separation distance between pair components, postulated in [53], we propose the model of luminescence mechanism in AlN ceramics, which can explain the above-mentioned features revealed by this material in PL, AGL, TL, and OSL. The diagram of the proposed luminescence mechanism is shown in Figure 9.16. This diagram describes processes in the defects, which produce the UV emission band. We believe that luminescence mechanism of the Blue band is similar and includes localized tunnel recombination, though the responsible luminescence centers are not shown in the scheme. 

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