Correlation between the Two Charge States of Zinc Vacancies and Deep-Level Emissions in ZnO

The debate surrounding defect research in ZnO has significantly hindered its widespread application. The specific transitions and origins of deep-level emissions, such as green luminescence (GL, peaking at 2.45 eV) and yellow luminescence (YL, peaking at 2.16 eV), remain unresolved. Traditional energy band models often mistakenly correlate emission peak energies with transition energy levels, neglecting lattice relaxation effects in deep-level transitions, which complicates the identification of defect origins. By combining the configuration coordinate diagram with first-principles calculations, the transitions between two charged states of zinc vacancies are illustrated. The GL and YL can be attributed to the thermodynamic transitions between these two charge states (−1/0) and (−2/–1) of zinc vacancies, respectively. Furthermore, this assignment coherently explains abnormal phenomena observed in photoluminescence, such as negative thermal activation energy, blue shift with temperature, and subthreshold excitation. These findings not only shed light on the origins of typical deep-level emissions but also provide new insights into the characteristics and transition processes of defect luminescence.