Rare-earth defects and defect-related luminescence in ZnS

Abstract

Structure and energetics of rare-earth (RE) defects and luminescence of RE and related defects in zincblende zinc sulfide (ZnS) are investigated using hybrid density-functional defect calculations. We find that europium (Eu) is stable predominantly as the divalent Eu2+ ion in bulk ZnS. The trivalent Eu3+ ion is structurally and electronically stable but energetically unfavorable compared to Eu2+ due to the presence of low-energy native defects and Eu2+-related defect complexes. Other RE dopants, dysprosium (Dy) and erbium (Er), are stable only as Dy3+ and Er3+, respectively. These results provide an explanation why it is difficult to realize Eu3+ in bulk ZnS. A non-negligible Eu3+/Eu2+ ratio might be achieved with Li co-doping under S-rich (and probably non-equilibrium) synthesis conditions. Optically, Eu-related defects can act as carrier traps for band-to-defect transitions and emit light in the visible range. To assist with experimental optical characterization of the RE defects, we include band-to-defect luminescence involving native defects (Zn vacancies) and/or non-RE impurities (Cu, Cl, and Al) that may also be present in Eu-doped ZnS samples and assign luminescence centers often observed in experiments to specific defect configurations.