Defect energy levels and persistent luminescence in Cu-doped ZnS

TL;DR

This study uses first-principles calculations to analyze defect energy levels in Cu-doped ZnS, revealing how native defects and impurities influence luminescence, especially green and blue emissions, with implications for phosphor design.

Contribution

It provides a detailed theoretical understanding of defect levels and luminescence mechanisms in Cu-doped ZnS, clarifying the roles of various impurities and defect complexes.

Findings

Cu_Zn acts as a deep acceptor, not contributing to p-type conductivity.

Isolated Cu_Zn causes green luminescence in ZnS.

Defect complexes like Cu_Zn-Al_Zn and Cu_Zn-Cl_S lead to blue luminescence.

Abstract

Zinc sulfide (ZnS) based materials are widely used in many applications. Yet, due to a lack of detailed knowledge of defect energy levels, the electrical properties and luminescence mechanisms in the materials still give rise to debate. Here, we report a first-principles study of native point defects and impurities in zincblende ZnS using hybrid density-functional calculations. We find that cation and anion vacancies and antisite defects introduce deep defect levels in the band gap and can act as donors or acceptors depending on the position of the Fermi level. The substitutional impurity Cu$_{\rm Zn}$ acts as a deep acceptor and thus does not contribute to p-type conductivity. Substitutional impurities Al$_{\rm Zn}$ and Cl$_{\rm S}$, on the other hand, are shallow donors. More importantly, we identify the isolated (i.e., unassociated) Cu$_{\rm Zn}$ as a source of the green luminescence observed in ZnS-based phosphors and Cu$_{\rm Zn}$$-$Al$_{\rm Zn}$ and Cu$_{\rm Zn}$$-$Cl$_{\rm S}$ defect complexes as sources of blue luminescence. The materials may have both green and blue emissions with the relative intensity dependent on the ratio between the unassociated defect and defect complex concentrations, which is also consistent with experimental observations.