Deep europium-bound exciton in a ZnS lattice

Eu-related recombination processes in ZnS are discussed on the basis of electron-spin-resonance (ESR) and optical studies. The absence of any Eu²⁺ and/or Eu³⁺ intra-ion emissions is explained as a consequence of the midgap position of Eu²⁺ in ZnS. A new Eu-related infrared emission is observed and attributed to a bound-exciton (BE) recombination. In the Eu-bound exciton the hole is strongly localized on the 4f shell of Eu, whereas the electron is either delocalized on the 12 nearest-neighbor Zn-cation sites (for isolated Eu) or trapped at a compensating ion (for Eu complexes). The BE dissociation energy is determined to be about 10 meV.