Nature of optical transitions in the charge-transfer region of ZnS:Co and ZnSe:Co

The cobalt center in ZnS:Co and ZnSe:Co has long been considered as a possible candidate for the observation of so-called bound-electron levels. The optical transitions in the vicinity of the charge-transfer transition from the ${\mathit{d}}^6$/${\mathit{d}}^7$ charge-transfer level to the conduction band are therefore investigated by their magneto-optical properties. These clearly reveal the identical origin of the observed transitions in the two materials. With the aid of the known charge-transfer energies the transitions are identified as internal ${\mathit{d}}^7$→${\mathit{d}}^7$* excitations. Possible assignments are discussed within the framework of ligand-field theory, which also delivers an explanation of why similar internal transitions were not yet observed in other II-VI or even III-V compound semiconductors doped with cobalt. A uniform picture of the cobalt center in these semiconductor host materials is thus derived. © 1996 The American Physical Society.