Polarization of the Self-Activated Luminescence in ZnS:Al Crystals

Abstract

The polarization characteristics of the blue self-activated luminescence in a cubic ZnS:Al (or Ga) single crystal grown from the melt were investigated. The polarization with particular azimuthal dependences is observed only when the polarized characteristic excitation is given. This indicates that the center has its own characteristic symmetry which is lower than that of the host lattice. The polarization spectra show two maxima with positive values and a minimum with negative values. The results are interpreted according to the associated center model proposed by Prener and Williams, as in the case of the self-activated luminescence of ZnS:Cl, i.e. the model that the center is a zinc ion vacancy associated with an aluminum ion at one of the nearest neighbor cation sites. Discussions on the luminescence transition are given on the basis of group theory. The luminescence is ascribed to a dipole lying in the mirror plane of the center (|| dipole) with the C_s symmetry.