Recombination processes in Yb-activated ZnS

Abstract

The occurrence of Yb ions in two charge states (trivalent and divalent) in ZnS is proved with use of the photostimulated electron-paramagnetic-resonance technique. ${\mathrm{Yb}}^{2+}$ is found to form a deep hole-trap center with energy level located at 1.65\ifmmode\pm\else\textpm\fi{}0.05 eV below the bottom of the ZnS conduction band. Efficient ${\mathrm{Yb}}^{3+}$ intrashell luminescence is observed under excitation in the region of the valence band to ${\mathrm{Yb}}^{2+}$ photoionization transition. Indirect recombination of a deep Yb-bound exciton with energy transfer to the 4f shell of ${\mathrm{Yb}}^{3+}$ is proposed to explain the photoluminescence excitation spectrum. Tentative assignment of the observed broad emission band with the maximum at 10 800 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ to direct Yb-bound excitation recombination is suggested.