Tb-F emission centers in ZnS:Tb,F thin-film electroluminescent devices

The relative amount of fluorine(F) to terbium(Tb), F/Tb, in a ZnS:Tb,F electroluminescent film deposited by rf-sputtering method sensitively varies with Tb concentration and preparation conditions. Values of F/Tb are about 1 for the films with low Tb concentrations below 0.5 at. % or the films annealed at 600 °C. The presence of a TbF3 molecular center is inconceivable except for the nonannealed films with relatively high Tb concentrations. It was clarified that variations of the emission spectra and the decay rate with Tb concentration are attributed to the change of F/Tb in the ZnS film. Furthermore, the surprising result is that the Tb-F center has the better emission efficiency, rather than the TbF3 center. A brightness exceeding 600 ft L was obtained under 1-kHz pulse excitation by adjusting the concentration ratio F/Tb to about 1. The real nature of the Tb-F center as well as the role of F− ions are discussed.