Electroluminescence and Photoluminescence of Thin Films of ZnS Doped with Rare-Earth Metals

Abstract

It has been shown recently that efficient electroluminescence may be obtained from thin films formed by the coevaporation of ZnS and rare-earth fluorides. In order to assess the importance of the fluorine ions in such systems, films have been produced by coevaporating ZnS and rare-earth metals (Tb, Sm, Nd). It is shown that large concentrations of well-dispersed rare-earth ions may also be obtained by this method. For ZnS: (Tb metal) the concentration which optimizes the electroluminescence power efficiency has about the same value as that for ZnS : TbF3 (2 at. % Tb), and the peak electroluminescence power efficiency is slightly lower than that obtained for ZnS : TbF3. In general, the metal-doped systems yield electroluminescence spectra which differ from their fluoride-doped counterparts at a similar concentration. This is taken as evidence that the fluorine ions remain in the vicinity of the rare-earth ion. In the case of ZnS : EuF3, it is further suggested that the fluorine ions may help to control the valence of the rare-earth ion to yield Eu3+ since independent evidence indicates that in ZnS crystals only Eu2+ is stable. Absolute photoluminescent power efficiency measurements on films of ZnS : (Tb metal) are used to show that a mechanism for electroluminescence based on electron-hole pair formation with subsequent transfer of this energy to the Tb3+ ion may be effectively ruled out; indirect evidence for direct impact excitation of the active ion is thus obtained. For purposes of application, it appears that systems in which the rare-earth ions are introduced by evaporation of the fluorides are superior, since the evaporation of these is simpler and the electroluminescence spectra of such devices are more reproducible.