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.