D.C. Electroluminescence in zinc sulphide and related compounds

For a considerable time, the study of electroluminescence in zinc sulphide was largely confined to the a.c. properties. Although the d.c. properties offer much greater scope for the eventual elucidation of the actual mechanism taking place, these were largely neglected. It has been demonstrated that with careful control of the material, high brightness and good efficiency are obtained. The electroluminescent phosphors showing these properties consist of fine powders of pure zinc sulphide as the host lattice, this is uniformly doped with manganese and coated with a copper rich phase. In the present review we will outline the development of d.c. electroluminescent phosphor and describe the recent developments which have improved our knowledge of the processes taking place. It will be shown, using copper coated powders and manganese activators, that a change in the host lattice to zinc selenide or even to gallium sulphide still yields phosphors showing electroluminescent processes; while the replacement of Mn by rare earth or other transition metals gives a variety of emission spectra from zinc sulphide. The future potential of ZnS in useful devices depends on two other factors; the life under operating conditions and the light emission under pulsed excitation. It will be demonstrated that contrary to various predictions, good maintenance in the ZnSMnCu system is possible and the recent work of Werring and Mears will be used to illustrate the high brightness characteristics that can be obtained at low duty cycle.