Dc electroluminescence in copper-free ZnS:Mn thin films. II. A dielectric breakdown theory of instability

Abstract

The occurrence of localized destructive breakdown (LDB) is a feature of direct coupled thin-film electroluminescence (DCTFEL) which has been attributed in the past to variations in applied field or threshold voltage caused by geometrical artifacts introduced during film deposition. However, in a companion paper it has been reported that this view of LDB is inconsistent with observations; instead there are strong indications that both LDB and DCTFEL are natural consequences of dielectric breakdown in ZnS:Mn. In this paper, the O’Dwyer theory of dielectric breakdown is applied to DCTFEL in ZnS:Mn with the aid of a simple computational equilibrium model. It is found that with reasonable assumed values for basic electronic parameters, current-controlled negative resistance (CCNDR) is expected in the normal DCTFEL current density regime. Localized (filamentary) conduction is, of course, a well-known consequence of CCNDR and the profound implications of this for EL are explored in some detail. It becomes clear that a number of poorly understood features of practical DCEL devices arise quite naturally from this dielectric breakdown theory; examples are premature brightness saturation at near maximum efficiency, the coexistence of DCTFEL and LDB, and the factors affecting the balance of that coexistence. It is concluded that while the dielectric breakdown model of DCTFEL presently rests precariously on assumed parameter values, the qualitative predictive power of the theory gives it significant weight. In view of this, some of the implications for DCTFEL device design are explored.