Control of point defects and space charge in electroluminescent ZnS:Mn thin films

Abstract

The mechanisms leading to improved brightness, efficiency, and stability of alternating-current thin-film electroluminescent (ACTFEL) ZnS:Mn phosphors have been studied. Previously we have shown that ex situ codoping of the sputter deposited ZnS:Mn active layer with K and Cl results in a 53% improvement in brightness, a 62% improvement in efficiency, and better 100 h accelerated aging stability. In this work, we demonstrate that these improvements result from a 75% increase in excitation efficiency for conduction electrons, combined with a small decrease in both light outcoupling and nonradiative recombination. Electrical properties data were used to determine that there is a reduced amount of static space charge in the codoped films, resulting in a larger average field, increased excitation efficiency, and increased charge multiplication. The reduced space charge is attributed to the addition of charge compensating zinc vacancy–chlorine complexes and isolated chlorine point defects, which are acceptor and donor defects, respectively, and a reduction of zinc vacancy deep hole traps. It is postulated that higher average fields results in sufficient electron multiplication or donor ionization such that current for EL excitation is limited by the phosphor resistance rather than capacitance or density of interface states. The possibility of using these mechanisms to increase the efficiency of other ACTFEL phosphors is discussed.