SLOW RESPONSE AND EU-2+-RELATED TRAPPING STATES IN CAS-EU THIN-FILM ELECTROLUMINESCENT DEVICES

In order to clarify causes of the slow buildup of the optical output, i.e., slow optical response, in red-emitting CaS:Eu thin-film electroluminescent devices, Eu2+ concentration and temperature dependence of the response time of transferred electrons were measured. It was found that the response time increased exponentially with the increase in Eu2+ concentration and temperature. To understand these results, trapping states in CaS:Eu phosphor layer were studied by measuring temperature dependence of the following electro-optical characteristics with Eu2+ concentration as a parameter; (i) decay profile of trapped electrons, (ii) photoluminescent intensity, and (iii) photoinduced charge density. From these measurements, it was found that there existed two kinds of shallow states in the band gap of CaS. One was electron trapping states located at approximately 0.15 eV below the conduction-band minimum. These states were considered to be generated by Eu2+ clusters. The other was located at 0.06-0.08 eV below the conduction-band minimum. These states were considered to be 5d-excited states of isolated Eu2+ activators. Based on these experimental results, an energy band model for CaS:Eu thin film was proposed and causes for the slow optical response were discussed.