ROLE OF EU-2+ LUMINESCENT CENTERS IN THE ELECTROOPTICAL CHARACTERISTICS OF RED-EMITTING CAS-EU THIN-FILM ELECTROLUMINESCENT DEVICES WITH MEMORY

In order to investigate the role of the Eu2+ luminescent centers in determining the electro-optical characteristics of red-emitting CaS:Eu thin-film electroluminescent (TFEL) devices with inherent memory, light irradiation effects on these characteristics were studied. Both the threshold voltage and the response time decreased by light irradiation at wavelengths of 400-600 nm. This wavelength region coincides with that of the direct-excitation spectrum of Eu2+. Furthermore, emission peak height at the trailing edge of the pulse drive voltage increased by light irradiation in the same wavelength region. Since this emission is assumed to be caused by recombination of diffusing electrons with holes located at the ionized Eu2+ luminescent centers, increase in the emission peak height at the trailing edge of the pulse drive voltage indicated an increase in the number of the ionized Eu2+ luminescent centers. From these arguments we conclude that ionized Eu2+ luminescent center is a possible source of positive space-charge, which is known to play a decisive role in the memory and response characteristics of TFEL devices.