1. Field Of The Invention
This invention relates to the art of the electroluminescence and more particularly, to an inorganic thin film electroluminescence device (hereinafter referred to simply as EL device) using inorganic fluorescent materials, which device is particularly suitable for used as a flat light source or a flat panel display.
2. Description Of The Prior Art
Inorganic thin film EL devices have recently drawn attention to their use as a flat panel display or flat light source. For this purpose, there has been heretofore used inorganic fluorescent materials, which comprise a matrix made of at least one of ZnS, CaS, SrS and the like and doped with not larger than 5 wt % of at least one element selected from Mn, Tb, Sn, Ce, Eu, Sm, Tm and the like as a light emission center.
Among these fluorescent materials, ZnS:Mn has been studied for use as an orange color fluorescent material. Because of its good luminance, life and the like characteristics, this fluorescent material has been utilized as a flat panel display. On the other hand, lead sulfides such as ZnS:Tb have been widely studied for use as a green color fluorescent material.
Other matrices which have been extensively studied include alkaline earth metal sulfide fluorescent material such as SrS:Ce as a blue light emitting fluorescent material, CaS:Eu as a red light emitting fluorescent material, a green light emitting fluorescent material, and the like.
The light emitting mechanisms of the fluorescent material or fluorescer of transition metals and rare earth elements are different from each other: with transition metals such as Mn, the light emission is caused by direct collision of the electrons of the matrix, whereas with rare earth elements, light emission depends greatly on the transition of an energy corresponding to the band gap of the matrix. The alkaline earth metal sulfides have a band gap energy ranging from 4.3 to 4.4 eV and ZnS has a band gap energy as small as 3.6 eV. In order to obtain blue to UV light emission which requires a high energy, these band gap energies are too small. Accordingly, for the blue to UV light emission, studies have been made on matrices which have a high band gap energy and include, for example, ZnF.sub.2 :Gd whose band gap energy ranges 7 to 8 eV (J. J. A. P. vol. 10 B (1991), pp. L1815 to 11816) and CaF.sub.2 :Eu (Appl. Phys. Lett. 41, 1982, p.462).
Inorganic fluorescers using these sulfides except ZnS:Mn are not always satisfactory for use as a flat panel display or flat light source with respect to the luminance, efficiency and life, and have never been applied as a flat panel color display at present.