Ternary compound phosphors for electroluminescent displays

For color EL display, high-luminance and wide color gamut were achieved with ternary compound phosphors such as BaAl2S4: Eu. As a green phosphor, CaGa2S4: Eu is a promising material. To obtain red EL, many kinds of phosphors were investigated. Alkalin-earth thioindionate phosphor film such as CaIn2S4 : Eu has high electron conductivity due to the strong oxidization. In this system, the emission band shifts to longer wavelength with increasing ionic radius of the IIIb ion. The ionic radius of the IIIa ion, such as Y, is larger than that of IIIb ion. Red electroluminescence was observed from (Sr, Ca) Y2S4: Eu thin-films. The luminance of (Sr, Ca)Y2S4: Eu is not enough for full color EL display. The manganese doped Ba2ZnS3 and ZnGa2S4 EL device were also investigated. From both devices, although pure red EL was observed, luminance levels were not satisfy for the requirement of color displays. So we investigated new ternary compounds as a red-emitting phosphor. It is well known that Mn doped ZnS is an efficient EL phosphor. To obtain a red emission from divalent Mn ion, crystal field strength of Mn ion is required to be larger than that of umber emitting ZnS: Mn phosphors. In many materials, we focused on the CuAl2S4 for the host material. The CuAl2S4 compounds is the widest band-gap member of the I-III-VI2 type chalcopyrite semiconductors with a direct energy gap of 3.55 eV, which is almost same band-gap energy of ZnS. The chalcopyrite structure is ternary analogs of the sphalerite modifications of binary compounds. The lattice constant (a) of CuAlS2 is smaller than that of ZnS, so red shift is expected. We observed pure red luminescence from CuAl2S4: Mn films.