| Co-Investigator(Kenkyū-buntansha) |
YOSHIYAMA Hideki Tottori University, Faculty of Engineering, Research Assistant, 工学部, 助手 (90191650)
TANAKA Shosaku Tottori University, Faculty of Engineering, Associate professor, 工学部, 助教授 (90032284)
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| Research Abstract |
Our purpose is to show that alkaline-earth sulfides, such as CaS and SrS, are potential host materials for full-color thin-film electoroluminescent (TFEL) devices. For this purpose we have investigated on thin-film growth method of CaS and SrS, and the possibility of multicoloring of rare-erath doped CaS and SrS TFEL devices. The results are summarized as follows.
1. The growth behavior of evaporated CaS (SrS) thin films has been investigated. The crystallinity of CaS (SrS) thin films is improved at higher substrate temperatures than 400 C. Sulfur coevaporation further helps to form a more perfect film.
2. Double insulated CaS:Eu, CaS:Ce and SrS:CeK, TFEL devices have been fabricated. They emit bright red, green and blue EL of three primary colors, respectively. Brightness levels, obtained from these EL devices, are 15 cd/m^2, 15 cd/m^2 and 36 cd/m^2, respectively, under 60 Hz excitation.
3. Inherent memory effect has been found in SrS:Ce,K and CaS: Eu TFEL devices. The origin of the memory effect can be attributed to native defects of the phosphor films.
4. EL waveforms of CaS:Eu and SrS:Ce,K EL devices has been measured. Three emission peaks appear for a period of pulse voltage excitation. First and third peaks appear when excitation voltage is so low to excite luminescent centers due to hot electrons. The excitation mechanism of the CaS:Eu and SrS:Ce,K TFEL devices is considered to be as follows. Luminescent centers are ionized by electric field, subsequently they capture conduction band electrons, and finally they give rise to luminescence.
5. A full-color TFEL device has been developed. The SrS:Ce,K blue-green emission is filtered to separate into blue and green emission, which are superimposed on the CaS:Eu red emission. The luminance white level of 7.5 cd/m^2 (60 Hz drive) has been achieved.
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