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Rare Earth-Doped Phosphors for White LEDs Chapter 278 41 1 0.5 ex. em 0 300 400 500 600 700 Wavelength (nm) Excitation and emission spectra of Eu2+-activated Ca-a-SiAlON phosphor. Redrawn from Xie, R.J., Hirosaki, N., Sakuma, K., Yamamoto, Y., Mitomo, M., 2004. Eu2+-doped Ca-alpha-SiAlON: a yellow phosphor for white light-emitting diodes. Appl. Phys. Lett. 84, 5404. The excitation spectrum of Eu2+-activated Ca-a-SiAlON consists of two broad bands covering the region from UV to visible (Fig. 37), indicating that the phosphor is suitable as color converter excitable by any wavelength in this range emitted from a primary light source. The phosphor has a broad yellow emission band ranging from 500 to 700 nm with a maximum at 581 nm, which is longer than that of YAG:Ce3+, meaning that high CRI can be pro- duced by combining Eu2+-activated Ca-a-SiAlON phosphor with a blue- LED chip. In addition, much efforts have recently been devoted to improve the emission color of this phosphor by substituting Ca2+ for other metal ions such as Li+, Mg2+, and Y3+. The position of the emission band can be adjusted by tailoring the composition of the host or by changing the values of m and n in the chemical formula (Sakuma et al., 2007a,b; Xie et al., 2006a,b). Further- more, Eu2+-activated Ca-a-SiAlON has an excellent thermal stability against temperature quenching, and the emission intensity of the phosphor at 150°C ($90% of the initial value at 25°C) is much higher than that of YAG:Ce3+ phosphor ($87% of the initial value at 25°C) as shown in Fig. 38. This is the typical and the most important feature of (oxy)nitride phosphors. 4.2.1.2 CaAlSiN3:Ce3+ Phosphors CaAlSiN3 is a famous host material used in white LEDs. In particular, Eu2+ -activated CaAlSiN3 is one of the best commercial red-emitting phosphors. Detailed discussion of its photoluminescence properties will be given in Section 4.2.1.3. Here, we mainly discuss the Ce3+-activated CaAlSiN3 phos- phors emitting yellow light. CaAlSiN3 has an orthorhombic structure with space group Cmc21 (Fig. 39), isostructural with ASi2N3 (A 1⁄4 Li and Na) FIG. 37 Normalized intensity (a. u.)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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