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Rare Earth-Doped Phosphors for White LEDs Chapter 278 55 properties of CASN were first reported by Uheda et al. (2006a,b). Thereafter, this phosphor has been investigated by many researchers whose goals were to enhance the luminescence efficiency and to improvement the synthesis for pro- ducing single phase CASN. This phosphor is generally prepared by a solid-state reaction method using a mixture of powder Si3N4, Ca3N2, and Eu2O3 or EuN at high temperatures, over 1600°C, and high pressures, about 0.5–0.9 MPa, in a nitrogen atmosphere. As described earlier, the Ce3+ activator substitutes Ca2+ sites where Ca2+ adopt fivefold nitrogen coordination to form a square pyramid with short Ca–N bond distances. A large crystal-field splitting of the d levels should then be observed when the activator ion substitutes the site which fea- tures a narrow space, because of large columbic interaction caused by shorter metal–anion distances and smaller coordination number. The large crystal-field splitting usually results in longer wavelengths of both excitation and emission, as shown in Fig. 6. The excitation spectrum of CASN shows an extremely broad band cover- ing the spectral region from 250 to 600 nm and having strong optical absorp- tion in the visible (Fig. 51). Under blue light excitation (lex 1⁄4 450 nm), CASN presents a broad deep-red emission with a peak at 655 nm and a FWHM value of 93 nm. The highest emission intensity is obtained for Ca0.99Eu0.01AlSiN3 which exhibits an external quantum efficiency of 70% (lex1⁄4450nm). In addition, the phosphor shows a small Stokes shift of 2200 cm1. In general, small Stokes shifts contribute to suppressing thermal quenching and to high conversion efficiency (Dorenbos, 2003). Temperature dependence of the emission intensity of CASN is presented in Fig. 52, normalized with respect to the value at 25°C. CASN has an excellent thermal stability against temperature quenching. When temperature increased 1 0.5 em 0 300 400 500 ex. 600 700 800 Wavelength (nm) Excitation and emission spectra of CaAlSiN3:Eu2+ (CASN). Data were collected from a phosphor synthesized by the authors. FIG. 51 Normalize intensity (a.u.)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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