Cerium Doped Garnet Phosphors for Application in White GaN-based LEDs
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Cerium Doped Garnet Phosphors for Application in White GaN-based LEDs Jennifer L. Wu1, Steven P. Denbaars2, Vojislav Srdanov3, Henry Weinberg4 1 Department of Chemical Engineering, 2Department of Materials, 3Center for Polymers and Organic Solids, University of California, Santa Barbara Santa Barbara, CA, 93106 4 Symyx Technologies, 3100 Central Expressway Santa Clara, CA, 95051 ABSTRACT Recently, renewed interest has emerged for the development of visible light, downconverting phosphors for application in white light emitting diodes (LEDs). In such devices, a blue GaN LED can act as a primary light source, serving as an efficient pump to excite photoluminescence in a phosphor with subsequent emission occurring at lower energies. The combination of blue light from the LED chip and emission from the phosphor(s) produces white light. It was recently reported that a combinatorial approach to synthesize and screen potential inorganic phosphors for use in white LEDs could aid in identifying improved phosphors for blue to yellow down conversion. Solid state thin-film arrays (libraries) based on the garnet structure (A1-x,Bx)3-z(C1-y,Dy)5O12:Ce3+z, where A, B = Y, Gd, Lu, La; C, D = Al, Ga; x and y = 0 to 1.0; and z = 0.03, were synthesized, and x-ray diffraction was used to select library samples of the crystalline garnet phase. Libraries of these various garnets were then characterized spectroscopically and their properties compared to traditionally prepared bulk powder phosphors of similar composition. Emission and excitation trends show that as larger cations are substituted for Y (A = Y), emission and excitation are red-shifted and as larger cations are substituted for Al (C = Al), emission and excitation are blue-shifted. If smaller cations are substituted for Y and Al an opposite trend is observed. Temperature dependence photoluminescence measurements and emission/excitation trends are also examined. INTRODUCTION Several methods have been used to develop white LEDs, which include use of conjugated polymers [1], organic dye molecules [2], and inorganic phosphors [3] as luminescent converters of blue LED light. In such a device, an InGaN blue LED can serve as the primary light source, acting as a pump to generate photoluminescence (PL) in organic or inorganic luminescent materials in which subsequent photon emission occurs at lower energies [1-5], cf., Fig. 1. Y3Al5O12:Ce3+ (YAG:Ce3+) emission is well suited for such applications, since when properly mixed its yellow emission under blue-light excitation yields white light [2,3]. The excitation and emission of YAG:Ce3+ have been well studied [6,7]. It is known that substitution of Gd3+ and Ga3+ for Y3+ and Al3+, respectively, in the garnet host shifts the emission of YAG:Ce3+ so that different shades of white light can be realized [2-5]. The addition of the larger ion Gd3+ for Y3+ red-shifts YAG:Ce3+ emission and substitution of Ga3+ for Al3+ tends to blue shift the characteristic yellow emission. A complex compositional space, such as the substituted YAG host or terna
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