White Light-Emitting Diode Lamps Using Oxynitride and Nitride ...
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INVITED PAPER
Special Section on Electronic Displays
White Light-Emitting Diode Lamps Using Oxynitride and Nitride Phosphor Materials Ken SAKUMA†a) , Member, Naoto HIROSAKI†† , Nonmember, Naoki KIMURA† , Member, Masakazu OHASHI† , Rong-Jun XIE†† , Yoshinobu YAMAMOTO†† , Takayuki SUEHIRO†† , Nonmembers, Kenichiro ASANO† , and Daiichiro TANAKA† , Members
SUMMARY White-light emitting diode lamps for general illumination can be realized by a combination of a blue light-emitting diode semiconductor die and phosphors. Newly developed oxynitride and nitride phosphors are promising candidates for this application because they have suitable excitation and emission wavelengths and stable optical properties in a high temperature environment. High brightness warm-white LED lamps have been realized using a yellowish-orange α-SiAlON oxynitride phosphor. High color-rendering index white LED lamps have been also realized using three color oxynitride/nitride phosphors. key words: light-emitting diode, phosphor, nitride, illumination
1.
Introduction
Blue light-emitting diode (LED) was a long-awaited technology, because a full-color LED display can be realized by use of blue LED lamps with conventional red and green LED lamps. The first p-n junction blue LED was demonstrated in 1989 and the first candela-class high-brightness nitride-based blue LED was commercialized in 1993 [1]– [3]. Nowadays full-color LED displays are in widespread use, which is installed on the wall of buildings [4]. The success of developing a blue LED also opened the gate to the new era of solid-state lighting. By replacing conventional lamps with semiconductor light sources, there are great hopes that solid-state lighting could save the electricity used for general illumination. The features of long lifetime and mercury free also would contribute to reduce environmental problems [5], [6]. The luminous efficacies of white LED lamps have already been superior to those of the incandescent lamps. At the present, we can see many kinds of white LED lamps in our daily life such as a desk lamp, a flashlight and so on. The efficacies of them are expected to be higher than that of the fluorescent lamp in near future, so that white LED lamps are supposed to be alternative to conventional lamps. There are several methods to generate white light from LED semiconductor dice. A primitive white LED lamp consists of three semiconductor LED dice of red, green and blue. By this method, any color can be produced. In addition to full-color display system, LED lamps of this type Manuscript received March 22, 2005. The authors are with Optics and Electronics Laboratory, Fujikura Ltd., Sakura-shi, 285-8550 Japan. †† The authors are with Advanced Materials Laboratory, National Institute for Materials Science, Tsukuba-shi, 305-0044 Japan. a) E-mail: [email protected] DOI: 10.1093/ietele/e88–c.11.2057 †
are used in the backlight unit for large size liquid crystal display (LCD), cellular phone illumination and so on. The disadvantage of this method is its high cost. The drive voltages of red, green and blue LEDs are different from each other. Thermal properties and degradation trends are also different. To obtain a white light, therefore, a detector and a feedback system is required for this method and the driver circuit tends to be complicated. An excellent solution to realize fixed chromaticity white LED lamps in low cost is to combine a blue LED die and a yellow phosphor which can be excited by blue light. Yellow light is complementary color of blue light, therefore this bi-chromatic LED lamp exhibits white light. Currently (Y,Gd)3 (Al,Ga)5 O12 :Ce yellow phosphor (YAG:Ce) is commonly used [4], [7]. Most of the backlight units for small size LCD of cellular phone adopts the white LED lamps of this type. Tri-chromatic or quad-chromatic LED lamps using blended phosphors have also been reported. For example, the combination of blue LED die, SrGa2 S4 :Eu2+ green phosphor and SrS:Eu2+ red phosphor was proposed [8]. Near ultraviolet (UV) LED die can be used as alternative excitation light source for phosphors in white LED lamps. The quadchromatic white LED lamp consists of a near UV LED die, orange, yellow, green and blue phosphor was also presented [9]. Many efforts have been done to develop new phosphors suitable for white LED lamps in past decade. Most of the investigated phosphors are sulfide or oxide. Even though their optical properties are suitable for the application, they often have some difficulties in reliability or high temperature stability. In the phosphor research field, oxynitride and nitride phosphors have attracted much attention in recent years. Some of these new phosphor materials have optimum characteristics for the white LED lamp application. The reliability and high temperature stability of them are better than conventional phosphors. In this paper, newly developed oxynitride and nitride phosphors for white LED lamps are introduced. The high brightness warm-white LED lamp and high color rendering index white LED lamps of various correlated color temperatures (CCT) are also presented. These LED lamps using oxynitride and nitride phosphors are expected to resolve the existing problems and they would lead the prevalence of LED light source in electronic displays and general illumi-
c 2005 The Institute of Electronics, Information and Communication Engineers Copyright
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Fig. 1 Oxynitride and nitride phosphors excited under blacklight (365 nm).
nation fields. 2.
Oxynitride and Nitride Phosphors
Figure 1 shows the oxynitride and nitride phosphors excited under blacklight at the wavelength of 365 nm. It can be clearly seen that all of them have strong emission under UV excitation. The red phosphor is CaAlSiN3 :Eu reported by K. Uheda et al. in 2003 [10], [11]. It has an extremely wide excitation band from UV to green visible region. The emission peak wavelength is about 650 nm and the emission intensity is very high compared to conventional red phosphors. The yellow one is Ca-α-SiAlON:Eu phosphor reported by R.-J. Xie et al. and J.W.H. van Krevel et al. independently in 2002 [12], [13]. Its excitation band is also wide from UV to blue visible region. The emission peak wavelength is about 585 nm and can be tailored widely in yellow and orange color region. A warm-white LED lamp can be realized by the combination with a blue LED die [14], [15]. The emission intensity of it is superior to a commercially available conventional yellow phosphor. The green one is β-SiAlON:Eu phosphor reported by N. Hirosaki et al. in 2005 [16], [17]. It also can be excited by blue light in addition to UV light. The emission peak wavelength is about 540 nm. These three phosphors have enough emission intensity with 450 nm blue light excitation, therefore they are suitable for white LED lamps. The blue phosphor which can be excited by UV light have also been realized. It can be applicable for white LED lamps using UV LED die. 3.
Fig. 2 Excitation and emission spectra of Ca-α-SiAlON:Eu and (Y,Gd)3 Al5 O12 :Ce.
Blue Yellow Bi-chromatic White LED Lamp
3.1 Yellowish Orange Ca-α-SiAlON:Eu Phosphor At the present, bi-chromatic white LED lamp consisting of a blue LED die and YAG:Ce is the best-selling product because of its high luminous efficacy. YAG:Ce is suitable for generating cool-white light and white light, the CCT of which is higher than 3000 K. On the other hand, sometimes the warm-white light of low CCT is demanded in general
Fig. 3
Preparation procedure of oxynitride and nitride phosphors.
illumination field. Ca-α-SiAlON:Eu is an optimal phosphor for this application, because its emission wavelength is longer than that of YAG:Ce. Figure 2 shows the excitation and emission spectra of a Ca-α-SiAlON:Eu yellowish orange phosphor. A fluorescence spectrophotometer (F-4500, Hitachi Ltd.) was used for the measurement. Spectral correction was done using Rhodamine-B, a light diffuser and a standard light source as references. The excitation and emission spectra of commercially available (Y,Gd)3 Al5 O12 :Ce yellow phosphor are also shown for comparison. The emission peak intensity of this Ca-α-SiAlON:Eu is about 135% compared to the conventional phosphor. 3.2 Preparation Procedure A preparation procedure of oxynitride and nitride phosphors is shown in Fig. 3. To obtain Ca-α-SiAlON:Eu, fine powders of Si3 N4 , AlN, CaCO3 and Eu2 O3 were used as starting materials. The element ratio of oxygen and nitrogen can be controlled when Ca3 N2 and EuN are used, but a glovebox with a nitrogen atmosphere is required in the case [18]. A planetary ball mill was used in mixing process. About the sintering process, Gas Pressure Sintering (GPS) was carried out at 1700–2000◦ C in a 0.5–1 MPa N2 atmosphere. Hot pressing is also available [12]. Recently, T. Suehiro et al. reported that Ca-α-SiAlON:Eu can be obtained by the
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reduction-nitridation process at relatively low temperatures of 1400–1500◦ C from oxide starting materials [19]. After synthesizing Ca-α-SiAlON:Eu, crushing and classifying processes were done to obtain the phosphor particles of suitable diameter for LED lamp packaging. 3.3 High Brightness Warm-White LED Lamps High brightness warm-white LED lamps of low CCT was realized by combination of a blue LED die and Ca-αSiAlON:Eu phosphor. The luminous efficacies, the value of which being total luminous flux divided by supplied electrical power, were 36.3 lm/W for the round shape through-hole LED lamp and 41.7 lm/W for the surface mount device (SMD) type chip LED, respectively [20], [21]. This value is two or three times higher than that of the conventional incandescent lamps. The luminous efficiencies of the radiation, calculated from the emission spectra, were 374 lm/W for the round shape one and 362 lm/W for the SMD type one, respectively. The LEDs were measured by a spectroradiometer (OL-770, Optronic Laboratories, Inc.) at the operating conditions of room temperature and a forward-bias current of 20 mA. Figure 4 shows the schematic diagram of the SMD type white LED. The phosphor powder is dispersed in a transparent resin and a blue LED die is coated by the resin. Figure 5 present the top view of the SMD type LED. The emission spectra of the SMD type LED is shown in Fig. 6. The chromaticity coordinates (x, y) on CIE1931
Fig. 4
Structure of a SMD type white LED.
Fig. 5
Top view of a SMD type white LED.
diagram is (0.453, 0.398) and CCT is 2700 K. This chromaticity is just in the range of class L which is defined in Japanese Industrial Standard (JIS) Z 9112 [22]. Class L is called the incandescent lamp color in other words. The general color rendering index (CRI) Ra is 57, which is slightly less than traditional fluorescent lamps of Ra 60. The general CRI Ra value is the average of the special CRI R1–R8 [23]. 3.4 Precision CCT Control For the general illumination purpose, the lamps of various CCTs are demanded, therefore it is very important to develop a method of precisely tailoring of the emission wavelength of phosphors for white LED lamps. It has been suggested that the emission wavelength of Ca-α-SiAlON:Eu phosphor could be tailored by changing its composition [12], [24], [25]. R.-J. Xie et al. have reported that the emission peak wavelengths of Ca-α-SiAlON:Eu are between 583 nm and 603 nm, corresponding to CCT range between 1900 K and 3300 K for white LED lamps [14], [18]. The general formula of Ca-α-SiAlON:Eu is described as Ca x Euy (Si,Al)12 (O,N)16 . To investigate the chromaticity dependence on composition, Ca-α-SiAlON:Eu of the compositions of x = 0.875 and y = 0.02–0.11 were prepared [26]. The dominant wavelengths of emission were varied from 578 nm to 583 nm with increasing the y value, Eu element concentration. The efficiencies of phosphors were high at y = 0.03–0.08. Concentration quenching was observed for y > 0.08. On the other hand, the Eu element concentration of y = 0.02 is too low to obtain enough emission. This dominant wavelength range luckily corresponds to the most CCT range of class L, when warm-white LED lamps are fabricated using these phosphors. The chromaticity coordinates of the two Ca-αSiAlON:Eu phosphors and two LED lamps are shown in Fig. 7. Two lines are connecting the chromaticity coordinates of 450 nm blue LED die and each phosphors. The chromaticity of fabricated LED lamps was on these lines depending on the amount of phosphors. By selecting optimum phosphor amount, the chromaticities were adjusted to blackbody locus. The CCT of the LED lamp using Ca0.875 Eu0.04 (Si,Al)12 (O,N)16 phosphor was 3020 K. For Ca0.875 Eu0.07 (Si,Al)12 (O,N)16 phosphor, it was 2640 K. The
Fig. 6
Emission spectra of a SMD type LED lamp.
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Fig. 7 Obtained CCT range of warm-white LED lamps by changing composition of Ca-α-SiAlON:Eu phosphor.
cially available class D LED lamp and the triangles represent a commercially available class L LED lamp. The Ca-αSiAlON:Eu class L LED lamp we fabricated is represented by circles. The black marks are the chromaticity coordinates at 25◦ C and the white marks are at 100◦ C, respectively. The chromaticity coordinates of the class D LED lamp varied from (0.317,0.312) to (0.307,0.305), and the class L LED lamp varied from (0.453,0.398) to (0.445,0.395). The chromaticity variation distances on CIE1931 diagram were 0.012 and 0.008, respectively. In contrast, the variation distance of the Ca-α-SiAlON:Eu class L LED lamp was only 0.002 from (0.438,0.396) to (0.437,0.398). The chromaticity of Ca-α-SiAlON:Eu LED lamp is extremely thermally stable compared to commercially available conventional white LED lamps. When temperature is raised, two phenomena are observed. One of them is red shift of the blue LED die and another is the temperature quenching of the phosphor. The temperature quenching of (Y,Gd)3 Al5 O12 :Ce yellow phosphor is relatively large and one kind of YAG:Ce phosphors like this may be used in conventional white LED lamps. The excitation band of (Y,Gd)3 Al5 O12 :Ce, as shown in Fig. 2, is not enough wide, therefore additional quenching may be caused by the red shift of the blue LED excitation light source. As a result, the chromaticity of conventional white LED lamp shifts toward blue and the variation become large. On the other hand, the thermal quenching of Ca-αSiAlON:Eu phosphor is negligible and the excitation band width of it is very wide. Hence the remarkable quenching of Ca-α-SiAlON:Eu phosphor would not be caused, even if the temperature of Ca-α-SiAlON:Eu warm-white LED lamp is raised to 100◦ C. This can explain the fact that the locus of chromaticity variation is almost parallel to the blue region of spectral locus (locus of red shift of blue LED die) and the variation distance is so small for the Ca-α-SiAlON:Eu class L LED lamp. 4.
Fig. 8 lamps.
Thermal stability of chromaticity coordinates of white LED
luminous efficacy of these LED lamps was as high as about 30 lm/W. 3.5 Chromaticity Stability at High Temperature α-SiAlON has been developed as a high temperature structural material [27] and the optical properties of Ca-αSiAlON:Eu are also stable at high temperature [15], [24]. The thermal stability of the chromaticity between the room temperature of 25◦ C and 100◦ C was investigated for three LED lamps and the chromaticity coordinates of them are shown in Fig. 8. The rectangles represent a commer-
High CRI Quad-chromatic White LED Lamps
Recent years, high color quality lamps with higher CRI Ra than 80 are recommended for lighting of various indoor work places [28]. The CRI Ra value is about 70–75 for conventional white and warm-white LED lamps including YAG:Ce and less than 60 for the Ca-α-SiAlON:Eu warmwhite LED lamp. The full width at half maximum (FWHM) of the emission spectra is 127 nm for (Y,Gd)3 Al5 O12 :Ce and 92 nm for Ca-α-SiAlON:Eu, respectively. As shown in Fig. 2, the problem is lack of red light and green light. In order to improve the color rendering, CaAlSiN3 :Eu red phosphor and β-SiAlON:Eu phosphor were added [29]. The normalized emission spectra of blue LED die and three phosphors are shown in Fig. 9. By combination of them, quadchromatic white LED lamps with high color rendering can be realized. Using the mixture of these three oxynitride and nitride phosphors, five white LED lamps with various CCT were fabricated. The chromaticity coordinates of these quad-chromatic LED lamps of class D, class N, class W,
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Fig. 12 minant. Fig. 9
Fig. 10
Emission spectra of class L LED lamp and A CIE standard illu-
Normalized emission spectra.
Chromaticity coordinates of white LED lamps with various CCT.
Table 1
High CRI LEDs.
D
N
W
WW
L
CIE x y
0.311 0.333
0.345 0.358
0.373 0.370
0.407 0.392
0.449 0.408
CCT [K]
6,580
5,010
4,160
3,470
2,840
CRI Ra R9 R15
81 95 93
82 97 97
83 78 93
86 99 99
88 97 98
LEfficacy [lm/W] LEfficiency [lm/W]
28 270
25 272
27 282
24 267
25 254
Fig. 13 Color space of oxynitride and nitride phosphor quad-chromatic LED lamps, NTSC and s-RGB on CIE1931 diagram.
Fig. 14 gram.
Fig. 11 Emission spectra of class D LED lamp and D65 CIE standard illuminant.
Decorative various color LED lamps placed on CIE1976 dia-
class WW and class L are shown in Fig. 10. The CCT of them is controlled by changing the mixing ratio of the phosphors and their chromaticities were adjusted to the blackbody locus by selecting the optimum amount of the phosphor mixture to coat the blue LED die.
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The chromaticity coordinates, the CCT, the CRI values of Ra, R9, R15, the luminous efficacy and the luminous efficiency of each LED lamps are shown in Table 1. The CRI Ra of them is higher than 80. The special CRI R9 and R15 of them are significantly high. R9 corresponds to strong red, the value of which for conventional white LED lamps is not good enough [30]. R15 corresponds to the color of the skin of the face of the Japanese women, which is very important for indoor lightings. The emission spectra of class D and class L LED lamps are shown in Fig. 11 and Fig. 12, respectively. The spectral distribution of CIE standard colorimetric illuminants are also shown [31]. 5.
Decorative Various Color Lamps
Not only white lamps just on the blackbody locus, but also decorative various color lamps can be realized by the quadchromatic oxynitride and nitride phosphor LED lamps. Figure 13 shows the quadrilateral color space on CIE1931 diagram, in which the chromaticity could be realized by the quad-chromatic LED lamp consisting of a blue LED die, CaAlSiN3 :Eu red phosphor, Ca-α-SiAlON:Eu yellow phosphor and β-SiAlON:Eu green phosphor. The extent of it is 82% of NTSC and 116% of s-RGB [32], [33]. The fabricated various color LED lamps are presented in Fig. 14. Each LED lamps are placed at corresponding chromaticity coordinates on CIE1976 diagram. These color lamps would be useful for applications such as decorative illuminations. Fixed design electronic display signs are also promising applications which would alternate conventional signs such as neon signs. 6.
Conclusions
The oxynitride and nitride phosphors which can be excited by blue visible light were introduced. The bi-chromatic high brightness warm-white LED lamp consisting of a blue LED die and Ca-α-SiAlON:Eu yellow phosphor of 41.7 lm/W luminous efficacy and 2700 K CCT was presented. The chromaticity of it was extremely thermally stable compared to conventional white LED lamps. The quad-chromatic high CRI various CCT white LED lamps consisting of a blue LED die, CaAlSiN3 :Eu red phosphor, Ca-α-SiAlON:Eu yellow phosphor and β-SiAlON:Eu green phosphor were demonstrated. The CRI Ra values were higher than 80. The LED lamps using oxynitride and nitride phosphors will play an important role in the coming era of solid-state lighting. References [1] H. Amano, M. Kito, K. Hiramatsu, and I. Akasaki, “P-type conduction in Mg-doped GaN treated with low-energy electron beam irradiation (LEEBI),” Jpn. J. Appl. Phys., vol.28, pp.L2112–L2114, 1989. [2] S. Nakamura, T. Mukai, and M. Senoh, “Candela-class highbrightness InGaN/AlGaN double-heterostructure blue-light-emitting diodes,” Appl. Phys. Lett., vol.64, pp.1687–1689, 1994. [3] I. Akasaki, S. Kamiyama, and H. Amano, “The evolution of nitridebased light-emitting devices,” IEICE Trans. Electron., vol.E85-C,
no.1, pp.2–9, Jan. 2002. [4] S. Nakamura, “Present performance of InGaN based blue/green/ yellow LEDs,” Proc. SPIE, vol.3002, pp.26–35, 1997. [5] The Japan Research and Development Center of Metals’ National Project on Light for the 21st Century, Year 2000 Report of Results. [6] D. Malakoff, “Lighting initiative flickers to life,” Science, vol.296, p.1782, June 2002. [7] K. Bando, K. Sakano, Y. Noguchi, and Y. Shimizu, “Development of high-bright and pure-white LED lamps,” J. Light Vis. Environ., vol.22, no.1, pp.2–5, 1998. [8] R. Mueller-Mach, G.O. Mueller, M.R. Krames, and T. Trottier, “High-power phosphor-converted light-emitting diodes based on IIInitrides,” IEEE J. Sel. Top. Quantum Electron., vol.8, no.2, pp.339– 345, 2002. [9] T. Taguchi, “Present status of white LED lighting technologies in Japan,” J. Light Vis. Environ., vol.27, no.3, pp.131–139, 2003. [10] K. Uheda, N. Hirosaki, H. Yamamoto, H. Yamane, Y. Yamamoto, W. Inami, and K. Tsuda, “The crystal structure and photoluminescence properties of a new red phosphor, calcium aluminum silicon nitride doped with divalent europium,” Abstract 2073, The Electrochemical Society 206th Meeting, Honolulu, HI, Oct. 2004. [11] K. Uheda, N. Hirosaki, H. Yamamoto, and R.-J. Xie, “Red phosphors for warm white light-emitting diodes,” The 305th Meeting Technical Digest, Phosphor Research Society, pp.37–47, Nov. 2004. [12] R.-J. Xie, M. Mitomo, K. Uheda, F.F. Xu, and Y. Akimune, “Preparation and luminescence spectra of calcium- and rare-earth (R = Eu, Tb, and Pr)-codoped α-SiAlON ceramics,” J. Am. Ceram. Soc., vol.85, no.5, pp.1229–1234, 2002. [13] J.W.H. van Krevel, J.W.T. van Rutten, H. Mandal, H.T. Hintzen, and R. Metselaar, “Luminescence properties of terbium-, cerium-, or europium-doped α-sialon materials,” J. Solid State Chem., vol.165, pp.19–24, 2002. [14] R.-J. Xie, N. Hirosaki, K. Sakuma, Y. Yamamoto, and M. Mitomo, “Eu2+ -doped Ca-α-SiAlON: A yellow phosphor for white lightemitting diodes,” Appl. Phys. Lett., vol.84, pp.5404–5406, 2004. [15] K. Sakuma, K. Omichi, N. Kimura, M. Ohashi, D. Tanaka, N. Hirosaki, Y. Yamamoto, R.-J. Xie, and T. Suehiro, “Warm-white lightemitting diode with yellowish orange SiAlON ceramic phosphor,” Opt. Lett., vol.29, pp.2001–2003, 2004. [16] N. Hirosaki, R.-J. Xie, K. Kimoto, T. Sekiguchi, Y. Yamamoto, and T. Suehiro, “Luminescent properties of green phosphor, βSiAlON:Eu2+ ,” 52th Spring Meeting, 2005, JSAP and Related Societies, 30a-YH-7, 2005. [17] N. Hirosaki, R.-J. Xie, K. Kimoto, T. Sekiguchi, Y. Yamamoto, T. Suehiro, and M. Mitomo, “Characterization and properties of greenemitting β-SiAlON:Eu2+ powder phosphors for white light-emitting diodes,” Appl. Phys. Lett., vol.86, 211905, 2005. [18] R.-J. Xie, N. Hirosaki, M. Mitomo, Y. Yamamoto, T. Suehiro, and K. Sakuma, “Optical properties of Eu2+ in α-SiAlON,” J. Phys. Chem. B, vol.108, pp.12027–12031, 2004. [19] T. Suehiro, N. Hirosaki, R.-J. Xie, and M. Mitomo, “Powder synthesis of Ca-α -SiAlON as a host material for phosphors,” Chem. Mater., vol.17, pp.308–314, 2005. [20] K. Sakuma, N. Hirosaki, N. Kimura, K. Omichi, Y. Yamamoto, R.-J. Xie, T. Suehiro, M. Ohashi, and D. Tanaka, “High luminous efficacy warm-white light-emitting diode lamp using α-SiAlON phosphor,” 65th Autumn Meeting, 2004, JSAP, p.1284, 2p-ZL-15, 2004. [21] N. Kimura, N. Hirosaki, K. Sakuma, K. Asano, and D. Tanaka, “High luminous efficiency warm white light-emitting diode using α-SiAlON phosphor,” Proc. IEICE Gen. Conf. 2005, C-9-1, 2005. [22] JIS Z 9112-1990, “Classification of fluorescent lamps by chromaticity and colour rendering property,” Japanese Standards Association. [23] JIS Z 8726-1990, “Method of specifying colour rendering properties of light sources,” Japanese Standards Association. [24] A. Ellens, G. Huber, and F. Kummer, “Illumination unit having at least one LED as light source,” United States Patent no.6,657,379, 2003.
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[25] R.-J. Xie, N. Hirosaki, M. Mitomo, Y. Yamamoto, T. Suehiro, and K. Sakuma, “α-SiAlON-based oxynitride/nitride phosphors: Synthesis, properties and applications,” Proc. 11th International Display Workshops (IDW’04), PH4-1 (Invited), pp.1105–1108, Niigata, Japan, 2004. [26] K. Sakuma, N. Hirosaki, N. Kimura, Y. Yamamoto, R.-J. Xie, T. Suehiro, K. Omichi, M. Ohashi, and D. Tanaka, “High brightness warm-white LED lamps using Ca-α-SiAlON phosphors,” Proc. 11th International Display Workshops (IDW’04), PHp-1, pp.1115–1118, Niigata, Japan, 2004. [27] G.Z. Cao and R. Metselaar, “α -sialon ceramics: A review,” Chem. Mater., vol.3, pp.242–252, 1991. [28] ISO 8995:2002, CIE S 008/E:2001, “Lighting of indoor work places,” Joint ISO/CIE Standard. [29] K. Sakuma, N. Hirosaki, N. Kimura, K. Masuko, Y. Yamamoto, R.J. Xie, T. Suehiro, K. Asano, and D. Tanaka, “High color-rendering index white light-emitting diode lamps using oxynitride phosphors,” 52th Spring Meeting, 2005, JSAP and Related Societies, 30a-YH-8, 2005. [30] M. Yamada, T. Naitou, K. Izuno, H. Tamaki, Y. Murazaki, M. Kameshima, and T. Mukai, “Red-enhanced white-light-emitting diode using a new red phosphor,” Jpn. J. Appl. Phys., vol.42, pp.L20–L23, 2003. [31] JIS Z 8781-1999, “CIE standard colorimetric illuminants,” Japanese Standards Association. [32] International Telecommunication Union Radiocommunication Sector Recommendation ITU-R BT.470-6 (1970–1998). [33] International Electrotechnical Commission (IEC) Standard no. 61966-2-1: 1999.
Ken Sakuma was born in Chiba, Japan in 1969. He received the B.E. and M.E. degrees in materials science and metallurgy from the University of Tokyo, Japan, in 1993 and 1995 respectively. Since joining Fujikura Ltd. in 1995, he had been engaged in research and development on the optical fiber testing systems, optical fiber arrays and planar lightwave circuit devices for optical fiber communication systems. He is currently a visiting researcher of National Institute for Materials Science (NIMS) since July 2003. His current research interest includes oxynitride and nitride phosphor materials for white LED lamps. He received the Outstanding Poster Paper Award at the 11th International Display Workshops (IDW’04) in 2004. He is currently a member of Optical Society of America, The Japan Society of Applied Physics, The Ceramic Society of Japan, and Phosphor Research Society.
Naoto Hirosaki is a senior researcher of National Institute for Materials Science (NIMS). He received a B.E., M.E., and D.E. degrees in industrial chemistry from Kyoto University in 1978, 1980, and 1990, respectively. He worked for 17 years at Nissan Motor Co. Ltd. He joined NIRIM in 1998. His research interests are sialon-based phosphor, synthesis of new nitride materials, and sintering process of silicon nitride.
Naoki Kimura was born in 1975. He received the B.E. and M.E. degrees in applied physics from Tohoku University, Sendai, Japan in 1998 and 2000, respectively. Since he joined Fujikura Ltd. in 2000, he has been engaged in research and development of optical fiber components.
Masakazu Ohashi was born in Tokyo, Japan in 1964. He received the B.E. and M.E. degrees in chemicals engineering from the Shinshu University, Nagano, Japan, in 1988, and 1990, respectively. In 1990, he joined Fujikura Ltd., where he has been engaged in research and development of industrial wires and optical devices.
Rong-Jun Xie was born in Jiangsu Province, China in 1969. He received the M.E. (Metallurgy) and PhD degrees (Ceramics) from Xi’an Jiaotong University and Shanghai Institute of Ceramics, Chinese Academy of Sciences, in 1995 and 1998, respectively. Since joining National Institute for Materials Science (NIMS) in 2003, he has been engaged in research and development of oxynitride phosphors for white light-emitting diodes. He is a member of The American Ceramic Society, The Japan Society of Applied Physics, and The Ceramic Society of Japan.
Yoshinobu Yamamoto worked at Nissan Motor Co. Ltd. till 1999, majored in chemical analysis and development of catalyst. Then moved to National Institute for Research in Inorganic Materials (NIRIM, currently NIMS) and worked on chemical analysis and high-temperature materials till 2003. Since 2004, he works on chemical analysis and hightemperature synthesis (sintering).
Takayuki Suehiro was born in 1974. He received his B.E. (1997), M.E. (1999), and Ph.D. (2002) in Materials Science from Yokohama National University. He joined NIMS as a postdoctoral researcher in 2002, working with Dr. Naoto Hirosaki. His research interests focus on the powder synthesis and characterization of nitride materials. He is currently a member of The American Ceramic Society and The Ceramic Society of Japan.
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Kenichiro Asano was born in 1966. He graduated in opto-electronics engineering from Technos International College. He joined Fujikura Ltd. in 1989 and has been engaged in research and development of optical devices.
Daiichiro Tanaka was born in Tokyo, Japan in 1961. He received the B.E. degree in electronics engineering from Chiba University, Chiba, Japan, in 1985. In 1985, he joined Fujikura Ltd., where he has been engaged in research work in optical fibers, optical fiber amplifiers and optical components. He is a member of the Illuminating Engineering Institute of Japan.
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INVITED PAPER
Special Section on Electronic Displays
White Light-Emitting Diode Lamps Using Oxynitride and Nitride Phosphor Materials Ken SAKUMA†a) , Member, Naoto HIROSAKI†† , Nonmember, Naoki KIMURA† , Member, Masakazu OHASHI† , Rong-Jun XIE†† , Yoshinobu YAMAMOTO†† , Takayuki SUEHIRO†† , Nonmembers, Kenichiro ASANO† , and Daiichiro TANAKA† , Members
SUMMARY White-light emitting diode lamps for general illumination can be realized by a combination of a blue light-emitting diode semiconductor die and phosphors. Newly developed oxynitride and nitride phosphors are promising candidates for this application because they have suitable excitation and emission wavelengths and stable optical properties in a high temperature environment. High brightness warm-white LED lamps have been realized using a yellowish-orange α-SiAlON oxynitride phosphor. High color-rendering index white LED lamps have been also realized using three color oxynitride/nitride phosphors. key words: light-emitting diode, phosphor, nitride, illumination
1.
Introduction
Blue light-emitting diode (LED) was a long-awaited technology, because a full-color LED display can be realized by use of blue LED lamps with conventional red and green LED lamps. The first p-n junction blue LED was demonstrated in 1989 and the first candela-class high-brightness nitride-based blue LED was commercialized in 1993 [1]– [3]. Nowadays full-color LED displays are in widespread use, which is installed on the wall of buildings [4]. The success of developing a blue LED also opened the gate to the new era of solid-state lighting. By replacing conventional lamps with semiconductor light sources, there are great hopes that solid-state lighting could save the electricity used for general illumination. The features of long lifetime and mercury free also would contribute to reduce environmental problems [5], [6]. The luminous efficacies of white LED lamps have already been superior to those of the incandescent lamps. At the present, we can see many kinds of white LED lamps in our daily life such as a desk lamp, a flashlight and so on. The efficacies of them are expected to be higher than that of the fluorescent lamp in near future, so that white LED lamps are supposed to be alternative to conventional lamps. There are several methods to generate white light from LED semiconductor dice. A primitive white LED lamp consists of three semiconductor LED dice of red, green and blue. By this method, any color can be produced. In addition to full-color display system, LED lamps of this type Manuscript received March 22, 2005. The authors are with Optics and Electronics Laboratory, Fujikura Ltd., Sakura-shi, 285-8550 Japan. †† The authors are with Advanced Materials Laboratory, National Institute for Materials Science, Tsukuba-shi, 305-0044 Japan. a) E-mail: [email protected] DOI: 10.1093/ietele/e88–c.11.2057 †
are used in the backlight unit for large size liquid crystal display (LCD), cellular phone illumination and so on. The disadvantage of this method is its high cost. The drive voltages of red, green and blue LEDs are different from each other. Thermal properties and degradation trends are also different. To obtain a white light, therefore, a detector and a feedback system is required for this method and the driver circuit tends to be complicated. An excellent solution to realize fixed chromaticity white LED lamps in low cost is to combine a blue LED die and a yellow phosphor which can be excited by blue light. Yellow light is complementary color of blue light, therefore this bi-chromatic LED lamp exhibits white light. Currently (Y,Gd)3 (Al,Ga)5 O12 :Ce yellow phosphor (YAG:Ce) is commonly used [4], [7]. Most of the backlight units for small size LCD of cellular phone adopts the white LED lamps of this type. Tri-chromatic or quad-chromatic LED lamps using blended phosphors have also been reported. For example, the combination of blue LED die, SrGa2 S4 :Eu2+ green phosphor and SrS:Eu2+ red phosphor was proposed [8]. Near ultraviolet (UV) LED die can be used as alternative excitation light source for phosphors in white LED lamps. The quadchromatic white LED lamp consists of a near UV LED die, orange, yellow, green and blue phosphor was also presented [9]. Many efforts have been done to develop new phosphors suitable for white LED lamps in past decade. Most of the investigated phosphors are sulfide or oxide. Even though their optical properties are suitable for the application, they often have some difficulties in reliability or high temperature stability. In the phosphor research field, oxynitride and nitride phosphors have attracted much attention in recent years. Some of these new phosphor materials have optimum characteristics for the white LED lamp application. The reliability and high temperature stability of them are better than conventional phosphors. In this paper, newly developed oxynitride and nitride phosphors for white LED lamps are introduced. The high brightness warm-white LED lamp and high color rendering index white LED lamps of various correlated color temperatures (CCT) are also presented. These LED lamps using oxynitride and nitride phosphors are expected to resolve the existing problems and they would lead the prevalence of LED light source in electronic displays and general illumi-
c 2005 The Institute of Electronics, Information and Communication Engineers Copyright
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Fig. 1 Oxynitride and nitride phosphors excited under blacklight (365 nm).
nation fields. 2.
Oxynitride and Nitride Phosphors
Figure 1 shows the oxynitride and nitride phosphors excited under blacklight at the wavelength of 365 nm. It can be clearly seen that all of them have strong emission under UV excitation. The red phosphor is CaAlSiN3 :Eu reported by K. Uheda et al. in 2003 [10], [11]. It has an extremely wide excitation band from UV to green visible region. The emission peak wavelength is about 650 nm and the emission intensity is very high compared to conventional red phosphors. The yellow one is Ca-α-SiAlON:Eu phosphor reported by R.-J. Xie et al. and J.W.H. van Krevel et al. independently in 2002 [12], [13]. Its excitation band is also wide from UV to blue visible region. The emission peak wavelength is about 585 nm and can be tailored widely in yellow and orange color region. A warm-white LED lamp can be realized by the combination with a blue LED die [14], [15]. The emission intensity of it is superior to a commercially available conventional yellow phosphor. The green one is β-SiAlON:Eu phosphor reported by N. Hirosaki et al. in 2005 [16], [17]. It also can be excited by blue light in addition to UV light. The emission peak wavelength is about 540 nm. These three phosphors have enough emission intensity with 450 nm blue light excitation, therefore they are suitable for white LED lamps. The blue phosphor which can be excited by UV light have also been realized. It can be applicable for white LED lamps using UV LED die. 3.
Fig. 2 Excitation and emission spectra of Ca-α-SiAlON:Eu and (Y,Gd)3 Al5 O12 :Ce.
Blue Yellow Bi-chromatic White LED Lamp
3.1 Yellowish Orange Ca-α-SiAlON:Eu Phosphor At the present, bi-chromatic white LED lamp consisting of a blue LED die and YAG:Ce is the best-selling product because of its high luminous efficacy. YAG:Ce is suitable for generating cool-white light and white light, the CCT of which is higher than 3000 K. On the other hand, sometimes the warm-white light of low CCT is demanded in general
Fig. 3
Preparation procedure of oxynitride and nitride phosphors.
illumination field. Ca-α-SiAlON:Eu is an optimal phosphor for this application, because its emission wavelength is longer than that of YAG:Ce. Figure 2 shows the excitation and emission spectra of a Ca-α-SiAlON:Eu yellowish orange phosphor. A fluorescence spectrophotometer (F-4500, Hitachi Ltd.) was used for the measurement. Spectral correction was done using Rhodamine-B, a light diffuser and a standard light source as references. The excitation and emission spectra of commercially available (Y,Gd)3 Al5 O12 :Ce yellow phosphor are also shown for comparison. The emission peak intensity of this Ca-α-SiAlON:Eu is about 135% compared to the conventional phosphor. 3.2 Preparation Procedure A preparation procedure of oxynitride and nitride phosphors is shown in Fig. 3. To obtain Ca-α-SiAlON:Eu, fine powders of Si3 N4 , AlN, CaCO3 and Eu2 O3 were used as starting materials. The element ratio of oxygen and nitrogen can be controlled when Ca3 N2 and EuN are used, but a glovebox with a nitrogen atmosphere is required in the case [18]. A planetary ball mill was used in mixing process. About the sintering process, Gas Pressure Sintering (GPS) was carried out at 1700–2000◦ C in a 0.5–1 MPa N2 atmosphere. Hot pressing is also available [12]. Recently, T. Suehiro et al. reported that Ca-α-SiAlON:Eu can be obtained by the
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reduction-nitridation process at relatively low temperatures of 1400–1500◦ C from oxide starting materials [19]. After synthesizing Ca-α-SiAlON:Eu, crushing and classifying processes were done to obtain the phosphor particles of suitable diameter for LED lamp packaging. 3.3 High Brightness Warm-White LED Lamps High brightness warm-white LED lamps of low CCT was realized by combination of a blue LED die and Ca-αSiAlON:Eu phosphor. The luminous efficacies, the value of which being total luminous flux divided by supplied electrical power, were 36.3 lm/W for the round shape through-hole LED lamp and 41.7 lm/W for the surface mount device (SMD) type chip LED, respectively [20], [21]. This value is two or three times higher than that of the conventional incandescent lamps. The luminous efficiencies of the radiation, calculated from the emission spectra, were 374 lm/W for the round shape one and 362 lm/W for the SMD type one, respectively. The LEDs were measured by a spectroradiometer (OL-770, Optronic Laboratories, Inc.) at the operating conditions of room temperature and a forward-bias current of 20 mA. Figure 4 shows the schematic diagram of the SMD type white LED. The phosphor powder is dispersed in a transparent resin and a blue LED die is coated by the resin. Figure 5 present the top view of the SMD type LED. The emission spectra of the SMD type LED is shown in Fig. 6. The chromaticity coordinates (x, y) on CIE1931
Fig. 4
Structure of a SMD type white LED.
Fig. 5
Top view of a SMD type white LED.
diagram is (0.453, 0.398) and CCT is 2700 K. This chromaticity is just in the range of class L which is defined in Japanese Industrial Standard (JIS) Z 9112 [22]. Class L is called the incandescent lamp color in other words. The general color rendering index (CRI) Ra is 57, which is slightly less than traditional fluorescent lamps of Ra 60. The general CRI Ra value is the average of the special CRI R1–R8 [23]. 3.4 Precision CCT Control For the general illumination purpose, the lamps of various CCTs are demanded, therefore it is very important to develop a method of precisely tailoring of the emission wavelength of phosphors for white LED lamps. It has been suggested that the emission wavelength of Ca-α-SiAlON:Eu phosphor could be tailored by changing its composition [12], [24], [25]. R.-J. Xie et al. have reported that the emission peak wavelengths of Ca-α-SiAlON:Eu are between 583 nm and 603 nm, corresponding to CCT range between 1900 K and 3300 K for white LED lamps [14], [18]. The general formula of Ca-α-SiAlON:Eu is described as Ca x Euy (Si,Al)12 (O,N)16 . To investigate the chromaticity dependence on composition, Ca-α-SiAlON:Eu of the compositions of x = 0.875 and y = 0.02–0.11 were prepared [26]. The dominant wavelengths of emission were varied from 578 nm to 583 nm with increasing the y value, Eu element concentration. The efficiencies of phosphors were high at y = 0.03–0.08. Concentration quenching was observed for y > 0.08. On the other hand, the Eu element concentration of y = 0.02 is too low to obtain enough emission. This dominant wavelength range luckily corresponds to the most CCT range of class L, when warm-white LED lamps are fabricated using these phosphors. The chromaticity coordinates of the two Ca-αSiAlON:Eu phosphors and two LED lamps are shown in Fig. 7. Two lines are connecting the chromaticity coordinates of 450 nm blue LED die and each phosphors. The chromaticity of fabricated LED lamps was on these lines depending on the amount of phosphors. By selecting optimum phosphor amount, the chromaticities were adjusted to blackbody locus. The CCT of the LED lamp using Ca0.875 Eu0.04 (Si,Al)12 (O,N)16 phosphor was 3020 K. For Ca0.875 Eu0.07 (Si,Al)12 (O,N)16 phosphor, it was 2640 K. The
Fig. 6
Emission spectra of a SMD type LED lamp.
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Fig. 7 Obtained CCT range of warm-white LED lamps by changing composition of Ca-α-SiAlON:Eu phosphor.
cially available class D LED lamp and the triangles represent a commercially available class L LED lamp. The Ca-αSiAlON:Eu class L LED lamp we fabricated is represented by circles. The black marks are the chromaticity coordinates at 25◦ C and the white marks are at 100◦ C, respectively. The chromaticity coordinates of the class D LED lamp varied from (0.317,0.312) to (0.307,0.305), and the class L LED lamp varied from (0.453,0.398) to (0.445,0.395). The chromaticity variation distances on CIE1931 diagram were 0.012 and 0.008, respectively. In contrast, the variation distance of the Ca-α-SiAlON:Eu class L LED lamp was only 0.002 from (0.438,0.396) to (0.437,0.398). The chromaticity of Ca-α-SiAlON:Eu LED lamp is extremely thermally stable compared to commercially available conventional white LED lamps. When temperature is raised, two phenomena are observed. One of them is red shift of the blue LED die and another is the temperature quenching of the phosphor. The temperature quenching of (Y,Gd)3 Al5 O12 :Ce yellow phosphor is relatively large and one kind of YAG:Ce phosphors like this may be used in conventional white LED lamps. The excitation band of (Y,Gd)3 Al5 O12 :Ce, as shown in Fig. 2, is not enough wide, therefore additional quenching may be caused by the red shift of the blue LED excitation light source. As a result, the chromaticity of conventional white LED lamp shifts toward blue and the variation become large. On the other hand, the thermal quenching of Ca-αSiAlON:Eu phosphor is negligible and the excitation band width of it is very wide. Hence the remarkable quenching of Ca-α-SiAlON:Eu phosphor would not be caused, even if the temperature of Ca-α-SiAlON:Eu warm-white LED lamp is raised to 100◦ C. This can explain the fact that the locus of chromaticity variation is almost parallel to the blue region of spectral locus (locus of red shift of blue LED die) and the variation distance is so small for the Ca-α-SiAlON:Eu class L LED lamp. 4.
Fig. 8 lamps.
Thermal stability of chromaticity coordinates of white LED
luminous efficacy of these LED lamps was as high as about 30 lm/W. 3.5 Chromaticity Stability at High Temperature α-SiAlON has been developed as a high temperature structural material [27] and the optical properties of Ca-αSiAlON:Eu are also stable at high temperature [15], [24]. The thermal stability of the chromaticity between the room temperature of 25◦ C and 100◦ C was investigated for three LED lamps and the chromaticity coordinates of them are shown in Fig. 8. The rectangles represent a commer-
High CRI Quad-chromatic White LED Lamps
Recent years, high color quality lamps with higher CRI Ra than 80 are recommended for lighting of various indoor work places [28]. The CRI Ra value is about 70–75 for conventional white and warm-white LED lamps including YAG:Ce and less than 60 for the Ca-α-SiAlON:Eu warmwhite LED lamp. The full width at half maximum (FWHM) of the emission spectra is 127 nm for (Y,Gd)3 Al5 O12 :Ce and 92 nm for Ca-α-SiAlON:Eu, respectively. As shown in Fig. 2, the problem is lack of red light and green light. In order to improve the color rendering, CaAlSiN3 :Eu red phosphor and β-SiAlON:Eu phosphor were added [29]. The normalized emission spectra of blue LED die and three phosphors are shown in Fig. 9. By combination of them, quadchromatic white LED lamps with high color rendering can be realized. Using the mixture of these three oxynitride and nitride phosphors, five white LED lamps with various CCT were fabricated. The chromaticity coordinates of these quad-chromatic LED lamps of class D, class N, class W,
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Fig. 12 minant. Fig. 9
Fig. 10
Emission spectra of class L LED lamp and A CIE standard illu-
Normalized emission spectra.
Chromaticity coordinates of white LED lamps with various CCT.
Table 1
High CRI LEDs.
D
N
W
WW
L
CIE x y
0.311 0.333
0.345 0.358
0.373 0.370
0.407 0.392
0.449 0.408
CCT [K]
6,580
5,010
4,160
3,470
2,840
CRI Ra R9 R15
81 95 93
82 97 97
83 78 93
86 99 99
88 97 98
LEfficacy [lm/W] LEfficiency [lm/W]
28 270
25 272
27 282
24 267
25 254
Fig. 13 Color space of oxynitride and nitride phosphor quad-chromatic LED lamps, NTSC and s-RGB on CIE1931 diagram.
Fig. 14 gram.
Fig. 11 Emission spectra of class D LED lamp and D65 CIE standard illuminant.
Decorative various color LED lamps placed on CIE1976 dia-
class WW and class L are shown in Fig. 10. The CCT of them is controlled by changing the mixing ratio of the phosphors and their chromaticities were adjusted to the blackbody locus by selecting the optimum amount of the phosphor mixture to coat the blue LED die.
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The chromaticity coordinates, the CCT, the CRI values of Ra, R9, R15, the luminous efficacy and the luminous efficiency of each LED lamps are shown in Table 1. The CRI Ra of them is higher than 80. The special CRI R9 and R15 of them are significantly high. R9 corresponds to strong red, the value of which for conventional white LED lamps is not good enough [30]. R15 corresponds to the color of the skin of the face of the Japanese women, which is very important for indoor lightings. The emission spectra of class D and class L LED lamps are shown in Fig. 11 and Fig. 12, respectively. The spectral distribution of CIE standard colorimetric illuminants are also shown [31]. 5.
Decorative Various Color Lamps
Not only white lamps just on the blackbody locus, but also decorative various color lamps can be realized by the quadchromatic oxynitride and nitride phosphor LED lamps. Figure 13 shows the quadrilateral color space on CIE1931 diagram, in which the chromaticity could be realized by the quad-chromatic LED lamp consisting of a blue LED die, CaAlSiN3 :Eu red phosphor, Ca-α-SiAlON:Eu yellow phosphor and β-SiAlON:Eu green phosphor. The extent of it is 82% of NTSC and 116% of s-RGB [32], [33]. The fabricated various color LED lamps are presented in Fig. 14. Each LED lamps are placed at corresponding chromaticity coordinates on CIE1976 diagram. These color lamps would be useful for applications such as decorative illuminations. Fixed design electronic display signs are also promising applications which would alternate conventional signs such as neon signs. 6.
Conclusions
The oxynitride and nitride phosphors which can be excited by blue visible light were introduced. The bi-chromatic high brightness warm-white LED lamp consisting of a blue LED die and Ca-α-SiAlON:Eu yellow phosphor of 41.7 lm/W luminous efficacy and 2700 K CCT was presented. The chromaticity of it was extremely thermally stable compared to conventional white LED lamps. The quad-chromatic high CRI various CCT white LED lamps consisting of a blue LED die, CaAlSiN3 :Eu red phosphor, Ca-α-SiAlON:Eu yellow phosphor and β-SiAlON:Eu green phosphor were demonstrated. The CRI Ra values were higher than 80. The LED lamps using oxynitride and nitride phosphors will play an important role in the coming era of solid-state lighting. References [1] H. Amano, M. Kito, K. Hiramatsu, and I. Akasaki, “P-type conduction in Mg-doped GaN treated with low-energy electron beam irradiation (LEEBI),” Jpn. J. Appl. Phys., vol.28, pp.L2112–L2114, 1989. [2] S. Nakamura, T. Mukai, and M. Senoh, “Candela-class highbrightness InGaN/AlGaN double-heterostructure blue-light-emitting diodes,” Appl. Phys. Lett., vol.64, pp.1687–1689, 1994. [3] I. Akasaki, S. Kamiyama, and H. Amano, “The evolution of nitridebased light-emitting devices,” IEICE Trans. Electron., vol.E85-C,
no.1, pp.2–9, Jan. 2002. [4] S. Nakamura, “Present performance of InGaN based blue/green/ yellow LEDs,” Proc. SPIE, vol.3002, pp.26–35, 1997. [5] The Japan Research and Development Center of Metals’ National Project on Light for the 21st Century, Year 2000 Report of Results. [6] D. Malakoff, “Lighting initiative flickers to life,” Science, vol.296, p.1782, June 2002. [7] K. Bando, K. Sakano, Y. Noguchi, and Y. Shimizu, “Development of high-bright and pure-white LED lamps,” J. Light Vis. Environ., vol.22, no.1, pp.2–5, 1998. [8] R. Mueller-Mach, G.O. Mueller, M.R. Krames, and T. Trottier, “High-power phosphor-converted light-emitting diodes based on IIInitrides,” IEEE J. Sel. Top. Quantum Electron., vol.8, no.2, pp.339– 345, 2002. [9] T. Taguchi, “Present status of white LED lighting technologies in Japan,” J. Light Vis. Environ., vol.27, no.3, pp.131–139, 2003. [10] K. Uheda, N. Hirosaki, H. Yamamoto, H. Yamane, Y. Yamamoto, W. Inami, and K. Tsuda, “The crystal structure and photoluminescence properties of a new red phosphor, calcium aluminum silicon nitride doped with divalent europium,” Abstract 2073, The Electrochemical Society 206th Meeting, Honolulu, HI, Oct. 2004. [11] K. Uheda, N. Hirosaki, H. Yamamoto, and R.-J. Xie, “Red phosphors for warm white light-emitting diodes,” The 305th Meeting Technical Digest, Phosphor Research Society, pp.37–47, Nov. 2004. [12] R.-J. Xie, M. Mitomo, K. Uheda, F.F. Xu, and Y. Akimune, “Preparation and luminescence spectra of calcium- and rare-earth (R = Eu, Tb, and Pr)-codoped α-SiAlON ceramics,” J. Am. Ceram. Soc., vol.85, no.5, pp.1229–1234, 2002. [13] J.W.H. van Krevel, J.W.T. van Rutten, H. Mandal, H.T. Hintzen, and R. Metselaar, “Luminescence properties of terbium-, cerium-, or europium-doped α-sialon materials,” J. Solid State Chem., vol.165, pp.19–24, 2002. [14] R.-J. Xie, N. Hirosaki, K. Sakuma, Y. Yamamoto, and M. Mitomo, “Eu2+ -doped Ca-α-SiAlON: A yellow phosphor for white lightemitting diodes,” Appl. Phys. Lett., vol.84, pp.5404–5406, 2004. [15] K. Sakuma, K. Omichi, N. Kimura, M. Ohashi, D. Tanaka, N. Hirosaki, Y. Yamamoto, R.-J. Xie, and T. Suehiro, “Warm-white lightemitting diode with yellowish orange SiAlON ceramic phosphor,” Opt. Lett., vol.29, pp.2001–2003, 2004. [16] N. Hirosaki, R.-J. Xie, K. Kimoto, T. Sekiguchi, Y. Yamamoto, and T. Suehiro, “Luminescent properties of green phosphor, βSiAlON:Eu2+ ,” 52th Spring Meeting, 2005, JSAP and Related Societies, 30a-YH-7, 2005. [17] N. Hirosaki, R.-J. Xie, K. Kimoto, T. Sekiguchi, Y. Yamamoto, T. Suehiro, and M. Mitomo, “Characterization and properties of greenemitting β-SiAlON:Eu2+ powder phosphors for white light-emitting diodes,” Appl. Phys. Lett., vol.86, 211905, 2005. [18] R.-J. Xie, N. Hirosaki, M. Mitomo, Y. Yamamoto, T. Suehiro, and K. Sakuma, “Optical properties of Eu2+ in α-SiAlON,” J. Phys. Chem. B, vol.108, pp.12027–12031, 2004. [19] T. Suehiro, N. Hirosaki, R.-J. Xie, and M. Mitomo, “Powder synthesis of Ca-α -SiAlON as a host material for phosphors,” Chem. Mater., vol.17, pp.308–314, 2005. [20] K. Sakuma, N. Hirosaki, N. Kimura, K. Omichi, Y. Yamamoto, R.-J. Xie, T. Suehiro, M. Ohashi, and D. Tanaka, “High luminous efficacy warm-white light-emitting diode lamp using α-SiAlON phosphor,” 65th Autumn Meeting, 2004, JSAP, p.1284, 2p-ZL-15, 2004. [21] N. Kimura, N. Hirosaki, K. Sakuma, K. Asano, and D. Tanaka, “High luminous efficiency warm white light-emitting diode using α-SiAlON phosphor,” Proc. IEICE Gen. Conf. 2005, C-9-1, 2005. [22] JIS Z 9112-1990, “Classification of fluorescent lamps by chromaticity and colour rendering property,” Japanese Standards Association. [23] JIS Z 8726-1990, “Method of specifying colour rendering properties of light sources,” Japanese Standards Association. [24] A. Ellens, G. Huber, and F. Kummer, “Illumination unit having at least one LED as light source,” United States Patent no.6,657,379, 2003.
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[25] R.-J. Xie, N. Hirosaki, M. Mitomo, Y. Yamamoto, T. Suehiro, and K. Sakuma, “α-SiAlON-based oxynitride/nitride phosphors: Synthesis, properties and applications,” Proc. 11th International Display Workshops (IDW’04), PH4-1 (Invited), pp.1105–1108, Niigata, Japan, 2004. [26] K. Sakuma, N. Hirosaki, N. Kimura, Y. Yamamoto, R.-J. Xie, T. Suehiro, K. Omichi, M. Ohashi, and D. Tanaka, “High brightness warm-white LED lamps using Ca-α-SiAlON phosphors,” Proc. 11th International Display Workshops (IDW’04), PHp-1, pp.1115–1118, Niigata, Japan, 2004. [27] G.Z. Cao and R. Metselaar, “α -sialon ceramics: A review,” Chem. Mater., vol.3, pp.242–252, 1991. [28] ISO 8995:2002, CIE S 008/E:2001, “Lighting of indoor work places,” Joint ISO/CIE Standard. [29] K. Sakuma, N. Hirosaki, N. Kimura, K. Masuko, Y. Yamamoto, R.J. Xie, T. Suehiro, K. Asano, and D. Tanaka, “High color-rendering index white light-emitting diode lamps using oxynitride phosphors,” 52th Spring Meeting, 2005, JSAP and Related Societies, 30a-YH-8, 2005. [30] M. Yamada, T. Naitou, K. Izuno, H. Tamaki, Y. Murazaki, M. Kameshima, and T. Mukai, “Red-enhanced white-light-emitting diode using a new red phosphor,” Jpn. J. Appl. Phys., vol.42, pp.L20–L23, 2003. [31] JIS Z 8781-1999, “CIE standard colorimetric illuminants,” Japanese Standards Association. [32] International Telecommunication Union Radiocommunication Sector Recommendation ITU-R BT.470-6 (1970–1998). [33] International Electrotechnical Commission (IEC) Standard no. 61966-2-1: 1999.
Ken Sakuma was born in Chiba, Japan in 1969. He received the B.E. and M.E. degrees in materials science and metallurgy from the University of Tokyo, Japan, in 1993 and 1995 respectively. Since joining Fujikura Ltd. in 1995, he had been engaged in research and development on the optical fiber testing systems, optical fiber arrays and planar lightwave circuit devices for optical fiber communication systems. He is currently a visiting researcher of National Institute for Materials Science (NIMS) since July 2003. His current research interest includes oxynitride and nitride phosphor materials for white LED lamps. He received the Outstanding Poster Paper Award at the 11th International Display Workshops (IDW’04) in 2004. He is currently a member of Optical Society of America, The Japan Society of Applied Physics, The Ceramic Society of Japan, and Phosphor Research Society.
Naoto Hirosaki is a senior researcher of National Institute for Materials Science (NIMS). He received a B.E., M.E., and D.E. degrees in industrial chemistry from Kyoto University in 1978, 1980, and 1990, respectively. He worked for 17 years at Nissan Motor Co. Ltd. He joined NIRIM in 1998. His research interests are sialon-based phosphor, synthesis of new nitride materials, and sintering process of silicon nitride.
Naoki Kimura was born in 1975. He received the B.E. and M.E. degrees in applied physics from Tohoku University, Sendai, Japan in 1998 and 2000, respectively. Since he joined Fujikura Ltd. in 2000, he has been engaged in research and development of optical fiber components.
Masakazu Ohashi was born in Tokyo, Japan in 1964. He received the B.E. and M.E. degrees in chemicals engineering from the Shinshu University, Nagano, Japan, in 1988, and 1990, respectively. In 1990, he joined Fujikura Ltd., where he has been engaged in research and development of industrial wires and optical devices.
Rong-Jun Xie was born in Jiangsu Province, China in 1969. He received the M.E. (Metallurgy) and PhD degrees (Ceramics) from Xi’an Jiaotong University and Shanghai Institute of Ceramics, Chinese Academy of Sciences, in 1995 and 1998, respectively. Since joining National Institute for Materials Science (NIMS) in 2003, he has been engaged in research and development of oxynitride phosphors for white light-emitting diodes. He is a member of The American Ceramic Society, The Japan Society of Applied Physics, and The Ceramic Society of Japan.
Yoshinobu Yamamoto worked at Nissan Motor Co. Ltd. till 1999, majored in chemical analysis and development of catalyst. Then moved to National Institute for Research in Inorganic Materials (NIRIM, currently NIMS) and worked on chemical analysis and high-temperature materials till 2003. Since 2004, he works on chemical analysis and hightemperature synthesis (sintering).
Takayuki Suehiro was born in 1974. He received his B.E. (1997), M.E. (1999), and Ph.D. (2002) in Materials Science from Yokohama National University. He joined NIMS as a postdoctoral researcher in 2002, working with Dr. Naoto Hirosaki. His research interests focus on the powder synthesis and characterization of nitride materials. He is currently a member of The American Ceramic Society and The Ceramic Society of Japan.
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Kenichiro Asano was born in 1966. He graduated in opto-electronics engineering from Technos International College. He joined Fujikura Ltd. in 1989 and has been engaged in research and development of optical devices.
Daiichiro Tanaka was born in Tokyo, Japan in 1961. He received the B.E. degree in electronics engineering from Chiba University, Chiba, Japan, in 1985. In 1985, he joined Fujikura Ltd., where he has been engaged in research work in optical fibers, optical fiber amplifiers and optical components. He is a member of the Illuminating Engineering Institute of Japan.
