Chinese Science Bulletin
? 2008 SCIENCE IN CHINA PRESS
https://www.doczj.com/doc/4510111791.html, | https://www.doczj.com/doc/4510111791.html, Chinese Science Bulletin | October 2008 | vol. 53 | no. 19 | 2923-2930R E V I E W C O N D E N S E D S T A T E P H Y S I C S
The development of silicate matrix phosphors with broad excitation band for phosphor-convered white LED
LUO XiXian?, CAO WangHe & SUN Fei
Optoelectronic Technology Institute, Department of Physics, Dalian Maritime University, Dalian 116026, China
This paper briefly reviews the recent progress in alkaline earth silicate host luminescent materials with broad excitation band for phosphor-convered white LED. Among them, the Sr-rich binary phases (Sr, Ba, Ca, Mg)2SiO4:Eu2+ and (Sr, Ba, Ca, Mg)3SiO5:Eu2+ are excellent phosphors for blue LED chip white LED. They have very broad excitation bands and exhibit strong absorption of blue radiation in the range of 450―480 nm. And they exhibit green and yellow-orange emission under the InGaN blue LED chip radiation, respectively. The luminous efficiency of InGaN-based (Sr, Ba, Ca, Mg)2SiO4:Eu2+ and (Sr, Ba, Ca, Mg)3SiO5:Eu2+ is about 70—80 lm/W, about 95%―105% that of the InGaN-based YAG:Ce, while the correlated color temperature is between 4600―11000 K. Trinary alkaline earth silicate host luminescent materials MO(M=Sr, Ca, Ba)-Mg(Zn)O-SiO2 show strong absorption of deep blue/near-ultraviolet radia-tion in the range of 370―440 nm. They can convert the deep blue/near-ultraviolet radiation into blue, green, and red emissions to generate white light. The realization of high-performance white-light LEDs by this approach presents excellent chromaticity and high color rendering index, and the application disadvantages caused by the mixture of various matrixes can be avoided. Moreover, the application prospects and the trends of research and development of alkaline earth silicate phosphors are also discussed.
white LED, broad excitation band, silicate matrix
As a new solid-state light source, the white light emit- ting diode (W-LED) has been widely used in lighting and displaying due to its low power consumption, envi- ronmental friendliness, long lifetime, compact size, etc. Among the technological strategies of obtaining W-LED, phosphor-converted white LED (pcW-LED, such as dichromatic pcW-LED, polychromatic pcW-LED) is particulaly attractive[1]. For example, white light can be generated via pumping a yellow phosphors (λem = 520―580 nm) using a GaInN blue LED (λem = 450―480 nm). And this method has become the most common strategy. The phosphors for pcW-LED material must present three characteristics at least: first of all, strong absorp-tion of blue or ultraviolet/violet radiation, namely broad excitation band in 360―480 nm region; second, excel-lent physical and chemical stability, no reaction with encapsulation resin and utravioletproof; third, high thermal stability and luminescence quenching tempera-ture (>150℃), since the radiation density of LED is about 200 W/cm2, which is about three orders of magni-tude higher than that of traditional fluorescent lamp. However, up to now, there are few yellow phosphors emitting efficiently under the 450―480 nm excitation range except YAG:Ce[2,3]. Therefore, investigation on materials for pcW-LED has become a leading project in Received January 13, 2008; accepted March 26, 2008
doi: 10.1007/s11434-008-0392-4
?Corresponding author (email: luoxixiandl@https://www.doczj.com/doc/4510111791.html,)
Supported by the High-Tech Research and Development Program of China (Grant Nos. 2004AA001530 and 2006AA03A137) and Dalian Maritime University Youth Teacher Foundation Program (Grant No. DLMU-ZL-200713)
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the field of luminescent material [4―
6].
Phosphor consists of matrix and activator. Some well- known activator ions, such as Eu 2+, Yb 2+, Ce 3+, etc., ex-hibit 5d → 4f transition which presents broad absorption and emission bands characteristic. The crystal field environment and nephelauxetic effect in different host lattices make the emission spectra vary from ultraviolet to infrared region, which means that they are suitable activators for pcW-LED phosphors. The luminous effi-ciency depends on the matrix. Any variations in host composition may alter the energy transfer, the strength of the crystal field and the covalency, thus influencing the luminous efficiency and property.
With wonderful chemical and thermal stability, as well as the low price of high-purity silicate, the silicate host phosphors are sighificant photoluminescence and CRT phosphors [7,8]. The ion radii of alkaline earth ions are similar to that of divalent europiums, such as [Eu 2+] = 0.112 nm, [Ca 2+] = 0.099 nm, [Sr 2+] = 0.112 nm, [Ba 2+] = 0.134 nm. Divalent europium is consequently more stable in alkaline earth silicate host and easier to diffuse into the alkaline earth silicate lattice site. Barry [9,10],
Blasse [11―
13] and Dorenbos [14], had investigated the lu-minescence of Eu 2+ in alkaline earth silicate systemati-cally. However, only excitation spectrum data below 350 nm is available in these early publications. There exist many binary and trinary compounds in the MO(M=Sr, Ca, Ba, Zn)-Mg(Zn)O-SiO 2 silicate system (Figure 1). In the study on long afterglow phosphors which can be excited by daylight, Xiao’s team [15,16] first found that Eu 2+ ions presents broad excitation band characteristic
(its excitation spectum reaches 500 nm) in these com-
Figure 1 The phase digram of MO(M=Sr, Ca, Ba, Zn)-Mg(Zn)O-SiO 2 system. pounds, and the emission peak is adjustable at 450—580 nm which is broader than that of YAG :Ce, indicating that it is a promising luminescent material for pcW- LED.
1 Binary silicate system
As shown in Figure 1, binary silicate compounds mainly consist of M 3SiO 5, M 2SiO 4, M 3Si 2O 7, MSiO 3, M 2Si 3O 8, M 5Si 8O 21, M 3Si 5O 13, MSi 2O 5(M ＝Mg, Ca, Sr, Ba, Zn), etc. But because of low quenching temperature, MSiO 3: Eu 2+ is unsuitable to be pcW-LED material. Thus more attention has long been paid to alkaline earth orthosili-cate matrix, and remarkable progress has been made [17]. In the alkaline earth orthosilicate [18,19], the structure of Ba 2SiO 4 is orthorhombic and isotypic with β-K 2SO 4, whereas Sr 2SiO 4 owns two type structures, viz. β- Sr 2SiO 4 and α’-Sr 2SiO 4. The former, low temperature phase of Sr 2SiO 4, is monoclinic and isostrucural with β-Ca 2SiO 4; and the latter that forms at temperature above 85 is orthorhombic, whose struct ℃ure is isotypic with Ba 2SiO 4. The stable phase of α’-Sr 2SiO 4 can also be obtained by partly substiting Sr with Ba at room temperature. For example, the pure phase of α’-Sr 2SiO 4 can be genarated at a Ba content as low as 2.5%.
There are two different cation sites in M 2SiO 4 (M = Ca, Sr, Ba) lattice: large size M(I) and small size M(II), coordinated by ten and nine oxygen ions, respectively [20]. Eu 2+ takes up the site of M(I), which forms a large bond length and weak crystal field, while Eu(II) is situated at the site of M(II), forming a short bond length and strong crystal field [21]. Therefore, as shown in Figure 2, M 2SiO 4: Eu 2+ present two emission bands. Eu(I) results in the short-wavelength band while Eu(II) results in the long- wavelength band. This phenomenon is particularly ob-vious in Sr 2SiO 4:Eu 2+ (λem1=495 nm and λem2=570 nm). Ba 2SiO 4:0.01Eu 2+ and Ca 2SiO 4:0.01Eu 2+ also show similar results at low temperature (4.2 K), whose emis-sion spectrum splits into two peaks with close intensity at 505 and 520 nm [22,23]. But the two peaks overlap into one (~500 nm) at room temperature.
Various emissions can be generated through altering composition of M 2SiO 4[25]. Figure 3 shows the depend-ence of emission peak position on species and content of alkaline earth ions in M 2SiO 4. The emission peaks at 515, 575 and 505 nm correspond to Ca 2SiO 4, Sr 2SiO 4 and Ba 2SiO 4, respectively. When mixing two kinds of
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Figure 2 Emission spectra of Ca2SiO4:Eu2+, Sr2SiO4:Eu2+ and Ba2SiO4: Eu2+ (λex = 370 nm)[24].
Figure 3 The emission peak wavelength and the alkaline earth in M2SiO4:0.02Eu (M=Ca, Sr, Ba).
alkaline earth ions randomly, the emission peaks can be changed readily, and we may accordingly design the composition of matrix so as to obtain the diserable emission.
Although M2SiO4:Eu shows favorable optical prop-erties, in fact, the excitation band of pure M2SiO4:Eu2+ is not broad enough and their excitation wavelength is not long enough. For example, the excitation peak of Sr2SiO4:0.03Eu2+ lies at about 370 nm, but the excita-tion efficiency declines dramatically with exitation wavelength beyond 390 nm, and show very weak ab-sorption in the range of 450―470 nm blue radiation (Figure 4(a) and (b)). Therefore, the pure M2SiO4:Eu2+ are only suitable for long-wavelength ultraviolet or short-wavelength blue light LED chip[26,27]. Currently, the key point is how to extend the excitation spectrum to blue region (450―480 nm). It is well-known that the d→f transition of Eu2+ is strongly affected by the around crystallography environment. On one hand, splitting of 5d level depends on the size and shape of coordination polyhedron, i.e. the effect of crystal field. On the other hand, the centriod of 5d level is related to the character of ions and ligand around. In other words, the center of gravity of 5d level depends on the character of chemical bond and the polarizability of the ligand: center of grav-ity decreasing with covalency increasing, which results in red shift of emission peak. Properly codoping with other alkaline earth ions will broaden the emission tail of Sr2SiO4:0.03Eu2+, intensify the absorption of 450—470 nm blue radiation, and improve its luminous efficiency. For example, the excitation peak of Sr1.95Ba0.05SiO4: Eu2+ shifts to a long-wavelength of about 390 nm, and the tail reaches 480 nm, which makes it possible to cre-ate white light from a combination of a blue LED emis-sion like the YAG phosphor (Figure 4(b))[28]
.
Figure 4 The excitation spectra of Eu2+ doped silicate host phosphor and YAG:Ce. (a) Ba2SiO4:Eu; (b) Sr1.95Ba0.05SiO4:Eu; (c) YAG:Ce; (d) optimized Sr2SiO4:Eu2+; (e) optimized Sr3SiO5:Eu2+; (f) Eu2+ doped alkaline earth silicon nitrides.
Sr2SiO4 and Ba2SiO4 can form continuous solid solu- tion. By partly replacing Ba by Sr, the excitation and emission spectra of Sr x Ba2?x SiO4:Eu2+ can be obviously widened. When the content of Sr ranges from 1.06 to 1.64, the excitation spectrum is 300―480 nm, while the position of emission peak is determined by Ba content as shown in Figure 3. In addition, the increase in the Ba2+ content also enhances the excitation and emission inten-sity, and two emission peaks overlap into one, but emis-sion peak presents blue shift, which is undesirable for pcW-LED. The chemical stability of Ba2SiO4:Eu2+ is worse than that of Sr2SiO4:Eu2+, but the chemical sta-
bility of solid solution Sr x Ba2?x SiO4 is better than that of single phase.
β-Ca2SiO4 and Sr2SiO4 can form continuous solid so-lution at 1200℃. Contrary to Ba, doping Ca ion in Sr2SiO4:Eu2+ greatly decreases the luminous efficiency; furthermore, with the increase of Ca, the absorption ef-ficiency in visible area decreases, and the excitation band becomes narrower. Besides, melting occurs above 1250℃ with the addition of Ca, which is detrimental to the preperation process.
The luminous efficiency of Eu2+ in Ca x Ba2?x SiO4 is quite low. Substituting Mg for partial Sr in Sr x Mg2?x SiO4 (0.5≤x≤1.8), the increase of Mg does not change the shape of excitation spectrum (280―440 nm), but it de-creases the excitation and emission intensity. Generally speaking, the Sr-rich Sr x Ba2?x SiO4:Eu2+ phase presents higher luminous efficiency which is, to a great extent, also determined by the preperation approaches and processes[29], such as the firing temperature and time. There exist other approaches to improving the excita-tion property of Sr2SiO4:Eu2+. Sr2?x?y Ba y SiO4:Eu x ex-hibits emission peak red shift with the increasing Eu2+ concentration[26]: [Eu2+]=0.005, λem = 519 nm, [Eu2+] = 0.03, λem = 531 nm, [Eu2+] = 0.05, λem = 536 nm, [Eu2+] = 0.1, λem = 543 nm. The increase in Eu2+ concentration shortens the distance between Eu2+ ions, thus leading to more chances of energy transfer. The possibility of Eu2+ staying at 5d higher level (this may result in energy transfer to 5d lower level) increases with the concentra-tion of Eu2+, resulting in red shift of emission spec-trum[30].
Moreover, emission band of Sr1.97?y Ba y SiO4:0.03Eu shifts to a longer wavelength with the increasing SiO2 content. When the ratio of Sr to Si changes from 2/0.5 to 2/0.8, 2/1.0, 2/1.3, the excitation spectrum consists of two bands peak around 332 and 382 nm (Sr/Si=2/0.5). These two bands have almost merged and broadened to appear as a broad single band, and the 382 nm excitation peak shifts to 384 nm (Sr/Si = 2/0.8), 387 nm (Sr/Si = 2/1.0), and 394 nm (Sr/Si = 2/1.3). The emission spec-trum is a single broad band with peak shifting from 523 nm (Sr/Si = 2/0.5) to 527 nm (Sr/Si = 2/0.8), 533 nm (Sr/Si = 2/1.0), 555 nm (Sr/Si = 2/1.3), and in the mean time, Stokes shift and crystal field splitting (CFS) both increase. The increase in SiO2 content enhances the de-gree of covalency, and weakens the interaction between the electrons. As a result, they spread out over wider orbitals[31,32]. Consequently, the energy difference be-tween ground state (4f7) and excited state (4f65d1) de-creases with the increase of covalency, and enhances the degree of the t2g and e g levels split, leading to larger CFS and red shift of emission peak.
In addition, codoping with suitable amount of Ba2+ and Mg2+ increases the excitation efficiency of Sr2- SiO4:Eu2+ in the range of 450―470 nm; thus the lumi-nous efficiency is increased. The Stokes shifts of Ba, Mg codoped and Ba-doped Sr2SiO4:Eu2+ are 3404 and 3984 cm?1 respectively; however, the Stokes shift of pure Sr2SiO4:Eu2+ is 5639 cm?1, which may be the reason of higher luminous efficiency of Ba, Mg codoped Sr2SiO4:Eu2+[33].
Using the methods mentioned above, the excitation peak band of Sr2SiO4:Eu2+ can be broadened to about 470 nm. Figure 4(d) shows the excitation spectrum of optimized Sr2SiO4:Eu2+ whose excitation spectrum shape between 450―480 nm is simialr to that of YAG: Ce. Adopting InGaN (470 nm chip)-based LED, the package luminous efficiency reaches 70―80 lm/W, and the correlated color temperature is between 4600 and 11000 K, which is comparable with that of the same chip based-YAG:Ce[34,35].
Another significant matrix is M3SiO5. Ba3SiO5 and Sr3SiO5 have tetragonal structure, and there exist two M ion sites with equal amount in the lattice. In the Eu2+ doped sample, Eu2+ ions occupy two different sites in the Ba3SiO5 lattice and give rise to two emission bands at about 504 and 566 nm respectively, and the peak of 566 nm is much stronger. Contrary to Ba2SiO4:Eu2+, with the increase of Eu2+ concentration, Ba3SiO5:Eu2+ shows different spectrum characteristics. Ba3SiO5:0.01 Eu2+ shows one broad emission band at 568 nm. As the concentration of Eu2+ increases, however, the emission band of Ba3SiO5:0.15Eu2+ phosphor is split into two emission bands with a main peak at 504 nm and a less intense band at 568 nm[36].
Sr3SiO5:Eu2+, as a yellow phosphor, can be excited by 450—470 nm blue light[37―39]. Furthermore, it pre-sents broader excitation spectrum and higher excitation efficiency than that of Sr2SiO4:Eu2+ (Figure 4(e))[34]. When the Eu2+ concentration is not more than 0.15 mol, the emission spectrum of Sr3SiO5:Eu2+ is the broadband with a peak at 570 nm. For Sr3SiO5:0.07Eu, the emis-sion intensity from λex = 460 nm has been estimated to
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be 93% by comparing the emission intensity from λex = 365 nm. And quantum efficiency of Sr 3SiO 5:Eu is 82%, higher than 70% of that for Zn 2SiO 4:Mn [38]. Also, emis-sion peak of Sr 3SiO 5:Eu 2+ shows red shift as the SiO 2 content increases. When the ratio of Sr to Si changes from 3/0.8 to 3/0.9, 3/1.0, 3/1.1, the 559 nm emission peaks shift to 564, 568, 570 nm, and Stokes shift and CFS increase with the increase of SiO 2 content.
Codoped with Ba 2+, emission of Sr 3SiO 5:Eu 2+ under 450―470 nm blue radiation shifts to longer wave-length [40]. Increasing the Ba 2+ content from 0 to 0.2 mol, the emission peak shifts from 570 to 585 nm (opposite to Sr 2SiO 4:Eu 2+). In Sr 2.93?x Ba x SiO 5:0.07Eu 2+, a differ-ence in the ionic radius between Ba 2+ ions and Sr 2+ ions results in the lattice parameter increasing with Ba 2+ con-tent. As the length of c axis increases by replacing part of the Sr 2+ by Ba 2+ ions, the effect of preferential orien-tation of Eu 2+ d orbital decreases, and at the same time, an increase in the Ba 2+ amount also decreases the octa-hedral symmetry at around Sr 2+ ion, so the Eu 2+ emis-sion shifts to a longer wavelength.
Another advantage of Sr 3?x Ba x SiO 5:Eu 2+ is that when luminous efficiency reaches the maximum, Sr 2?x Ba x SiO 4: Eu 2+ presents emission peak wavelength in green region (about 530 nm) and undesirable color rendering (Ra: 70― 76). However, emission peak wavelength of Sr 3?x Ba x SiO 5: Eu 2+ is about 570 nm when luminous efficiency is the highest, which helps to improve color rendring. Efficient pcW-LED can be prepared by single Sr 2SiO 4:Eu 2+ phosphor (lack of orange-red color) or (BaSr)3SiO 5:Eu 2+ phosphor (lack of green color), but the color rendering is quite low. Through integration of the InGaN blue LED chip and the two-phosphor blends (Sr 2SiO 4:Eu 2+ yellow phosphor + (BaSr)3SiO 5:Eu 2+ yellow-orange phosphor), the red region in spectrum is strengthened, and desired general illumination warm white pcW-LED is achieved with correlated color temperature in the range of 2500—5000 K and a good color rendering of over 85 is also obtained, compared to the same InGaN-based YAG :Ce, whose color temperature is 6500 K.
Other improvements are as follows: (1) Some other matrixes are achieved except MSiO 3, M 2SiO 4 and M 3SiO 5 in Figure 1 (such as Ca 3Si 2O 7:Eu 2+). (2) Re-placement or addition of other cations. Part of the alka-line earth ions are replaced by alkaline and/or rare earth elements, such as Li 2CaSiO 4:Eu 2+[41], Li 2SrSiO 4:Eu 2+[42,43],
CaLa 4Si 3O 13:Eu 2+, or substituting other elements, such
as Al, B, Ge, P, for Si [44,45]. (3) Replacement of anion. Anion can be replaced by halogen, such as Ca 3SiO 4Cl 2: Eu 2+[46,47] and SrLiSiO 4F [48]. Part or all of O can also be replaced by N, forming a new oxynitride pcW-LED phosphor, whose excitation spectrum is further broad-ened and emission spectrum reaches red region, being the best red pcW-LED phosphor. (4) Improvement of activator is also in progress [49,50].
Temperature change of LED junction affects light output, wavelength and width of spectrum, so tempera-ture-dependent property is significant for LED phos-phor [51]. With temperature increasing, two emission bands of Sr 2SiO 4:Eu 2+ exhibit red shift, emission band widening and emission intensity decreasing, and color rendering coordinate shifts to red region [24]. Compared to 4.2 K, emission intensity of Ba 2SiO 4:0.01Eu 2+ is halved at 430 K, while emission intensity decreases by 90% at 550 K. Sr 2SiO 4:Eu 2+ has better temperature- dependent property than that of Ba 2SiO 4:Eu 2+, while the temperature-dependent property of solid solution Sr x Ba 2?x SiO 4:Eu 2+ is better than that of single phase. The emission intensity of BaSrSiO 4:0.02Eu 2+ at 100℃ decreases to less than 5% (compared to room tempera-ture). In the range of 25―250℃, Sr 3SiO 5:Eu has better temperature-dependent property than YAG :Ce. Emis-sion intensity of YAG :Ce dramatically decreases with the temperature increasing, whereas there is a slight in-crease in emission intensity of Sr 3SiO 5:Eu, rather than decrease.
2 Trinary silicate system
The investigation of trinary silicate system phosphor mainly focuses on disilicate and magnesium containing orthosilicate. Many efficient phosphors are Eu 2+-acti- vated alkaline earth silicate, trinary silicate compound (Figure 1). Mg forms trinary silicate compound with alkaline earth ions more easily than Zn.
Poort et al.[52,53] reported that there are alkaline earth ion chains in matrixes of Eu 2+-doped (Ca,Sr)2MgSi 2O 7, BaMgSiO 4, CaMgSiO 4, SrLiSiO 4F, etc. Due to the pref-erential orientation of d orbital, Eu 2+ shows long wave-length emission in these hosts, which can be used for pcW-LED. The emission peaks of Ca 2MgSi 2O 7:Eu 2+ and Sr 2MgSi 2O 7:Eu 2+ are situated at 535 nm and 470 nm, respectively. And their excitation spectrum is a
broadband with tail reaching blue-green region area (≤480 nm)[54―56]. In (Ca,Sr)2MgSi2O7:Eu2+, when Ca ions are replaced by some Sr ions, lattice parameters increase, the effect of preferential orientation of d orbital in this chain is weakened, and the emission of Eu2+ shifts to a shorter wavelength. The emission spectrum of BaMgSiO4:Eu2+ at 4.2 K is composed of a narrowband at 440 nm and a broadband at 560 nm. At room tem-perature, the broadband at 560 nm changes into a tail at 440 nm. Emission of CaMgSiO4:Eu2+ at 4.2 K has a peak at 470 nm, and a tail at 550 nm. No evident changes in emission are observed with the temperature increasing, and the excitation spectrum at 550 nm emis-sion is very broad. Therefore they can be employed as phosphors for pcW-LED.
Generally speaking, trinary silicate matrix phosphors do not satisfy the blue GaN-pumped yellow phosphor pcW-LED, because of the low excitation efficiency un-der 450―480 nm blue radiation. The pcW-LED through the integration of blue LED chip and yellow phosphor shows that white emitting color changes with different input power, low color rendering index and color repro-ducibility. To solve these problems, white light output can be realized by combining deep blue LED or ultra-violet LED chips with red, green, blue phosphors[57,58]. This type of W-LED has the advantages of high toler-ance to UV chip’s color variation and excellent color rendering index. Tinary silicate phosphors present promising application in this aspect. A codoped single silicate host phosphor, such as Ba3MgSi2O8:mEu2+, nMn2+[59―61], emits blue (442 nm), green (505 nm), and red (620 nm) light simultaneously excited by UV chip (λem = 400 nm). The emission color can be adjusted by concentration changes of m, n activators in Ba3MgSi2O8: m Eu2+, n Mn2+. The fabricated W-LED integrating 400-nm-emitted chip with a Ba3MgSi2O8:m Eu2+, n Mn2+ phosphor shows warm white light and higher color ren-dering index and higher color stability against input power than a commercial blue-pumped YAG:Ce. Simi-larly, 375 nm emitting InGaN UV chip and Sr3MgSi2O8: 0.02Eu2+ (blue, yellow light) and Sr3MgSi2O8:0.02Eu2+, 0.05Mn2+ (blue, yellow, and red light) are integrated into pcW-LED. At 5892 K color temperature[62], the color rendering coordinations are x=0.32, y=0.33, and CRI is 83(Sr3MgSi2O8:0.02Eu2+); at 4494 K color temperature, the color rendering coordinations are x=0.35, y=0.33, and CRI is 92 (Sr3MgSi2O8:0.02Eu2+, 0.05Mn2+). This pcW-LED presents high color reproducibility, high color stability on forward-bias current and excellent color rendering index.
3 Conclusions
The alkaline earth silicate matrix phosphor, as a new material for pcW-LED, has attracted extensive attention. So far, more than ten patents are applied, including Luminglight (ZL98105078, US6093346, EP0927915, KR0477347), GE/Gelcore (US6255670, US2004007961, WO2005004202), Toyoda Gosei (US6809347, US6943- 380, WO02054503), Matsushita Electric Industrial (WO- 03021691), Lumileds (WO03/080763), Philips (US2005- 200271), Phosphortech (WO2004111156), and Inte-matix (US20060027781, US20060027785, US200600- 28122). Prominent progress has been made and the lu-minous efficiency of White LED using InGaN blue LED chip combined with the Eu2+-activated silicate phos-phors is about 95%―105% of that of the same chip based YAG:Ce. They can meet the need of practical application and is likely to surpass YAG:Ce in the near future. Moreover, the chemical composition of silicate is quite complicated and can be altered over a wide range, resulting in a wide adjustable range of luminescence properties. Therefore they can be employed not only as yellow phosphor for InGaN/GaN blue LED chip (λem = 440―480 nm), but also as red, green and blue phosphor for deep blue/ultraviolet LED chip, avoiding applicant difficulties caused by combination of different matrixes. In addition, the easy preparation process (firing tem-perature is about 200 lower than
℃that of YAG:Ce) and the low price of raw materials are benifit to practical application.
Further studies and exploration, however, are still re-quired on the alkaline earth silicate matrix phosphor to pave the way to wider applications. The phase composi-tion of host material is one of the most crucial factors determining luminous efficiency. It is not a easy task to prepare single alkaline earth silicate phase with good crystallinity due to the complicated silicate compolisi-tion and structure, and the inertia silicate. Another direc-tion is to explore other silicate compounds and investi-gate the mechanism underlying excitation band boardening and excitation efficiency improvement. What is more, further refinement of the preparation is
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also needed because the defeats in host are easily gener-ated during preparation process. As the quantum effi-ciency of alkaline earth silicate phosphor is still lower than that of YAG :Ce (near 100%), there are plenty of scopes for improvement. However, due to the excellent visible light transparency of this phosphor, the output of White LED using blue LED chip encapsulated with this phosphor is equivalent to that of the same chip based- YAG :Ce. If the quantum efficiency is further improved, the total luminous efficiency may exceed that of YAG : Ce, for the quantum efficiency of YAG :Ce is ap-proaching high-point.
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