Synthesis, photoluminescence and thermal quenching of YAG:Ce phosphor for white light emitting diodes

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1 Indian Journal of Pure & Applied Physics Vol. 49, May 2011, pp Synthesis, photoluminescence and thermal quenching of YAG:Ce phosphor for white light emitting diodes Arunachalam Lakshmanan #, R Satheesh Kumar, V Sivakumar, Preema C Thomas & M T Jose + Saveetha Engineering College, Thandalam, Chennai , India + Radiological Safety Division, Indira Gandhi Center for Atomic Research, Kalpakkam , India # E mail: arunachalamlakshmanan@yahoo.com Received 25 October 2010; revised 28 January 2011; accepted 10 February 2011 The synthesis route at low temperature for Ce-doped yttrium aluminium garnet (YAG:Ce) phosphor is described. Its photoluminescence efficiency and quenching temperature dependence characteristics are found to be comparable to the imported one. YAG:Ce shows least thermal quenching among the several blue LED phosphors [CaS:Eu 2+, (Y,Gd) 3 Al 5 O 12 :Ce 3+, SrS:Eu 2+ and SrGa 2 S 4 :Eu 2+ ]. Keywords: YAG:Ce, Blue and white LED, Phosphor, Photoluminescence, Thermal quenching 1 Introduction Solid state lighting based on white light emitting diodes (LED) could save electricity and reduce environmental problems 1. Indigenous development of luminescence phosphors for blue LED is the subject matter of the present study. After a decade of intense research, a bright blue LED based on GaN with emission maximum near nm and blue-violet LED based on InGaN with emission maximum near nm were successfully produced by Nakamura, Nichia Chemical Company, Japan. There is a strong increase in the use of phosphor converter LEDs (PCLED) for white light illumination applications. Presently, the most popular method is to use a blue LED (~450 nm band emission) and a yellow-emitting phosphor, such as Ce-doped yttrium aluminium garnet (YAG:Ce). Here, the blue-light activates the phosphor. Two colours mix to perceive white. These LEDs had a life time 100,000 h and luminescence efficiency more than 50 lm/w. Unlike traditional filament based bulbs that radiate away 90% of waste heat, LEDs must be conductively cooled using package systems. Most LEDs are low-power devices as they typically dissipate only up to 1 W of power and are limited to a junction temperature of 125 C. In addition, LED lifetime is inversely proportional to junction temperature. An important requirement of LED phosphor is that it must withstand temperatures up to 150 C without significant reduction in luminescence efficiency. Conventional phosphors used in Hg based fluorescent lamps excited at 254 nm UV light are not ideal for solid-state lighting because they have poor absorption for the LED light (near UV or blue). Bandto-band transitions in direct band gap semiconductors such as ZnS, ZnSe, ZnO and CdS offer the strongest absorption to LED light. But the main difficulty of their real value in phosphor convertor LED arises from strong temperature dependence. Strong absorption can also be expected from dipole-allowed electron transitions in dopant ions, the best examples being Ce 3+ and Eu 2+, which can be crystal field shifted in the spectral location of their absorption and emission lines. However, combination of such yellow and blue colours gives insufficient colour rendering index (CRI) due to the lack of red component in the emission spectra. Some efficient nitride-based phosphors such as CaAlSiN 3 :Eu 2+ have been developed which meet some of the spectral requirements of LED lighting. However, synthesis problems and stability of nitride based phosphors still remain a big challenge with them 2. Considerable effort has been put for shifting the emission spectrum of YAG:Ce into the red spectral region by co-doping with other RE ions such as Tb 3+, Gd 3+ etc 3. A red shift in Ce 3+ emission could also be achieved by increased covalency, for instance by substitution of Al 3+ by Mg 2+ -Si 4+ pair on the octahedral and tetrahedral sites, respectively 4. Y 3 Mg 2 AlSi 2 O 12 :Ce 3+ synthesized by sol-gel route has been reported to show a strongly red shifted phosphor with maximum emission at 600 nm while conventional YAG:Ce emits at 560 nm. However, strong concentration quenching has been reported which was attributed to increased Stokes shift 5.

2 304 INDIAN J PURE & APPL PHYS, VOL 49, MAY 2011 Phase pure YAG phosphors are usually synthesized by solid-state diffusion reaction techniques at very high temperatures (1500 C) despite the addition of flux materials 6 such as BaF 2 or by several other techniques, such as sol-gel, co-precipitation, hydrothermal and microwave combustion methods 7. This work describes a low temperature synthesis, photoluminescence and thermal quenching properties of YAG:Ce phosphor for application in white LEDs. 2 Experimental Details The raw materials used for the synthesis of YAG:Ce were analytical grade reagents Y 2 O 3, Al 2 O 3, CeO 2, NH 4 NO 3 and suitable flux materials which include MgCl 2, BaF 2, BaCO 3, Li 2 CO 3, SiO 2 and NaF. YAG:Ce phosphor was prepared using pyrolysis technique in which the constituent chemicals decompose and fuse by the action of heat at relatively low external temperatures ( 1000 C) in air with adequate ventilation so that the gaseous products released during decomposition reactions escape and the fusion process is complete, the details of which are described in Ref. (8). When urea (fuel) was additionally added as an ingredient products caught fire due to combustion but the reproducibility of results obtained was poor. While in combustion, the reaction products catch fire, in pyrolysis the decomposition results in smoke and not fire. After wet mixing, the reagents in porcelain crucibles, initial firing was carried out in air at 600 C for 1 h in a muffle furnace with its door kept partially open in the initial stages of firing to let out the smoke created during the pyrolysis reaction. An exhaust fan served to let the smoke out of the room. After this treatment, the powder samples were ground mildly and kept in alumina crucibles before firing in reducing conditions around 1000 C for 1 h. Carbon black powder was used to obtain reducing atmosphere. The samples were allowed to cool inside the furnace itself overnight and removed the next morning. The material obtained was once again hand ground mildly to obtain grain sizes below 30 µm. Ce can stabilize in 3+ (4f 1 ) as well as 4+(4f 0 ) form. In oxidizing condition at high temperature Ce 4+ is stable. Therefore, reducing atmosphere was found essential to reduce Ce 4+ to Ce 3+ at high temperatures. YAG:Ce samples were also made with solid state reaction route with NaF flux (melting point = 993 C). Physical mixture of Y 2 O 3, Al 2 O 3, CeO 2 and NaF was fired at 1000 C in carbon black atmosphere. The photoluminescence (PL) emission spectra of different LED phosphors (Table 1) usually coated on the blue (450 nm emission) LED to obtain white light, were recorded using the Fluorolog Jobin Yvon-Spex spectrofluorimeter which consists of two monochromators, one for excitation and the other for emission measurements. A 400W Xe-lamp was used as the excitation source. For thermal quenching studies, the emission intensities under 450 nm excitation at different temperatures were recorded at the peak emission wavelengths of the respective phosphors using a separate attachment. It consisted of a kanthal heating strip whose temperature could be varied using a heating programmer. Phosphor powder samples were spread uniformly over the heating strip. The monochromatic excitation light from the fluorimeter to the phosphor sample as well as the fluorescent light emitted by the sample to the fluorimeter were transmitted using a light guide. XRD patterns of the powder samples were recorded with a German Make STOE X-ray diffractometer with Cu Kα radiation (λ = nm). 3 Results and Discussion 3.1. Photoluminescence and XRD Table 2 compares the relative PL intensities corresponding to the respective peak emission wavelengths under 450 nm excitation of different LED phosphors. Among all the phosphors studied, S.No. Company Table 1 Commercial LED phosphors LED/Phosphor combination 1 Nichia Blue LED + YAG:Ce 2 Cree nuvled + DC phosphors 3 Osram Blue, Green, nuv LEDs + Ce or Tb doped garnet (TAG:Ce) or Sulphursubstituted garnet 4 HP SrS:Eu (λexi = 460 nm; λemi = Red) Sr thiogallate (λexi = 460 nm; λemi = 550 nm green) 5 Toyoda Gosei Blue or UV LED with alkaline earth orthosilicates doped with Eu phosphors Table 2 Relative PL intensities (respective emission peak intensities) under 450 nm excitation of different LED phosphors S.No Phosphor Peak emission wavelength (nm) Relative PL intensities 1 CaS:Eu SrS:Eu Thiogalate, SrGa 2 S 4 :Eu YAG:Ce (Y,Gd) 3 Al 5 O 12 :Ce YAG:Ce (indigenous)

3 LAKSHMANAN et al.: SYNTHESIS, PHOTOLUMINESCENCE AND THERMAL QUENCHING OF YAG:Ce PHOSPHOR 305 SrGa 2 S 4 :Eu 2+ showed the maximum fluorescence efficiency under 450 nm excitation. Fig. 1 shows the PL excitation (a, λ exi = 450 nm) and emission (b, λ emis = 534 nm) spectra of SrGa 2 S 4 :Eu 2+ (Make: Phosphor Technology, England). Its broad excitation peak corresponds to band-to-band transitions and emission in the green region corresponds to the 5d-4f Eu 2+ transition. Figure 2 shows the comparison of the PL excitation and emission spectra of indigenous YAG:Ce with that of commercial Y 3 Al 5 O 12 :Ce 3+. The former is 20% more sensitive than the latter. In both samples, the PL excitation spectra show two bands, a weak one localized at 350 and a strong broad band in the range nm with a maximum at 450 nm due to transitions from the 4f 1 ground state ( 2 F 5/2 and 2 F 7/2 ) to the excited 5d 1 levels ( 2 D) of Ce 3+. In accordance to the literature 9,10, the green-yellow broad band emission in the range nm peaking around nm is ascribed to the overlap of two emission transitions from the excited state of 2 D J to the ground states of 2 F 5/2 and 2 F 7/2 of Ce 3+ ions in the YAG host crystal. The broad yellow emission band centered around 540 nm actually consists of the superposition of two emission bands centered around 509 and 548 nm corresponding to 2 D J 2 F 5/2 and 2 D J 2 F 7/2 transitions, respectively. At room temperature, the two emission bands overlap. Gaussian fitted curves have confirmed such a doublet structure of Ce 3+ emission band and the branching ratio of the two emissions was about 27% and 73%, respectively 11. The XRD spectra of Y 3 Al 5 O 12 (JCPDS NO: ) which has a bcc structure with space group la3d (230) and cell parameters a=b=c = 12Å shown in Fig. 3 matches very well with the commercial Y 3 Al 5 O 12 :Ce 3+ (Phosphor Technology England) and indigenous YAG:Ce 3+ phosphors. This shows that phase-pure YAG:Ce could be prepared by using pyrolysis technique at relatively low temperatures (1000 C). 3.2 Effect of Y 2 O 3 to Al 2 O 3 molar ratio Table 3 presents the effect of Y 2 O 3 to Al 2 O 3 molar ratio on the PL efficiency of YAG:Ce under 450 nm excitation. The Y 2 O 3 to Al 2 O 3 molar ratio for Y 3 Al 5 O 12 (YAG) is However, optimal PL sensitivity in the present recipe could be achieved only when Y 2 O 3 to Al 2 O 3 molar ratio is This shows that in the pyrolysis technique when multiple fluxes (which include NH 4 NO 3 MgCl 2, BaF 2, BaCO 3 / Fig. 1 PL excitation (a, λ exi = 450 nm) and emission (b, λ emis = 534 nm) spectra of SrGa 2 S 4 :Eu 2+ (Phosphor Technology, England) Fig. 2 PL excitation (a,c λ exi = 450 nm) and emission (b,d λ emis = 538 nm) spectra of indigenous YAG:Ce 3+ (a,b) and Y 3 Al 5 O 12 :Ce 3+ (c,d) (Phosphor Technology England) Fig. 3 XRD spectra of Y 3 Al 5 O 12 (JCPDS NO: ), a Y 3 Al 5 O 12 :Ce 3+ (Make: Phosphor Technology England) and b indigenous YAG:Ce 3+

4 306 INDIAN J PURE & APPL PHYS, VOL 49, MAY 2011 Table 3 Effect of Y 2 O 3 to Al 2 O 3 molar ratio on the PL efficiency of YAG:Ce under 450 nm excitation. The Y 2 O 3 to Al 2 O 3 molar ratio for Y 3 Al 5 O 12 (YAG) is 0.60 S. No Ingradients Y 2 O 3 to Al 2 O 3 molar ratio Relative PL 1 pyrolysis with multiple fluxes do do do solid state reaction with NaF flux 6 -do Table 4 Effect of Ce concentration on the PL efficiency of YAG:Ce under 450 nm excitation S,No Ce concentration (wt%) λ emi (nm) Relative PL Ingradients without NH 4 NO without NH 4 NO without NH 4 NO with NH 4 NO with NH 4 NO with NH 4 NO with NH 4 NO with NH 4 NO with NH 4 NO 3 Li 2 CO 3, SiO 2 and NaF) are used, an excess of Al 2 O 3 is needed to arrive at optimal sensitivity. This might be associated with the reactions between different compounds used during the synthesis. With a simple solid state diffusion reaction involving Y 2 O 3, Al 2 O 3, NaF and CeO a relatively higher Y 2 O 3 to Al 2 O 3 molar ratio of 0.45 yielded a higher PL efficiency. 3.3 Effect of Ce concentration Table 4 presents the effect of Ce concentration on the PL efficiency of YAG:Ce under 450 nm excitation. In the absence of NH 4 NO 3, PL sensitivity increases with increasing Ce concentration up to the level (8.8 mol %) studied. When NH 4 NO 3 was used as an oxidizer, optimal PL sensitivity was, however, achieved at the Ce concentration of 4.8 mol% and higher Ce concentrations quenches luminescence efficiency. 3.4 Thermal quenching Phosphors involving optically active anions possess low quenching temperature, eg. BaWO 4 fluoresces at 77 K but not at RT. Phosphors having rare earth (RE) cations/activators have high quenching temperatures as their energy levels do not intersect and are shielded from the crystal Fig. 4 Thermal quenching of various LED phosphors under 450 nm excitation. (*): YAG:Ce 3+ (λ emi = 537 nm), ( ):Y 3 Al 5 O 12 :Ce 3+ (λ emi = 538 nm), ( ): (Y,Gd) 3 Al 5 O 12 :Ce 3+ (λ emi = 551 nm), ( ): CaS:Eu 2+ (λ emi = 651 nm), ( ): SrS:Eu 2+ (λ emi = 610 nm) and ( ): SrGa 2 S 4 :Eu 2+ (λ emi = 534 nm) environment by 5s 2 5p 6 electrons that form completely filled shells. The temperature dependence of luminescence is determined by intersection of the configuration curves and the energy difference between the configuration curve minimum of the excited state and its intersection with the ground state, through which a transition to ground state can be made without luminescence. At low temperatures, there is very little change in brightness with temperature, but at elevated temperatures the luminescence efficiency decreases rapidly. This is interpreted as meaning that thermal vibrations become sufficiently intense to raise the system to the point where the excited vibrational energy state intersects with that of ground state, from which the system can fall to the ground state by emitting a small amount of heat, or infrared radiation. It does not give off luminescence. This is known as thermal quenching. One practical consequence of this effect, important in lighting (eg., CFL or high pressure Hg lamps) and display phosphors (eg., LEDs), is to use only those phosphors having high quenching temperatures. Figure 4 shows that among the several commercial blue LED phosphors studied, YAG:Ce phosphors show least thermal quenching whereas all sulphide based LED phosphors (CaS:Eu, SrS:Eu, thiogalate SrGa 2 S 4 :Eu 2+ ) exhibit significant thermal quenching. For instance, the emission intensity of SrS:Eu at 100 C is just 60% of that at RT. At 200 C, the emission intensities of SrS:Eu, CaS:Eu and SrGa 2 S 4 :Eu 2+ are only 40% of their respective intensities at RT. As stated earlier, to improve the colour rendering of YAG:Ce masked blue LED (it shows a yellow halo), Gd is added to YAG: Ce to red shift its emission spectrum. However, as seen in Fig.4, (Y,Gd) 3 Al 5 O 12 :Ce 3+ shows higher thermal

5 LAKSHMANAN et al.: SYNTHESIS, PHOTOLUMINESCENCE AND THERMAL QUENCHING OF YAG:Ce PHOSPHOR 307 quenching when compared to that of YAG:Ce. For instance at 200 C, the luminescence efficiency of the former is about 40% lower while that of the latter is only 15% lower than their corresponding efficiencies at RT. Therefore, further efforts are required to arrive at a high sensitive phosphor for blue LED which shows higher colour rendering coupled with thermal stability. 4 Conclusions A low temperature synthesis route using pyrolysis with multiple flux materials for YAG:Ce blue LED phosphor has been developed. Its PL efficiency and quenching temperature dependence characteristics are found to be superior to the imported one. The XRD spectra of Y 3 Al 5 O 12 (JCPDS NO: ) and that of the commercial and indigenous YAG:Ce 3+ phosphors are found to be identical. This shows that phase-pure YAG:Ce could be prepared by using this technique. Among the phosphors studied, SrGa 2 S 4 :Eu 2+ showed the maximum fluorescence efficiency under 450 nm excitation. However, all sulphide based LED phosphors exhibit significant thermal quenching. At 200 C, the emission intensities of SrS:Eu, CaS:Eu and SrGa 2 S 4 :Eu 2+ are only 40% of their respective intensities at RT. YAG:Ce shows least thermal quenching among all the blue LED phosphors studied. Since the colour rendering of YAG:Ce masked blue LED is poor (it shows a yellow halo), Gd is added to YAG:Ce to red shift its emission spectrum. However, (Y,Gd) 3 Al 5 O 12 :Ce 3+ shows higher thermal quenching when compared to that of YAG:Ce. For instance at 200 C, the luminescence efficiency of the former is about 40% lower while that of the latter is only 15% lower than their corresponding efficiencies at RT. This shows that to obtain white LEDs with higher colour rendering coupled with thermal stability, further efforts are required to arrive at a high sensitive phosphor for blue LED excitation. Acknowledgement We are grateful to Dr ( Mrs) S Kalavathy, Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam for XRD measurements. We also thank Dr R Venkatasamy, Prinicipal, Saveetha Engineering College for the encouragement. References 1 Lakshmanan A, Luminescence & Display Phosphors: Phenomena & Applications, (Nova Publishers, New York, USA, ISBN (2008). 2 Uheda K, Hirosaki N, Yamamoto Y, Naito A, Nakajima T & Yamamoto H, Electrochem Solid State, 9 (2006) H22. 3 Jang H S, Im W B, Lee D C, Jeon D Y & Kim S S, J Lumin, 126 (2007) Robertson JM, van Tol MW, Smits W H & Heynen JPH, Philips J Res, 36 (1981) Katelnikovas A, Bettentrup H, Uhlich D, Sakirzanovas S, Jüstel T & Kareiva A, J Lumin, 129 (2009) Han R, Wang L, Chen K & Yang S, Mat Sci & Eng B, 166 (2010) Fu Y P, J Alloys & Comp, 402 (2005) Lakshmanan A, Bhaskar R S, Thomas P C, Satheesh Kumar R, Sivakumar V & Jose M T, Mat Lett, 64 (2010) Yang H & Kim Y S, J Lumin, 128 (2008) Blasse G & Grabmaier B C, Luminescence Materials (Springer-Verlag, Berlin), Huili L, Xuejian L, Qitu Z & Liping H, J Rare Earths, 25 (2007) 401.

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