Crystal Structures and Luminescence Properties of Aln-Deficient Eu2+

Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010, pp. 990∼993 Crystal Structures and Luminescence Properties of AlN-deficient Eu2+-activated Ca-α-SiAlON Phosphor Sung-Soon Park, Bo-Yun Jang∗ and Joo-Seok Park Korea Institute of Energy Research, Daejeon 305-343 Sahn Nahm Department of Materials Science and Engineering, Korea University, Seoul 136-701 (Received 15 January 2010, in final form 5 July 2010) Eu2+-doped Ca-α-SiAlON phosphor was synthesized by normal pressure sintering with conven- <ul style="display: flex;"><li style="flex:1">tional tube furnace, and its crystal structures and luminescence properties were investigated. Eu2+ </li><li style="flex:1">-</li></ul>doped Ca-α-SiAlON phosphor exhibited an orange light peaking at 583 nm. During the annealing, some of the raw materials evaporated, which resulted in unreacted AlN in the final product. In order to obtain a final product with a single α-phase, we prepared samples with various AlN-deficient compositions, and we studied the effects on crystal structures and luminescence properties. For the sample with 15 mol% AlN-deficient composition, 18% enhancement of the intensity was obtained with the disappearance of excess AlN. In addition, the internal quantum efficiency (IQE) was measured using a specially designed integrating sphere, and 56.82% IQE was gained from Eu2+-doped Ca-α-SiAlON phosphor. PACS numbers: 42.70.-a Keywords: α-SiAlON, Europium, Phosphor, Luminescence, Crystal Structure DOI: 10.3938/jkps.57.990 I. INTRODUCTION shows excellent chemical and mechanical stability [15– 17]. SiAlON is known to be stabilized structurally by doping with Y, Li, Ca and rare earth element. Krevel et al. have been reported the use of Ce3+- and Eu2+-doped M-α-SiAlON (M = Y, Ca) phosphors as green and yellow White light emitting diodes (LEDs) have been the subjects of enormous research as next-generation illumination sources because of their high efficiency, long life time and low power consumption [1–3]. The yellow phosphor Y3Al5O12:Ce3+, so-called YAG, packaged with a blue chip (emission at around 460 nm) has been widely used for first-generation LEDs [2, 3]. However, the color rendering index (CRI) of the YAG phosphor is too low (∼75%) to be applied for general illumination sources. There are two approaches to increase the CRI. The first one is packaging the blue chip with green and red phosphors. The other is based on an ultraviolet (UV) chip (emission at around 405 nm) packaged with blue (B), green (G), and red (R) phosphors. Especially, an UV chip with RGB phosphors has been investigated for next-generation illumination because the emission power of the UV chip is higher than that of a blue chip [4,5]. Many nitride and oxy-nitride phosphors have been reported for LED applications because these phosphors can absorb light with wavelengths of 390 ∼ 460 nm [6– 14]. As an orange phosphor, Eu2+-doped Ca-α-SiAlON is a promising phosphor because this crystal structure <ul style="display: flex;"><li style="flex:1">phosphors, respectively [6]. Xie et al. synthesized Ce3+ </li><li style="flex:1">-</li></ul>or Eu2+-doped α-SiAlON using hot pressing (HP) and gas pressure sintering (GPS) [7–10]. Fabrication of this phosphor with a blue chip was conducted by Sakuma, and a luminous efficiency of 25.9 lm/W was achieved at room temperature with a forward-bias current of 20 mA [11]. Eu2+-doped β-SiAlON was also studied, and green light was emitted [12]. Recently, Eu2+-doped Ca, Y-stabilized α-SiAlON with an emission at over 580 nm was reported [13,14]. However, those phosphors required a relatively complex process, such as GPS or carbothermal reduction nitridation (CRN), to obtain a single α phase. In this study, Eu2+-doped Ca-α-SiAlON phosphor was synthesized by conventional solid state reaction method with a horizontal tube furnace, and its crystal structures and luminescence properties were analyzed. Especially, the effects of nonstoichiometric compositions on the luminescence properties were investigated. ∗E-mail: [email protected]; Fax: +82-42-860-3133 -990- <ul style="display: flex;"><li style="flex:1">Crystal Structures and Luminescence Properties of AlN-deficient · · · – Sung-Soon Park et al. </li><li style="flex:1">-991- </li></ul>Fig. 1. Schematic diagram of the system for measuring the internal quantum efficiency. II. EXPERIMENTS AND DISCUSSION Ca0.8−xEuxSi9.2Al2.8O1.2N14.8 (x = 0.08, 0.12, 0.16) Fig. 2. Excitation and emission spectra for various amounts of Eu2+ doping in samples fired at 1680 ◦C for 2 hrs in 30% H2 - 70% N2 atmosphere. was prepared by the solid state reaction method. CaCO3 (Acros, 99.9%), AlN (Kojundo, 99.9%), α-Si3N4 (LC- 12, Starck, α content 90%, oxygen content <1.2%) and Eu2O3 (Kojundo, 99.9%) were used as the as-received raw materials. After the sample powders had been weighed in appropriate amounts, they were mixed and ground in high-purity isopropyl alcohol us◦ing ball milling for 2 hrs. After having been dried at 90 C, the sample powders were transferred into an alumina boat. Those mixtures were fired in a horizontal tube furnace at 850 ◦C for 4 hrs in N2 atmosphere to evaporate CO2 from CaCO3. Subsequently, a second annealing was carried Ls η = ,Lb − Lu =<ul style="display: flex;"><li style="flex:1">R</li><li style="flex:1">R</li></ul><ul style="display: flex;"><li style="flex:1">where Ls </li><li style="flex:1">Is(λ)dλ, Lb </li><li style="flex:1">=</li></ul>λ Ib(λ)dλ, Lu = Iu(λ)dλ, with λη being the internal quantum efficiency, Isλ, Ib, and Iu being the intensities from the light emitted by the sample, the light from the base lamp and the light unabsorbed by the sample, respectively, Ls, Lb, and Lu being the photon fluxes from the light emitted by the sample, the light from the base lamp and the light unabsorbed by the sample, respectively, and λ being the wavelength. R◦out at 1680 C under flowing 30% H2 - 70% N2 mixed gas controlled by mass flow controllers. After firing, the samples were gradually cooled down in the furnace. X-Ray diffraction (XRD, Rigaku, DMAX 2000, Japan) pattern analysis was conducted using Cu-Kα radiation (λ = 1.5418 angstrom). For the analysis of luminescence characteristics, the excitation and emission spectra were achieved using luminescence spectrometer (Perkin-Elmer, LS50B, USA) equipped with a 200 W Xe lamp and red-sensitive photomultipliertube (Hamamatsu, R928, Japan). To evaluate the possibility of its use as an orange phosphor for a white LED light source, we measured the IQE using the system shown in Fig. 1. LED (LS395, Oceanoptics, USA) with excitation at 395 nm was used as a reference light source. A 2048 × 64 pixel CCD array detector (Maya2000pro, Oceanoptics, USA) was connected to the integrating sphere using an optical fiber. The sample was placed at the center of an integrating sphere with two baffles. At first, the base spectrum (Ib) from the LED light source was measured without the sample. Then, a quartz tube filled with the phosphor was placed into the integrating sphere, and the sample spectrum was measured. The sample spectrum was divided into two spectra, which were attributed to the light unabsorbed by the sample (Iu) and the light emitted by the sample (Is). Therefore, the internal quantum efficiency was calculated using the following equation: Excitation and emission spectra were obtained from Ca0.8−xEuxAl2.8Si9.2O1.2N14.8 phosphor with various amounts of doped-Eu2+ ions, as shown in Fig. 2. All the emission spectra were measured at an excitation wavelength of 405 nm, and the excitation spectra were measured with the maximum emission wavelength of each sample. As shown in the emission spectra, broad emission bands peaking from 580 to 586 nm were observed. With increasing Eu2+ doping concentration, the peak shifted toward longer wavelength. This shift stopped when x exceeded 0.16. The intensity of the emission band changed slightly with x. but that change was small. Generally, emission from the 4f - 5d transition of Eu2+ ion is known to have a very sharp and narrow peak because only line-to-line transitions between energy levels shielded by the outer 5d-electron shell [18]. The broad emission band in α-SiAlON phosphor might be attributed to the large energy level splitting due to the strong covalent bonding between the Eu2+ ions and neighboring atoms. This strong covalent bonding was one of the characteristics of nitride or oxy-nitride materials. In the case of excitation, many broad bands were detected from 300 to 550 nm. The sharp excitation <ul style="display: flex;"><li style="flex:1">-992- </li><li style="flex:1">Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010 </li></ul>Fig. 3. X-ray diffraction pattern of Eu2+-doped Ca-α- Fig. 4. X-ray diffraction patterns of AlN-deficient Eu2+ doped Ca-α-SiAlON fired at 1680 ◦C for 2 hrs in 30% H2 70% N2 atmosphere. -- SiAlON fired at 1680 ◦C for 2 hrs in 30% H2 - 70% N2 atmosphere. peak at 390 nm has been reported to be a representative peak of Eu2+ ions [18]. In SiAlON phosphor, however, there were additional bands that might also be due to the strong covalent bonding of Eu2+ ion. It must be pointed out that four small peaks from 425 to 460 nm were dummy ones from the xenon light source because those peaks were detected for all phosphors. There wide excitation bands allow α-SiAlON phosphor to be applied for UV, as well as blue, LEDs. In this study, evaporation was always observed in the raw materials during the annealing process. At the outlet of the alumina tube in the furnace, the evaporated elements, which were proven to be mainly SiO2 by SEM and XRD measurements, were deposited. Although annealing was conducted in strong a reducing atmosphere, oxidation always occurred due to the residual oxygen inside and outside the raw materials. Thus, SiO2 was formed and easily evaporated during annealing at temperatures Fig. 5. Normalized intensities and peak emission wavelengths of AlN-deficient Eu2+-doped Ca-α-SiAlON fired at 1680 ◦C for 2 hrs in 30% H2 - 70% N2 atmosphere. ◦higher than 1600 C. As a result, a secondary phase was detected in the final product. Figure 3 shows the XRD patterns of α-SiAlON phosphor (x = 0.12) fired at 1680 ◦C for 2 hrs in 30% H2 - 70% N2 atmosphere. Main peaks were matched those from Ca0.8Al2.8Si9.2O14.8 (JCPDF #33-0261). Even though the raw materials had been mixed in a stoichiometric composition, unreacted AlN was detected in the final product, which was obviously attributed to the evaporation of Si from the raw materials. Generally, unreacted elements caused a degradation of the luminescence properties. Therefore, to obtain a single α-phase, we synthesized AlN-deficient samples, and their properties were observed. A single α-phase was achieved from the samples when the deficient amount of AlN was from 10 to 30 mol% in the starting composition. In addition, unreacted Si3N4 appeared when AlN deficiency was 35 mol%. As a result, preparing the starting materials with AlN-deficient compositions was needed to gain single-phase α-SiAlON. To investigate the luminescence properties of the above phosphors, we measured the PL spectra. Figure 5 shows the normalized intensities and peak emission wavelengths of 0 ∼ 30 mol% AlN-deficient phosphors fired at 1680 ◦C for 2 hrs in 30% H2 - 70% N2 atmosphere. As the deficient amount of AlN increased, the PL intensity also increased. The maximum intensity was gained when the deficient amount of AlN was 15 mol%. The intensity decreased when the deficient amount of AlN exceeded 15 mol%. An intensity enhancement of 18% was obtained by preparing the sample in 15 mol% AlN-deficient com- Figure 4 shows the XRD patterns of AlN-deficient ◦phosphors (x = 0.12) fired at 1680 C for 2 hrs in 30% H2 -70% N2 atmosphere. There was no significant crystal structure change from that of α-SiAlON. However, a single α-phase without unreacted AlN was achieved. <ul style="display: flex;"><li style="flex:1">Crystal Structures and Luminescence Properties of AlN-deficient · · · – Sung-Soon Park et al. </li><li style="flex:1">-993- </li></ul>α-SiAlON was synthesized in atmosphere at normal pressure, SiO2 always evaporated from the raw materials, resulting in some AlN remaining in the final phase. Therefore, the samples with AlN-deficient compositions were sintered. From the crystal structural analysis, unreacted AlN disappeared when the deficient amount of AlN was from 10 ∼ 30 mol%. The maximum intensity was obtained from the 15 mol% AlN deficient sample. An internal quantum efficiency of 56.83% was obtained from that sample. As a consequence, high-efficiency Eu2+-doped Ca-α-SiAlON could be achieved using normal pressure sintering by preparing the starting materials in AlN- deficient composition. <ul style="display: flex;"><li style="flex:1">Fig. 6. Emission spectra of 15 mol% AlN-deficient Eu2+ </li><li style="flex:1">-</li></ul>-REFERENCES doped Ca-α-SiAlON fired at 1680 ◦C for 2 hrs in 30% H2 70% N2 atmosphere: (a) peak from the source light, (b) peak from the light unabsorbed by the sample, and (c) peak from the light emitted by the sample. [1] S. Nakamura, T. Mukai and M. Senoh, Appl. Phys. Lett. 64, 1687 (1994). [2] S. Narukawa and G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers (Springer, Berlin, 1997), p. 216. [3] P. Schlotter, J. Baur, Ch. Hielscher, M. Kunzer, H. Obloh, R. Schmidt and J. Schneider, Mater. Sci. Eng., B. 59, 390 (1999). [4] W. Tews, G. Roth and S. Tews, Advanced Silicate Phos- phors for Improved White LED: Proceeding of the Phos- phor Global Summit (Seoul, 2007), p. 5. [5] Y. Fukuda, Luminescence of Eu-doped Sr-SiAlON Phosphor, Proceeding of the Phosphor Global Summit (Seoul, 2007). position. There was no change in the emission peak’s wavelength, except for the 30 mol% AlN-deficient sample. This result indicates that no crystal field changes occurred until the deficient amount of AlN was lower than 30 mol%. According to previous research, a highpressure atmosphere is required to synthesize nitride or oxy-nitride phosphors. From the results of this research, however, an oxy-nitride phosphor of α-SiAlON could be obtained by using a normal-pressure sintering process. In addition, the raw materials needed to be prepared with AlN-deficient compositions for the synthesis of α-phase SiAlON phosphor by normal pressure sintering. [6] J. W. H. van Krevel, J. W. T. van Rutten, H. Mandal, H. T. Hintzen and R. Metselaar, J. Solid State Chem. 165, 19 (2002). [7] R. J. Xie, M. Mitomo, K. Uheda, F. F. Xu and Y. Akimune, J. Am. Ceram. Soc. 85, 1229 (2002). [8] R. J. Xie, N. Hirosaki, M. Mitomo, Y. Yamamoto and T. Suehiro, J. Phys. Chem. B. 108, 12027 (2004). [9] R. J. Xie, N. Hirosaki, M. Mitomo, Y. Yamamoto, T. Suehiro and N. Ohashi, J. Am. Ceram. Soc. 87, 1368 (2004). [10] R. J. Xie, N. Hirosaki, K. Sakuma, Y. Yamamoto and M. Mitomo, Appl. Phys. Lett. 84, 26 (2004). [11] K. Sakuma, K. Omichi, N. Kimura, M. Ohashi and D. Tanaka, Opt. Lett. 29, 17 (2004). To evaluate α-SiAlON phosphor in this study for LED applications, we measured the internal quantum efficiency. Figure 6 shows the emission spectra of 15mol% AlN-deficient phosphor fired at 1680 ◦C for 2 hrs in 30% H2 - 70% N2 atmosphere. Peak (a) was from the source light, the peak (b) was from the light unabsorbed by the sample and peak (c) was from the light emitted by the sample. The measured internal quantum efficiency was 56.83%. Consequently, α-SiAlON phosphor in this research should be a good candidate for UV, as well as blue LED applications. [12] N. hirosaki, R. J. Xie, K. Kimoto, T. Sekiguchi, Y. Yamamoto, T. Suehiro and M. Mitomo, Appl. Phys. Lett. 86, 211905 (2005). [13] T. Suehiro, N. Hirosaki, R. J. Xie and M. Mitomo, Chem. Mater. 17, 308 (2005). [14] K. Sakuma, N. Hirosaki, R. J. Xie, Y. Yamamoto and T. Suehiro, Mater. Lett. 61, 547 (2007). III. CONCLUSION AlN-deficient Eu2+-doped Ca-α-SiAlON was synthesized by normal pressure sintering with a conventional alumina tube furnace, and its crystal structures and luminescence properties were investigated. In the PL spectra, broad emission and excitation bands were observed, and those broad bands were due to a strong crystal field caused by highly-covalent bonding of Eu2+ ions. When [15] J. W. T. van Rutten, H. T. Hintzen and R. Matselaar, J. Eur. Ceram. Soc. 16, 955 (1996). [16] H. Mandal, J. Eur. Ceram. Soc. 19, 2349 (1999). [17] C. Santos, K. Strecker, J. V. C. de Souza, O. M. M. Silva and C. R. M. da Silva, Mater. Lett. 58, 1792 (2004). [18] G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer Verlag, Berlin, 1994), p. 25.

Crystal Structures and Luminescence Properties of Aln-Deficient Eu2+

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