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 conventional tube furnace, and its crystal structures and luminescence properties were investigated. Eu2+- 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 or- der 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 White light emitting diodes (LEDs) have been the sub- al. have been reported the use of Ce3+- and Eu2+-doped jects of enormous research as next-generation illumina- M-α-SiAlON (M = Y, Ca) phosphors as green and yellow tion sources because of their high efficiency, long life time phosphors, respectively [6]. Xie et al. synthesized Ce3+- and low power consumption [1–3]. The yellow phos- or Eu2+-doped α-SiAlON using hot pressing (HP) and 3+ phor Y3Al5O12:Ce , so-called YAG, packaged with a gas pressure sintering (GPS) [7–10]. Fabrication of this blue chip (emission at around 460 nm) has been widely phosphor with a blue chip was conducted by Sakuma, used for first-generation LEDs [2, 3]. However, the color and a luminous efficiency of 25.9 lm/W was achieved rendering index (CRI) of the YAG phosphor is too low at room temperature with a forward-bias current of 20 (∼75%) to be applied for general illumination sources. mA [11]. Eu2+-doped β-SiAlON was also studied, and There are two approaches to increase the CRI. The green light was emitted [12]. Recently, Eu2+-doped Ca, first one is packaging the blue chip with green and red Y-stabilized α-SiAlON with an emission at over 580 nm phosphors. The other is based on an ultraviolet (UV) was reported [13,14]. However, those phosphors required chip (emission at around 405 nm) packaged with blue a relatively complex process, such as GPS or carbother- (B), green (G), and red (R) phosphors. Especially, an mal reduction nitridation (CRN), to obtain a single α UV chip with RGB phosphors has been investigated for phase. next-generation illumination because the emission power In this study, Eu2+-doped Ca-α-SiAlON phosphor was of the UV chip is higher than that of a blue chip [4,5]. synthesized by conventional solid state reaction method Many nitride and oxy-nitride phosphors have been re- with a horizontal tube furnace, and its crystal structures ported for LED applications because these phosphors and luminescence properties were analyzed. Especially, can absorb light with wavelengths of 390 ∼ 460 nm [6– the effects of nonstoichiometric compositions on the lu- 14]. As an orange phosphor, Eu2+-doped Ca-α-SiAlON minescence properties were investigated. is a promising phosphor because this crystal structure

∗E-mail: [email protected]; Fax: +82-42-860-3133 -990- Crystal Structures and Luminescence Properties of AlN-deﬁcient ··· – Sung-Soon Park et al. -991-

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 2+ ◦ was prepared by the solid state reaction method. CaCO3 amounts of Eu doping in samples fired at 1680 C for 2 (Acros, 99.9%), AlN (Kojundo, 99.9%), α-Si3N4 (LC- hrs in 30% H2 - 70% N2 atmosphere. 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 Ls ground in high-purity isopropyl alcohol using ball milling η = , Lb − Lu for 2 hrs. After having been dried at 90 ◦C, the sample R R powders were transferred into an alumina boat. Those where Ls = Is(λ)dλ,Lb = Ib(λ)dλ,Lu = R λ λ mixtures were fired in a horizontal tube furnace at 850 λ Iu(λ)dλ, with η being the internal quantum efficiency, ◦ C for 4 hrs in N2 atmosphere to evaporate CO2 from Is,Ib, and Iu being the intensities from the light emitted CaCO3. Subsequently, a second annealing was carried by the sample, the light from the base lamp and the light ◦ out at 1680 C under flowing 30% H2 - 70% N2 mixed unabsorbed by the sample, respectively, Ls,Lb, and Lu gas controlled by mass flow controllers. After firing, the being the photon fluxes from the light emitted by the samples were gradually cooled down in the furnace. sample, the light from the base lamp and the light un- X-Ray diffraction (XRD, Rigaku, DMAX 2000, Japan) absorbed by the sample, respectively, and λ being the pattern analysis was conducted using Cu-Kα radia- wavelength. tion (λ = 1.5418 angstrom). For the analysis of lu- Excitation and emission spectra were obtained from minescence characteristics, the excitation and emission Ca0.8−xEuxAl2.8Si9.2O1.2N14.8 phosphor with various spectra were achieved using luminescence spectrometer amounts of doped-Eu2+ ions, as shown in Fig. 2. All the (Perkin-Elmer, LS50B, USA) equipped with a 200 W emission spectra were measured at an excitation wave- Xe lamp and red-sensitive photomultipliertube (Hama- length of 405 nm, and the excitation spectra were mea- matsu, R928, Japan). To evaluate the possibility of its sured with the maximum emission wavelength of each use as an orange phosphor for a white LED light source, sample. As shown in the emission spectra, broad emis- we measured the IQE using the system shown in Fig. 1. sion bands peaking from 580 to 586 nm were observed. LED (LS395, Oceanoptics, USA) with excitation at 395 With increasing Eu2+ doping concentration, the peak nm was used as a reference light source. A 2048 × 64 shifted toward longer wavelength. This shift stopped pixel CCD array detector (Maya2000pro, Oceanoptics, when x exceeded 0.16. The intensity of the emission USA) was connected to the integrating sphere using an band changed slightly with x. but that change was optical fiber. The sample was placed at the center of an small. Generally, emission from the 4f - 5d transition integrating sphere with two baffles. At first, the base of Eu2+ ion is known to have a very sharp and nar- spectrum (Ib) from the LED light source was measured row peak because only line-to-line transitions between without the sample. Then, a quartz tube filled with the energy levels shielded by the outer 5d-electron shell [18]. phosphor was placed into the integrating sphere, and the The broad emission band in α-SiAlON phosphor might sample spectrum was measured. The sample spectrum be attributed to the large energy level splitting due to was divided into two spectra, which were attributed to the strong covalent bonding between the Eu2+ ions and the light unabsorbed by the sample (Iu) and the light neighboring atoms. This strong covalent bonding was emitted by the sample (Is). Therefore, the internal quan- one of the characteristics of nitride or oxy-nitride ma- tum efficiency was calculated using the following equa- terials. In the case of excitation, many broad bands tion: were detected from 300 to 550 nm. The sharp excitation -992- Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010

Fig. 3. X-ray diffraction pattern of Eu2+-doped Ca-α- Fig. 4. X-ray diffraction patterns of AlN-deficient Eu2+- ◦ ◦ SiAlON fired at 1680 C for 2 hrs in 30% H2 - 70% N2 atmo- doped Ca-α-SiAlON fired at 1680 C for 2 hrs in 30% H2 - sphere. 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, oxi- Fig. 5. Normalized intensities and peak emission wave- dation always occurred due to the residual oxygen inside lengths of AlN-deficient Eu2+-doped Ca-α-SiAlON fired at ◦ and outside the raw materials. Thus, SiO2 was formed 1680 C for 2 hrs in 30% H2 - 70% N2 atmosphere. and easily evaporated during annealing at temperatures higher than 1600 ◦C. As a result, a secondary phase was detected in the final product. Figure 3 shows the XRD A single α-phase was achieved from the samples when patterns of α-SiAlON phosphor (x = 0.12) fired at 1680 the deficient amount of AlN was from 10 to 30 mol% in ◦ C for 2 hrs in 30% H2 - 70% N2 atmosphere. Main peaks the starting composition. In addition, unreacted Si3N4 were matched those from Ca0.8Al2.8Si9.2O14.8 (JCPDF appeared when AlN deficiency was 35 mol%. As a re- #33-0261). Even though the raw materials had been sult, preparing the starting materials with AlN-deficient mixed in a stoichiometric composition, unreacted AlN compositions was needed to gain single-phase α-SiAlON. was detected in the final product, which was obviously To investigate the luminescence properties of the above attributed to the evaporation of Si from the raw mate- phosphors, we measured the PL spectra. Figure 5 shows rials. Generally, unreacted elements caused a degrada- the normalized intensities and peak emission wavelengths tion of the luminescence properties. Therefore, to obtain of 0 ∼ 30 mol% AlN-deficient phosphors fired at 1680 ◦C a single α-phase, we synthesized AlN-deficient samples, for 2 hrs in 30% H2 - 70% N2 atmosphere. As the de- and their properties were observed. ficient amount of AlN increased, the PL intensity also Figure 4 shows the XRD patterns of AlN-deficient increased. The maximum intensity was gained when the phosphors (x = 0.12) fired at 1680 ◦C for 2 hrs in 30% deficient amount of AlN was 15 mol%. The intensity de- H2 -70% N2 atmosphere. There was no significant crys- creased when the deficient amount of AlN exceeded 15 tal structure change from that of α-SiAlON. However, mol%. An intensity enhancement of 18% was obtained a single α-phase without unreacted AlN was achieved. by preparing the sample in 15 mol% AlN-deficient com- Crystal Structures and Luminescence Properties of AlN-deficient ··· – Sung-Soon Park et al. -993-

α-SiAlON was synthesized in atmosphere at normal pressure, SiO2 always evaporated from the raw materials, re- sulting in some AlN remaining in the final phase. There- fore, 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.

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