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Optical Review https://doi.org/10.1007/s10043-018-0431-6

REGULAR PAPER

Effect of sintering temperature of  Ce3+-doped Lu3Al5O12 phosphors on light emission and properties of crystal structure for white-light-emitting diodes

Hiroshi Koizumi1,3  · Junya Watabe2 · Shin Sugiyama2 · Hideaki Hirabayashi3 · Tetsuya Homma1

Received: 3 December 2017 / Accepted: 16 March 2018 © The Optical Society of Japan 2018

AbstractThe effect of the sintering temperature of Ce3+-doped Lu3Al5O12 (Ce-LuAG) phosphors on the emission and properties of the crystal structure was studied. A cathodoluminescence peak at 317 nm, which was assigned to lattice defects, was exhibited in addition to emission peaks at 508 and 540 nm for the Ce-LuAG phosphors. The intensities of the 317 nm emission peak for the phosphors with mean particle diameters of 5.0 and 10.0 µm formed at a low sintering temperature of 1430 °C were higher than those for the phosphors with mean particle diameters of 18.0 and 20.5 µm formed at a high sintering temperature of 1550 °C. In contrast, the electroluminescence spectra for fabricated white-light-emitting diodes (LEDs) using the phos-phors revealed that the intensity of the peak at 540 nm was strong for the mean particle diameters of 18.0 and 20.5 µm. The intensity of the 540 nm peak, which is attributed to the 4f→5d transition of the Ce3+ activator, showed a dependence on the sintering temperature. The relationship between the optical properties and the lattice defects is discussed.

Keywords Solid-phase synthesis · Luminescence · Green aluminate garnet · Ce3+ · Lattice defect · White LEDs

1 Introduction

White-light-emitting diodes (LEDs) have attractive charac-teristics such as a long operating lifetime, small size, low electricity consumption, and no use of toxic substances. Therefore, they are expected to be used for next-generation lighting and display systems in which high luminescence efficiency is required [1]. White LEDs are composed of two emission layers. One is a gallium nitride layer (GaN-LED) that emits blue light, and the other is a phosphor layer that absorbs blue light (420–480 nm) and then emits yellow and/or green light [2, 3]. These phosphors are dispersed in a

highly transparent resin on the GaN-LED. If the phosphors are not dispersed uniformly, the performance of white LEDs is degraded because of the shifting color temperature. The settling velocity of larger particle size phosphors is faster according to Stokes’ law [4, 5]. If a phosphor contains a mixture of small and large particles, it is difficult to dis-tribute the particles uniformly. Therefore, phosphors with a small and uniform particle size are expected to be more effective. Thus, we have developed phosphors with a small and uniform particle size and high luminescence efficiency for LED systems, whose brightness and color properties can be optimized by designers for future lighting systems [1, 2].

Ce3+-doped lutetium aluminate garnet (Ce-LuAG; Lu3Al5O12: Ce3+) phosphor has high-efficiency yel-low–green emission at a wavelength around 540 nm. This phosphor has excellent thermal stability and high optical transmittance over a wide spectral region [1–3, 6]. Its appli-cation to scintillators and LEDs has also been reported. However, it is necessary to improve its emission proper-ties for practical use in white LEDs. Moreover, the relations among the emission properties, the conditions of solid-phase synthesis, and the lattice defects for Ce-LuAG phosphor have not yet been clarified [6–11].

* Hiroshi Koizumi hiroshi.koizumi@toshiba.co.jp

1 Postgraduate Courses of Functional Control Systems, Shibaura Institute of Technology, 3-7-5, Toyosu, Koto-ku, Tokyo 135-8548, Japan

2 Nemoto Lumi-Materials Co., Ltd., 4-1 Shinmachi, Hiratsuka-shi, Kanagawa 254-0076, Japan

3 Process Technology Research Department, Corporate Manufacturing Engineering Center, Toshiba, 33 Shin-Isogo-cho, Isogo-ku, Yokohama-shi, Kanagawa 235-0017, Japan

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In this work, the dependence of the light emission proper-ties of Ce-LuAG phosphors on the sintering temperature has been studied using a fabricated white LED, and the cause of the dependence is discussed.

2 Experimental

Figure 1 shows a flowchart of the synthesis process for the Ce-LuAG phosphors. Four Ce-LuAG samples with various particle sizes were prepared by solid-phase synthesis, where the mean diameters of the particles were 5.0, 10.0, 18.0, and 20.5 µm. High-purity Lu2O3 (0.97 mol), CeO2 (0.03 mol), and Al2O3 (1.5833 mol) were used as precursors, and were combined to adjust to the formula to (Lu0.97Ce0.03)3Al5O12. BaF2 was added to the precursors as a flux material. The precursors were mixed and then mechanically milled using a ball mill. The resultant mixture was placed in a boron nitride (BN) crucible and heated to 1550 °C for 8 h in an atmos-phere of steam-free 97% nitrogen and 3% hydrogen. After the mixture was cooled slowly to room temperature, it was milled again, and then washed with pure water and an acid solution. Next, the mixture was dried at 70 °C in a drying oven for 12 h in air [12]. The synthesized phosphors were separated into two particle sizes using 20-µm mesh sieves. The particles were separated into those with mean diameters

of 18.0 and 20.5 µm. The phosphors with the other two par-ticle sizes were prepared by sintering at 1430 °C under the same synthesis conditions as above. Then the synthesized phosphors were separated into those with mean diameters of 5.0 and over 10.0 µm using 10-µm mesh sieves. These particles were measured using a particle size measurement system based on a diffraction method using a diode laser with a wavelength of 680 nm as a light source (Shimadzu SALD-2200). A non-doped LuAG sample was also prepared by sintering at 1430 °C and the same synthesis conditions as for the other Ce-LuAG phosphors.

Figure 2 shows a cross-sectional picture of the fabricated white LED obtained using a surface-mounted device (SMD). Four white LEDs were fabricated using the synthesized Ce-LuAG phosphor particles with mean diameters of 5.0, 10.0, 18.0, and 20.5 µm. The phosphor particles were dispersed in a resin with 99% visible-light transmittance on a high-power GaN-LED as a 450 nm excitation light source. Subsequently, the phosphor particles in the resin were deposited on the GaN-LED by centrifugation to form a phosphor layer. This layer was used for the semi-quantitative measurement of the electroluminescence (EL) intensity and correlated color temperature (CCT) of the fabricated white LEDs. Each of the phosphor layers was formed with a uniform thickness of 100 µm on the GaN-LEDs.

The crystal phases of the synthesized particles were iden-tified by X-ray diffraction (XRD) analysis in the range of 15–100° in the θ–2θ mode using Cu-Kα (λ1 = 0.1541837 nm) radiation at 45 kV and 40 mA. A PANalytical X’pert diffrac-tometer was used for the XRD analysis. The intensity data were recorded by continuous scanning with a step width of 0.0084°. The diffraction profiles were analyzed using the Inorganic Material Database [13]. Photoluminescence (PL) spectra were measured using the 325 nm line of a He–Cd laser as an excitation light source at room temperature. Omnichrome Series 56 with an LC-500 He–Cd laser con-troller was used for the PL measurement. For the semi-quan-titative measurement of the XRD profiles and PL intensities of the synthesized particles, a thin film of 500 µm thickness with a flat surface was formed on a SiO2 substrate using a certain quantity of the samples [14, 15]. Cathodolumines-cence (CL) spectra measurements were conducted with an

Fig. 1 Flowchart of the synthesis process for Ce-LuAG phosphor Fig. 2 Cross-sectional picture of fabricated white LED using SMD

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acceleration voltage of 5 kV at room temperature using a scanning electron microscopy (SEM) system equipped with a CL system. A Hitachi S-4300SE/N system and a Horiba Jobin Yvon MicroHR CCD-3500V CL collector were used for the SEM observation and CL measurement, respec-tively. Qualitative elemental distribution maps and single-spot analysis results were obtained using a field-emission electron probe microanalyzer (EPMA; JEOL JXA-8530F). The conditions for elemental mapping were an acceleration voltage of 10 kV and a beam current of 5 × 10−9 A. The dwell time per pixel was set at 10 ms for the spot beam using in the mapping. The conditions for qualitative spot analysis were an acceleration voltage of 10 kV, a beam cur-rent of 1 × 10−8 A, and a beam diameter of 1.0 µm. Both the mapping and single-spot analyses were performed in the wavelength-dispersive spectrometry (WDS) mode using two spectrometers employing thallium acid phthalate (TAP) and lithium fluoride (LIF) analyzing crystals. The EL and CCT of the fabricated white LEDs were measured under a driv-ing current of 350 mA and a luminescence time of 100 ms. A Gooch & Housego OL770 High-Speed LED Test and Measurement System spectroradiometer with an integrat-ing sphere was used for the EL and CCT measurements.

3 Results and discussion

Figure 3 shows XRD profiles for all the phosphors and the non-doped LuAG particles, compared with the database profile for the LuAG structure [13]. The profiles for all

particles were the same as the database profile. Therefore, all the particles have the same crystal structure as LuAG. Ce3+ ions, included in the Ce-LuAG phosphor as an activa-tor, are substituted for Lu3+. However, no difference was observed between Ce-LuAG and the non-doped LuAG in our XRD measurements. The crystallite size D was derived from the full width at half maximum (FWHM) of the XRD diffraction peaks using the Scherrer equation.

where θ, K, λ2, and β are the Bragg angle, Scherrer constant (0.94), X-ray wavelength, and FWHM, respectively. Figure 4 shows the crystallite sizes calculated from the (420) diffrac-tion peak at 2θ = 33.607° using Eq. (1). The crystallite sizes varied from 56.0 nm for the 5.0 µm particles to 70.5 nm for the 20.5 µm particles [14]. The crystallite sizes for the 18.0 and 20.5 µm particles sintered at 1550 °C are larger than those for the 5.0 and 10.0 μm particles sintered at 1430 °C. The crystallite size was smallest for the 5.0 µm particles. Taking the size of the 5.0 µm particles as 1.0, the crystallite sizes were 1.03, 1.16, and 1.13 for the 10.0-, 18.0, and 20.5 µm particles, respectively. It is considered that larger crystal-lites can improve the crystal quality [14].

Figure 5 shows CL spectra for (a) wavelengths ranging from 200 to 800 nm and (b) wavelengths ranging from 200 to 450 nm for all the particles, where the intensities were normalized by the highest CL intensity at 508 nm. The CL spectra exhibited emission peaks at 508 and 540 nm. In addi-tion to these peaks, the CL spectra also showed a peak at 317 nm. Although the CL spectra of the non-doped LuAG exhibited an emission peak at 317 nm, emission peaks were not observed at 508 and 540 nm. Therefore, it is considered

(1)D = K�2∕� cos �

Fig. 3 XRD profiles for all Ce-LuAG phosphors, and non-doped LuAG samples, compared with LuAG database [13]

Fig. 4 Estimated crystallite size of Ce-LuAG samples calculated from the diffraction peak at 2θ = 33.607°

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that the peaks at 508 and 540 nm can be separated into two peaks, which characterize the electron transition from the 5d state to the two levels of the 4f state (2F5/2 and 2F7/2) of the Ce3+ activator [2, 3, 9, 14–17]. The emission peak at 317 nm originated from the LuAG host lattice. This broad 317 nm emission peak was exhibited for all the particles in the range from 210 to 450 nm, as shown in Fig. 5b. The emission intensity of the peak at 317 nm, normalized by the highest CL intensity at 508 nm, showed the lowest value for the 20.5 µm particles. Taking the intensity for the 20.5 µm particles as 1.0, the intensities were 96, 391, and 1837 for the parti-cles with diameters of 18.0, 10.0, and 5.0 µm, respectively. From these results, it was found that the intensities of the

317 nm emission peak for the 5.0 and 10.0 μm particles were higher than those for the 18.0 and 20.5 µm particles. Mean-while, upon Gaussian fitting, an emission peak at 366 nm was exhibited in addition to the 317 nm peak. An evaluation of the emission peaks at 317 and 366 nm from α-Al2O3, YAG, and LuAG crystals has already been reported. These crystals have similar emission peaks [2, 14, 17–20], and the emission peaks at 317 and 366 nm are derived from lattice defects such as antisite defects (ADs), which means that the Lu cation occupies the position of the Al cation, and vacancy-type defects [17–20, 23]. Therefore, it is consid-ered that these emission peaks observed for all the phosphor particles originate from same lattice defects as those in the LuAG host crystal. Moreover, the intensities of these peaks for the 20.5 µm particles were reduced. This reduction sug-gests a decrease in the number of lattice defects.

Figure 6 shows the PL spectra for all the particles meas-ured by excitation using the 325 nm line of the He–Cd laser. The PL spectra showed an emission peak at 540 nm for all the particles, and the PL intensity at 540 nm exhibited the lowest value for the 5.0 µm particle. Taking the intensity for the 5.0 µm particles as 1.0, the intensities for the 10.0, 18.0, and 20.5 µm particles were 1.2, 1.9, and 2.2, respectively. From these results, the intensity at 540 nm increased with increasing particle size. In contrast, the non-doped LuAG particle did not exhibit the PL peak at 540 nm. Hence, the peak at 540 nm is due to electron transition from the 5d to the 4f state of the Ce3+ activator [2, 9, 14–17].

This peak was similar to the CL peaks at 508 and 540 nm, as shown in Fig. 5a. However, the PL peak at 540 nm did not split into two peaks. This is because the emission peak is broad owing to the large Franck–Condon offset, which is a characteristic of the 4f→5d transition, and the influ-ence of the scattering irradiation of the photon energy [2,

Fig. 5 CL spectra of all Ce-LuAG particles. The spectra are ranging from a 200 to 800 nm and b 200 to 450 nm

Fig. 6 PL spectra of Ce-LuAG phosphor particles measured by exci-tation of 325 nm line at room temperature

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9]. This phenomenon can be explained by an energy level diagram, as shown in Fig. 7. In the case of excitation by the 325 nm line of the He–Cd laser, an electron is excited to the 5d2 state, and then the excited electron transits to the 5d1 state. Therefore, the peaks at 508 and 540 nm are due to multiphonon relaxation from the lowest level of the 5d1 state to the 4f state. The emission peak at 508 nm is due to the 5d1→2F5/2 transition of the 4f state, and that at 540 nm is due to the 5d1→2F7/2 transition of the 4f state of

the Ce3+ activator [17–23]. As mentioned above, the emis-sion intensity of the PL peak at around 540 nm due to the Ce3+ activator was higher for the 18.0 and 20.5 µm particles, while the CL peak at 317 nm, which is derived from the number of lattice defects, was lower. This phenomenon sug-gests an improvement of the crystal quality and is consistent with the calculated crystallite size, as shown in Fig. 4. In the case that the lattice defect leads the 4f electron to the defect level (366 nm, 3.39 eV) before the transition to the 5d level, the radiative transition ratio of the peak at around 540 nm decreases with increasing non-radiative deactivation ratio. This phenomenon is considered to involve the trap-ping of excited electron by ADs and vacancy-type defects around Ce3+ before the excitation from 4f to 5d2 (325 nm, 3.81 eV), and the electron transits from the 5d2 to 5d1 states [17, 21–23]. Therefore, for the 5.0 and 10.0 µm particles, the intensity of the peak at 540 nm is lower owing to the increased number of lattice defects.

Figure 8 shows EPMA elemental distribution maps with a pixel size of 112 × 90 µm2 for the 5.0 and 20.5 µm par-ticles. The EPMA maps show distributions of the Lu, Al, and Ce elements in the Ce-LuAG phosphor for both the 5.0 and 20.5 µm particles. The X-ray diffraction intensity for Ce was lower than those for Lu and Al because of its lower concentration, as shown in Fig. 8a, b. The EPMA maps also showed a strong X-ray diffraction intensity for Al for the 5.0 µm particles, as shown in Fig. 8a. However, a strong intensity of Al was not exhibited for the 20.5 µm particles, as shown in Fig. 8b.

Figures 9 and 10 show backscattered electron images (BEIs) and the results of EPMA qualitative spot analysis for all the Ce-LuAG particles, respectively. The energy spectra of the EPMA spots exhibited Lu and Al peaks as the prin-cipal components of Ce-LuAG. In addition, a weak X-ray Fig. 7 Energy level diagram for Ce-LuAG phosphor

Fig. 8 EPMA elemental distri-bution maps for a 5.0 µm, and b 20.5 µm particles

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peak of Ce was also exhibited for point 1 (P1). In contrast, the 5.0 and 10.0 µm particles exhibited a strong Al peak for point 2 (P2). In addition, a weak X-ray peak of Lu was also exhibited, as shown in Figs. 9a, b and 10a, b. Moreover, the X-ray peak intensity of Al for the 5.0 µm particles for P2 was 1.5 times higher than that for P1, while the X-ray peak

intensity of Lu for P2 was less than 0.1 times that for P1, as shown in Figs. 9a and 10a. The EPMA profile for the 10.0 μm particles was almost the same as that for the 5.0 µm particles, as shown in Figs. 9b and 10b. This difference is suggested to be generated by the segregation of Al owing to the lower sintering temperature of 1430 °C. Moreover, for

Fig. 9 BEI analysis results for a 5.0 µm, b 10.0 μm, c 18.0 μm, and d 20.5 µm particles

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the 5.0 µm particles, the BEI showed the existence of parti-cles with diameter less than 1.0 µm (corpuscles), as shown in Fig. 9a. In contrast, no corpuscles were observed for the 18.0 and 20.5 µm particles, as shown in Fig. 9c, d, respectively.

Figure 11 shows SEM images of particles sintered at the lower temperature of 1430 °C and the higher temperature of 1550 °C before they were separated according to diameter. Reaggregated corpuscles were observed for the particles sin-tered at 1430 °C, as shown in Fig. 11a. This phenomenon is considered to be due to the solid-phase reaction stopping at the middle of the diffusion state. When the precursors were

mixed and mechanically milled using the ball mill, they were pulverized to corpuscles generating lattice defects. Subse-quently, the corpuscles were heated and diffused during the solid-phase reaction. For the particles sintered at 1430 °C, it is considered that insufficient diffusion occurred owing to the reduced activity due to the lower sintering tempera-ture. Therefore, it is considered that lattice defects remained in the diffused corpuscles. In addition, Al remained in the segregated particles because the lattice energy of Al2O3 is higher than those of the other precursors. In contrast, for the particles sintered at 1550 °C, large particles with a mean diameter of about 10.0–30.0 µm were observed. Moreover, large particles of size 30.0 µm were also observed during the diffusion, as shown in Fig. 11b. This phenomenon suggests that the synthesized particles became larger and the number of lattice defects decreased owing to the higher activity of the solid-phase diffusion at the higher sintering temperature. From these results, it is considered that the crystal quality was improved and large particles were obtained [14, 24]. In the case that the number of lattice defects decreases between the transition from the 4f to 5d2 states, the excited electron transient ratio of the 5d2→5d1 transition and the energy relaxation ratio of the 5d1→4f transition are increased. The PL intensity of the peak at around 540 nm is also increased [14, 23, 24]. Therefore, it is clarified that the lattice defects, generated by insufficient solid-phase diffusion, act as killer centers for visible-light emission. The number of defects can be decreased by optimizing the sintering conditions in the solid-phase synthesis.

Figure 12 shows EL spectra in the wavelength range from 400 to 800 nm for the fabricated white LEDs at room temperature, where the intensities were normalized by the 450 nm peak originated from the GaN-LED. The peak around 450 nm showed a similar profile for all the fabri-cated white LEDs. In addition, the EL spectra also showed a 540 nm peak with broad emission due to Ce3+, which is a characteristic of the garnet phosphor [2]. The EL intensity of the 540 nm peak exhibited the lowest value for the particle size of 5.0 µm. Taking this intensity for the 5.0 µm particles as 1.0, the EL intensities for the 10.0, 18.0, and 20.5 µm par-ticles were 1.2, 1.8, and 2.2, respectively. From the results, the EL intensity at 540 nm increased with increasing phos-phor particle size in the phosphor layer. These properties are consistent with the PL spectra shown in Fig. 6.

Figure 13 shows the CCTs as a function of particle size for the fabricated white LEDs at room temperature. The CCTs were around 5000 K, corresponding to the color of white light prescribed by American National Standards Institute (ANSI) C78.377-2008, for all the fabricated white LEDs [1]. Moreover, the CCTs were also dependent on the EL intensity and the phosphor particle sizes. The CCTs were 4623 and 4792 K at high EL intensity for the 20.5 and 18.0 μm particles, respectively. This indicates a shift

Fig. 11 SEM images of particles generated by a lower sintering tem-perature 1430 °C and b higher sintering temperature 1550 °C, where the acceleration voltage was 5 kV

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from the white to yellow-green region. Meanwhile, the CCTs were 5268 and 5610 K at a low EL intensity for the 10.0 and 5.0 µm particles, respectively. This also indicates a shift from the white to blue region. This phenomenon suggests that if the 20.5 and 18.0 μm particles are effective for absorbing the emission at 450 nm, then the intensity at the 540 nm peak becomes broad owing to the increased emission of Ce3+ [25, 26].

4 Conclusion

In this work, the dependence of the light emission and crys-tal properties of Ce-LuAG phosphors on the sintering tem-perature has been studied. The 18.0 and 20.5 µm particles were formed at a high sintering temperature of 1550 °C, and the 5.0 and 10.0 μm particles were formed at a low sinter-ing temperature of 1430 °C by solid-phase synthesis. It was clarified that the crystallite size of the particles increased with increasing particle size. It was found that the PL peak at 540 nm is derived from the 4f→5d transition of Ce3+ and that the PL intensity increased with increasing particle size, while the intensities of the CL peak at 317 and 366 nm due to the decrease of the number of lattice defects. In addi-tion, a strong peak of Al was observed in EPMA qualitative analyses, originating from the segregation, for the 5.0 and 10.0 μm particles due to insufficient solid-phase diffusion at the low sintering temperature. Moreover, it was clarified that the EL intensity for the fabricated white LED also increased with increasing particle size.

Consequently, Ce-LuAG phosphor contains lattice defects, which act as visible-light emission killer centers. The defect density can be reduced by improving the sintering conditions in the solid-phase synthesis. Furthermore, it is effective to improve the light emission properties for high-power white LEDs. To further improve the emission inten-sity of Ce-LuAG phosphors with small particles for white LEDs, it is necessary to develop a low-temperature synthesis method while decreasing the number of lattice defects.

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