Highly Efficient and Thermally Stable Blue-Emitting AlN:Eu2+

May 1, 2009 - (17, 21) Another more flexible method to create white light is to pump red, green, and blue phosphors by a UV InGaN LED.(29-31) To achie...
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J. Phys. Chem. C 2009, 113, 9392–9397

Highly Efficient and Thermally Stable Blue-Emitting AlN:Eu2+ Phosphor for Ultraviolet White Light-Emitting Diodes Kazuo Inoue, Naoto Hirosaki, Rong-Jun Xie,* and Takashi Takeda Nitride Particle Group, Nano Ceramics Center, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan ReceiVed: February 13, 2009; ReVised Manuscript ReceiVed: April 8, 2009

An Eu2+-activated AlN phosphor was synthesized by firing the powder mixture of AlN, R-Si3N4, and Eu2O3 at 1500-2050 °C for 4 h under 1.0 MPa N2. The phase purity, photoluminescent properties, thermal quenching, and quantum efficiency of the fired samples were investigated. A single AlN wurtzite phase was formed at low doping concentrations of Eu2+ (e0.1 mol %) and Si (e2.2 mol %). The introduction of Si is essential for the solubility of Eu2+ in the AlN lattice. Intense blue luminescence with a peak emission wavelength of 465 nm was observed in AlN:Eu2+, when Si was doped simultaneously. This blue phosphor shows a small thermal quenching, retaining the luminance of 90% at 150 °C. The absorption and external quantum efficiencies of AlN:Eu2+ are 63%µ and 46% upon 365 nm excitation, respectively. These results indicate that AlN:Eu2+ has great potential as a blue phosphor for white light-emitting diodes (LEDs) utilizing UV chips as the light source. Introduction White light-emitting diodes (LEDs), the next-generation solid state lighting, have attracted much attention due to their superior features such as quite low power consumption, high efficiency, small volume, low maintenance, and being mercury-free. White LEDs have been widely used as backlights for electronic devices, and are anticipated to replace traditional fluorescent lamps for general illumination as their efficiency and color rendering properties are significantly improved. The most common white LED is obtained by combining a blue InGaN chip with a yellow cerium doped yttrium aluminum garnet.1 However, this type of white light has poor color rendering caused by the color deficiency in the red- and blue-green regions. To overcome this problem, green and red phosphors were combined with blue LEDs.2-7 Recent work by several groups points to the suitability of rare earth nitride phosphors in white LEDs.8-22 The 5d orbit of Eu2+ or Ce3+ in nitride compounds with high covalent chemical bonds is significantly split into several levels under the strong crystal field, which yields the downshift of the lowest position of the excited state of rare earth ions, and finally results in blue-light excitable phosphors.8,9,17,18,23-27 However, searching for phosphors that can be excited by blue GaN LED chips is still a difficult and time-consuming task. Up to now, only a few nitride phosphors are developed for white LEDs, which include β-sialon:Eu2+,13,28 SrSi2O2N2:Eu2+,6,14 R-sialon:Eu2+,8-11 M2Si5N8:Eu2+ (M ) Ca, Sr, Ba),16,19,20,23 and CaAlSiN3:Eu2+.17,21 Another more flexible method to create white light is to pump red, green, and blue phosphors by a UV InGaN LED.29-31 To achieve this goal, a blue emitting phosphor is required. A common blue phosphor for ultraviolet white LED is BaMgAl10O17:Eu2+ (BAM).32 This phosphor has a very high efficiency of luminous, but degrades significantly upon heating,33 which would reduce the lifetime and efficiency of white LEDs. Therefore, it is necessary to develop novel blue phosphors that have high efficiency and small thermal quenching. * To whom correspondence should be addressed. E-mail: xie. [email protected].

Eu2+-doped phosphates, such as LiSrPO4 and KSrPO4, have shown potential as blue phosphors for white LEDs, which have intense luminescence under UV light excitation.34,35 The KSrPO4: Eu2+ phosphor also exhibits a small thermal quenching.35 In addition, we have shown that nitride compounds emit blue colors when they are doped with Eu2+ or Ce3+. These phosphors, including R-sialon:Ce3+,22,24,27 SrSiAl2O3N2:Eu2+,36 La3Si8O4N11: Ce3+,37 and LaAl(Si6-zAlz)(N10-zOz):Ce3+,31 are potential blue down-conversion materials in UV white LEDs. Takahashi et al. reported high color rendering white LEDs (Ra > 95), by using the blue LaAl(Si6-zAlz)(N10-zOz):Ce3+ and other phosphors.31 Recently, we reported a blue phosphorsEu2+-doped AlNsfor full color field-emission displays (FEDs).38 This phosphor shows intense blue cathodoluminescence (CL) with a broad emission band centered at 465 nm, under the excitation of electronic beams. Moreover, it exhibits high brightness, high color purity, and high stability against electronic beam irradiations. The excellent CL properties of AlN:Eu2+ make us believe that it would also show interesting photoluminescence and have novel potential applications besides FEDs. In a previous paper, we studied the low acceleration voltage CL properties of AlN:Eu2+.38 In this work, we investigate the photoluminescence properties, thermal quenching and quantum efficiency of AlN:Eu2+, and discuss the influence of the Sidoping on the phase purity and photoluminescence of the phosphors. We demonstrate that AlN:Eu2+ is a highly efficient and thermally stable blue-emitting phosphor for ultraviolet white LEDs. Experimental Section Materials. AlN (Type F, Tokuyama Corp., Japan), R-Si3N4 (SN-E10, Ube Industries, Japan), and Eu2O3 (Shin-Etsu Chemical Co. Ltd., Japan) were used as received. Synthesis. Powder samples, with the compositions of (1 - x - y)AlN:xEu, ySi (0 e x e 0.0048, 0 e y e 0.0078), were prepared by mixing the appropriate amount of AlN, R-Si3N4, and Eu2O3. The powder mixture was packed into a boron nitride crucible and fired in a gas-pressure sintering furnace (FVPHR-

10.1021/jp901327j CCC: $40.75  2009 American Chemical Society Published on Web 05/01/2009

Blue-Emitting AlN:Eu2+ Phosphor

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Figure 1. XRD patterns of AlN with varying Si concentrations and a fixed Eu2+ concentration of 0.10 mol %. Open and closed circles represent unknown impurity phases.

R-10, FRET-40, Fujidempa Kogyo Co. Ltd., Osaka, Japan) with a graphite heater. The samples were heated at a constant heating rate of 600 deg/h under 10-2 Pa in vacuum from room temperature to 800 °C. At 800 °C, a nitrogen gas (99.999% purity) was introduced into the chamber, and simultaneously the temperature was raised to 1500-2050 °C. The samples were heated at that temperature for 4 h under a nitrogen gas pressure of 1.0 MPa. After firing, the power was shut off, and the samples were cooled with the furnace. Characterization. The phase products were identified by a powder X-ray diffraction (XRD) (Ultima III, Rigaku, Japan), using Cu KR radiation. The morphology of phosphor particles was observed by a scanning electron microscope (SEM). The Si, Al, and Eu contents of the sample were measured by an induction coupled plasma method (ICP). The oxygen and nitrogen contents of the sample were analyzed by the selective hot-gas extraction method (TC-436, CS-444LS, LECO Co.). Photoluminescent Properties. The photoluminescence spectra were measured at room temperature with a fluorescent spectrophotometer (F-4500, Hitachi Ltd., Japan) with a 200 W Xe-lamp as an excitation source. The emission spectrum was corrected for the spectral response of a monochromator and Hamamatsu R928P photomultiplier tube by a light diffuser and tungsten lamp (Noma, 10 V, 4A). The excitation spectrum was also corrected for the spectral distribution of the xenon lamp intensity by measuring Rhodamine-B as reference. The temperature-dependent luminescence was measured at 25-300 °C by an intensified multichannel spectrometer (MCPD-7000, Otsuka Electronics with a monochromatic xenon lamp as the light source). Quantum Efficiency. External (η0) and internal (ηi) quantum efficiencies (QEs) were calculated by using the following equations:39

η0 )

∫ λ · P(λ) dλ ∫ λ · E(λ) dλ

ηi )

∫ λ · P(λ) dλ ∫ λ{E(λ) - R(λ)} dλ

where E(λ)/hν, R(λ)/hν, and P(λ)/hν are the number of photons in the spectrum of excitation, reflectance, and emission of the phosphor, respectively. The luminescence spectra for QE measurement were recorded by an intensified multichannel spectrometer (MCPD-7000). The reflection spectrum of Spectralon diffusive white standards is used for calibration. Results and Discussion Phase Purity and Microstructure of AlN:Eu2+. Figure 1 shows the effect of the Si-doping on the phase purity of AlN

Figure 2. XRD patterns of AlN with varying Eu2+ and Si concentrations. Closed circles and open triangles represent unknown impurity phases.

doped with 0.10 mol % of Eu2+. As seen, the sample without the Si-doping contains not only a major phase of AlN, but also a trace amount of an unknown impurity phase. This indicates that the Eu atoms are not dissolved in the AlN lattice totally, and the impurity phase must be Eu-containing compounds. The impurity phase disappears with the doping of Si, and the single AlN phase is obtained when the Si concentration is less than 2.2 mol %. This means that the Si-doping is essential to achieve the entire dissolution of Eu2+ into the AlN lattice. However, the mechanism for the dissolution of Eu and Si into AlN is not clear yet, and great efforts are being made to elucidate it. One possible explanation is that the Si-doping yields some defect structures such as stacking faults that accommodate or stabilize Eu. When the Si concentration increases up to 2.9 mol %, a small shoulder appears around the (002) diffraction peak. This shoulder finally develops into an intense peak at ∼35.30° as the Si concentration reaches 5.4%, indicating the formation of an impurity phase in heavily doped samples. Furthermore, the broadness of AlN phase peaks is observed when the Si concentration exceeds 2.9 mol %, indicative of poor crystallinity caused by the excess Si-doping. In addition, two small peaks around the (002) diffraction peak are clearly seen in the sample doped with 0.19 mol % of Eu2+, as shown in Figure 2. This demonstrates that the solubility of Eu2+ in AlN is less than 0.1 mol % when 2.2 mol % of Si is codoped. Typical SEM images of the microstructure of the AlN:Eu2+ sample containing 0.24 mol % of Eu and 2.9 mol % of Si are presented in Figure 3. As seen, the particles coarsen as the firing temperature increases. The particle size increases from ∼0.6 µm for the starting powder to ∼3 µm for the sample fired at 1900 °C, and finally to ∼5 µm at 2050 °C. Moreover, the firing temperature obviously affects the particle morphology of the products. The AlN particles show a uniform and spherical morphology with smooth surfaces when the sample was synthesized at 2050 °C, whereas the particles have a plate-like shape and a broad size distribution when they are fired at 1900 °C. The chemical analysis by ICP (Table 1) indicates that the loss of Si, Eu, and O occurs when the sample is fired at high temperatures, making the real composition of the fired sample slightly deviate from the nominal one. Photoluminescence Spectra of AlN:Eu2+. Figure 4 shows the excitation and emission spectra of the sample with 2.9 mol

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Figure 4. Excitation and emission spectra of the sample doped with 2.9 mol % of Si and 0.24 mol % of Eu2+. The emission spectra were measured under the 290 nm excitation, and the excitation spectra were recorded by monitoring the emission of 465 nm.

Figure 3. SEM images of the sample fired at (a) 1950 and (b) 2050 °C.

Figure 5. Emission spectra of samples doped with dilute Eu2+ concentrations. The spectra were recorded under the 260 nm excitation

TABLE 1: Chemical Analysis of the Sample Fired at Varying Temperatures (mol %) nominal composition fired at 1950 °C fired at 2050 °C

Si

Al

O

N

Eu

2.86 2.07 1.54

46.4 47.2 47.9

0.24 0.56 0.10

50.2 49.9 50.3

0.24 0.24 0.14

% of Si and 0.24 mol % of Eu2+. The phosphor exhibits an intense blue emission upon UV light excitation. The emission spectrum consists of a single and symmetric broadband centered at 465 nm. The full width at half-maximum (fwhm) of the band is about 52 nm. The blue emission can be ascribed to the allowed 4f65d f 4f7 transitions of Eu2+ because the undoped AlN was reported to give UV emission,40-43 as discussed below. No red line emissions from Eu3+ are observed, indicating that the valence of Eu ions in AlN powders is divalent. The excitation spectrum, monitored at 465 nm, covers a broad range of 250-450 nm. Several peaks or a shoulder at 280, 290, and 350 nm are observed. The structure in the excitation spectrum is due to the crystal field splitting of the 5d level of Eu2+ ions. The number of splitting levels is determined by the local symmetry around Eu2+. Unfortunately, the uncertainty of the location of Eu in AlN makes it difficult to understand the local symmetry and electronic structure around Eu2+. To understand the luminescence mechanism of Eu2+ in AlN, we prepared several samples with dilute Eu2+ concentration. Figure 5 gives the emission spectra of these samples. It is noted that the undoped AlN sample shows a broad emission band centered at 400 nm under the excitation at 260 nm. It has been addressed that the origin of this UV emission is ascribed to the radiative recombination processes involving oxygen related impurity and Al vacancies.40-43 An extra peak at 465 nm appears for the sample with 0.005 mol % of Eu2+, which is attributed

Figure 6. Effect of the Eu2+ concentration on the luminance of samples doped with 2.9 mol % of Si. The samples were fired at 2050 °C for 4 h.

to 4f65d f 4f7 transitions of Eu2+. Further increase of the concentration of Eu2+ results in the weakness of the UV emission but the intensity of the blue emission. The sample with 0.05 mol % of Eu2+ only shows the blue emission band. This suggests that the UV emission related to the oxygen impurity is suppressed by the energy transfer from the donor level of the oxygen impurity to the Eu2+ emission center when the Eu2+ concentration increases. Concentration Quenching. Figure 6 shows the emission intensity of samples containing 2.9 mol % of Si as a function of the Eu2+ concentration. The emission data were collected under the 290 nm excitation. As seen, starting at a low doping level, the emission intensity reaches the maximum at about 0.10 mol % of Eu2+ and falls again at higher doping levels. The decrease of emission intensity is generally due to the concentration quenching, which is usually explained by the energy transfer between Eu2+ ions.9 When the concentration of Eu2+ increases,

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Figure 7. Excitation and emission spectra of the sample only doped with 0.10 mol % of Eu2+. The emission spectrum was measured under 336 nm excitation, and the excitation spectrum was monitored at 545 nm. The sample was fired at 2050 °C for 4 h.

the distance between Eu2+ ions is shortened, and thus the probability of energy transfer increases. In the present case, however, the solubility of Eu2+ in AlN is quite low (e0.10 mol %), implying that the possibility of energy transfer is also lowered. In other words, the energy transfer mechanism is not a completely convincing explanation for the concentration quenching of AlN:Eu2+. As mentioned previously, Eu2+ is hardly dissolved in AlN, and the excess amount of Eu2+ will reduce the crystallinity and volume fraction of AlN. Therefore, we consider that the lowered crystallinity and volume of AlN in heavily doped samples also accounts for the decrease of emission intensity. The increase of emission intensity for the diluted samples results from the increased absorption of Eu2+. No red-shift of the emission is observed as the activator concentration increases, which is again attributable to the low solubility of Eu2+ in AlN that hardly changes the local environment of the luminescence center. Role of the Si-Doping. We have addressed that the Si-doping dramatically improves the CL intensity, and ascribed it to the increase of the electronic conduction and the charge neutrality when Eu2+ dissolves into the AlN lattice.38 In this work, we find that the Si-doping also enhances the photoluminescence properties of AlN:Eu2+, which is shown and discussed below. Figure 7 shows the excitation and emission spectra of the sample without the Si-doping. The sample contains 0.10 mol % of Eu2+. Differing from the sample with 2.9 mol % of Sidoping, which has a blue emission (see Figure 4), this sample shows a green emission band centered at 545 nm. The green emission strength, however, is only a few percent of the blue one. The green emission of Eu2+ in AlN was also observed in combustion synthesized samples that do not contain Si.44 This thus means that a very few percent of Eu2+ is able to enter into the AlN lattice even without the Si-doping. The difference in emission color indicates that there are some changes in local structure, symmetry, and coordination number of Eu2+ in samples with and without the Si-doping. Figure 8 shows the effect of the Si-doping on the photoluminescence of samples with 0.10 mol % of Eu2+. The emission spectra were recorded upon the 290 nm excitation, and the excitation spectra were measured by monitoring the 465 nm emission. It is clearly seen that the blue emission is enhanced significantly with an increase in the Si concentration. The optimal Si concentration is found at about 2.2 mol %. The decrease in emission intensity of heavily doped samples is attributable to the poor crystallinity and the presence of impurity phases, as shown in XRD in Figures 1 and 2. Furthermore, the emission band tends to shift toward the long wavelength side as the Si concentration increases. The shape of the excitation spectrum remains unchanged with the Si-doping, indicating that

Figure 8. (a) Emission and (b) excitation spectra of samples with varying Si concentrations and a fixed Eu2+ concentration of 0.10 mol %. The emission spectrum was measured under the 290 nm excitation, and the excitation spectrum was recorded by monitoring the emission at 465 nm.

the local environment of Eu2+ is similar in all samples. The red-shift is therefore due to the increased Stokes shift with the Si-doping. The role of the Si-doping on the luminescence of AlN:Eu2+ is still in argument because the exact location of Si and Eu is still under investigation. Obviously, one of the roles of the Sidoping is to enhance the solubility of Eu into AlN and guarantee the phase purity, as seen from the XRD patterns in Figures 1 and 2. In addition, the different emission colors of samples with and without Si indicate that the local structure of Eu is altered in both samples. The introduction of Si is thus believed to change the symmetry, crystal-field strength, and coordination of Eu2+ in AlN powders. We suppose that Si plays a crucial role in the formation of some local structures that are similar to the layer structure containing Sr(O,N)12 cuboctahedron and (Si,Al)(O,N)4 tetrahedron in the Sr-containing AlN polytypoid.45 The latter one also shows a similar blue emission when Eu2+ is doped.46 The analysis of the fine structure, coordination, valence, and bonding status of the Si-doped AlN:Eu2+ is carried out now by means of Transmission Electronic Microscope (TEM), Extended X-ray-Absorption Fine Structure (EXAFS), and X-ray Photoelectron Spectroscopy (XPS), and the results will be published elsewhere. Effect of Firing Temperature. Figure 9 shows the emission spectra of samples fired at varying temperatures. The sample was doped with 0.10 mol % of Eu and 2.2 mol % of Si. As seen, the firing temperature has a great influence in the luminescence of Eu2+. No emission is observed below 1600 °C, whereas very intense blue emission is achieved at 2050 °C. The emission intensity continuously increases with the temperature, which originates from (i) the increase of the solubility of Eu in AlN, (ii) particle coarsening, and (iii) the improvement

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Figure 11. Emission spectra of the sample doped with 0.10 mol % of Eu2+ and 2.2 mol % of Si, measured at varying temperatures. Figure 9. Effect of the temperature on the luminance of the sample doped with 0.10 mol % of Eu2+ and 2.2 mol % of Si. The emission spectrum was measured under the 290 nm excitation.

Figure 10. Temperature dependence of the luminance of the sample doped with 0.10 mol % of Eu2+ and 2.2 mol % of Si.

of the crystallinity of the particles. In addition, the emission band blue-shifts with increasing the temperature. The blue-shift is attributed to the loss/evaporation of Eu and Si at high temperatures, as shown in Table 1. Thermal Quenching. Phosphors must sustain emission efficiency at temperatures of about 150 °C over a long-term when they are used in white LEDs. It is thus required that the thermal quenching of phosphors should be small for achieving exceptional lifetimes of white LEDs, typically for high-power ones. The thermal quenching of AlN:Eu2+ was evaluated by measuring the temperature-dependent emission intensity. Figure 10 shows the emission-peak intensity as a function of temperature for the sample doped with 2.2 mol % of Si and 0.10 mol % of Eu2+. No thermal hysteresis is observed during the heating and cooling cycles. The blue phosphor shows a very small thermal quenching. At 150 °C, the emission intensity of AlN: Eu2+ maintains 90% of that measured at room temperature. In comparison with other nitride and phosphate phosphors reported previously,11,19,28,35 AlN:Eu2+ has superior thermal quenching behavior. Moreover, the emission band does not shift with increasing temperature (see Figure 11), indicative of the chromatic stability of the blue nitride phosphor against temperature. Quantum Efficiency. Figure 12 shows internal quantum efficiency (ηi), external quantum efficiency (η0), and absorption (R) as a function of the excitation wavelength. The values of ηi, η0, R for AlN:Eu2+ are 76%, 46%, and 63%, respectively, when it is excited at 365 nm. The quantum efficiency is greatly affected by the probability of the nonradiative transition process. To prohibit the nonradiative recombination, the phosphor particles should be perfect in crystallinity and free of surface defects. We believe that the quantum efficiency can be enhanced through modifying the processing conditions or annealing the powder several times. The high absorption and quantum

Figure 12. Internal quantum efficiency (ηi), external quantum efficiency (η0), and absorption (R) of the sample doped with 0.10 mol % of Eu2+ and 2.2 mol % of Si as a function of the excitation wavelength.

efficiency of AlN:Eu2+ at the excitation wavelength of 365 nm indicate that it matches well with the emission wavelength of InGaN UV-LED chips and is a promising blue-emitting downconversion phosphor in white LEDs. Conclusions Eu2+ and Si codoped AlN:Eu2+ phosphors were synthesized via the solid state reaction of Si3N4, AlN, and Eu2O3 at high temperatures under a nitrogen gas pressure of 1.0 MPa. A phasepure AlN was achieved when the doping concentration of Eu2+ and Si is less than 0.1 and 2.2 mol %, respectively. The AlN: Eu2+ phosphor shows a broad emission band centered at 465 nm upon irradiations of the UV light (280-380 nm). The luminance of the phosphor was significantly enhanced by increasing the firing temperature and the Si-doping, which is due to the increases of the solubility of Eu2+, phase purity, crystallinity, and particle size. The blue phosphor has a small thermal quenching, retaining its luminance of 90% at 150 °C. The absorption and external quantum efficiency of the product is 63% and 46% at an excitation wavelength of 365 nm, respectively. The excellent luminescent properties and thermal stability enable AlN:Eu2+ to be used an a blue-emitting downconversion phosphor in white LEDs utilizing InGaN UV-LED chips. References and Notes (1) Nakamura, S.; Fasol, G. The Blue Laser Diode: GaN Based Light Emitters and Lasers; Springer: Berlin, Germany, 1997. (2) Yamada, M.; Naitou, T.; Izuno, K.; Tamaki, H.; Murazaki, Y.; Kameshima, M.; Mukai, T. Jpn. J. Appl. Phys., Part 2 2003, 42, L20. (3) Xie, R.-J.; Hirosaki, N.; Kimura, N.; Sakuma, K.; Mitomo, M. Appl. Phys. Lett. 2007, 90, 191101. (4) Sakuma, K.; Hirosaki, N.; Kimura, N.; Ohashi, M.; Yamamoto, Y.; Xie, R.-J.; Suehiro, T.; Asano, K.; Tanaka, D. IEICE Trans. Electron. 2005, E88-C, 2057.

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