The temperature-dependent luminescence properties

The temperature-dependent luminescence properties of BaAl2xSixO4xNx:Eu2+ and its application in yellowish-green light emitting diode Mei Zhang Ministry of Education Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, Guangdong 510275, People’s Republic of China; and Institute of Functional Materials, Wuyi University, Jiangmen, Guangdong 529020, People’s Republic of China

Baohong Li, Jing Wang,a) Zhiyang Zhang, Qiuhong Zhang, and Qiang Sub) Ministry of Education Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, Guangdong 510275, People’s Republic of China (Received 22 June 2008; accepted 22 September 2008)

The influences of (SiN)+ and Eu2+ concentration on the optical properties of BaAl2xSixO4xNx:Eu2+ were investigated. The lifetime results show that there are two different cation sites occupied by Eu2+ ions and the energy transfer occurs between them. The Huang–Rhys factor and the Stokes energy shift were determined, and thermal quenching with increasing temperature was observed. Finally, intense yellowish-green light emitting diodes (LED) with the color coordinate of (0.2936, 0.4483) under a forwardbias current of 20 mA was successfully fabricated on the basis of a structure consisting of BaAl2xSixO4xNx:Eu2+ phosphor and near-ultraviolet (395 nm) GaN chip. I. INTRODUCTION

It is well known that the invention of white light emitting diodes (LEDs) has brought another revolution to the illumination of this century to supersede conventional incandescent or fluorescent lamps because of its excellent properties such as high brightness, reliability, lower power consumption, and long lifetime.1,2 At present, there are several ways to make the white LEDs.3,4 One significant scheme is phosphor-converted white LED (pc-wLED), which is further classified into two approaches: blue (440–470 nm) and near (n)-UV (390– 410 nm) InGaN chips combined with down-converting phosphors. For the blue InGaN chip, the commonly used down-converting phosphor is yellow YAG:Ce3+. However, such white LEDs encounter low color-rendering index (Ra < 80) because of the scarcity of red emission. Besides, the white LEDs based on blue InGaN chip encounter low color reproducibility in mass manufacture process. The n-UV LED is considered to be more stable and efficient with higher output.5 In addition, the n-UV LED generally emits at wavelengths shorter than 400 nm. It has little effect on the chromaticity coordinate of the pc-wLED, which is generally determined by the visible radiation distribution of phosphor between 380 and Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2009.0325 J. Mater. Res., Vol. 24, No. 8, Aug 2009

730 nm.6 Thus, the n-UV pc-wLEDs are expected to have great application potential in the field of solid-state lighting. The basic requirement for phosphor used in the white LED based on n-UV GaN chip is that the dominant excitation band of the phosphor should be matched well with the maximum emission band of n-UV GaN chip in the wavelength range of 380 to 420 nm. Nowadays, many phosphors have been investigated in oxides and sulfides including orthosilicates, akermanites, aluminates, molybdates, thioaluminates, and thiogallates.7–11 Recently, the research interest worldwide has been focused on the compounds containing nitrogen element, nitrides, and oxynitrides. The key point is that N3 shows higher formal charge and larger nephelauxetic effect (covalence), both of which contribute to the strengthening crystal-field splitting of the 5d levels and the red shift of the center of gravity of the 5d states of rare earths (Eu2+,Ce3+) than in an analogous oxygen environment. Consequently, oxynitride and nitride phosphors doped with rare earth ions are anticipated to show longer excitation and emission wavelengths than their oxide counterparts.12 The spectroscopic properties of Eu2+ doped BaAl2O4 phosphors were first investigated in the 1960s.13 Thereafter, many researchers have paid more attention to its longlasting phosphorescence properties and mechanism.14–17 The excitation and absorption bands of BaAl2O4:Eu2+ dominate in the UV range. Therefore, BaAl2O4:Eu2+ does not match with the UV–blue emission (370–460 nm) from © 2009 Materials Research Society

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M. Zhang et al.: Temperature-dependent luminescence properties of BaAl2xSixO4xNx:Eu2+ and its application in yellowish-green LEDs

GaN-based LEDs, suggesting that it is not suitable for white LEDs applications. Recently, Li et al. introduced the silicon and nitrogen atoms into the MAl2O4 (M = Ca, Sr, and Ba), e.g., (AlO)+ replacement by (SiN)+ and systematically investigated the effect of the substitution of (SiN)+ for (AlO)+ on the phase formation and crystal structure by x-ray powder diffraction combined with Rietveld refinement. The absorption and photoluminescent excitation and emission of MAl2xSixO4xNx:Eu2+ (M = Ca, Sr, Ba) and the dependence of luminescence properties on Eu2+ concentration in BaAl2xSixO4xNx at room temperature were briefly investigated. It was concluded BaAl2xSixO4xNx:Eu2+ could be efficiently excited in the range of 390 to 440 nm radiation, which makes this material attractive as conversion phosphor for white LED lighting applications.18 However, there is little research concerning the fundamental knowledge about the optical properties of the Eu2+ ion in Ba Al2xSixO4xNx, its temperature-dependent luminescence, and the fabrication and the optical properties of pc-LEDs by n-UV LEDs and Ba Al2xSixO4xNx:Eu2+. In this paper, the main aim is to enlarge the fundamental knowledge about the optical properties of the Eu2+ ion in Ba Al2xSixO4xNx lattice by means of xray diffraction pattern, the lifetime and the excitation and emission spectra at room temperature, and the temperature-dependent luminescence (10–425 K). The Huang– Rhys factor, the Stokes shift, and the thermal-quenching temperature are evaluated. The yellowish-green LEDs were also fabricated by combining Ba Al2xSixO4xNx: Eu2+ phosphors with 395 nm GaN chips. II. EXPERIMENTAL A. Syntheses

performed using a Fluorolog-3 spectrofluorometer (Jobin Yvon Inc./specx) equipped with a 450 W Xe lamp and double-excitation monochromators. The room-temperature lifetime measurement and temperature-dependent luminescence properties in the temperature range of 10 to 450 K were measured by a FLS920-combined Time Resolved Steady State Fluorescence Spectrometer (Edinburgh Instruments) equipped with a 450 W xenon lamp and a DE-202S closed-cycle helium cryostat (Advanced Research Systems, Inc.). C. Characterization of yellowish-green LED

The green LEDs were fabricated by combining InGaNbased ultraviolet LEDs (Cree Inc., C395-MB290, 12 mW, lem = 395 nm) and the as-synthesized phosphors. Their optical properties were evaluated by a LED-1100 Spectral/ Goniometric Analyzer (Labsphere Inc.) under a direct current of 20 mA at room temperature. All measurements were operated at room temperature except the temperature-dependent experiments. III. RESULTS AND DISCUSSION A. Phase Identification

Figure 1 shows the XRD patterns in the range of 10 < 2y < 80 . It can be seen that the XRD data of the series of samples generally confirm a single hexagonal phase and agree well with JCPDS 17-0306. However, there is a trace amount of impurity phase for x = 0.5, suggesting that the maximum solubility of (SiN)+ in BaAl2O4 is about 0.5. In addition, the substitution of Ba2+ by Eu2+ ion has no obvious influences on the host structure even if the concentration of Eu2+ is higher, which is due to the same valence and relatively smaller

Powder samples with a general formula of Ba1yEuy Al2xSixO4xNx (x = 0, 0.1, 0.3, 0.5, y = 0.015, 0.03, 0.06, 0.08, 0.1, 0.15, 0.25) were synthesized by solidstate reaction at high temperature. The raw materials were BaCO3(A.G.), Al2O3(A.G.), aSi3N4(99.5%), and Eu2O3(99.99%). The stoichiometric mixtures of the corresponding raw materials were thoroughly grounded and then prefired at 773 to 1173 K for 2 to 6 h in ambient atmosphere. Finally, they were sintered at 1473 to 1673 K for 4 to 8 h in a reductive H2 atmosphere. B. Characterization

The phase purity of the as-prepared phosphors was investigated by x-ray powder diffraction (XRD) with a Rigaku D/max 2200 vpc x-ray diffractometer with Cu Ka radiation at 40 kV and 30 mA. The XRD patterns were collected in the range 10 2y 80 . The photoluminescence (PL) and photoluminescence excitation (PLE) spectra at room temperature were 2590

FIG. 1. XRD patterns of Ba1yEuyAl2xSixO4xNx (x = 0, 0.1, 0.3, 0.5, y = 0, 0.015, 0.06, 0.10, 0.25) phosphors.

J. Mater. Res., Vol. 24, No. 8, Aug 2009


ion radius of Eu2+ ion. These results are consistent with those reported by Li.18 B. Photoluminescent properties of BaAl2xSixO4xNx:yEu2+ phosphors at room temperature

Figure 2 presents the PLE and PL spectra of Ba0.9Eu0.1Al2xSixO4xNx (x = 0, 0.1, 0.3, 0.5) phosphors at room temperature. The PLE spectra of (Ba0.9Eu0.1Al2xSixO4xNx) phosphors consist of several broad excitation bands with high absorption efficiency in the wavelength range of 350 to 420 nm, because of the 4f ! 5d transitions of Eu2+. It is obvious that the absorption band of Eu2+ ions at longer wavelength shows a red shift with an increasing amount of (SiN)+. Monitoring the excitation light at 370 nm, one emission band dominating at 498 to 512 nm is observed, due to the 5d ! 4f transitions of Eu2+ ion. The increasing amount of (SiN)+ induces a red shift of the emission of Eu2+ ion, from 498 nm (x = 0) to 512 nm (x = 0.5). Table I shows the luminescent data of Ba0.9Eu0.1Al2x SixO4xNx. Since the excitation spectra are not well

FIG. 2. The excitation and emission spectra of Ba0.9Eu0.1Al2xSix O4xNx (x = 0, 0.1, 0.3, 0.5) phosphors (lem = 370 nm, lex = 498– 512 nm). TABLE I. The emission position (lem), the position of the lowest 5d excited level (Eabs), the Stokes’ shift (Ds), the lowering (D) of the fdtransition energy of Eu2+ ion in Ba0.9Eu0.1Al2xSixO4xNx (x = 0, 0.1, 0.3, 0.5). Ba0.9Eu0.1Al2xSixO4xNx x lem (nm) Eabs (cm1) D (cm1) Ds (cm1) D + Ds (cm1)

0 498 25,580 8,275 5,500 13,775

0.1 495 24,840 9,016 4,640 13,656

0.3 503 24,460 9,396 4,580 13,976

0.5 512 24,040 9,816 4,510 14,326

resolved, the position of the lowest 5d excited level of Eu2+ (Eabs) is generally estimated by using the mirrorimage relationship between the emission and the excitation spectra.19,20 The Stokes shift (Ds) can be estimated by taking twice the energy difference between the zero phonon line and the energy of emission maximum. The position of the zero phonon line is taken to be the intersection point of the excitation and emission spectra.21 D represents the lowering of the fd-transition energy of Eu2+ when it is brought from the gaseous state into BaAl2xSixO4xNx. It can be seen in Table I that the value of Eabs gradually decreases from 25,580 to 24,040 cm1 and at the same time the value of D increases from 8275 to 9816 cm1 with the increasing amount of (SiN)+. In general, the lowering of the fd-transition energy of Eu2+ is the result of a combined effect of the centroid shift (Ec) and the crystal field splitting (Ecfs). The centroid shift is commonly associated with the nephelauxetic effect that is often attributed to the covalency between the 5d orbital and the p orbital of the anions. The increase in polarizability of the anions as well as covalency between anion and Eu2+ leads to larger centroid shift.22,23 In our case, the increasing amount of (SiN)+ would lead to larger polarizability of the anions as well as larger covalency between anion and Eu2+, because of higher formal charge and smaller electronegative. Therefore, it is reasonable to observe the red shift of the fd-transition energy of Eu2+ with the increasing amount of (SiN)+. In addition, the red shift of the emission of Eu2+ with the increasing amount of (SiN)+ can be explained by considering the combined effect of D and Ds. It can be seen in Table I that although the Stokes shift decreases gradually from 5500 to 4510 cm1, the sum of D and Ds increases. Therefore, it is reasonable to observe the red shift of the emission of Eu2+ with the increasing amount of (SiN)+. The influence of the Eu2+ concentration was also investigated as shown in Fig. 3(a). The Ba1yEuyAl1.5 Si0.5O3.5N0.5 phosphors show several broad excitation bands with high absorption efficiency in the wavelength range of 350 to 450 nm and their dominating emissions shift from 504 (y = 0.015) to 528 nm (0.25) with the increasing amount of Eu2+ ions. Table II shows the luminescent data of Ba1yEuyAl1.5 Si0.5O3.5N0.5. It can be seen in Table II that the value of Eabs initially fluctuates up to y = 0.03 and then increases from 23,660 cm1 (y = 0.06) to 24,450 cm1 (y = 0.25), and at the same time the value of D decreases from 10,126 to 9406 cm1 with the increasing amount of Eu2+. As discussed previously, the lowering of the fdtransition energy of Eu2+ is the result of a combined effect of the centroid shift (Ec) and the crystal field splitting (Ecfs). Since the amount of (SiN)+ is fixed at 0.5, the nephelauxetic effect attributed to the covalency between the 5d orbital and the p orbitals of the anions is


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considered to be same. That is to say, the centroid shift would keep constant. Therefore the decrease in the value of D should be caused by the decreasing crystal field splitting with the increasing amount of Eu2+. These

results are not consistent with those reported by Li.18 In addition, the influence of the Eu2+ concentration on the red shift of the emission of Eu2+ can be explained by considering the combined effect of D and Ds. It can be seen in Table II that although the value of D generally decreases, the Stokes shift increases gradually from 3890 to 5510 cm1, resulting in the total increase in the sum of D and Ds. Therefore, it is concluded that a larger Stokes shift, not a larger crystal-field splitting as reported by Li,18 contributes the red shift of the emission of Eu2+ with the increasing amount of Eu2+. The photoluminescence decay curves of Ba1yEuy Al1.5Si0.5O3.5N0.5 (lex = 370 nm, y = 0.015, 0.03, 0.06, 0.08, 0.10, 0.15, 0.25) were measured and the data are presented in Table III. Figure 3(b) shows the representative photoluminescence decay curves of Eu2+ in Ba0.9Eu0.1Al1.5Si0.5O3.5N0.5 phosphor. All the decay curves can be well fitted by a double exponential equation: IðtÞ ¼ I0 þ Aet=t1 þ Bet=t2

;

ð1Þ

where I0 and I are the luminescence intensities at time 0 and t, respectively, A and B are constants, and t1,t2 are the lifetimes for the exponential components, respectively.6 For x = 0.5 and y = 0.015, the lifetimes of Eu2+ ion are determined to be 0.22 and 0.69 ms. Also for x = 0.5 and y = 0.25, the lifetimes for Eu2+ ion are 0.11 and 1.16 ms. These results are reasonable for the 5d–4f allowed transition of Eu2+ ions because the lifetime of Eu2+ ion is usually in the range of 0.2 to 2 ms.24,25 These results indicate that there are two lattice sites occupied by Eu2+ ions, which is supported by the structure information of BaAl2O4.26 In addition, it is found that the value of t1 generally decreases and the value of t2 gradually increases with increasing Eu2+ ion concentration, as shown in Table III. This behavior suggests that the energy transfer between two Eu2+ sites occurs.27 C. Thermal quenching process FIG. 3. (a) The excitation and emission spectra of Ba1yEuyAl1.5 Si0.5O3.5N0.5 (y = 0.015, 0.03, 0.06, 0.08, 0.1, 0.15, 0.25) phosphors (lem = 370 nm, lex = 504–528 nm). (b) The photoluminescence decay curves of Eu2+ in Ba0.9Eu0.1Al1.5Si0.5O3.5N0.5 (lex = 370 nm, lem = 515 nm).

The temperature dependence of the emission characteristics of Ba0.9Eu0.1Al1.5Si0.5O3.5N0.5 was investigated in the temperature range of 10 to 425 K, as shown in Fig. 4. The integrated emission intensity of Eu2+ ion gradually decreases with the temperature increasing up

TABLE II. The emission position (lem), the position of the lowest 5d excited level (Eabs), the Stokes’ shift (Ds), the lowering (D) of the fdtransition energy of Eu2+ ion in Ba1yEuyAl1.5Si0.5O3.5N0.5 (y = 0.015, 0.03, 0.06, 0.08, 0.10, 0.15, 0.25). Ba1yEuyAl1.5Si0.5O3.5N0.5 y lem (nm) Eabs (cm1) D (cm1) Ds (cm1) D + Ds (cm1)

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0.015 504 23,730 10,126 3,890 14,016

0.03 506 23,810 10,046 4,050 14,096

0.06 512 23,660 10,196 4,130 14,326

0.08 515 23,690 10,166 4,270 14,436


0.1 515 23,690 10,166 4,270 14,436

0.15 518 23,790 10,066 4,490 14,556

0.25 528 24,450 9,406 5,510 14,916


TABLE III. The lifetime of Ba1yEuyAl1.5Si0.5O3.5N0.5 (lex = 370 nm, y = 0.015, 0.03, 0.06, 0.08, 0.10, 0.15, 0.25). Ba1yEuyAl2xSixO4xNx x y lem (nm) t1 (ms) t2 (ms)

0.015 504 0.22 0.69

0.03 506 0.20 0.71

0.06 512 0.17 0.77

FIG. 4. The temperature dependence of the integrated emission intensity and the emission FWHM of Ba0.9Eu0.1Al1.5Si0.5O3.5N0.5 phosphors (lem = 370 nm).

to 425 K. The quenching temperature, at which the initial PL intensity at 10 K is halved, is about 350 K. The decrease in emission intensity with the increase in temperature can be explained by the configurational coordinate diagram.28 The temperature dependence of the full width at half-maximum (FWHM) can be described by using the configuration coordinate model and the Boltzmann distribution. Assuming hn is the same for the 4f7 ground state and the 4f65d excited state, the temperature dependence of the FWHM is expressed by: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffi pffiffiffi hn ; ð2Þ FWHMðTÞ ¼ 8ln2 hn S coth 2kT where hn is the phonon energy, S is the Huang-Rhys parameter, and k is the Boltzmann constant.29 The best fit is obtained with S = 3.9 0.1 and hn = 88 1 meV. These values enable us to evaluate the Stokes shift at 0.59 0.02 eV (4750 170 cm1) according to: Ds ¼ ð2S 1Þhv :

ð3Þ 1

This value is consistent with those (4270 cm ) estimated previously by taking twice the energy difference between the zero phonon line and the energy of emission maximum.

0.5 0.08 515 0.16 0.89

0.10 515 0.16 0.89

0.15 518 0.13 1.02

0.25 528 0.11 1.16

FIG. 5. The CIE 1931 chromaticity diagram of Ba0.9Eu0.1Al2xSix O4xNx (X, x = 0, 0.1, 0.3, 0.5), Ba1yEuyAl1.5Si0.5O3.5N0.5 (D, y = 0.015, 0.03, 0.06, 0.08, 0.1, 0.15, 0.25) phosphors and the asfabricated yellowish LEDs under DC = 20mA (H, UV LED + Ba0.9Eu0.1Al1.5Si0.5O3.5N0.5).

D. Fabricate LED with Ba0.9Eu0.1Al1.5Si0.5O3.5N0.5 phosphors

The excitation and emission spectra suggest that Ba1yEuyAl2xSixO4xNx are a series of efficient phosphors with broad absorption bands, matching well with the widely used commercial near-UV LED chips (395 nm). Figure 5 shows the CIE 1931 chromaticity diagram of the Ba1yEuyAl2xSixO4xNx phosphors. It can be seen that the emission color is tunable in visible region from bluish green to green by controlling the amount of (SiN)+ and from green to yellowish green by adjusting the concentration of Eu2+ ions. In addition, the thermal quenching behavior of phosphors is also an important factor that has great influence on light output, color render index, and the correlated temperature of pc-wLED, especially for high-power LED device, the joint temperature of which is generally around 420 K. As shown in Fig. 4, the emission intensity of Eu2+ at 425 K is about 71% as intense as that of Eu2+ at 300 K (RT) and the FWHM of Eu2+ at 425 K is only about 1.05 times as wide as that of Eu2+ at 300 K (RT). These results mean that the Ba1yEuyAl2xSixO4xNx phosphors are promising


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are analyzed. The lifetime data support that there are two Eu2+ sites, between which the energy transfer occurs. The quenching temperature is estimated at 350 K and the Huang–Rhys factor is about 3.9 0.1 meV. Finally, the yellowish-green phosphor-converted LEDs were successfully fabricated by precoating BaAl2xSixO4xNx: Eu2+ phosphors onto 395 nm emitting Ga(In)N chips. ACKNOWLEDGMENTS

We acknowledge the financial support from the National Nature Science Foundation of China (20501023), the Nature Science Foundation of Guangdong Province (5300527), and the Science and Technology Project of Guangzhou (2005Z2-D0061). FIG. 6. The electroluminescence spectra of the UV-LED and the as-fabricated yellowish-green LED based on UV chip and Ba0.9Eu0.1Al1.5Si0.5O3.5N0.5 phosphors under DC =20 mA.

candidates for color tunable phosphors potentially used in pc-wLEDs. Figure 6 shows the intense green LEDs fabricated with UV chip (395 nm) and the Ba0.9Eu0.1Al1.5Si0.5O3.5N0.5 phosphor under IF = 20 mA and V = 3.6 V. It can clearly be seen that there are two emission bands around 395 and 520 nm, respectively. The ultraviolet emission at 395 nm is caused by the electroluminescence of UV InGaN chip. The green emissions are generally consistent with the PL spectra of Ba0.9Eu0.1Al1.5Si0.5O3.5N0.5 as shown in Fig. 3. It is observed that the near-violet light of the naked chip at 395 nm decreases dramatically from 3650 (curve 1) to 350 mW/sr-nm (curve 2), which is down-converted into an intensive green light around 520 nm simultaneously. The luminous intensity of the fabricated LED is about 3030 mcd under IF = 20 mA and V = 3.6V. The remaining 395 nm emission of Ga(In)N chip can still be observed as shown in Fig. 6. It is advantageous to feed other phosphors to obtain the white LED by combining red/green/ blue tricolor phosphors with an n-UV Ga(In)N chip.5 As shown in Fig. 5, the CIE chromaticity coordinate calculated from the emission spectrum of Ba0.9Eu0.1Al1.5 Si0.5O3.5N0.5 phosphor is (0.2624, 0.4952), and that of the as-fabricated LED is (0.2936, 0.4483) from curve 2 (Fig. 6), respectively. Therefore, we obtain the yellowish-green LED to the naked eyes. The difference in color coordinate between phosphor and yellowish-green LED is small, supporting that the remaining n-UV has little effect on the color balance of the as-fabricated LED. IV. CONCLUSIONS

In the present work, Eu2+-activated BaAl2xSix O4xNx phosphors were synthesized by solid-state reaction. The role of the (SiN)+ and Eu2+ concentration on the red shift of the emission of Eu2+ in the present lattice 2594

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