Luminescence properties of Eu2+- and Ce3+-doped CaAl2S4 and application in white LEDs

Article abstract of DOI:10.1016/j.jssc.2007.12.038

The Eu²⁺- and Ce³⁺-doped CaAl₂S₄ phosphors were comparatively synthesized by conventional solid-state reaction and the evacuated sealed quartz ampoule. The X-ray diffraction (XRD) patterns show that the sample with better crystalline quality was prepared by the evacuated sealed quartz ampoule, resulting in the enhancement of the emission intensity of Eu²⁺ ion by a factor of 1.7. The intensive green LEDs were also fabricated by combining CaAl₂S₄:Eu²⁺ with near-ultraviolet InGaN chips (λ_em=395 nm). The dependence of as-fabricated green LEDs on forward-bias currents shows that it presents good chromaticity stability and luminance saturation, indicating that CaAl₂S₄:Eu²⁺ is a promising green-emitting phosphor for a near-UV InGaN-based LED. In addition, the optical properties of CaAl₂S₄:Ce³⁺ were systematically investigated by means of diffuse reflectance, photoluminescence excitation and emission, concentrating quenching and the decay curve.

Full text of DOI:10.1016/j.jssc.2007.12.038

ARTICLE IN PRESS
Journal of Solid State Chemistry 181 (2008) 658–663
www.elsevier.com/locate/jssc
2
+
3+
Luminescence properties of Eu - and Ce -doped CaAl S and
2
4
application in white LEDs
Ã
Ã
Ruijin Yu, Jing Wang , Jianhui Zhang, Haibin Yuan, Qiang Su
MOE 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, PR China
Received 6 October 2007; received in revised form 28 November 2007; accepted 30 December 2007
Available online 12 January 2008
Abstract
2
+
3+
The Eu - and Ce -doped CaAl
evacuated sealed quartz ampoule. The X-ray diffraction (XRD) patterns show that the sample with better crystalline quality was
2 4
S phosphors were comparatively synthesized by conventional solid-state reaction and the
2
prepared by the evacuated sealed quartz ampoule, resulting in the enhancement of the emission intensity of Eu ion by a factor of 1.7.
+
2
+
The intensive green LEDs were also fabricated by combining CaAl
dependence of as-fabricated green LEDs on forward-bias currents shows that it presents good chromaticity stability and luminance
2
saturation, indicating that CaAl S :Eu is a promising green-emitting phosphor for a near-UV InGaN-based LED. In addition, the
2
S
4
:Eu
with near-ultraviolet InGaN chips (lem ¼ 395 nm). The
+
2
3
4
+
2 4
optical properties of CaAl S :Ce were systematically investigated by means of diffuse reflectance, photoluminescence excitation and
emission, concentrating quenching and the decay curve.
r 2008 Elsevier Inc. All rights reserved.
Keywords: Luminescence; Calcium thioaluminate; Europium; Cerium
1
. Introduction
Recently, the ternary compounds M M (S,Se) doped
34-in thick dielectric electroluminescent (TDEL) screen
using the color-by-blue (CBB) technique [10]. In order to
make the CBB approach function effectively, it is essential
to develop efficient green and red down-converting
phosphors well matching with the efficient thin-film blue-
emitting EL device.
II III
2
4
with rare earth ions have attracted much attention for
optoelectronic devices such as field emission display
(
FED), inorganic thin-film electroluminescent (iTFEL)
2
+
devices, and phosphor converted white light emitting
diodes (pc-WLED) [1–7].
CaAl S :Eu
2
is expected to be the promising green
4
candidate for full-color applications [11]. In the past
decade, research interest was concerned with the prepara-
tion method and fundamental information of CaAl S :
2
+
In 1992, Le Thi et al. [8] systematically studied the Eu
-
doped MS-Al S (M ¼ Ca, Sr, Ba) systems and noted that
2
+
3
2 4
2
2+
BaAl S :Eu
2
phosphor had a high efficiency of blue
emission. Thereafter, Miura demonstrated the high-lumi-
nance blue TFEL device with a new blue-emitting EL
Eu . Le Thi et al. [8] used alkaline earth sulfides, EuS and
Al2S3 as starting materials to prepare thioaluminate
powders performed in silica tubes sealed under vacuum.
Oh et al. [12] prepared CaAl2S4 single crystals by a
chemical transport reaction method in a closed system
using high-purity iodine as a transport agent. In addition,
4
2
phosphor BaAl S :Eu prepared by the two targets pulse-
+
2
4
electron-beam evaporation [9]. This blue phosphor with
suitable color coordinates, high-enough luminance and
efficiency made the full-color device of EL display possible
and accelerated iFire Company to announce a full-color
2
+
3+
thin-film phosphors of CaAl S :Eu ,Gd
2
with a con-
4
2
siderably high luminance of 4200 cd/m at 260 V were also
electron-beam deposited on substrates using of simulta-
2
+
Ã
neous deposition from two source materials, CaS:Eu
3
,
Corresponding authors. Fax: +86 20 84111038.
E-mail addresses: ceswj@mail.sysu.edu.cn (J. Wang),
Suqiang@mail.sysu.edu.cn (Q. Su).
+
Gd
made use of CaS, Al S and EuF as raw materials to
and Al S [11]. Recently, Van Haecke et al. [13]
2 3
2
3
3
0022-4596/$ - see front matter r 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.12.038

ARTICLE IN PRESS
R. Yu et al. / Journal of Solid State Chemistry 181 (2008) 658–663
659
2
+
synthesize the target compound CaAl S :Eu
under
continuous flow of H S and systematically investigate the
equipped with a double out-of-plane littrow monochrom-
eter, using BaSO as a standard reference in the measure-
2
4
2
4
radiative properties of the phosphor by means of a single-
configuration coordinate model.
ments. The photoluminescence (PL), photoluminescence
+
2
excitation (PLE) spectra of CaAl S :RE (RE ¼ Eu
,
2
4
3
+
In this paper, cheap and stable aluminum powder was
used as the starting material instead of Al S to prepare
Ce ) were measured by a Fluorolog-3 spectrofluorometer
(Jobin Yvon Inc/specx) equipped with a 450 W Xe lamp
and double-excitation monochromators. The decay curves
were recorded on an Edinburgh FLS 920 spectrofluorom-
eter, equipped with a 450 W xenon lamp, a 150 W nF900
nanosecond flash lamp with a pulse width of 1 ns and a
pulse repetition rate of 40–100 kHz. The above measure-
ments were carried out at room temperature.
2
3
2
+
CaAl S :Eu under a continuous flow of H S and under
2
4
2
vacuum in sealed silica tubes, respectively. The structure
and optical properties of as-synthesized phosphors by two
methods were comparatively investigated. The intense
green LEDs were first fabricated by combining CaAl S :
2
4
2
+
Eu
phosphors with 395 nm InGaN UV chips and the
dependence of the optical properties of the green LEDs on
different forward-bias currents was investigated. Further-
3
+
3. Results and discussion
more, the optical properties of Ce
in calcium thioalu-
minate were systematically investigated for the first time by
means of diffuse reflectance, photoluminescence excitation
and emission spectra, concentration quenching and the
decay curve.
3
.1. Phase characterization
Fig. 1 shows the XRD patterns of CaAl S :RE
2
4
2
+
3+
(RE ¼ Eu ,Ce ) by two different methods. Obviously,
a mixture of CaAl S , CaS and Al O was formed by
2
4
2
3
2
. Experimental
method (b). Similar results were also reported in prepara-
tion of CaAl2S4 by using Al2S3 as starting materials
sintered in a H S atmosphere [13]. It is inevitable to
2
.1. Samples preparation
2
introduce the trace amount of air into the sintering system
in a flow of freshly prepared H S stream. The aluminum
The starting sulfide materials CaS and EuS were pre-
2
prepared by a solid-state reaction method at high
temperature in horizontal tube furnaces. CaS was prepared
from CaCO (A.R.) under flowing H S gas at 1000 1C for
powder is very sensitive to oxygen at high temperature. The
presence of very stable Al O is most probably due to
2
3
3
2
oxidation of a small part of the aluminum powder in the
sintering reaction.
The pure CaAl S phase can be attained by method (a) in
2
h. EuS was prepared from Eu O (99.99%) with CS
2 3 2
reducing atmosphere at 1200 1C for 2 h. Here, CaAl S :RE
2
4
2
+
3+
2
4
(
two different methods:
RE ¼ Eu ,Ce ) phosphor powders were prepared by
evacuated quartz ampoules by heating the stoichiometric
amounts of CaS, Al, EuS or Ce S , Na CO with 50
2
3
2
3
Method (a): By synthesis from stoichiometric amounts of
CaS, Al(A.R.) and EuS or Ce S , Na CO (A.R.) and 50
mass% excess S at 1050 1C for 5 h. No other impure phases
2
3
2
3
such as CaS, Al S or Al O were detected, which is in
good agreement with the JCPDS card (No. 77–1186). The
lattice constants of phosphors are calculated to be
2
3
2
3
mass% excess S(A.R.) in sealed quartz ampoules at
050 1C for 5 h. The starting materials (about 0.6 g total
mass) were placed in the quartz ampoules (150 mm length,
10 mm inner diameter) and then evacuated to 1 ꢀ 10 Torr
and sealed.
1
3
˚
˚
˚
˚
ꢁ
6
a ¼ 20.003 A, b ¼ 20.588 A, c ¼ 11.909 A, V ¼ 4904.56 A
Method (b): By solid-state reaction from mixtures of
CaS, Al(A.R.) and EuS or Ce S , Na CO (A.R.) in
2
3
2
3
stoichiometric quantities, maintained for 2 h at 1050 1C in
a flowing H S stream. In the initial and the final heat-
treatment stages, the H S was replaced by Ar to prevent the
2
2
possible oxidation of the mixture at temperatures below
7
00 1C.
2
.2. Characterization and optical measurements
The structure of the final products was examined by
X-ray powder diffraction using a Rigaku D/max 2200 vpc
X-ray Diffractometer with CuKa radiation at 40 kV and
3
0 mA. The X-ray diffraction (XRD) patterns were
collected in the range 101p2yp701 with a scan rate of
4
1/min.
The diffuse reflection spectra of the samples were
measured by a Cary 5000 UV–Vis–NIR spectrophotometer
2
+
Fig. 1. XRD patterns of the CaAl
(2) prepared by method (b); (3) JCPDS card (No. 77–1186).
2 4
S :Eu : (1) prepared by method (a);

ARTICLE IN PRESS
R. Yu et al. / Journal of Solid State Chemistry 181 (2008) 658–663
660
2
+
˚
˚
for CaAl S :Eu
2
;
a ¼ 19.892 A, b ¼ 20.644 A, c ¼
300 nm partially originated from the 4f-5d transition of
4
3
3+
+
2+
Eu . The host absorption edge is determined to be
˚
˚
1
1.892 A, V ¼ 4883.46 A for CaAl S :Ce , Na , which
2
4
are consistent with the reported lattice constants a ¼
around 300 nm (4.13 eV), essentially due to the valence to
conduction band transitions of the host.
The excitation and emission spectra of CaAl S :10
3
˚
˚
˚
˚
2
CaAl S [14]. Considering the ionic radii of Ca (112 pm),
0.163 A, b ¼ 20.052 A, c ¼ 12.026 A, V ¼ 4862.21 A of
2+
2
+
4
2 4
2
3+
2+
Eu (125 pm) and Ce (114 pm) ions in site of CN ¼ 8, it
mol%Eu
phosphor prepared by two different methods
is reasonable that the unit cell of CaAl S expands when
2
are shown in Fig. 3. For the sample prepared by method
(a), the excitation spectrum of curves exhibits several broad
bands around 280, 332, 384 and 454 nm, which are
4
2
+
2+
3+
Ca ions are substituted by Eu ions or Ce ions. This
result implies that the dopant ions are incorporated into
the structure of CaAl S . In summary, aluminum powder
7
8
attributed to the host absorption and the 4f ( S7/2)-
2
4
6
7
1
2+
can be used as a cheap and stable starting material for
preparation of CaAl S instead of expensive and weakly
hygroscopic Al S and the method (a) by evacuated quartz
4f ( F)5d transitions of Eu ion, and consistent with the
diffuse reflectance spectra as presented in Fig. 2. For the
emission spectrum, a predominating band with the full-
width at half-maximum (FWHM) about 37 nm is around
2
4
2
3
ampoules used in the present case is necessary to access
excellent crystalline CaAl S powders.
6
7
1
7 8
516 nm, ascribed to the 4f ( F)5d -4f ( S7/2) allowed
2
4
2
+
transition of Eu
method (b), there are some impurities such as CaS and
Al in the sintered powder. However, a small amount of
CaS:Eu in the sintered powder has a negligible emission
ions. For the sample prepared by
3
.2. The luminescence properties of CaAl S :RE
2 4
2
+
3+
O
2 3
(
RE ¼ Eu ,Ce )
2
+
2
+
at 650 nm, as it has a low efficiency at Eu concentrations
higher than 0.2 mol%. Al
Fig. 2 presents the diffuse reflectance and the absorption
2
3+
:Eu powder is characterized
+
3+
2
O
emission lines [13]. Both the spectra character-
3
spectra of CaAl S , CaAl S :Eu
+
and CaAl S :Ce
,
phosphors. The absorption spectra F(R) were
2
4
2
4
2
4
3+
by Eu
istics of CaS:Eu
Na
2
+
3+
and Al O :Eu
were not observed in
curve (2) of Fig. 3. So the presence of impurities in the
2
3
obtained from the diffuse reflection spectra by using the
Kubelka–Munk function [15]:
powder has no influence on the spectrum shape of the
2+
CaAl S :Eu phosphor.
2 4
It can be comparatively observed in Fig. 3 that the
phosphor prepared by method (a) presents more intense
excitation bands in the wavelength range from 250 to
2
FðRÞ ¼ ð1 ꢁ RÞ =2R ¼ K=S,
where R, K and S are the reflectivity, the absorption
coefficient and the scattering coefficient, respectively. It is
clear that the CaAl S host shows a platform of nearly
5–70% diffuse reflectance in the wavelength range of
00–650 nm (curve 1). When europium and cerium ions are
doped into the host, several obvious broad bands are
induced in the wavelength range of 260–520 nm, due to the
(1)
2
4
5
2
00 nm, especially the host absorption band around
2+
6
5
80 nm. And the emission intensity of Eu
ion in the
sample synthesized by method (a) is by a factor of 1.7 as
2+
strong as that of Eu
ion in the sample prepared by
method (b). The distinct optical features are ascribed to the
improved crystalline quality as mentioned in the phase
characterization.
2
+
3+
4
coefficient of CaAl S :Eu
f-5d transition of Eu and Ce ions. The absorption
2
+
around 300 nm (curve 3 in
Fig. 2) is larger than that of the host material. This implies
2
4
2+
that the absorption of Eu -doped CaAl2S4 around
Fig. 2. The diffuse reflectance and the absorption spectra of CaAl
2+
:0.10Eu and CaAl
2
S
4
,
Fig. 3. PL emission (lex ¼ 395 nm) and excitation (lem ¼ 516 nm) spectra
3+ +
:0.03Ce , 0.03Na phosphors prepared
2+
2 4
of CaAl S :Eu (10 mol%) prepared by the method (a) for curve (1) and
2
CaAl S
4
2
S
4
by the method (a).
method (b) for curve (2).

ARTICLE IN PRESS
R. Yu et al. / Journal of Solid State Chemistry 181 (2008) 658–663
661
3
+
3+
and Ce -doped CaAl S compounds exhibit obvious blue
2 4
The emission and excitation spectra of Ce in CaAl2S4
are presented in Fig. 4 at room temperature. It is obvious
shift compared to CaGa S compounds. Such a blue shift is
2
4
3
+
that Ce
ion shows two well-resolved emission bands at
mainly ascribed to the difference in the distance of the
Ca–S bond in these compounds. The compounds CaAl2S4
and CaGa2S4 are isomorphic, both of which show an
orthorhombic crystal structure. The average distance of the
–
36 nm (22936 cm ) and 477 nm (20964 cm ), correspond-
1
–1
4
ing to the transition of 5d-excited state to F and F of
2
2
5
/2
7/2
3
+
ꢁ1
Ce
excitation spectra of Ce
broad bands at around 277, 320 and 404 nm in the range of
40–420 nm, monitoring at 436 nm emission. They are
ion. The energy difference is about 1972 cm . The
3
+
˚
in CaAl2S4 consist of three
Ca–S bond in CaAl2S4 is 3.023 A, longer than that
2
+
3+
˚
(3.015 A) in CaGa S [14]. Therefore, Eu
or Ce
in CaAl2S4 have weaker crystal fields rather than in
ions
2
4
2
3
+
attributed to the 4f-5d transition of Ce and consistent
with the diffuse reflectance spectra. The value of the red
shift (D) expressed by the energy difference of the lowest
CaGa S , indicating that the lowest 5d-excited level of
2
4
2
+
3+
Eu
or Ce
ions is higher in CaAl2S4 than that in
CaGa S . The same blue shift behavior is also observed in
2
4
3
+
2+
II III
2+
5
the free ion of Ce
d-excited levels of Ce in the present host compared with
ꢁ1
Eu -doped M M S . The crystal field splitting of Eu
2 4
5d orbitals by the S
compounds decreases with the decreasing size of M
(In, Ga, Al) and with the increasing size of the M cation
(Ca, Sr, Ba) [19,20].
3
+
2ꢁ
II III
is 24588 cm . The Stokes shift is
3+
anions in these M M S
2 4
–
1
calculated to be 1636 cm . It is weaker in CaAl S :Ce
III
2
4
3
+
II
compared to the value found in CaGa S :Ce
4
2
–
1
(
S ¼ 0.24 eV ¼ 1935 cm ) [16].
Furthermore, Dorenbos [17] also proposed a relation-
3
ship correlating the 4f–5d energies of Ce and Eu ions
The influence of the europium and cerium concentration
on the luminescence process was investigated as shown in
Fig. 5. The PL intensity increases with the doped ions content
increasing until it reaches a maximum intensity at about 3
+
2+
in the same lattice site of a specific host:
EðEu Þ ¼ ð0:64 ꢂ 0:02Þ ꢀ EðCe3þÞ þ ð0:53 ꢂ 0:06Þ eV;
2þ
3+
2+
and 10 mol% for Ce and Eu , respectively. And then it
decreases because of concentration quenching. The observed
optimum europium concentration for Ca1 Eu Al S is
(2)
ꢁx
x
2 4
2
+
3+
where E(Eu ) and E(Ce ) are the energies of the lowest
generally identical with previous data [8,13].
The fluorescence decay times of Eu
2
+
3+
2+
3+
f–d transition absorption of Eu and Ce , respectively.
Using this equation, the lowest 5d crystal-field splitting
and Ce
emissions at room temperature are shown in Fig. 6, the
decay curves can be well fitted by a single exponential
equation:
2
+
component of Eu
2
in CaAl2S4 is estimated to be
.5070.12 eV, i.e. in the range of 473–521 nm, showing
an agreement with the results obtained by using the
mirror–image relationship between the emission and the
excitation spectrum [13].
It ¼ I0 expðꢁt=tÞ, (3)
where I and I are the luminescence intensities at time t
t
0
It is reported that the emission peaks are located at
and 0, respectively, and t is the decay time. The values of t
are calculated to be 0.48 ms and 24.2 ns from the fitted
curves for the samples Ca0.90Eu0.10Al S and Ca0.94Ce0.03
2 4
2
53 nm and 468, 517 nm for Eu - and Ce -doped
+
3+
5
CaGa S , respectively [16,18]. In the present case, Eu
2
+
-
2
4
Fig. 4. PL emission [(1), lex ¼ 404 nm] and excitation [(5), lem ¼ 436 nm]
3
+
+
(3 mol%), Na (3 mol%) prepared by the
spectra of CaAl
2
S
4
:Ce
Fig. 5. Dependence of PL intensities of Ca1ꢁxEu
x
Al
Al
2
S
4
(lex ¼ 395 nm,
(lex ¼ 404 nm,
2
+
method (a) [(2), the fitted curve of curve (1); (3) and (4), the deconvoluted
curves of curve (1)].
l
em ¼ 516 nm) on Eu
content and Ca1ꢁ2xCe
x
Na
x
2 4
S
3
+
lem ¼ 436 nm) on Ce content prepared by the method (a).

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R. Yu et al. / Journal of Solid State Chemistry 181 (2008) 658–663
662
Fig. 7. The electroluminescence spectra of a naked UV LED (curve 1) and
2
+
the fabricated PC-LED (curve 2) combined with CaAl
prepared by the method (a) under 20-mA forward-bias current.
2 4
S :Eu phosphor
Fig. 6. The decay curves of Ca0.90Eu0.10Al
2
S
4
(bottom: lex ¼ 395 nm,
(upper:
ex ¼ 404 nm,
l
em ¼ 516 nm) and Ca0.94Ce0.03Na0.03Al
2
S
4
l
lem ¼ 436 nm) prepared by the method (a).
2
+
Na0.03Al S , respectively. The lifetime value of Eu
2
in
is comparable to the reported value
4
2
+
CaAl S :0.10Eu
2
4
3
+
0
.40 ms in Ref. [13]. The lifetime value of Ce
in
Ca1 Ce Na Al S is in the range of nanosecond, which
ꢁ2x
is also reasonable for the 5d–4f-allowed transition of
x
x
2 4
3+
Ce [21].
2
.3. Fabricated LEDs with CaAl S :Eu phosphor
+
3
2
4
As shown in Figs. 3 and 4, the broad excitation bands of
2
CaAl S :RE (RE=Eu ,Ce ) phosphors match well with
+
3+
2
4
n-UV light emitted by the InGaN LED, indicating a
potential application of these phosphors in an n-UV LED
chip. It is necessary to investigate the optical properties of
these phosphor in a device combined with an n-UV
2+
2 4
Fig. 8. The CIE coordinates of PC-LEDs based on CaAl S :Eu
3
+
Ga(In)N chip. Compared with the CaAl S :Ce
phos-
phosphor shows much more
emission intensity in our experiments. For this purpose, a
prepared by the method (a) under IF ¼ 5.0, 10, 20, 30, 40 and 50 mA in
the CIE 1931 chromaticity diagram. The inset shows the dependence of the
emission intensity on forward-bias currents.
2
4
2
+
phor, the CaAl S :Eu
2
4
3
LED based on CaAl S :Eu
95-nm Ga(In)N chip is used to fabricate the single-color
+
2
phosphor. Fig. 7 is the
electroluminescence spectra of the naked n-UV LED and
fabricated LED shift slightly as shown in Fig. 8 with the
circle symbols. The X value of color coordinates decreases
slightly, approximately difference percentage of ꢁ1.3%,
and the Y value of color coordinates decreases a little,
approximately difference percentage of ꢁ6%. The FWHM
of the emission bands are 42, 42, 42, 43, 43 and 44 nm,
respectively. As is shown in the inset, the relative intensity
of the fabricated LED increases with the forward-bias
current increasing from 5 to 50 mA, it tends to no
significant saturation or quenching phenomenon, even
the operation current is over 50 mA. The dependence of
as-fabricated green LED on forward-bias currents shows
that it presents good chromaticity stability and luminance
2
4
as-fabricated PC-LED at I ¼ 20 mA. Two distinct emis-
F
sion bands around 395 and 516 nm are clearly observed in
the emission spectrum of the PC-LED. From two curves, it
is obviously observed that the near-violet light of the naked
n-UV-LED chip at 395 nm is absorbed dramatically by the
2
+
CaAl S :Eu
(curve 2) and simultaneously down-con-
verted into an intensive green light around 516 nm. Its
2
4
color coordinates are about (0.158, 0.658) at I ¼ 20 mA.
F
Fig. 8 shows the dependence of the color coordinates of
the as-fabricated green LED on different forward-bias
currents (I ¼ 5.0, 10, 20, 30, 40 and 50 mA) plotted in the
F
2
+
CIE 1931 chromaticity diagram. As the drive current
increases, there are a few changes in color coordinates and
in the FWHM of the emission. The color coordinates of the
saturation, indicating that CaAl S :Eu
2
is a promising
green-emitting phosphor for a near-UV InGaN-based
white LED.
4

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663
4
. Conclusions
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[5] P. Benalloul, C. Barthou, J. Benoit, J. Alloy. Compd. 275–277 (1998)
09–714.
7
2
+
,
In this work, the phosphors CaAl S :RE (RE ¼ Eu
2
4
7
3
+
Ce ) have been comparatively synthesized by conventional
solid-state reaction and the evacuated sealed quartz
ampoule. The samples synthesized by the evacuated sealed
quartz ampoule show better crystalline quality and stronger
emission intensity. The optimal concentration for Ce and
+
Eu in CaAl S is about 3 and 10mol%, respectively. The
[
[
6] R. Mueller-Mach, G.O. Mueller, Proc. SPIE 3938 (2000) 30–41.
7] Y.R. Do, K.Y. Ko, S.H. Na, et al., J. Electrochem. Soc. 153 (2006)
H142–H146.
[
[
8] K.T. Le Thi, A. Garcia, F. Guillen, et al., Mater. Sci. Eng. B—Solid
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3
+
9] N. Miura, M. Kawanishi, H. Matsumoto, et al., Jpn. J. Appl. Phys.
Part 2 38 (1999) L1291–L1292.
2
2
4
intensive green LEDs were fabricated by combining the
[
[
[
[
[
[
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1
Acknowledgments
5
This work was supported by the National Nature
Science Foundation of China (20501023), the Nature
Science Foundation of Guangdong for Doctorial Training
base (5300527) and the Science and Technology Project of
Guangzhou (2005Z2-D0061).
(
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