Advances in Phosphors for Light-emitting Diodes


Advances in Phosphors for Light-emitting Diodes
Advances in Phosphors for Light-emitting Diodes
Chun Che Lin and Ru-Shi Liu*
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
ABSTRACT: Light-emitting diodes (LEDs) are excellent candidates for
general lighting because of their rapidly improving efficiency, durability, and
reliability, their usability in products of various sizes, and their environmentally
friendly constituents. Effective lighting devices can be realized by combining
one or more phosphor materials with chips. Accordingly, it is very important
that the architecture of phosphors be developed. Although numerous phosphors have been proposed in the past several years, the range of phosphors that
are suitable for LEDs is limited. This work describes recent progress in our
understanding of the prescription, morphology, structure, spectrum, and
packaging of such phosphors. It suggests avenues for further development
and the scientific challenges that must be overcome before phosphors can be
practically applied in LEDs.
ollowing an increasing awareness of climate change and
environmental issues, people are looking for alternatives to
fossil fuels as energy sources that do not emit carbon dioxide.13
White light-emitting diodes (WLEDs) are extensively used and
are very important as they significantly reduce global power
requirements and the use of fossil fuels.4 They have attracted
substantial attention owing to their extraordinary luminous
efficiency, low power consumption, reliability, and environmental friendliness. The quest for phosphors for lighting is one of the
most important and urgent challenges to be met by advanced
science and high technology, and novel and acceptable fluorescent-material-based solid-state lighting (SSL) must be developed.5
As presented in Tables 1 and 2, the intensity, width, durability,
and thermal quenching of commercial phosphors for lightemitting diodes (LEDs) suggest two exciting approaches of
UV-LED chip (with the wavelength of 380420 nm) and
blue-LED chip (with the wavelength of 450480 nm) for
accelerating this development. One involves mixing the emissions from red, green, and blue (RGB) phosphors with a UVLED chip, as in a device with the schematic structure that is
shown in the left inset in Figure 1A. The disadvantages of such a
device are low efficiency of the red phosphors (due to the large
Stokes shift) and the need for complex coating technology (e.g.,
problems of sedimentation and uniformity distribution of phosphors in silicon resin). Furthermore, mixing powders and finding
high-efficiency compounds are more difficult for this type device.
However, there are several benefits, including high color rendering index, high luminous efficiency, and stable light color that are
almost independent of the changed current. According to the
relevant literature,6,7 all such devices use inorganic phosphors
that are mixed with quantum dots, which act as an alternative to a
down-converter in WLEDs. Therefore, we propose a novel
mixture of variously colored quantum dots (InP) and silicon
resin as a color-converting material, which can be applied to a
UV-LED chip, as displayed in the right inset in Figure 1A. In the
r XXXX American Chemical Society
case of nontoxic InP QDs,8 the full color emission wavelengths
can be easily adjusted by controlling the particle size (quantum
confinement effect), and such QDs can be dispersed uniformly in
silicon resin. This fact can perhaps be exploited to solve the
problems of the efficiency and coating technology of UV-LED
The other method for lighting devices involves Ce3þ-doped
yttrium aluminum garnet (YAG/Ce)-based blue-LED chips,
which has been commercialized. The left inset in Figure 1B
presents such a device. It can offer advantages such as easy
fabrication and low cost. Nevertheless, the unstable light color
results in the halo phenomenon under the different output
current. The light from YAG/Ce-based WLEDs is colder and
bluer than that from a traditional incandescent lamp. Its color
rendering is poor owing to the red deficiency of the yellow
phosphor. This problem has attracted the attention of many
researchers, who have sought to improve the color rendering
property of phosphor-converted (pc) WLEDs. Coreshells or
coremultishells of CdSe-based core QDs have been widely
utilized to improve the color rendering index (CRI) of WLEDs.9,10
The right inset in Figure 1B shows a new combination of broad
emission Lu3 Al5O12/Ce3þ (LuAG/Ce) yellowgreen phosphors and highly efficient red InP QDs. Moreover, Figure 1C
exhibits the efficiency curves of different LED chips plotted as
a function of correlated color temperature (CCT). To realize
a low CCT of a blue-LED chip, a very thick phosphor layer
must be formed to reduce the transparency of blue light. A
very thick phosphor layer limits brightness because it prevents fluorescent light from traveling from the LED side to
the front of the device. If a violet LED is used to yield low
CCT, then the phosphor layer can be optimized to pass light
February 23, 2011
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Table 1. Examples of WLEDs that Incorporate UV-LEDs Excitable Phosphors. (O: Good; 4: Medium; : Bad)
emission characteristics
violet LED
blue phosphor
green phosphor
red phosphor
chemical composition
thermal quenching
3.5MgO 3 0.5MgF2 3 GeO2/Mn
Table 2. Examples of WLEDs that Incorporate Blue-LEDs Excitable Phosphors. (O: Good; 4: Medium; : Bad)
emission characteristics
blue LED
green phosphor
yellow phosphor
red phosphor
chemical composition
thermal quenching
from the LED side to the front of the device. The low CCT
can be obtained just by adjusting the mixing ratio of the RGB
phosphors even without adjusting the thickness of the phosphor layer. As expected from the device scheme in Figure 1,
semiconductor nanocrystals or QDs are a promising alternative to down-converting materials in WLEDs because of
their attractive properties, including size-tunable optical
characteristics, broad absorption spectrum, narrow emission
band, high quantum yields, and low light scattering. 11,12 The
criteria that must be applied and the approach that must be
taken to enable inorganic phosphors to be used in LEDs are of
On the basis of the above mentioned, rare-earth ions, like
Eu2þ, Ce3þ, Tb3þ, and Sm3þ, which work as luminescence
centers in most inorganic phosphors, tend to be very expensive.
Moreover, a large number of productions were usually synthesized under high temperatures, high pressures, or reducing
conditions. The development of available approaches for fluorescent materials without using rare-earth ions at relatively low
temperature under air atmosphere is agreeable to apply in the
future lighting. We will introduce some non-rare-earth-based
phosphors in this Perspective. Notably, the blue-color-emitting
GaZnON materials have not yet been reported in previous
literature. This Perspective highlights some characteristics of
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Figure 1. Structures for generating white light from fluorescent materials, based on (A) UV chip and (B) blue chip. (C) Efficiency of LED chips as a
function of correlated color temperature.
phosphors and provides an overview of the subject area for
WLEDs in the future.
Combinatorial chemistry is used to
optimize novel phosphors for application in light-emitting diodes.
YVO4/Bi 3þ,Eu3þ: Spectral Characteristics and Formula. Some
intelligent algorithms, such as genetic algorithms, Monte Carlo
techniques, simulated annealing, and artificial neural networks
algorithms, can be used to improve the efficiency of new materials
and to optimize the objective compounds.1316 Recently, Sohn
et al.17 employed solid-state combinatorial chemistry to synthesize
AE2Si5N8/Eu2þ (AE = alkaline earth elements) phosphors and
analyze their composition. Liu et al.18 screened (YxLu1xy)3Al5O12/Ce3y greenyellow phosphors using the same method
(combi-chem). Figure 2A schematically depicts the developed
drop-on-demand inkjet delivery system. A driving circuit and a
computer that controls the motor drive the piezoelectric inkjet
heads and the XY stage of a substrate. The emission spectra of
the samples in the library were measured using an automatic
system that was comprised of a Hg Lamp, a portable optical fiber
spectrometer (Ocean Optics, Inc., model SD2000), and an XY
stage. Figure 2B displays the composition map of the
Y1stVO4/Bi3þs,Eu3þt combinatorial library and a luminescent
photograph obtained under 365 nm excitation. The first column
clearly reveals faint red emission from Eu3þ at various concentrations (t = 00.060) without Bi3þ codoping (s = 0). The first
row clearly reveals that without Eu3þ codoping (t = 0), the strong
green emission of Bi3þ increases with Bi3þ concentration from
s = 0 to 0.050; the increase becomes negligible when the Bi3þ
content exceeds s = 0.025. Furthermore, the emission colors in all
other rows (t = 0.005, 0.015, 0.030, 0.045, and 0.060) remain
almost constant with different Bi3þ concentrations, but the
brightness increases with Bi3þ concentration from s = 0.005 to
0.040 and then declines as the Bi3þ concentration increases
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Figure 2. (A) Drop-on-demand inkjet delivery system. (B) Composition map of the (Y1stBisEut)VO4 combinatorial library and luminescent
photograph under 365 nm excitation. (C) Emission spectra of samples with s = 00.050 and t = 0.005 in the (Y1stBisEut)VO4 combinatorial library
under 365 nm excitation. (D) Emission spectra of samples with s = 0.040 and t = 00.060 in the (Y1stBisEut)VO4 combinatorial library under 365 nm
excitation. (E) Emission spectrum of a warm-white LED lamp. (Adapted from ref 16.)
further. These results suggest that Eu3þ cannot emit efficiently
without Bi3þ codoping under 365 nm excitation but that excessive
Eu3þ codoping severely reduces Bi3þ luminescence. Figure 2C
shows the emission spectra of the samples at t = 0.005 and s =
00.050 in the combinatorial library under 365 nm excitation.
The emission lines at 592, 618, 650, and 702 nm are attributed to
the 5D0 f 7FJ (J = 1, 2, 3, 4) transitions of Eu3þ, and the broad
emission band with a peak at 545 nm is assigned to the 3P1 f 1S0
transition of Bi3þ.16 The emission intensity of Eu3þ increases
continuously with Bi3þ concentration from s = 0 to 0.050,
indicating the transfer of energy between Bi3þ and Eu3þ. Meanwhile, the emission intensity of Bi3þ increases with concentration. The inset in Figure 2C plots the variation of the relative
height of the Bi3þ emission peaks at 545 nm and the Eu3þ
emission peaks at 618 nm with Bi3þ concentration. The rates of
increase of Eu3þ and Bi3þ emissions are similar to each other at
s = 00.025, but the rate of increase of Eu3þ emission exceeds
that of Bi3þ at s = 0.0250.050. The energy transfer from Bi3þ to
Eu3þ does not influence Bi3þ emission if the concentration of
Bi3þ does not exceed s = 0.040 and that of Eu3þ is less than
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Figure 3. (A) PL spectra of (Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with various fluxes (excited at 370 nm). (B) Scanning electron micrographs of
(Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with various fluxes. (C) Conductivity of (Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with various fluxes as a function of
time. (D) PL spectra of as-received and hydrated (Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with AlF3 and H3BO3 fluxes (excited at 370 nm). (Adapted from
refs 21.)
t = 0.005. However, Bi3þ emission is clearly degraded by the
shortening of the mean distance between Bi3þ and Eu3þ, and
Eu3þ then efficiently captures energy from Bi3þ. The results in
Figure 2D agree well with those in Figure 2C when the emission
of Bi3þ declines gradually as the Eu3þ content increases from t =
0 to 0.060, indicating that the shortening of the distance between
Bi3þ and Eu3þ results in the transfer of energy from Bi3þ to Eu3þ
to an extent that increases gradually with Eu3þ concentration.
The above results concerning the photoluminescence that is
excited by UV light (365 nm) are consistent with the results of
conventional-scale synthesis, revealing that the combinatorial
chemistry technique is fast, reliable, and reproducible. Therefore,
a novel prescription of phosphors can be found rapidly and
exhaustively by using a combinatorial chemistry approach. A
practical white LED lamp was fabricated by coating a NUV-LED
chip with commercially available Sr3MgSi2O8/Eu2þ (3128) blue
phosphor and self-optimized (Y0.956Bi0.040Eu0.004)VO4 phosphor. Figure 2E presents the emission spectrum of the prepared
LED lamp, with a Ra of up to 90.3.
Fluxes considerably improve the
stability of phosphors against
BaMgAl10O17/Eu,Mn: Moisture Measurement and Morphology.
The flux plays an important role in improving crystallinity, promoting grain growth. It is even involved in controlling the morphology
and size of a phosphor.19 On the basis of previous investigations,
the addition of a flux enhances the luminescence intensity and
reduces the reaction temperature of fluorescent materials. The
stability of the moisture content and the emission intensity of
BaMgAl10O17/Eu,Mn phosphors vary with various fluxes, such
as AlF3, BaF2, and H3BO3. Figure 3A shows the PL spectra of
(Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with various fluxes. The
spectra have similar shapes but various emission intensities.
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Figure 4. (A) Calculated density of states (DOS) of (a) pure KSrPO4 and KSr1xPO4/Eux (x = 3.125%) in the (b) non-spin-polarized state,
(c) spin-polarized state, and (d) spin-polarized state with on-site Coulomb interaction. (B) Majority spin orbital structure and possible mechanism of
electronic transition in the KSrPO4/Eu system. (Adapted from ref 24.)
The fluxes significantly enhance the luminescence intensity and
color saturation. Accordingly, the phosphor that is synthesized
with AlF3 has a higher relative emission intensity (116%) and
color purity than commercial BAM phosphor. The involvement
of fluxes in the mechanism of BAM particle growth has not been
elucidated in detail. However, fluxes are broadly agreed to be
compounds of alkali or alkaline earth metals with low melting
points; when fluxes melt, the surface tension of the liquid helps
particles coagulate and facilitates their sliding and rotation,
providing more opportunities for particleparticle contact and
promoting particle growth. Recent work has established that
when fluorides are used as fluxes, BAM particle growth may
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proceed by the following mechanism20
BaCO3 þ MgO þ 4=3AlF3 f BaMgF4 þ 2=3Al2 O3 þ CO2
BaMgF4 þ 17=3Al2 O3 f BaMgAl10 O17 þ 4=3AlF3
MgO þ 2BaF2 f BaMgF4 þ BaO
BaMgF4 þ 5Al2 O3 þ 2BaO f BaMgAl10 O17 þ 2BaF2
As revealed by these chemical equations, the fluoride reacts with
some of the raw material to produce BaMgF4 at approximately 1000 °C, which melts at a temperatures of over 1000 °C.
BaMgAl10O17 will begin to be created in BaMgF4 solutions at
about 1200 °C. At around 1600 °C, all of the BAM are created.
Figure 3B displays the SEM morphology of (Ba0.85Eu0.15)
(Mg0.7Mn0.3)Al10O17 with various fluxes. The widely dispersed
BAM phosphor has a plate-shaped morphology because of its
crystallographic characteristics. A comparison with a phosphor
prepared without flux demonstrates that the growth of particles
in the presence of flux is extensive; the flux improves their crystallinity, and the phosphor particles with a fluoride-based flux
have a hexagonal surface morphology. A sample that is prepared
by H3BO3 flux is semispherical and not smooth. The results also
indicate that the morphology of phosphor particles affects their
luminous properties and stability.21 Mishra et al.22 observed that
moisture can easily interact with the intermediate planes of the
aluminates in a β-alumina structure, degrading the BAM. Chemical stability is regarded as an important parameter in the application of phosphors in WLEDs, plasma display panels, and fluorescent lamps. Figure 3C plots the conductivity of (Ba0.85Eu0.15)
(Mg0.7Mn0.3)Al10O17 as a function of time with various fluxes. The
conductivity increases with time because some ions of the
phosphor host are dissolved in water. Specifically, the conductivity
varies only a little in the synthesis of BAM with AlF3 or BaF2
fluoride-based fluxes, enhancing the stability against moisture. This
result is obtained because phosphor particles with a fluoride-based
flux, which have a highly crystalline hexagonal surface morphology,
cannot easily interact with the moisture. However, the sample with
H3BO3 as the flux has the lowest stability owing to the absence of
highly crystalline particles and the smoothness of the surface.
Figure 3D shows the PL spectra of the as-received and hydrated
(Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with AlF3 and H3BO3 as
fluxes. The emission spectra indicate a reduction in the emission
intensity of both of the water-treated samples. The total integrated
radiance of the hydrated samples with AlF3 and H3BO3 as fluxes is
9 and 22% less than that of fresh powder, respectively. Hence,
synthesis with fluxes can enhance the stability against moisture;
AlF3 outperforms BaF2 or H3BO3. Furthermore, the selection of
suitable fluxes in the synthesis of any compounds can improve the
stability against moisture. Chemical stability is an important
requirement of phosphors for use in LEDs.
The mechanism of electronic transition was confirmed by theoretical
Chen et al.23 calculated the electronic structure of RMO4 (R =
Y, Gd; M = P, V) and elucidated its physical properties and
Figure 5. (A) Temperature-dependent emission spectra of Y3Al5O12/
Ce3þ and CaAlSiN3/Eu2þ obtained under 460 nm excitation. (B)
Configuration coordinate diagram. (Adapted from ref 3.)
interatomic interactions by employing density functional theory
(DFT). Although various densities of states (DOSs) of undoped
hosts have been reported, the DOS of doped phosphors had not
been calculated before the KSrPO4/Eu system was developed.24
Figure 4 shows the DOS of, and a possible mechanism of electron
transition in, the KSrPO4/Eu system. The results reveal that pure
KSrPO4 has a direct band gap of approximately 5.09 eV at point Γ
in (a) of Figure 4A. The band gap is shifted only slightly when the
800 eV cutoff energy for the plane wave basis and the geometrical
optimization of the KSrPO4 are considered, thus suggesting that
these parameters have only a weak effect on the electronic
structure of KSrPO4 systems. The reference energies are set to
the highest-energy electron-occupied state. The upper and lower
rows in (c) and (d) of Figure 4A concern the majority spin (spinup) and minority spin (spin-down), respectively. The electronic
structure of KSrPO4 after it is doped with dilute Eu is presented
below; band gap values of around 1.3 (b) and 2.5 eV (c) are
obtained for non-spin-polarized and spin-polarized electrons,
respectively. With respect to the spin polarization and the on-site
Coulomb interaction of Eu 4f electrons, the majority spin Eu 4f
states are fully occupied, and the main peak of the Eu 5d states
appears at about 3.0 eV above the Fermi level in (d) of Figure 4A.
Remarkably, the separation of 3.0 eV between the Eu 4f5d main
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peaks, corresponding to the wavelength of 414 nm, is very
close to the measured emission wavelength of 424 nm. Therefore, the calculations herein suggest a possible mechanism of
excitation from Eu 4f to the conduction band (CB) via Eu 5d,
followed by the main emission from Eu 5d to Eu 4f. On the
basis of the above discussion, Figure 4B presents a proposed
mechanism of electron transition in the KSrPO4/Eu system.
Initially, the Eu 4f f CB excitation pumps electrons to the
delocalized CB via Eu 5d. Then, nonradiative relaxation brings
the electron to the main peak of Eu 5d close to the lower band
edge of the CB. Finally, photoemission may be associated with
the on-site Eu 5d4f transition.
The thermal stability of phosphors
can affect color coordinates of
lighting for practical packaging.
The thermal stability of phosphors that are used in WLEDs
must be understood. Figure 5A plots the relationship between
the emission intensity and the environmental temperature of
Y3Al5O12/Ce3þ and CaAlSiN3/Eu2þ, measured under 460 nm
excitation. The emission intensity of the two samples declined as
the temperature increased because the nonradioactive transition
Figure 6. Developed scheme of phosphors for WLEDs.
from the excited states to the ground state increased by the
crossing point (F), as shown in the configurationally coordinate
diagram in Figure 5B; this effect is called thermal quenching. As a
result, the emission peaks of YAG/Ce3þ were red-shifted from
560 to 570 nm, which is explained with reference to the Varshini
equation for the temperature dependence of energy25
EðTÞ ¼ E0 aT 2
where E(T) is the energy difference between the excited states
and the ground states at temperature T; E0 is the corresponding
energy difference at 0 K, and a and b are fitting parameters.
Increasing the temperature reduces the transition energy and red
shifts the emission peak. Additionally, the configurationally
coordinate (red line) is slightly right-shifted from that of the
original coordinate (black line), reducing the activation energy
(ΔE = F00 to G). Sohn et al.26 found that a lower energy emission
of CaAlSiN3/Eu2þ, which was attributed to an Al-rich local
environment, made the polyhedron around the Eu2þ activator
smaller, while a Si-rich local environment resulted in a larger
polyhedron and a higher energy emission. Consequently, the
emission peaks of CaAlSiN3/Eu2þ in Figure 5A were blue-shifted
in a manner determined by the configuration coordinate (blue
line), increasing the activation energy (ΔE = F0 to G).
Prospects for Phosphors in WLEDs. As described above, in the
search for suitable phosphors, significant effort has been made to
develop highly efficient LED devices. Figure 6 presents the
developed scheme of phosphors for WLEDs. In recent years,
several highly luminous YAG/Ce or silicate-based blue-LED
chips have been fabricated, but they have a low color-rendering
index and low thermal stability. Although nitride and phosphate
phosphors have high thermal stability, they have very low
efficiency. The optimal mixture of Sr3MgSi2O8 (blue), LuAG
(yellowgreen), and CaAlSiN3 (red) was recently proposed for
use in a high-efficiency device. Additionally, considering the cost
of precursors and synthetic processes, non-rare-earth-based
phosphors maybe are excellent candidates for replacing doped
materials in the preparation of LEDs. Okuyama et al.27 have
reported full-color-emitting BCNO phosphors, which are produced by a one-step liquid process at relatively low temperature
(800 °C). The peak positions of emission spectra shift from the
UV (387 nm) to visible (571 nm) under different conditions,
Figure 7. (A) Diffuse reflection spectra for Ga2O3, ZnO, and GaZnON. (B) Excitation (λem = 450 nm) and emission (λex = 390 nm) spectra for
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including the polyethylene glycol/boron ratio, reaction temperature, and heating time. Nanda et al.28 have successfully developed
nanorod Ga2O3 that been used as a multilight emitter for WLEDs.
They also explained the possible growth mechanism and luminescence mechanism for this compound. The bluish-green
emission was ascribed to the oxygen vacancy, while the nitrogen
formed the red emission. Especially, the solid-solution GaZnON
was fabricated without doping rare-earth ions at considerably low
temperatures (below 500 °C) under ambient atmospheric pressure. Figure 7A presents the reflection spectra of the precursors
Ga2O3 and ZnO and the production of GaZnON. It clearly
indicates that the optical band gap was estimated to be 4.9 and 3.3
eV for Ga2O3 and ZnO, respectively. The band gap of GaZnON,
which has stronger absorption (380450 nm) than others, was
measured to be 3.7 eV. Figure 7B shows the excitation and
emission spectra of the GaZnON phosphor. The excitation
wavelength, which is consistent with the reflection spectra, is
suitable for the UV-chip- and blue-chip-based LEDs. The PL
spectrum exhibits a pure blue emission band from 400 to 550 nm
centered at 450 nm ascribed to the oxygen vacancy and urea. The
chromaticity coordinate of this compound was found to be
(0.1614, 0.1439), in the higher blue color purity region. We will
perform more studies to verify the luminescence mechanism and
the distribution of different elements in the near future. Briefly,
considerable effort will be devoted to the development of the
non-rare-earth-based phosphors for exercise in WLEDs. This
Perspective highlights the development and tendency of phosphors and provides some methods that can readily be scaled up
for industrial applications.
Corresponding Author
*Tel: þ886-2-33661169. Fax: þ886-2-23636359. E-mail: rsliu@
Chun Che Lin received his B.S. degree in chemistry from
Chung Yuan Christian University in 2005. He received his M.S.
degree in chemistry from National Taiwan University in 2007.
He is currently working on his Ph.D. in inorganic chemistry,
focusing on the synthesis of fluorescent materials for particular
applications. His current research interests include synthesis of
phosphors and quantum dots for LEDs and bioapplications.
Ru-Shi Liu is a professor at the Department of Chemistry,
National Taiwan University. He obtained two Ph.D. degrees in
chemistry, one from National Tsing Hua University in 1990 and
another from the University of Cambridge in 1992. He worked at
the Industrial Technology Research Institute from 1983 to 1985.
The authors would like to thank the National Science Council
of the Republic of China, Taiwan, for financially supporting this
research under Contracts NSC 97-2113-M-002-012-MY3, and
NSC 97-3114-M-002-002.
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