1. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &
ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME
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ISSN 0976 – 6553(Online)
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IJEET
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LUMINESCENCE PROPERTIES OF THE SiOx-COATED YELLOW
PHOSPHOR FOR WHITE LED USING LAYER DEPOSITION
In Gu Kang1,
Byung Ho Choi2
1,2
(School of Advanced Materials and Systems Eng., Kumoh National University of Technology,
1Yangho-Dong, Gumi 730-701, Korea)
ABSTRACT
An investigation is reported for the growth of nanoscaled SiOx films on Mg2+ Ba2+ co-doped
Sr2SiO4:Eu2+ yellow phosphors using atomic layer deposition. Silicon oxide films were prepared at
the 30℃ using Si(OC2H5)4, C2H5N and H2O as precursor, catalyst and reactant gas, respectively.
XPS analysis showed the surface composition of coated phosphor powder revealed silicon oxide.
TEM analysis showed the uniform and nanoscaled SiOx films and the growth rate was about 0.26
Å/cycle. Zeta potential analysis showed that dispersibility of coated phosphors was higher than that
of uncoated, due to the electrostatic repulsion between the SiOx-coated layers on the surface of
yellow phosphors. The photoluminescence intensity for coated phosphors showed 12.36~24.3 %
higher than that of uncoated.
Keywords: Atomic Layer Deposition, Mg2+ Ba2+ co-doped Sr2SiO4:Eu2+ Yellow Phosphor,
Silicon Oxide Coating, Photoluminescence
INTRODUCTION
White light generation using light-emitting diodes has a number of advantages over the
existing incandescent lamps. As a result, they are widely used in applications such as automotive
displays and traffic signals. The light-conversion phosphors for solid-state lighting have attracted
much attention in recent years [1]. One of them, Sr2SiO4:Eu2+ and Sr3SiO5:Eu2+ phosphors show
stronger yellow emission than the conventional light-conversion phosphor YAG:Ce [2]. The
introduction of Mg2+ or Ba2+ into Sr2SiO4:Eu2+ was found to increase the efficiency of Sr2SiO4:Eu2+
and to extend its excitation band to longer wavelengths [3,4]. Therefore, Eu2+ activated silicates are
also expected to be suitable luminescent materials for LEDs. However, phosphors are found to be
unstable and degraded because the luminance decrease due to oxidation [5]. One of solving the
problems is to passivate the surface of phosphors with oxide. The coating of oxide on the ZnS
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2. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME
phosphors was frequently applied to enhance the chemical stability of sulfide phosphors. Several
coatings, such as SiO2 [6], In2O3 [7] and TiO2 [8] have been investigated using sol-gel or CVD
process. In sol-gel or CVD process, the optical intensity of coated phosphor decreased normally than
that of uncoated due to oxide absorption and the aggregation of phosphor powder was also a main
barrier for applications. However, ALD films are grown by a respective process of a single layer(or
less than a layer) deposition sequence. Each sequence consists of several gas-surface interactions that
are all self-limiting [9, 10].
In the present work, the thin silicon oxide coating on Mg2+ Ba2+ co-doped Sr2SiO4:Eu2+
yellow phosphors was investigated. Silicon oxide films grown by atomic layer deposition was
prepared using Si(OC2H5)4 , C2H5N and H2O as precursor, catalyst and reactant gas, respectively.
The effect of silicon oxide films on the structural and optical properties of Mg2+ Ba2+ co-doped
Sr2SiO4:Eu2+ yellow phosphors was investigated as a function of the film thickness.
2. EXPERIMENTAL PROCEDURE
The growth process was carried out in a vertical flow-type ALD reactor [11]. Si(OC2H5)4
was evaporated from a boat at 70℃ and was transpired with Ar carrier gas. The heating line was
maintained at 100℃ to prevent recondensation of Si(OC2H5)4. The chamber temperature was about
25℃. The working pressure in the reactor was about 1 torr. C2H5N was used as a catalyst, H2O as a
reactant gas and Ar as carrier and purge gas. The opening and closing sequences of the air valves
were controlled by using a personal computer. To improve Mg2+ Ba2+ co-doped Sr2SiO4:Eu2+ yellow
phosphor (Intematix, SY-450, ~15 um) to adsorb the precursor and the reactant gas, the multi-pulsed
sequence process was applied to the chamber. The exposure time per one cycle was 54s. Fig. 1
shows the process sequence of the gas valve.
Fig. 1: The Process Sequence of the Gas Valve
The composition of the films grown on phosphor powders was examined by X-ray
Photoelectron Spectroscopy (XPS, ESCALAB 210). Field Emission Scanning Electron Microscopy
(FESEM, JSM-6500F-Jeol) and Transmission Electron Microscope (TEM, H-7600, Hitachi) were
used to investigate the surface morphology and the thickness of the films, respectively. The
photoluminescence was measured using Spectroradiometer (PL, CS1000, Minolta) at room
temperature. A mercury lamp was used for excitation of the phosphor in the UV(254nm) region.
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3. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME
3. RESULTS AND DISCUSSION
In sol-gel process, there are many process variables, such as concentration of precursors, pH
and temperature of solutions, which can affect the surface morphology. In ALD process, however,
the rough surface of uncoated phosphor became smoother and clearer as the number of ALD cycle
increased from 100 to 400 cycles, as show in Fig. 2 On the contrary, it was reported that in sol-gel
process, the surface of coated phosphor became rougher than that of uncoated [6].
Fig. 2: FE-SEM photographs of yellow phosphor powders:
(a) uncoated, (b) SiOx coated by 200 cycles and (c) 400 cycles
Fig. 3 shows XPS peak intensity of yellow phoshors grown with nanoscaled films by 300
ALD cycle. The two peaks at ~101.3eV, and ~ 104.7eV correspond to SiOx and SiO2, respectively.
Therefore, nanoscaled films can be confirmed to be the silicon oxide.
Fig. 3: XPS spectrum of yellow phosphor coated by SiOx at ALD 400 cycles
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4. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME
Fig. 4 shows TEM image of a silicon oxide-coated BAM particle. The film was deposited by
400 cycles. The TEM image reveals that the surface of the silicon oxide film is extremely uniform
and the thickness is about ~10nm. As a result, the growth rate is ~ 0.26 A per one cycle.
Fig. 4: TEM photograph of yellow phosphors coated by SiOx film at ALD 400 cycles
Fig. 5 shows Zeta potential decreases as ALD cycles increase. These results suggested that
the SiOx coating could modify the surface characteristics of yellow phosphors and improve the
dispersibility of yellow phosphors due to the electrostatic repulsion between the SiOx-coated layers
on the surface of yellow phosphors.
Fig. 5: Zeta potential of uncoated and SiOx coated yellow phosphors at various ALD cycles
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5. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME
Fig. 6 shows that the photoluminescence intensity of coated phosphor was 172.4~190.9 cd/m2
as a function of ALD cycles. These values were 12.36~24.3 % higher than that of uncoated
phosphor. This means that the reactive surface is coated with the stable silicon oxide and also the
oxide absorption is almost negligible due to ultra thin films ~10nm up to 400 ALD cycles. Surface
has high free energy due to the abrupt discontinuation of the bulk. The excess free energy reduces by
rearrangement of the silicon oxide. This phenomenon may attribute to higher PL intensity [12].
Until now, it was generally reported that initial intensity of uncoated phosphor was higher than that
of coated phosphor [6]. This inverse effect is probably due to ALD growth mechanism. It was also
convinced that the films grown with ALD were more uniform, continuous and free of surface defects
than that that of sol-gel or CVD.
Fig. 6: PL spectra of yellow phosphors at various ALD cycles
4. CONCLUSION
ALD process for oxide coating phosphors showed a remarkable improvement of PL intensity.
The dispersibility of coated phosphors was also higher than that of uncoated,..In sol-gel or CVD
process, the PL intensity of coated phosphor decreased normally than that of uncoated due to the
oxide absorption and the aggregate of phosphor powders was also a main barrier for application. In
ALD system, however, the ultra thin and uniform film can be controlled without the aggregate of
phosphor powders.
ACKNOWLEDGEMENT
The study has been supported by Kumoh National Institute of Technology.
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6. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 4, Issue 6, November - December (2013), © IAEME
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