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This paper reports the optical effects of a two-dimensional (2D) SiNx photonic crystal layer (PCL) on Y3Al5O12:Ce3+ (YAG:Ce) yellow ceramic plate phosphor (CPP) in order to enhance the forward emission of YAG:Ce CPP-capped high-power white light-emitting diodes (LEDs). By adding the 2D SiNx PCL with a 580 nm lattice constant, integrated yellow emission was improved by a factor of 1.72 compared to that of a conventional YAG:Ce CPP capped on a blue LED cup. This enhanced forward yellow emission is attributed to increased extraction of yellow emission light and improved absorption of blue excitation light through Bragg scattering and/or the leaky modes produced by the 2D PCLs. The introduction of 2D PCL can also reduce the wide variation of optical properties as a function of both ambient temperature and applied current, compared to those of a high-power YAG:Ce CPP-capped LED.
©2011 Optical Society of America
1. Introduction
Developing high-efficiency, high-power white LEDs for application to solid-state lighting has attracted considerable attention for the last decade. Among the various methods to fabricate white LEDs, a facile combination of wide-bandgap III-V nitride blue LEDs and color-converted phosphors has led to the commercial production of white phosphor-converted light-emitting diodes (pc-LEDs) [1–3]. A conventional white pc-LED is commonly composed of a blue LED chip and a yellow [Y3Al5O12:Ce3+ (YAG:Ce)] powder phosphor coating with a silicone resin on the LED chip surface to obtain white emission [1,4]. However, presently available white pc-LEDs provide limited conversion efficiency of emission from the powder phosphor layer. This is due to high scattering and reflecting loss of the emission from the coated micro-sized phosphor particles [5,6]. Additionally, heat accumulation on the phosphor matrix, which is coated onto blue LED chips, is becoming a serious problem as the size and operation temperature of LED chips are increased with increased output power for high-power LEDs. This results in decreased luminous efficacy and lifetime of white LEDs. This also causes wide variations of optical properties of high-power powder-based white LEDs with applied current and ambient temperature.
In efforts to address these problems, new types of white LEDs using YAG:Ce glass ceramic phosphor (GCP) [7–9] and transparent polycrystalline YAG:Ce ceramic plate phosphor (CPP) [10, 11] recently have been reported. However, the light extraction of both GCPs and transparent CPPs is relatively low in accordance with trapping of light by total internal reflection (TIR) and the waveguide effect, because both the GCPs and transparent CPPs have high transparency and a planar film-type form with a smooth surface [12–14]. The low extraction efficiency of the transparent, smooth, film-type phosphor layer is the most significant drawback for application in high-power solid-state LED lighting. Therefore, it is necessary to improve the light extraction efficiency of film-type phosphors so that they can replace powder phosphors by CPPs in high-power white LEDs. Here, we propose the introduction of 2D SiNx photonic crystal layers (PCLs) on YAG:Ce CPPs to suppress the low extraction efficiency at the interface of YAG:Ce CPP and air, with the ultimate goal of replacing YAG:Ce powder phosphors with 2D SiNx PCL-assisted YAG:Ce CPPs. This study also reports a simple optical structure, results, and analyses of the 2D SiNx PCL-assisted YAG:Ce CPP-capped white LED with the aim of improving the light output of high-power white LEDs and reducing the wide variation of the optical properties of a high-power, flat CPP-capped LED with ambient temperature and applied current.
2. Experimental methods
YAG:Ce CPP was purchased from Baikowski Japan Ltd. The preparation of YAG:Ce CPP was similar as described in previous publications [15, 16], where the average grain size of about 10 µm, volume of pores of about 1 ppm and the width of grain boundaries of about 1 nm were reported. The top surface of YAG:Ce CPP was coated with 2D SiNx PCLs using a nanosphere lithography (NSL) process. The 2D PCL with triangular-lattice SiNx nanohole arrays was fabricated via a NSL process as described in detail elsewhere [17–19]. Briefly, a 400-nm-thick SiNx layer was deposited on the topside of a 0.5 cm x 0.5 cm YAG:Ce CPP (0.1 mm thickness) by plasma enhanced chemical vapor deposition (PECVD). A monolayer of polystyrene (PS) beads was coated on the SiNx/YAG:Ce CPP substrate as a deposition mask using a self-assembly process. The monolayer of PS nanospheres formed on the substrate was thinned with O2 plasma until the diameter of the nanospheres reached the desired size [20, 21]. The substrate was immediately loaded into a thermal evaporation system for deposition of a 25 nm thick Cr metal mask. After removing the PS nanospheres, a Cr mask with triangular-lattice 2D Cr nanohole arrays was then clearly defined across the surface of the substrate. Finally, the 2D Cr nanohole mask was transferred to the SiO2 layer by carrying out CF4-based reactive ion etching (RIE) on the SiNx/YAG:Ce CPP substrate. After this pattern transfer, the 2D Cr nanohole mask layer was removed by a Cr etchant. Subsequently, YAG:Ce CPP coated with a 2D SiNx nanohole PCL was formed.
To fabricate a white pc-LED, the 0.5 cm x 0.5 cm YAG:Ce CPP and 2D SiNx PCL-assisted CPP were attached on the top of a blue LED cup.
The crystal structure of the YAG:Ce CPP was measured by the X-ray diffraction (XRD) method. The XRD patterns of the YAG:Ce CPP were obtained using an X-ray diffractometer (X’pert system, Philips) with Cu Kα1 radiation. The diffraction patterns were obtained over a range of 10° - 80° 2θ at a scan rate of 1° 2θ/min. Cross-sectional and surface images of the conventional YAG:Ce CPP and YAG:Ce CPP coated with the 2D PCL were obtained by a field-emission typed scanning electron microscopy (FE-SEM) (JSM 7401F, JEOL) operated at 10 kV. The surface morphology of the conventional YAG:Ce CPP was measured by atomic force microscopy (AFM) (Seiko instrument model SPA 400) operated in contact mode (Si cantilever). The optical transmittance of the obtained YAG:Ce CPP samples was measured over the wavelength region from 380 to 1100 nm using a UV/Vis spectrophotometer (OPTIZEN 2120UV, MECASYS). The integrated and normally directed photoluminescent (PL) emissions spectra, as well as the angular dependence of the spectra from the conventional YAG:Ce CPP and 2D SiNx PCL-assisted YAG:Ce CPP were measured under 445nm LED excitation using a spectrophotometer (PSI Co. Ltd. Model Darsa II). The electroluminescent (EL) properties of YAG:Ce CPP and 2D SiNx PCL-assisted YAG:Ce CPP capped on the top of blue InGaN LEDs cup were also measured using a spectrophotometer with integrated spheres as a function of the applied current and ambient temperature.
3. Results and discussion
The optical, structural, and morphological properties of the YAG:Ce CPP (0.1mm thickness) are shown in Fig. 1 . The optical transmittance of YAG:Ce CPP is ~80% at 550 nm, as shown in Fig. 1(a). The absorption at 460 nm originates from the 5d←4f transition of Ce3+. Transparency of the YAG:Ce CPP above 500 nm is high enough to show a possibility of reducing the scattering and/or reflecting loss of powder-typed phosphors. The XRD patterns shown in Fig. 1(b) indicate that a phase of the YAG:Ce CPP coincides with that of the previously reported conventional YAG:Ce CPP. The observed patterns are well matched with the lines specified in the JCPDS files (No. 33-0040) [10]. The two-dimensional AFM image shown in Fig. 1(c) shows that the root-mean-square (rms) roughness of the polished YAG:Ce CPP is approximately 0.42nm. The surface roughness of the YAG:Ce CPP is sufficiently smooth from the standpoint of light scattering of the film-type phosphor through the surface grains [12]. Top view SEM image also visualize grains with hardly distinguishable boundaries. Side and top view SEM images of the YAG:Ce CPP shown in Fig. 1(d) confirm that the surface of the YAG:Ce CPP is sufficiently smooth and planar without scattering loss. In addition, the thickness of YAG:Ce CPP (0.1mm) coincides with the thickness shown in Fig. 1(d). This thickness is sufficient to address the low PL intensity resulting from the small photon-solid interaction volume of the thin film-type phosphors. It is thus evident from these images that YAG:Ce CPP can serve as an ideal film-type phosphor with high brightness and large volume despite having low extraction efficiency due to its TIR and a waveguide effect.
Fig. 1 (a) Optical transmission spectra, (b) XRD pattern, (c) Two-dimensional AFM image, (d) Side and top view FE-SEM images of an ideally smooth YAG:Ce CPP.
In order to observe the effects of the 2D PCL on the conventional thick-film type YAG:Ce CPP, a 2D SiNx nanohole PCL was fabricated on the top surface of the YAG:Ce CPP by combination of a NSL process and RIE process. Figure 2 shows FE-SEM images of the YAG:Ce CPP coated with the 2D SiNx nanohole array PCL. A magnified plan and tilted side images of the locations marked by the circle at a point of the YAG:Ce CPP coated with 2D PCL are also provided in Figs. 2(b) and (c). These pictures indicate that the YAG:Ce CPP coated with the 2D SiNx nanohole array was successfully fabricated by the NSL and RIE processes.
Fig. 2 (a) Cross-sectional view FE-SEM images of a YAG:Ce CPP coated with a 2D PCL nanohole arrays. The magnified FE-SEM images of a 2D PCL of the locations marked by the circle at a point of a YAG:Ce CPP coated with a 2D SiNx PCL nanoholes.
As previously reported, in the film-type CPP, a limited fraction of light emitted at an angle less than the critical angle θc can escape from the smooth surface of the YAG:Ce CPP [12–14]. All other light trapped within the CPP is waveguided, and is eventually either absorbed or emitted from the edge of the plate. Here we introduce a 2D PCL on top of the CPP in an attempt to convert the light confined in the layer into light that can freely propagate toward the surrounding air, even though the effect of the 2D PCL on the thicker film-type phosphor layer is relatively small compared with that of a 2D PCL on a thinner film-type phosphor layer [22]. It is well known that this improvement of emitted light is caused by the combined effect of increased extraction of emission light and increased absorption of blue excitation light by the leaky and/or Bragg scattering produced by the 2D PCL [23, 24].
First, it is essential to study the effects of 2D SiNx nanoholes on the total enhanced extraction efficiency of YAG:Ce CPPs. It is also necessary to analyze the blue transmittance and enhanced PL efficiency to compare the portion of both the enhanced extraction efficiency and the enhanced absorption efficiency by the Bragg and/or leaky modes of 2D PCLs. The degree to which the introduction of the 2D SiNx nanohole arrays onto the YAG:Ce CPPs enhances their outcoupling efficiencies is determined by several parameters. One of the most interesting is the lattice parameter of the regularly arranged nanoholes with the same height and filling factor. Figure 3(a) shows the changes in the enhancement ratios of the integrated PLs as a function of the lattice parameters of the 2D triangular-lattice SiNx nanohole PCL. Here, the height and dielectric-area filling ratio were fixed to 400 nm and 0.40~0.45, respectively. This figure suggests that the enhancement ratio of the 580 nm 2D array-coated YAG:Ce CPP is an optimum value among 2D SiNx PCL-assisted samples with four lattice constants. The integrated PL emission spectra of a conventional YAG:Ce CPP and the 2D SiNx PCL-assisted YAG:Ce CPPs with four lattice constants on top of a blue LED cup were measured under 445nm LED excitation, and are shown in Fig. 3(b). This figure indicates that the integrated yellow emission is improved by a factor of 1.72 compared to that of the conventional YAG:Ce CPP capped on the blue LED cup, by adding a 2D SiNx PCL with 580 nm lattice constant. Otherwise, the large portion of transmitted blue LED light through 2D SiNx PCL-coated YAG:Ce CPP is reduced by Bragg scattering of the 2D PCL. To separate the enhancement of the extraction and absorption efficiency, we normalized the PL emission with respect to the absorption. The absorption and PL emission were measured in an integrating sphere, as described in the literature [21]. From the separation of the absorption and extraction factor, as seen in Fig. 3(a), a 1.34 fold enhancement of blue absorption and a 1.29 fold enhancement of extraction light are obtained with incorporation of the 2D SiNx PCL on top of YAG:Ce CPP. Hence, a large enhancement of yellow emission (1.72 fold) can be achieved in a YAG:Ce CPP on a blue LED cup due to Bragg scattering and/or leaky modes of emission and excitation by 2D PCLs composed of 580 nm-sized SiNx nanoholes. The structural parameters of the 2D SiNx nanohole PCL used in this study are as follows: 580 nm lattice constant, 400 nm height, and ~0.41 dielectric-area filling ratio. The 0.5 cm x 0.5 cm plate-typed CPPs are attached on the top of a blue LED cup for comparison of its properties with those of a conventional YAG:Ce CPP-capped white LED and a 2D PCL-assisted YAG:Ce CPP-capped white LED.
Fig. 3 (a) Enhancement ratio of the integrated PL intensities as a function of the lattice constant of the 2D SiNx nanohole PCL-assisted YAG:Ce CPPs. (b) The integrated PL emission spectra of a conventional YAG:Ce CPP and the 2D SiNx PCL-assisted YAG:Ce CPPs with four lattice constants (350, 420, 580, and 720 nm) on top of a blue LED cup under 445nm LED excitation.
Figures 4(a) and (b) show schematic diagrams of two different types of white LEDs compared in this study. Both the YAG:Ce CPP-capped white LED and the 2D PCL-assisted CPP-capped white LED are phosphor-on-cup type LEDs, unlike the phosphor-in-cup type of a conventional powder-based white LED. YAG:Ce CPPs are made of pure Y3Al5O12:Ce nanophosphors to reduce the possibility of inhomogeneous crystal quality by changing the host composition when fabricating a ceramic plate. Figure 5 shows the electroluminescent (EL) emission spectra of two different white LEDs at equal-current (350 mA, rated current). As a result of introducing the 2D PCL on top of the CPP, the intensity of the yellow emission and luminance of the 2D PCL-assisted YAG:Ce CPP-capped white LED become higher than those of the flat CPP-capped white LED. The enhanced EL luminance indicates that the 2D PCL-assisted YAG:Ce CPPs can be used in high-power white LEDs instead of using conventional flat CPPs.
Fig. 4 Schematic diagrams of (a) a conventional YAG:Ce CPP-capped LED and (b) a 2D SiNx PCL-assisted YAG:Ce CPP-capped LED.
Fig. 5 The blue-conversion-white EL spectra of a conventional YAG:Ce CPP-capped LED and a 2D SiNx PCL-assisted YAG:Ce CPP-capped LED.
In addition, the detailed EL properties are compared as functions of both applied current and ambient temperature to analyze the suitability of the 2D PCL-assisted YAG:Ce CPPs to be applied to high-power LEDs. A plot of both luminous efficacy (lm/W) and correlated color temperature (CCT) on two different typed LEDs as a function of the applied current are shown in Figs. 6(a) and (b) . The luminous efficacy of the conventional and 2D PCL-assisted CPP-capped LED was decreased from 72 lm/W to 30 lm/W and from 107 lm/W to 46 lm/W with an increase of applied current from 50 to 600 mA, respectively. The enhancement ratio of luminous efficacy by the 2D PCL is almost constant over all applied current ranges. The decreased trend of a 2D-PCL-assisted CPP-capped white LED is caused by degradation of the luminous efficacy of the blue LED chip itself with an increase of current, because the CPP does not come into contact with the blue LED. By the same reason, the CCT of the 2D PCL-assisted CPP-capped LED remains almost unchanged at ~5500K regardless of the applied current, as shown in Fig. 6(b). However, the CCT of the conventional flat CPP-capped white LED exhibits a large change with an increase of the applied current. This occurs because the enhancement of the yellow emission from CPP with an increase in the applied current is lower due to TIR and the waveguide effect compared to that of the blue excitation from a LED chip. Therefore, the introduction of the 2D PCL changes the bluish white color temperature (~11,000K) of the CPP-capped white LED to desirable white colors (~5,500K). Figures 6(c) and (d) compare the efficacy stability and CCT variations at equal current (350mA) with an increase of ambient temperature. The temperature stability of the efficacy levels in conventional and 2D PCL-assisted CPP-capped white LEDs are nearly identical regardless of the introduction of the 2D PCL, as shown in Fig. 6(c). However, Fig. 6(d) shows that the introduction of the 2D SiNx PCL on the YAG:Ce CPP-capped white LED reduces the temperature variations of the CCTs and prevents the CCTs from changing. These outcomes are due to increased out-coupling emission from the YAG:Ce CPPs by 2D PCL, which can reduce the thermal loss. Therefore, the improved current and temperature stabilities confirm the suitability of the 2D PCL-assisted YAG:Ce CPPs for use in a high-power white LED.
Fig. 6 (a) Luminous efficacy (lm/W) and (b) correlated color temperature (CCT) of a conventional YAG:Ce CPP-capped LED and a 2D SiNx PCL-assisted YAG:Ce CPP-capped LED as a function of the applied current. (c) The normalized luminous efficacy and (d) CCT of a conventional YAG:Ce CPP-capped LED and a 2D SiNx PCL-assisted YAG:Ce CPP-capped LED as a function of the ambient temperature.
The angular dependence of the emission spectrum was also examined, as large variations in the intensity and color with viewing angle are undesirable for lighting applications. Figure 7 plots the measured relative lumens and correlated color temperatures (CCT) as a function of the viewing angle for two different LEDs taken under identical excitation conditions (current at 350 mA). The conventional YAG:Ce CPP-capped LED shows a strong edge-emission pattern. This strong edge-emission pattern is due to the strong wave-guide effect at the interface between the surface of the flat CPP and air. Compared with this pattern, the figure indicates that the normally directed emission of the 2D PCL-assisted LED is much stronger than that of the conventional CPP-capped LED. As a result, the normally directed emission of the 2D PCL-coated CPP-capped LED is approximately 2.48 times higher than that of the flat CPP-capped LED. It can be seen that the enhancement ratio of normally directed light emitted from the 2D PCL-assisted YAG:Ce CPP-capped LED is higher than that of the integrated light extracted from the 2D PCL-assisted YAG:Ce CPP-capped LED. The 2D PCLs help focus more emitted light toward the viewers of white lighting devices. Moreover, reduced variation of CCTs with viewing angle is observed from the introduction of the 2D PCL on the CPP-capped LED. Figure 7(b) shows that the angular dependence of CCTs from the 2D PCL-assisted YAG:Ce CPP-capped LED is more stable than that of the conventional CPP-capped LED. Therefore, the improvement in the normally directed EL emission and the improved stability of the CCT with viewing angle are additional advantages of the 2D PCL-coated YAG:Ce CPP-capped LED over conventional flat CPP-capped LEDs.
Fig. 7 (a) The normalized lumens and (b) CCT as a function of the viewing angle for a conventional YAG:Ce CPP-capped LED and a 2D SiNx PCL-assisted YAG:Ce CPP-capped LED taken under identical excitation conditions (current at 350 mA).
4. Conclusions
The 2D PCL-assisted YAG:Ce CPP-capped white LED was proposed and demonstrated with the aim of achieving high-efficiency and stable white LEDs. A 1.72 fold enhancement of forward efficacy of a 2D SiNx PCL-coated YAG:Ce CPP-capped white LED was documented, and is attributed to a combination of a 1.34 fold enhancement of blue absorption and a 1.29 fold enhancement of extraction light. The color temperature of the white emission of the YAG:Ce CPP-capped LED was improved from bluish white (~11,000 K) to daylight (~5,500 K) by incorporating a 2D SiNx nanohole PCL. The luminous efficacy of the 2D SiNx PCL-coated YAG:Ce CPP-capped white LED is 68.8 lm/W at 350 mA, which is higher than that of conventional flat CPP-capped white LEDs (44.2 lm/W). Furthermore, the addition of the 2D SiNx PCL reduces the wide variation of luminous efficacy and CCT as functions of both applied current and ambient temperature, compared to conventional flat CPP-capped white LEDs. The improvement in the normally directed EL emission and the improved stability of CCT with viewing angle are additional advantages of the 2D PCL-coated YAG:Ce CPP-capped white LED over conventional flat CPP-capped white LEDs. It is expected that 2D SiNx PCL-coated YAG:Ce CPP-capped white LEDs will be good candidates for use in high-power white LEDs for headlights of automobiles.
Acknowledgments
This work was supported by the National Research Foundation (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) of Korea (no. 2011-0017449 and Nano Fields, no. 2008-03573).
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