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We present a facile fabrication process to directly fabricate cone-shaped microwells arrays on single crystal Y3Al5O12:Ce3+ (YAG:Ce) ceramic phosphor platelets (CPPs) by short-pulse laser direct patterning. Compared to unpatterned YAG:Ce CPP with smooth surface, the forward-to-total ratio of emission photons of patterned YAG:Ce CPPs was enhanced from 53.2% up to 78.2%, and the total emission within 4-π degree is 6% higher. The fabricated patterns are also beneficial in increasing the color conversion efficiency of YAG:Ce CPPs by 7.6%. The patterned YAG:Ce CPPs display much better correlated color temperature (CCT) uniformity under varied currents. The angular correlated color temperature uniformity (ACU) of patterned YAG:Ce CPPs reaches as high as 0.933 compared to 0.730 of the unpatterned one. These results suggest that laser patterning of YAG:Ce CPP could effectively manipulate its luminance, chromaticity and illumination pattern, which may lead to further technological advancements for diversified applications of film-type CPPs in highly efficient white LEDs.
© 2016 Optical Society of America
1. Introduction
Currently, the commercial white light emitting diodes (LEDs) are generally packaged by coating GaN-based LEDs with phosphor powder embedded in organic resins [1,2]. However, a significant portion of the phosphor-converted light traveling back towards the LED chip is absorbed by the chip and the other structures within the package [3–5]. Such scattering happens at the boundary of the phosphor particle and the silicone matrix. It was reported that the portion of backscattered photons by phosphor is up to 60% [3,4]. Since the scattering intensity is the sixth power of grain size or grain boundary [5,6], the significant scattering loss from grain boundary of phosphor powders is also inevitable. The absorption of backscattered photons by LED chip raises the temperature, which causes the degradation of luminous intensity and the shifting of emission color of white LEDs [7,8]. Recently, Y3Al5O12:Ce3+ (YAG:Ce) single crystal or polycrystalline ceramic phosphor platelets (CPPs) as an alternative photon converter has been applied due to its properties of reduced photon scattering, high transparency, good mechanical performance and excellent chemical stabilities [9–11]. However, due to the large refractive index differences between YAG:Ce CPPs (n ~1.8) and the air, most of the converted light is trapped due to guided modes, normally referred as total internal reflection (TIR) [12]. The trapped light is either emits from the side of the platelet or re-absorbed then converted to heat [13,14], both of which decrease the forward light emission. It severely degrades the angular color uniformity, and leads to poor white light qualities such as “yellow ring” effect [15]. Besides general lighting applications, YAG:Ce CPP-capped white LEDs have also been widely used in highly directional lighting applications that rely on the forward emission of light, such as projectors, automobile headlamps, and fiber-coupled light sources. Therefore, it is particularly desirable to increase the etendue and directionality of the light emitted from the YAG:Ce CPPs. However, fabricating YAG:Ce CPPs with high-efficient forward emission and high color spatial uniformity utilizing a simple and robust method remains challenging. Due to the high mechanical strength and excellent chemical stability of YAG crystal [16–18], it is extremely difficult to be reformed by conventional dry etching or wet etching methods. It has previously reported that layers of foreign materials such as SiO2, TiO2 and SiNx were deposited on the YAG:Ce CPPs. The deposited layers are then fabricated into various micro/nano structures such as nanosphere arrays, nanohole arrays, nanobowl arrays, nanopillars [10,19–23] by means of nanoimprint or polystyrene (PS) sphere self-assembly lithography to enhance the outcoupling efficiency. Most of these methods require combination of multi-step processes, such as film deposition, lithography masks, self-assembly, hardmask deposition and dry etching or wet etching. The whole processing steps are complicated, time consuming and expensive. On the other hand, those foreign structures fabricated on the surface of YAG:Ce CPPs are sometimes not chemically or physically stable, and suffer from hostile external environment conditions (temperature, humidity, acidity and alkalinity, etc.) are likely to be eroded or fall off, which leads to poor stability and limited lifetime.
Short-pulse laser-solid interaction has always been an interesting research topic over the decades, which is frequently used for non-contact, chemical-free material processing [24–27]. This technique allows direct machining of physically robust, chemical resistant materials. In this paper, a mask-free technique to fabricate microwells array by short-pulse laser patterning is presented. The optical properties of the patterned YAG:Ce CPPs were analyzed.
2. Experiments
The commercially available transparent Ce:YAG single crystal flat CPPs with thickness of 300 μm, 5 mm × 5 mm in size, and Ce3+ ions doping concentration of 0.3 at% were used in the experiment. A 355 nm diode pump solid state (DPSS) Q-switched nanosecond laser was used to ablate the sample surface in scanning mode. The pulse duration is about 40 ns, and the pulse repetition rate is 1 kHz. The laser power used in the experiment was tuned from 0.04 W up to 0.2 W with the laser spot size of about 10μm in diameter.
The schematic diagram of laser patterning process is shown in Fig. 1(a). The cleaned YAG:Ce CPP was put on a two-dimensional transition platform. Then the short-pulse laser beam was focused by an objective lens on the surface of the YAG:Ce CPP. By exposing the YAG surface to an ultrahigh energy laser pulse in very short time, the irradiated YAG materials can be effectively removed. By repeating such process in a fast scanning mode, arrays of holes were quickly formed on the YAG after the laser scanning across the surface of the whole CPP. After laser ablation, the YAG:Ce CPP was soaked in sulfuric acid solution (H2SO4, 98 wt%) at 70 °C to remove the debris and reduce the edge roughness of the microwells. The width and depth of the microwells array were manipulated by laser ablation power density and the distance of laser focus point to the surface of YAG:Ce CPP. The spacing of microwells was controlled by the scanning speed of platform and laser pulse repetition rate, and the spacing between two adjacent scanning lines was set artificially. 6.4 × 105 microwells were made on a 5 mm × 5 mm YAG:Ce CPP by laser patterning in less than 15 minutes.
Fig. 1 (a) Schematic diagram of the fabrication of microwells array on YAG:Ce CPP by laser direct patterning method. Schematic diagrams of the (b) forward emission and (c) total emission measurement methods.
Three samples with different size of patterns were designed. The detailed geometrical parameters of different samples are listed in Table 1. Samples were obtained by varying the laser power from 0.04 W to 0.20 W. The pattern with higher laser irradiation power has larger width and depth, and simultaneously higher depth-to-width ratio (aspect ratio). Cracks appear on the YAG:Ce CPP surface when laser power exceeds 0.20 W due to the material bulk heating. Microwell patterns of all samples were kept with the same filling factor of about 76%. Different sizes of holes were made by shifting the laser focal point towards and away from the sample surface.
Table 1. Fabrication parameters and geometric parameters of the fabricated microwells array patterns
All the YAG:Ce CPPs were excited by a surface-mounted LED package with size of 5 mm × 5 mm. A 1 × 1 mm2 vertical structure GaN blue LED chip is located in the center of the reflector cup with peak wavelength of 450 nm. The surface of the LED chip is roughed and wire-bonded to the lead frame. In the measurements, the YAG:Ce CPPs were attached above the LED cup by a double-sided adhesive tape on the edge of the package without silicone encapsulation, the laser patterned side of each CPP was placed away from the LED chip surface. The spectral power distribution (SPD) was measured using an integrating sphere. The forward emission and total emission SPDs of unpatterned and patterned YAG:Ce CPPs were analyzed separately. As shown in Fig. 1(b), the YAG:Ce CPP-capped package was placed in the entrance of the integrating sphere in the forward emission spectra measurement. It was properly placed so that only forward emission light can be collected by the integrating sphere. As shown in Fig. 1(c), the total emission spectra were obtained by putting the whole YAG:Ce CPP-capped package into integrating sphere to collect total emission light from the CPPs. The morphology of the fabricated microwells array pattern was characterized by a field-emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). The dependence of luminous efficacy and correlated color temperature (CCT) on various currents was also measured by gradually changing the driving current. The angular-dependent spectra and colorimetric performance were measured using a goniophotometer. The illumination far-field pattern images of different YAG:Ce CPPs capped LED at 350 mA were measured by placing the LED against a white wall and photographing the projected light by a digital camera.
3. Results and discussion
3.1 Surface morphology of the microwells array patterned on YAG:Ce CPPs
Figure 2(a) shows the appearance of unpatterned and patterned YAG:Ce CPP respectively. Due to the scattering properties of the microwells array, the appearance of YAG:Ce CPP with microwell-patterned surface shows less transparency compared to unpatterned YAG:Ce CPP with smooth surface. Figure 2(b) shows the optical microscopy image of the patterned sample, which displays uniform shape and dimensions of the fabricated microwells array formed on the YAG:Ce CPP surface.
Fig. 2 (a) Appearance of unpatterned YAG:Ce CPP with smooth surface and microwell-patterned YAG:Ce CPP. (b) Optical microscopy of the fabricated microwells array patterned on the YAG:Ce CPP surface.
Figure 3(a) and 3(b) show the SEM images of microwells array fabricated on the YAG:Ce CPPs surface at a laser power of 0.03 W. The spacing of adjacent microwells was set as 7.1 µm. The microwells array shows uniform size with smooth side wall. Figure 3(c) and 3(d) show the AFM image and depth profile of the microwells array, which reveal the cone-shaped bottom of the microwell with the depth of about 1.5 µm. The formation of the inverted triangular cone-shaped bottom should be attributed to the laser ablation and acid cleaning processes. Laser-induced chemical modification occurred in the edge of the laser ablated zone. Then the edge of microwells was slightly crystallography-etched by sulfuric acid in the subsequent cleaning process [24,28,29]. The depth profile of the microwell shows a slight asymmetry due to the imperfect central symmetry of the energy distribution of the laser spot used in the experiment.
Fig. 3 (a) Top-view and (b) 45° titled-view SEM images of the fabricated microwells array patterned on the YAG:Ce CPP surface at laser power of 0.03 W. Inset shows a single microwell. (c) A three-dimensional AFM image and (d) depth profile of the fabricated microwells array pattern.
3.2 Optical performance of the YAG:Ce CPPs with unpatterned and patterned surface
Figure 4(a) and 4(b) show the measured SPDs of forward and total emission at an injection current of 350 mA respectively. Compared to the unpatterned sample, the integrated yellow light intensities of patterned sample 3 increased by 117.0% and 22.2% for forward emission and total emission respectively. The integrated blue light intensities of patterned sample 3-decreased by 47.2% and 48.0% for forward and total emission respectively. The patterned YAG:Ce CPPs show overall higher intensities of yellow emission and stronger blue light absorption for both types of emission. Among which, the intensity of yellow light, especially emitted from the front surface of patterned CPPs is greatly enhanced. For the patterned samples, the incident blue photons undergo multiple reflections in the platelet by the micowells array pattern, which largely increases the probability of blue photons being absorbed by Ce3+ ions and converting to yellow photons. Therefore, the enhancement of yellow emission can be ascribed to the increased extraction efficiency for the converted yellow radiation and increased re-absorption efficiency by recycling more of blue radiation.
Fig. 4 Measured SPDs of (a) forward emission and (b) total emission and (c) total emission photons number enhancement, color conversion efficiency and the ratio of the forward emission photons number to the total emission photons number for all the samples at an injection current of 350 mA.
The total emission photons enhancement, color conversion efficiency and the ratio of forward-to-total emission photons number are drawn in Fig. 4(c). It shows that the total emission photons enhancement of patterned samples is nearly 6% higher than the unpatterned sample. The color conversion efficiency was calculated by dividing the number of emitted yellow photons to the number of absorbed blue photons. Obviously, the patterned samples has better blue-to-yellow conversion efficiency, among which sample 3 shows the highest color conversion efficiency of 73.1% compared to that of unpatterned sample of 65.5%. For the unpatterned sample, the ratio of forward-to-total emission photons number is 53.2%, which reveals that the side emission photons accounts for nearly half of the total number of photons emitted out of the CPP. However, the forward-to-total emission ratios of the patterned samples are significantly increased to 78.2% for sample 3. This means that by patterning high aspect ratio microwells array on YAG:Ce CPP, more converted yellow light can radiate out from the patterned front surface of CPPs other than via side emission or being trapped. Therefore, the microwells array pattern on CPP does not only increase the conversion efficiency and overall light extraction, but also re-shapes the emission light distribution of CPP.
Figure 5(a) and 5(b) show the measured luminous efficacy, CCT and CIE chromaticity coordinate distributions for YAG:Ce CPPs covered blue LED at an injection current of 350 mA. Sample 3 shows the highest luminous efficacy of 102 lm/W and the lowest CCT of 4661 K, while for the unpatterned sample they are 92.6 lm/W and 5267 K, respectively. Such difference is the results of the increased overall light extraction efficiency and color conversion efficiency together. As shown in Fig. 5(b), the blue-to-yellow ratio decreases owing to enhanced yellow light emission, so the CIE chromaticity coordinates of patterned samples move toward the yellow region in the chromatogram map. It should be noted that the luminous efficacy was calculated based on total emission. If only consider the forward emission luminous efficacy enhancement, sample 3 is about 117% higher than that of the unpatterned sample. The excellent optical performance indicates that the laser patterned YAG:Ce CPPs can be used in white LEDs as a replacement for flat CPPs.
Fig. 5 Measured (a) luminous efficacy (lm/W) and CCT and (b) CIE chromaticity coordinate distributions for the unpatterned and patterned YAG:Ce CPPs with different aspect ratios by integrating sphere at an injection current of 350 mA (insert shows the magnified chromaticity coordinates).
The luminous efficacy and lumen flux of the unpatterned and patterned YAG:Ce CPPs under varied driven current were evaluated, as shown in Fig. 6(a). Microwells array patterned YAG:Ce CPPs have higher lumen flux and luminous efficacy in the entire current range, and the enhancement ratio of luminous efficacy remains at an almost constant value. The maximum enhancement ratio for sample 3 is about 9.7% on average compared to that of the unpatterned sample. The evolution of luminous efficacy for all samples is consistent with that of the quantum efficiency of the blue LED chip [30]. There is no sign of extra sudden drop occurs for all the samples indicating the absence of heating effect between the chip and YAG:Ce CPPs [8]. Figure 6(b) shows the CCT variations of unpatterned and patterned samples as a function of current. The range of CCT variation (∆CCT = CCTmax - CCTmin) for sample 3 (30 K) is much smaller than that of the unpatterned sample (167 K), which indicates that more stable optical performance can be achieved for samples with high aspect ratio patterns in large current range. Detailed statistical data is summarized in Table 2. For the unpatterned sample, as the blue absorption of Ce3+ ions in the area directly above the LED chip tending to saturate under higher driven currents, so the higher blue-to-yellow ratios and higher CCT values were observed. On the contrary, because of the weakened waveguide effect and the TIR of the patterned YAG:Ce CPPs, a larger enhancement of yellow emission is obtained with the enhancement of blue excitation. Therefore, the blue-to-yellow ratio can be maintained at a relatively stable value, thus a smaller variation of CCT with the increase of driven current was obtained.
Fig. 6 Variation of (a) Luminous efficacy (lm/W) and lumen flux and (b) CCT for the unpatterned and patterned YAG:Ce CPPs with different aspect ratios driven at current range from 10 mA to 700 mA.
Table 2. CCT and CIE chromaticity coordinate variations of samples at the driven current range of 10 mA-700 mA
The far-field emission patterns for the unpatterned and microwells array patterned YAG:Ce CPPs with different aspect ratios at an injection current of 50 mA were measured (Fig. 7(a)). The far-field emission pattern of unpatterned YAG:Ce CPP shows strong side-emission properties, while the intensity distributions for the patterned YAG:Ce CPPs become more uniform in the forward directions and have much stronger intensities. Moreover, as shown in Fig. 7(b), the patterned YAG:Ce CPPs exhibit smaller divergent angles (50% of the full luminosity) compared to that of unpatterned YAG:Ce CPP. The relatively more collimated emission properties of patterned YAG:Ce CPPs are resulted from the enhanced forward emission as well as the suppression of the side emission.
Fig. 7 (a) Far-field emission patterns and (b) the divergent angles for the unpatterned and patterned YAG:Ce CPPs with different aspect ratios at an injection current of 50 mA. The far-field emission intensity unit is candela.
Figure 8(a) and 8(b) show the variations of CCT and CIE chromaticity coordinates at different viewing angles from 0° to 90°. Obviously, the patterned YAG:Ce CPPs have much smaller deviation of CCT with viewing angle than the unpatterned one. The angular correlated color temperature uniformity (ACU) is calculated using following equation [31]:
where the α denotes the view angles ranging from 0° to 90°, Tα is the CCT value at α, Tavg is the average value of CCT, n stands for the number of view angles. The higher the ACU, the smaller the CCT deviation and the better color spatial uniformity. The ACU of the unpatterned sample is 0.730 and that of the patterned samples is 0.933 for sample 3, respectively. Such color spatial uniformity improvement is consistent with the smaller deviations of CIE chromaticity coordinates distribution shown in Fig. 8(b). Figure 8(c) illustrates the illumination patterns of all the samples at equal current of 350 mA. It is found that the unpatterned sample suffers from severe yellow ring problem with most of the yellow light is distributed at the edge of the platelet. For the patterned samples, more yellow light is distributed on the forward side of the platelets. Thus more uniform color-mixing of blue and yellow light is achieved, especially for the samples with higher aspect ratio patterns.Fig. 8 Variations of (a) CCT (insert shows ACU of all the samples) and (b) CIE chromaticity coordinates as a function of the viewing angle for the unpatterned and patterned YAG:Ce CPPs with different aspect ratios at an injection current of 350 mA. (c) Illumination patterns for the unpatterned and patterned YAG:Ce CPPs.
Figure 9(a) and 9(b) show the relative sum of blue and yellow photons for different samples as a function of viewing angle, respectively. It is further shown that the improved optical performance of patterned YAG:Ce CPPs could be attributed to strong enhancement of yellow light forward emission, which is realized by combination of extracting more yellow light emitted from front surface of CPPs and converting more blue light to yellow light. On the other hand, the blue-to-yellow ratio of emission spectra shows highly dependence on the aspect ratio of the patterns, so we can readily manipulate the CCT values and CIE chromaticity coordinates of patterned samples by changing the laser power.
Fig. 9 Angle dependence of the relative sum of (a) blue photons and (b) yellow photons emission for the unpatterned and patterned YAG:Ce CPPs with different aspect ratios at an injection current of 350 mA.
4. Conclusion
In this paper, we demonstrate a chemical-free approach to fabricate microwells array on YAG:Ce CPPs surface by short-pulse laser patterning. Laser patterning of YAG:Ce CPPs can significantly enhance the forward light extraction. More stable optical performance and better color spatial uniformity can also be obtained for the patterned YAG:Ce CPPs. Furthermore, the chromaticity coordinates of patterned YAG:Ce CPPs can be conveniently controlled by adjusting the aspect ratio of patterns. In addition, the fabricated patterns inherit all the excellent properties of YAG single crystal, such as high mechanical strength and anti-radiation and resistance to corrosion, oxidation and radiation damage. The stable physical and chemical properties make the patterns capable of long-term reliability. The whole experimental process is straightforward, time-saving and reproducible. In particular, without the need to use any lithographic processing, this approach is suitable for large scale mass production.
Acknowledgments
This work was supported by the National High Technology Research and Development Program of China under Grant No. 2014AA032608 and National Natural Science Foundation of China (Grant No. 61404101).
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