nanopatterned yttrium aluminum garnet phosphor incorporated film for high-brightness gan-based white...
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Nanopatterned yttrium aluminum garnet phosphor incorporated film forhigh-brightness GaN-based white light emitting diodes
Joong-yeon Cho a, Sang-Jun Park a, Jinho Ahn b,⁎, Heon Lee a,⁎a Department of Materials Science and Engineering, Korea University, Seoul 136-713, South Koreab Department of Material Science & Engineering, Hanyang University, Seoul 133-791, South Korea
⁎ Corresponding authors.E-mail addresses: [email protected] (J. Ahn), heon
http://dx.doi.org/10.1016/j.tsf.2014.03.0650040-6090/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: J. Cho, et al., Nanopalight emitting diodes, Thin Solid Films (2014
a b s t r a c t
a r t i c l e i n f oAvailable online xxxx
Keywords:White light emitting diodesMoth-eye structuresDirect printingYAG phosphor incorporated filmSpin-on glass
In this study, we fabricated high-brightness white light emitting diodes (LEDs) by developing a nanopatternedyttrium aluminum garnet (YAG) phosphor-incorporated film. White light can be obtained by mixing blue lightfrom a GaN-based LED and yellow light of the YAG phosphor-incorporated film. If white light sources can be fab-ricated by exciting proper yellow phosphor using blue light, then these sources can be used instead of the conven-tional fluorescent lamps with a UV source, for backlighting of displays. In this work, a moth-eye structure wasformed on the YAG phosphor-incorporated film by direct spin-on glass (SOG) printing. The moth-eye structureshave been investigated to improve light transmittance in various optoelectronic devices, including photovoltaicsolar cells, light emitting diodes, and displays, because of their anti-reflection property. Direct SOG printing,which is a simple, easy, and relatively inexpensive process, can be used to fabricate nanoscale structures. Afterdirect SOG printing, the moth-eye structure with a diameter of 220 nm was formed uniformly on the YAGphosphor-incorporated film. As a result of moth-eye patterning on the YAG phosphor-incorporated film, thelight output power of awhite LEDwith a patterned YAG phosphor-incorporated film increased to up to 13% higherthan that of a white LED with a non-patterned film.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
In 1996, Nichia Co.manufacturedwhite light emitting diodes (LEDs)with blue chips (indium gallium nitride; InGaN) and yellow yttriumaluminum garnet (YAG; Y3Al5O12:Ce) phosphors [1]. After further de-velopments and improvements over several decades, white LEDs haveattracted much attention because of their various utilizations for light-ing and displays owing to their lowered energy consumption, improvedlifetime, lowered driving voltage, enhanced response time, and alsobecause they are highly environmentally friendly [2,3]. However, the ef-ficiency of the currentwhite LEDs should be improved further for betterapplicability to general lighting source [4]. The external quantum effi-ciency of the currentwhite LEDs ismainly limited by the light extractionefficiency. This efficiency is reduced by total internal reflection (TIR)that is caused by large difference in the refractive index between theLED structure and air [5–7].
Moth-eye structures, i.e., nano-scale periodic pattern arrays ofcones, with a diameter of 100 to 300 nm, have fascinated attention assolution, which can reduce the TIR caused at the interface betweenthe LED structure and air. They have anti-reflection properties, because
[email protected] (H. Lee).
tterned yttrium aluminum ga), http://dx.doi.org/10.1016/j
they can reduce the TIR of light by a gradual change in the average re-fractive index of the media [8–10]. In order to gain effective moth-eyeanti-reflection effects, high-density nano-structures and patterns witha tapered profile are needed. However, fabrication of tapered nano-scale moth-eye patterns with a high density of the patterns by theconventional nano-patterning technique is difficult.
In a prior work, a direct hydrogen silsesquioxane (HSQ, refractiveindex ~ 1.4) printing technique using a poly(dimethylsiloxane) (PDMS)mold as the transfer medium was reported [11]. This technique allowsthe direct fabrication of silica-based nanostructures by a simple printingand annealing process. In addition, the fabricated silica nanostructureshave the equal morphology as themaster template. Therefore, the directHSQ printing might be appropriate for fabricating silica moth-eye struc-tures with a high robustness and transmittance. The silicamoth-eye pat-terns have a high mechanical strength and durability, and improved thetransmittance in the visible light region of the spectrum. Thus, thesesilica moth-eye patterns were applied to a white LED device in order toenhance its light output efficiency.
In this study, we fabricated the 220 nm scaled moth-eye pattern onthe YAG phosphor incorporated film to increase the light output powerof the GaN based white LED with YAG film by direct HSQ patterningtechnique. Silica moth-eye patterns were formed on the YAGphosphor-incorporated film of a top emitting GaN-based blue LED, inorder to improve the light output power of the resulting white LED, asshown in Fig. 1.
rnet phosphor incorporated film for high-brightness GaN-basedwhite.tsf.2014.03.065
Fig. 1. Schematic diagram of (a) the conventional LED device and (b) the LED device with the silica moth eye patterns on the encapsulating glass, respectively.
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2. Experiments
First, epitaxial layers of a GaN-based blue LED were grown on ac-plane patterned sapphire substrate by metal oxide chemical vapordeposition and 300 × 300 μmblue LED chips were fabricated using con-ventional photolithography and an etching process. Details of the GaNepi-layer growth and fabrication of the LED device have been shownelsewhere [12].
Fig. 2, shows the schematic diagram of the fabrication of the HSQmoth-eye pattern on the YAG phosphor-incorporated film by usingthe direct HSQ printing technique. The Ni master template with moth-eye pattern was fabricated by laser interference lithography andelectroforming, as shown in Fig. 2(a). The moth-eye structure on thetop surface of the Ni master mold consists of a diameter of 220 nmand a height of 200 nm. Then, the 220 nm-diameter and 200 nm-height moth-eye pattern was replicated from a Ni master mold to aPDMS mold, as shown in Fig. 2(b)–(c). The PDMS mold was used as aflexible polymeric mold for the direct HSQ printing, because it canabsorb the solvent from the HSQ solution. The PDMS mold has a lowsurface energy and high gas permeability, and it provides conformal
Fig. 2. Schematic diagram of overall fabrication of mot
Please cite this article as: J. Cho, et al., Nanopatterned yttrium aluminum galight emitting diodes, Thin Solid Films (2014), http://dx.doi.org/10.1016/j
contact with the substrate. Therefore, this PDMS mold is suitable forthe direct HSQ printing. In this study, the PDMS mold was fabricatedby mixing Sylgard 184A (Dow Corning) as the precursor and Sylgard184B (Dow Corning) as the cross-linker at a ratio of 10:1. Then, thePDMS mold was diluted with toluene at a ratio of 6:4 in order to en-hance the formability of the PDMS mold. Then, the diluted mixturewas spin-coated onto a Ni master template at 3000 rpm for 30 s andcured at 80 °C for 30 min. After the first curing process, a moderateamount of the non-diluted mixture was poured onto the PDMS/Nimold and cured at 80 °C for 1 h. Thus, the PDMS mold with the moth-eye patterns could be fabricated [13,14].
Fig. 2(d)–(e) shows that themoth-eye patternwas fabricated on theYAG phosphor-incorporated film by the direct HSQ printing process. AFox-16 solution, containing 22 wt.% HSQ in methyl isobutyl ketone sol-vent, was used as the HSQ solution for the formation of the HSQ layer.The HSQ solution was spin-coated onto the PDMS mold. During spin-coating, the thickness of the HSQ layer could be modulated by control-ling the spin speed. In this study, spin-coating of the HSQ solution wascarried out at 3000 rpm for 30 s. After spin-coating, the PDMS moldwith the HSQ layer was placed on the YAG phosphor-incorporated
h-eye patterned YAG phosphor-incorporated film.
rnet phosphor incorporated film for high-brightness GaN-basedwhite.tsf.2014.03.065
Fig. 3. (a) SEM image of theHSQmoth-eye patterns on the YAGphosphor-incorporatedfilm, and (b) itsmagnification, (c) AFM image of theHSQmoth-eye patterns on the YAGphosphor-incorporated film, and (d) its height profile (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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film, and the direct printing process was carried out at 5 × 105 Pa for3 min. Finally, the PDMS mold was detached from the YAG film, asshown in Fig. 2(f).
3. Result and discussion
After direct SOG printing process, HSQmoth-eye patterns formed onthe YAG phosphor-incorporated film was observed and measured byfield emission scanning electron microscopy (FE-SEM, Horiba EX-200)and atomic force microscopy (AFM, Park Scientific InstrumentXE-100) system. As shown in Fig. 3, the nano-scale HSQmoth-eye pat-terns were uniformly formed on the YAG phosphor-incorporated filmafter the direct HSQ printing process. As shown in Fig. 3(a) and (b),transferred HSQ moth-eye patterns consisted of hexagonal cone arrayswith a diameter of 200 nm and a height of 210 nm. In order to achievean anti-reflective effect using a moth-eye structure, it is necessary foreach nanostructure in the cone array to have a tapered profile and
Fig. 4. (a) Transmittance of YAG film with and without moth-eye structures on the YAG pho
Please cite this article as: J. Cho, et al., Nanopatterned yttrium aluminum galight emitting diodes, Thin Solid Films (2014), http://dx.doi.org/10.1016/j
high aspect ratio. The direct HSQ printing technique enables the easyfabrication of 3-D nanostructures such as moth-eye structures.
Fig. 4 shows the transmittance graphs of the YAG phosphor-incorporated films with and without the moth-eye nanostructure layer.A UV–vis spectrometer (JASCO, V-670) was used to measure the trans-mission at room temperature. The transmittance wasmeasured with re-spect to normal incident light in the visible wavelength, in order toanalyze the anti-reflection effects of the HSQ moth-eye patterns on theYAG phosphor-incorporated film and the influence of the subsequentprocesses on their anti-reflection properties. As shown in Fig. 4, the pres-ence of the HSQ moth-eye patterns enhanced the transmittance of theYAG phosphor-incorporated film at wavelengths above 350 nm to upto 3–4% higher than that of un-patterned YAG phosphor-incorporatedfilm. These results, confirm that the transmittance of the YAGphosphor-incorporated film increased because of the presence of thetransferred HSQ moth-eye patterns on the YAG phosphor-incorporatedfilm. Antireflection effect of moth-eye formed on the YAG phosphor-incorporated film works because the moth-eye patterns are smaller
sphor-incorporated film, and (b) schematic diagram of measurement of transmittance.
rnet phosphor incorporated film for high-brightness GaN-basedwhite.tsf.2014.03.065
Fig. 5. (a) EL intensity of the GaN-based blue LED (inset shows the image of GaN-based blue LED during emission), (b) EL intensity of GaN-based white LED with YAG phosphor-incorporated film with and without moth-eye pattern (inset shows the image of white LED during emission). (c) Optical power according to the injection current of GaN-based whiteLED with YAG phosphor-incorporated film with and without moth-eye pattern after packaging process. (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)
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than the wavelength of visible light, so the light sees the surface as hav-ing a continuous refractive index gradient between the air and themedium, which decreases total internal reflection by effectivelyremoving the YAG phosphor-incorporated film and the air.
Fig. 5(a) shows that the electroluminescence (EL) intensity of GaN-based blue LEDwithout the YAG phosphor-incorporated film. The emis-sion peak of EL intensity of the GaN-based LEDwas about 460 nm. Afteradding the YAG film, to confirm the effect of the moth-eye pattern onlight extraction, we measured the EL intensity of the GaN-based whiteLED devices with YAG phosphor-incorporated film, as a function of theinjection current, as shown in Fig. 5(b). In the case of the GaN-basedLED and YAG phosphor-incorporated film are clearly resolved at460 nm and at around 530 nm, respectively. Compared to the LED devicewith unpatterned YAG phosphor-incorporated film, the EL intensityof the LED device with the moth-eye patterned YAG phosphor-incorporated film was higher by up to 14.9%. After packaging process,the optical power according to injection current of the GaN based whiteLED with the YAG film was measured. In Fig. 5(c), the optical power ofthe LED with the patterned YAG film was enhanced up to 13% than thatof the LED with the unpatterned YAG film. It was caused by reduction oftotal internal reflection due to the moth-eye patterned formed on theYAG film, because they can reduce the total internal reflection of lightby a gradual change in the average refractive index of the media [8–10].
4. Conclusion
In this study, HSQ-based moth-eye patterns with a diameter of220 nm were formed on a YAG phosphor-incorporated film in orderto enhance the light extraction efficiency of GaN-based white LEDs.The direct HSQ printing technique was used to form the moth-eye pat-terns on the YAG phosphor-incorporated film. This technique has ad-vantages such as high throughput, large-area patterning, and low costfor fabricating nanoscale structures. The light output power of a whiteLED with a moth-eye-patterned YAG film was up to 14.9% higher thanthat of a white LED with an unpatterned YAG film. Thus, we fabricateda high-brightness GaN-based white LED by a cost-effective and mass-production-compatible direct HSQ printing process.
Please cite this article as: J. Cho, et al., Nanopatterned yttrium aluminum galight emitting diodes, Thin Solid Films (2014), http://dx.doi.org/10.1016/j
Acknowledgments
This research was supported by the Nano Material Technology De-velopment Program through the National Research Foundation ofKorea (NRF), funded by the Korean Ministry of Education, Science andTechnology (2012M3A7B4035323) and LG Innotek—Korea UniversityNano-Photonics Program.
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