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Nano Energy (2014) 8, 78–83
http://dx.doi.org/12211-2855/& 2014 E
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RAPID COMMUNICATION
Enhanced light extraction of light-emittingdiodes via nano-honeycomb photonic crystals
Po-Han Fua, Guan-Jhong Lina, Hsin-Ping Wanga,Kun-Yu Laib,n, Jr-Hau Hea,c,nn
aInstitute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan, ROCbDepartment of Optics and Photonics, National Central University, Chung-Li 32001, Taiwan, ROCcDepartment of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC
Received 1 April 2014; received in revised form 10 May 2014; accepted 12 May 2014Available online 9 June 2014
KEYWORDSLight emitting diodes;Multiple quantumwells;Photonic crystals;Light extraction;Nanospherelithography
0.1016/j.nanoen.2lsevier Ltd. All rig
thor.uthor at: Institutewan University, Ta: [email protected] (J.-H. He).
AbstractWe present an effective light extraction scheme for GaN-based multi-quantum-well (MQW) lightemitting diodes with periodic SiO2 nano-honeycomb arrays fabricated by natural lithographyand reactive ion etching. The nano-honeycombs significantly boost light output by providingadditional light extraction channels, not only guiding the internal modes into air but alsoalleviating the sever droop effect at high input power. At the driving current of 400 mA, lightoutput power through the nano-honeycombs is 77.8% higher than that of the bare device.In addition, the output power is particularly enhanced at the diffraction angle around 651,which is attributed to the intensive first order diffraction on the honeycombs. Simulationsbased on finite-difference time-domain method are also carried out to reveal the fielddistribution across device interfaces.& 2014 Elsevier Ltd. All rights reserved.
Introduction
III-Nitride light-emitting diodes (LEDs) have drawn intensiveresearch efforts due to the advantages of high brightness, low
014.05.006hts reserved.
of Photonics and Optoelec-ipei 10617, Taiwan, ROC.(K.-Y. Lai),
power consumption, broad spectral range, and superior re-liability as compared with conventional lightings [1]. Althoughthe efficiencies have been significantly improved by advancedgrowth techniques, many challenges still remain to be tackled.For instance, light extraction efficiency is greatly hindered bythe limited photon escape cone from the device, which mainlycomes from the abrupt refractive index transition from GaN toair [2]. The undesired total internal reflections within thedevice not only put a limitation of light output but contributeto heat generation. To address this issue, a number of lightextraction schemes have been developed, such as surface
79Nano-honeycomb photonic crystals
roughening [3], microlen arrays [4], graded-index materials[5], and piezo-phototronic effect [6,7]. However, most ofthe strategies reported to date typically result in randomgeometry, forcing the light to escape the LED at arbitraryangles. Although the optical output power of the LEDs can beeffectively enhanced, the behavior of the emission lightcannot be well controlled.
Recently a variety of the photon management schemeshave been carried to enhance the efficiency of the optoelec-tronic devices, such as photodetectors [8,9], solar cells[10,11], and LEDs [12]. To harness the directionality of lightextraction, photonic crystal is considered as a promisingapproach for LEDs. According to coupling theory, the guidedmodes within the LEDs can be radiated to the air with newwave vectors by gaining an additional lattice vector providedby the periodically structured surface, which is called dif-fracted modes [13]. The enhanced light extraction efficiencycan be attained via the diffracted modes leaking to the airwith specific angles. The diffraction behavior of the extractedlight is related to wavelength, propagating direction, andpitch of the photonic crystal. To maximize extraction effi-ciency, these correlated parameters should be tuned such thatthe guided modes in all directions are coupled to the air.Accordingly, two-dimensional (2-D) photonic crystal nanostruc-tures are preferred to one-dimensional (1-D) gratings consid-ering the additional diffraction channels in 2-D cases [13].
2-D photonic crystals can be fabricated by various techni-ques, such as electron beam lithography [14], nano-imprintlithography [15], anodic aluminum oxide templating [16], andelectrochemical etching technique [17]. However, these tech-niques either require expensive facilities or only apply to verylimited areas. Periodic self-assembly polystyrene (PS) nano-sphere lithography is considered as an alternative due to itssimplicity, low-temperature processes and the capability toreach large-area uniformity [18,19]. Although antireflectivenanorods fabricated by PS nanosphere lithography has effec-tively enhanced light extraction for LEDs [20], it has beenfound that the hole-like nanostructure supports even largerdensity of waveguide modes and exhibits superior mechanicalrobustness as compared to the rod-like nanostructure with thesame filling factor [21,22]. Despite these promising traits, thefabrication of hole-like nanostructures employing PS nano-sphere lithography is scarcely reported to date.
In this letter, periodic honeycomb-like nanohole arrays arefabricated on GaN-based MQW LEDs by PS nanosphere litho-graphy and reactive ion etching (RIE) techniques. SiO2 isemployed in light of its low absorption coefficient in visiblewavelengths and the intermediate refractive index betweenair and GaN, which are expected to improve the opticaltransmission across the device/air interface. External quan-tum efficiency of the LED without the nanostructured surfaceonly hovers around 31%, which is mainly attributed to thesevere total internal reflection trapping photons within thedevice. With the nano-honeycomb, optical power of the LED issignificantly increased by 77.8%. In particular, the measuredradiation pattern shows that the light extraction with the SiO2
nano-honeycombs is mostly enhanced at the diffraction anglearound 651 due to the strong interaction of the guided modeswith the 2-D periodic nanostructure. The enhanced lightextraction is also supported by the theoretical analysis basedon finite-difference time domain (FDTD) method. Such a nano-honeycomb structure would offer an attractive solution to
increase the efficiency of a variety of optical devices. Thepresented concept and the manufacturing technique of light-extraction nano-honeycomb photonic structures represent aviable, promising path toward high-efficiency LEDs and shouldbenefit many other types of optoelectronic devices.
Experimental section
The nitride LEDs were grown by metal-organic chemicalvapor deposition on c-plane sapphire substrates. Detailedinformation of layer structures and fabrication steps isdescribed in Fabrication and methods. The LED structureexhibits a peak wavelength at 520 nm in electrolumines-cence (EL) spectrum. SiO2 nano-honeycombs were fabri-cated by PS nanosphere lithography and RIE techniques.As denoted in Figure 1, the PS nanospheres are self-assembledwith hexagonal closed-packed arrangement in order tomaintain the lowest surface energy. To create the spacefor the subsequent RIE process, O2 plasma was used toshrink the nanospheres. During the RIE process, PS nano-spheres are treated as the etching masks, and the shape/distribution of the shrunk PS nanospheres play a vital role inthe formation of honeycomb-like nanohole arrays. With theshrunk nanospheres, CHF3 gas was flowed into the chamberfor SiO2 etching. As the etching rate of PS is much lowerthan that of SiO2, PS nanospheres protect the SiO2 filmunderneath from being etched. After the SiO2 honeycombswere formed, high-concentration O2 plasma was flowed toremove the remaining nanospheres. The effective area ofthe monolayer nanospheres employed in this study is over100 um2, as presented by the secondary electron micro-scopy (SEM) image in Supporting Information.
Figure 2 shows top-view and cross-sectional SEM images ofthe SiO2 nano-honeycombs. The nano-honeycombs are�350 nm in height and �450 nm in periodicity. The fillingfraction of the nano-honeycombs, defined as the area ratio ofSiO2 to the entire surface, is 45%. This value is deliberatelyattained to follow the ideal one reported by Wiesmann et al.,who found that the nano-honeycomb with the filling fractionof �45% is expected to maximize the diffraction efficiency ofthe photonic crystal for all in-plane k-vectors [23].
Result and discussion
Figure 3a shows the current-voltage (I–V) curves of the LEDswith and without SiO2 nano-honeycombs. In the figure, the twocurves nearly overlap each other with the same forward voltage(Vf) of 2.81 V at 20 mA, and the reverse currents of the devicewith the nano-honeycombs are less than 10�7 A, showing thatthe nanostructured surface does not severely deteriorate theelectrical properties. EL spectra of the devices with andwithout nano-honeycombs can be found in SupportingInformation. Covering the LED with nano-honeycombs slightlyshifts the emission wavelength from 520 nm to 515 nm. Thelimited difference in peak wavelength caused by the photoniccrystal structure is due to the particularly selected period ofthe nano-honeycombs, which is designed to enhance the lightextraction in the green spectrum. Figure 3b presents the lightoutput intensity-current (L–I) characteristics of the LEDs withand without SiO2 nano-honeycombs under the driving currentfrom 10 to 400 mA. The luminescence intensities of the LEDs
Figure 2 Top-view and cross-sectional SEM images of the SiO2
nano-honeycomb photonic crystals.
Figure 1 Fabrication processes of the SiO2 nano-honeycomb photonic crystals.
P.-H. Fu et al.80
with the nano-honeycombs are enhanced over a wide range ofcurrent injections. Compared to the bare device, the powerenhancement reaches 77.8% at the driving current of 400 mA.The improved performances can be attributed to the extralight extraction of the modes originally guided within thedevice and the reduced interface reflections, both of whichcontribute to the significantly alleviated efficiency droop athigh injection currents. In the figure, the output intensity ofthe bare LED starts to saturate at the driving currents above150 mA whereas the intensity of the nano-honeycomb LED stillexhibits the increasing tendency. The saturated light output atincreased driving currents are commonly observed with nitrideLEDs and referred as the droop effect [24], which can beattributed to various mechanisms, such as poor injectionefficiency [25], Auger non-radiative recombination [26], etc.Here, the alleviated droop seen with the nano-honeycomb LEDis believed to be caused by the improved injection efficiencydue to the reduced joule heating within the device as the
extraction of optical output power is enhanced [27]. On thebare LEDs, a great portion of generated light reflects back atthe interfaces and is eventually re-absorbed by the semicon-ductor and transferred to heat, which leads to increasedamount of carriers captured by the lattice defects (non-radiative recombination centers) in the MQW, and thussacrificed internal quantum efficiency [28].
The radiation patterns of the devices with and without SiO2
honeycombs under the current injection of 100 mA are pre-sented in Figure 4a. One can observe that the LED with thehoneycombs exhibits superior light extraction as compared tothe bare device. In addition, the viewing angle increases from1261 to 1401, which is attributed to the combined effect of theintensive first-order diffraction and the surface scatteringcaused by the nano-honeycombs. To gain insight into thediffraction behaviors induced by the nano-honeycombs, wedefine the enhancement factors of light output as (IH–IB)/IB,where IH and IB are the output intensities of the LEDs with andwithout nano-honeycombs, respectively. As shown inFigure 4b, an obvious lobe at �651 indicates the stronginteraction of the guided modes with the photonic crystals.The observed lobe can be explained by the coupling theory ofphotonic crystals [29]. In the LED with nano-honeycombs, theguided modes within the device are radiated to the air bygaining an extra lattice vector mG
,provided by the periodic
nanostructure, where m is an integer and jG,j ¼ 2πa is the
reciprocal lattice vector of the nano-honeycombs with thepitch of a. The diffraction angle (θm) of the modes radiated tothe air can be determined by the relationship:
θm ¼ sin �1 k,rad
k,0
ð1Þ
where k,rad and k
,0 are respectively the in-plane and the total
wave vectors in the air. The values of k,rad and k
,0 can be
obtained through the equations:
k,
0 ¼2πλ
ð2Þ
Figure 3 (a) Current–voltage (I–V) characteristics of the LEDs with bare surface and the nano-honeycombs. (b) Light-outputintensities as a function of injection current of the two LEDs.
Figure 4 (a) Radiation patterns of the LEDs with and withoutSiO2 nano-honeycombs under the injection current of 100 mA.(b) Light-output enhancement factors of the nano-honeycombLED, where IH and IB are the output intensities with and withoutnano-honeycombs, respectively.
81Nano-honeycomb photonic crystals
k,
rad ¼ jk,jj þmG,j ð3Þ
with λ the emitting wavelength in the air and k,jj the in-plane
wave vector originally guided within the device. In general,the diffraction behavior of each optical wave should bedetermined by taking into account the spontaneously emittedphotons propagating in all directions [16]. For our case, sincethe transmission energy of the high-order diffraction modes isnegligibly small [30], we only consider the first order diffrac-tion of the fundamental mode along the in-plane direction.With λ=520 nm, a=450 nm, m=�1, and the refractive indexn_ITO=2.05, the following vectors are attained: k
,0=
12.083 μm�1, k,jj=24.770 μm�1, G
,=13.962 μm�1, and k
,rad=
10.808 μm�1, rendering θm ¼ 63.441, which is close to theangle of the radiation lobe observed in Figure 4b. Eqs. (1)–(3)suggest that the diffraction angle from the nano-honeycombscan be harnessed via the period of the nanoholes. For
example, the directionality of nano-honeycomb LEDs can beenhanced with a smaller value of a, which leads to adecreased diffraction angle (θm).
For a further investigation of the light propagationbehaviors across the interfaces, the distributions of electro-magnetic fields are simulated by FDTD analysis. The excita-tion source, with the same width (10 μm) as that of thesimulated device structure, is placed in InGaN MQW region.The wavelength for all simulations is selected to be 520 nm.Details of the simulated structures and the optical para-meters are described in Figure S1 in Supplementary data.Time-averaged TE-polarized electric field distributions, |Ez|,of the LEDs without and with SiO2 nano-honeycombs areshown in Figure 5a and b, respectively. All of the calculatedvalues are normalized to the ones of the excitation source.From the figures, one can see that the emitted fieldintensities are enhanced with the SiO2 nano-honeycombs.Aside from the light extraction due to the smoothed indextransition from the ITO layer to air, the SiO2 nano-honeycombs also broaden the field distribution by increasinglight scattering on the surface. In the region of SiO2 nano-honeycombs, one can see the strong field intensity betweennano-honeycombs caused by the constructive wave inter-ferences, indicating that the nano-honeycombs act aseffective scattering centers. Figure 5c presents the normal-ized optical power as a function of time, which is obtainedby integrating the power intensities within the time monitorregion indicated in Figure 5a and b. The steady-state powervalues for the devices without and with SiO2 nano-honeycombs are respectively 0.627 and 0.731, which arein line with the experimental observations.
Conclusion
The nano-honeycombs markedly enhance light extraction ofthe LED by acting as an effective scattering center andproviding an additional momentum in the reciprocal space.The 77.8% enhancement of light output power is achievedwith a particular diffraction angle at around 651, whichagrees with the analysis based on coupling theory. Theconcepts presented in this study should benefit the photon-management development for a wide variety of optoelec-tronic devices.
Figure 5 Time-averaged and normalized TE electric field distribution, |Ey|, simulated by FDTD analysis with different surfaceconditions: (a) bare surface (b) SiO2 nano-honeycombs. (c) Normalized optical power as a function of time for the LEDs withdifferent surface conditions.
P.-H. Fu et al.82
Fabrication and methods
Fabrication of the LEDsThe MQWs consist of nine periods of undoped In0.3Ga0.7N(3 nm)/GaN(17 nm) layers, sandwiched by 2.5-mm n-typeand 0.2-mm p-type GaN layers. In device fabrication, ITOwas deposited by electron beam evaporation on p-GaN toform transparent Ohmic contacts. The 1� 1 mm2 diodemesas were then defined by chlorine-based plasma etching.The metal contact scheme consists of interdigitated Ti/Al/Ni/Au grids deposited on the ITO and the n-GaN.
Preparation of SiO2 nano-honeycombsThe RIE process was performed using 50-sccm O2 (coilpower: 50 W; chamber pressure: 5 Pa) as the etching gasfor shrinking the PS nanospheres. 30-sccm CHF3 (coil power:90 W; chamber pressure: 1.3 Pa) was adopted to etch SiO2
and realize the photonic crystal structure.
SEMThe SEM images are recorded by JEOL JSM-7001F with theacceleration voltage of 15 kV and the probing current of1 μA.
I–V curveThe current–voltage curves were measured by a Keithley4200 source meter with the DC bias sweeping from to �3 to3 V.
L–I curveLight output intensities of the LEDs were collected by OceanOptics USB2000+ Spectrometer with the input currentsweeping from 10 mA to 400 mA supplied by the Keithley4200 source meter.
SimulationThe FDTD simulation was performed with the FullWAVE toolintegrated in the commercial software package Rsoft CAD.Detailed descriptions on the simulation procedures can befound in Supporting Information.
Acknowledgments
The research was supported in part by National ScienceCouncil Taiwan (102-2221-E-008-074, 102-2628-M-002-006-MY3 and 101-2221-E-002-115-MY2), National Taiwan Univer-sity (103R7823), the Aim for the Top University Project ofNational Central University (103G903-2), and Energy Tech-nology Program for Academia, Bureau of Energy, Ministry ofEconomic Affairs (102-E0606).
Appendix A. Supporting information
Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.nanoen.2014.05.006.
References
[1] D.A. Steigerwald, J.C. Bhat, D. Collins, R.M. Fletcher,M.O. Holcomb, M.J. Ludowise, P.S. Martin, S.L. Rudaz, IEEEJ. Sel. Top. Quant. 8 (2002) 310–320.
[2] E.F. Schubert, Light Emitting Diodes, second ed., CambridgeUniversity Press, Cambridge, England, 2006.
[3] T. Fujii, Y. Gao, R. Sharma, E.L. Hu, S.P. DenBaars,S. Nakamura, Appl. Phys. Lett. 84 (2004) 855.
[4] Y.-K. Ee, R.A. Arif, N. Tansu, P. Kumnorkaew, J.F. Gilchrist,Appl. Phys. Lett. 91 (2007) 221107.
[5] J.K. Kim, A.N. Noemaun, F.W. Mont, D. Meyaard, E.F. Schubert,D.J. Poxson, H. Kim, C. Sone, Y. Park, Appl. Phys. Lett. 93(2008) 221111.
[6] Q. Yang, W. Wang, S. Xu, Z.L. Wang, Nano Lett. 11 (2011)4012–4017.
[7] Q. Yang, Y. Liu, C. Pan, J. Chen, X. Wen, Z.L. Wang, Nano Lett.13 (2013) 607–613.
[8] D.S. Tsai, C.A. Lin, W.C. Lien, H.C. Chang, Y.L. Wang, J.H. He,ACS Nano 5 (2011) 7748–7753.
[9] C.Y. Hsu, D.H. Lien, S.Y. Lu, C.Y. Chen, C.F. Kang, Y.L. Chueh,W.K. Hsu, J.H. He, ACS Nano 6 (2012) 6687–6692.
[10] L.K. Yeh, K.Y. Lai, G.J. Lin, P.H. Fu, H.C. Chang, C.A. Lin,J.H. He, Adv. Energy Mater. 1 (2011) 506–510.
[11] H.P. Wang, T.Y. Lin, C.W. Hsu, M.L. Tsai, C.H. Huang, W.R. Wei,M.Y. Huang, Y.J. Chien, P.C. Yang, C.W. Liu, L.J. Chou, J.H. He,ACS Nano 7 (2013) 9325–9335.
83Nano-honeycomb photonic crystals
[12] Y.H. Hsiao, C.Y. Chen, L.C. Huang, G.J. Lin, D.H. Lien,J.J. Huang, J.H. He, Nanoscale 6 (2014) 2624–2628.
[13] A. David, H. Benisty, C. Weisbuch, J. Disp. Technol. 3 (2007)133–148.
[14] S.A. Boden, D.M. Bagnall, Appl. Phys. Lett. 93 (2008) 133108.[15] Z.N. Yu, H. Gao, W. Wu, H.X. Ge, S.Y. Chou, J. Vac. Sci.
Technol. B 21 (2003) 2874–2877.[16] Z. Fan, J.C. Ho, Int. J. Nanoparticle 4 (2011) 164–183.[17] I. Montanari, A.F. Nogueira, J. Nelson, J.R. Durrant, C. Winder,
M.A. Loi, N.S. Sariciftci, C. Brabec, Appl. Phys. Lett. 81 (2002)3001–3003.
[18] P.H. Fu, G.J. Lin, C.H. Ho, C.A. Lin, C.F. Kang, Y.L. Lai,K.Y. Lai, J.H. He, Appl. Phys. Lett. 100 (2012) 013105.
[19] Y.R. Lin, K.Y. Lai, H.P. Wang, J.H. He, Nanoscale 2 (2010)2765–2768.
[20] J.I. Sim, B.G. Lee, J.W. Yang, H.-d. Yoon, T.G. Kim, Jpn.J. Appl. Phys. 50 (2011) 102101.
[21] S.E. Han, G. Chen, Nano Lett. 10 (2010) 1012–1015.[22] K.Q. Peng, X. Wang, L. Li, X.L. Wu, S.T. Lee, J. Am. Chem. Soc.
132 (2010) 6872–6873.[23] C. Wiesmann, K. Bergenek, N. Linder, U.T. Schwarz, Laser
Photon. Rev. 3 (2009) 262–286.[24] D.S. Meyaard, Q. Shan, J. Cho, E. Fred Schubert, S.-H. Han,
M.-H. Kim, C. Sone, S. Jae Oh, J.K. Kim, Appl. Phys. Lett. 100(2012) 081106.
[25] I.V. Rozhansky, D.A. Zakheim, Phys. Status Solidi A 204 (2007)227–230.
[26] Y.C. Shen, G.O. Mueller, S. Watanabe, N.F. Gardner,A. Munkholm, M.R. Krames, Appl. Phys. Lett. 91 (2007)141101–141103.
[27] D.-S. Wuu, S.-C. Hsu, S.-H. Huang, C.-C. Wu, C.-E. Lee,R.-H. Horng, Jpn. J. Appl. Phys. 43 (2004) 5239–5242.
[28] M.-H. Kim, M.F. Schubert, Q. Dai, J.K. Kim, E.F. Schubert,J. Piprek, Y. Park, Appl. Phys. Lett. 91 (2007) 183507.
[29] J.J. Wierer, A. David, M.M. Megens, Nat. Photon. 3 (2009)163–169.
[30] R.G. Hunsperger, Integrated Optics, sixth ed., Springer,New York, 2009.
Po-Han Fu received his B.S. degree in 2010and M.S degree in 2012 from the NationalTaiwan University. He is currently a Ph.D.student at the Institute of Photonics andOptoelectronics in National Taiwan Univer-sity. His current research focuses on inte-grated silicon photonics.
Guan-Jhong Lin received his M.S. degree inPhotonics and Optoelectronics from theNational Taiwan University in 2012. Hisresearch focuses on the efficiency enhance-ment of nitride- and silicon-based optoelec-tronic devices.
Hsin-Ping Wang received her M.S. degree(2011) from the Graduate Institute ofPhotonics and Optoelectronics at theNational Taiwan University, Taipei, Taiwan.She is now a Ph.D. student in Dr. Jr-Hau He'sgroup and currently works in UCSD and UCBerkeley as a visiting scholar. Her researchinterests include theoretical and experi-mental research on optical properties ana-lysis of nanostructure, varied types of solar
cells (including conventional Si solar cell, thin-film solar cell, a-Sisolar cell, and Si heterojunction solar cell), and solar water splittingcells.
Kun-Yu Lai is currently an assistant profes-sor in the Department of Optics and Photo-nics at the National Central University(NCU) in Taiwan. Dr. Lai received his Ph.D.degree in Electrical Engineering from theNorth Carolina State University in 2009.After working as the postdoctoral fellow inthe Graduate Institute of Photonics andOptoelectronics at National Taiwan Univer-sity from 2009 to 2011, he joined the
faculty in the Department of Optics and Photonics at NCU, wherehe specialized in the growth/fabrication of novel III-nitrides optoe-lectronic devices and the optical properties of low-dimensionalstructures, such as quantum wells and nanowires.
Jr-Hau He received his B.S. and Ph.D.degrees from the National Tsing Hua Uni-versity, Hsinchu, Taiwan, in 1999 and 2005,respectively. He is currently an AssociateProfessor at the Institute of Photonics andOptoelectronics and the Department ofElectrical Engineering, National Taiwan Uni-versity, Taipei, Taiwan. He is involved in thedesign of new nanostructured architecturesfor nanophotonics and the next generation
of nanodevices, including photovoltaics, and resistive memory.Prof. He is a recipient of the Outstanding Young Electrical EngineerAward from the Chinese Institute of Electrical Engineering (2013),the Outstanding Youth Award of the Taiwan Association for Coatingand Thin Film Technology (2012), the Youth Optical EngineeringMedal of the Taiwan Photonics Society (2011), and has wonnumerous other awards and honors with his students in professionalsocieties and conferences internationally.
