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Light-Emitting Diodes www.pss-a.com

ORIGINAL PAPER

GaN based UV-LEDs with Ni/Au Nanomeshes asTransparent p-type Electrodes

Moheb Sheikhi, Houqiang Xu, Jie’an Jiang, Sudong Wu, Xi Yang, Zhenhai Yang,Mingdun Liao, Wei Guo,* and Jichun Ye*

Ni/Au nanomesh (NM) is utilized as the p-type contact in the ultraviolet(UV)-LEDs for improving the light extraction efficiency. The substitution ofconventional planar Ni/Au p-type electrode with highly ordered, large scalableNi/Au NMs results in enhanced light transmission in the p-type contactregion of the UV-LEDs. Transmission over 75% is demonstrated for Ni/Au-NM thin film coated sapphire substrate, whereas light is totally blocked byconventional planar Ni/Au thin film. Linearized I–V characteristic betweenp-GaN and Ni/Au NMs is achieved by appropriate thermal annealing of thecontact, and specific contact resistivity of 0.90Ω cm2 is reported. UV-LEDincorporated with Ni/Au NMs exhibits higher output power than that withplanar contact due to enhanced light transmission through the metal networkstructure. Finally, simulation results suggest good agreement with experimen-tal data, illustrating that Ni/Au NMs can serve as a potential candidate inthe development of high efficiency UV optoelectronic devices.

1. Introduction

AlGaN based UV-LEDs have been intensively studied in the pastdecade due to their wide applications in UV curing, airsterilization, and waste water treatment.[1–3] However, theexternal quantum efficiency (EQE) of UV-LEDs is still relativelylow compared to the InGaN-based blue LED counterpart. One ofthe main reasons is low light extraction efficiency (LEE) due tostrong UV absorption by the p-type electrode and p-GaN contactlayer, which dramatically limits the output power of UV-LEDs.

Progress has been made to mitigate this problem, forexample, the usage of indium tin oxide (ITO) as a transparentconductive electrode (TCE) for both carrier distribution and lightextraction thanks to its high electrical conductivity and opticaltransparency above 365 nm.[4,5] ITO has been deposited on p-

M. Sheikhi, H. Xu, J. Jiang, Dr. S. Wu, Dr. X. Yang, Z. Yang, M. Liao,Prof. W. Guo, Prof. J. YeNingbo Institute of Materials Technology and EngineeringChinese Academy of SciencesNingbo, Zhejiang 315201, ChinaE-mail: guowei@nimte.ac.cn; jichun.ye@nimte.ac.cn

M. Sheikhi, H. Xu, J. Jiang, Prof. J. YeUniversity of Chinese Academy of SciencesBeijing 100049, China

J. JiangShanghaiTech UniversityPudong, Shanghai 201210, China

DOI: 10.1002/pssa.201800684

Phys. Status Solidi A 2018, 1800684 © 21800684 (1 of 8)

GaN of the LED structures acting as TCE.Even though ITO on p-GaN was electricallypoor and non-ohmic, but with the intro-duction of composite structure such asoxidized NiAu/ ITO or Ag/ITO, electricalproperty and optical transparency of theLED device were improved.[6–8] However,the cost of ITO is relatively high due toexsitence of rare-earth element indium. Asa good replacement of ITO, the utilizationof indium-free TCE such as Al-doped zincoxide (AZO)[9] or Ga-doped zinc oxide(GZO)[10] have been investigated, especiallywhen Mg was doped into the system due tothe wider tunability in bandgap[11] andbetter performance in LEDs compared withthat of conventional Ni/Au contact.[12,13]

For instance, Jang et al. investigated Ga-doped MgZnO (GMZO) in the applicationof near UV-LED. Transmission of 84% wasachieved in UVA region. The external

quantum efficiency of the LED with GMZO as TCL was 7.7and 2.7% higher than the LEDs with Au/NiOx and ITO TCL,respectively, indicating that GMZO is promising in replacingtraditional ITO.[14] Borysiewicz et al. discussed the application ofanother quaternary compound Al doped MgZnO (AMZO) as aTCL in UV-LEDs. By replacing the Ni/Au contact with AMZO,250% enhanced in irradiated power was reported in a 385 nmemission LED.[15] Despite of great progress made in the area ofoxide-based TCE, they all face technical challenges like highturn-on voltages due to poor contact properties, and thebrittleness nature of the above oxide-based TCE makes themnot suitable for flexible optoelectronic devices.[16]

Aiming for flexible and broad-band light transmissionproperty, alternative concepts in realizing transparency electrodeis the usage of ultrathin metal contact or metal nanowires suchas Ag, Au, or Cu.[17–21] For example, Yang et al. demonstrated9 nm ultra-thin Ag thin film as TCE with sheet resistance7.39 ohm sq�1, and transmission exceeding 90% at wavelengthof 500 nm.[20] Park et al. reported the AlGaN based near-UVLEDs with Ag nanowires with enhanced light output powercompared to ITO based devices.[22] But the instability to scratchesand resistance to thermal annealing of both ultrathin metalcontact or nanowire based electrode remain a significant hurdleand currently precludes the large application in UV-LEDs. Fornanowire electrodes especially, the challenge also lie in the highcontact resistance at the junctions of nanowires, which are easilyoxidized. And in the case of AlGaN based UV-LEDs, both Ag and

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Cu cannot form good ohmic contact with p-GaN, placing metalnanowires as p-contact impractical.

Recently, metallic thin films with nanohole structures havedrawn tremendous interests among researchers due to theirsuperior optical transmission and sheet resistance, which arecomparable to ITO films.[23–25] Au nanomeshes (NMs) withtransmission larger than 80%and sheet resistance of 20 ohmsq�1

has been successfully demonstrated.[26] Ag NM structures withtransmittance of 88% and sheet resistance of 10 ohmsq�1[27] havealso been reported. Metal NMs have been explored in theapplication of optoelectronic devices such as LEDs. Li et al.reported the utilization of Pt(Au) nanomesh on top of p-GaNepitaxial layer of the LED structure.[28] Transmission and currentspreading length both increase compared to Pt layer due to theformation of stable NMs. A balance between optical transparencyand electrical behavior was also reported. Furthermore, Min et al.also reported a combination of ITO and metal meshes acting asTCLintheNUV-LED.Lowsheet resistance (�4), improvedturn-onvoltage (3.65V) as well as 40% enhanced light output powercompared to conventional 150nm ITO-incorporated LED.[29] As atrade-offbetweentheopticalandelectricalpropertiesofmetalNMsacting as TCE, thickness and structural morphologies of the NMs

Figure 1. Schematic process flow in fabricating Ni/Au NMs on top of p-Gananohole diameter of 2.05 μm (b) and 1.74 μm (c).

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needs to be carefully designed. But unfortunately, this not beenaddressed so far. In addition, although the incorporation of metalNMs can enhance light transmission, but concern onwhether thistechnique can improve the light output power of UV-emitters stillremains, which requires further in-depth investigations.

As p-type contacts in AlGaN based UV-LEDs, especially, thechoices are very limited. Traditionally, Nickel serves as a commonp-GaN contact due to its appropriate fermi level of 5.04 eVcompared to Mg-doped GaN.[30,31] Ni/Au bilayer metal stack hasusually been deposited on top of p-GaN as ohmic contact forreducedcontact resistivity and improvedsurfaceuniformity.[32] Yetunfortunately, Ni is neither aUV transparentmetal nor a goodUVreflector. Therefore, small area contact is necessary, whichinevitably leading to nonuniformities in current flow and evenelectrical breakdown. This poses a challenge to many practicalapplications in UV emitters and necessitates the development ofhigh transparency and low-resistance p-type contact. In order torealize high-efficiency UV-LEDs with both superior electricalconductivity andphoton extraction capability, Ni/Au inNMdesignstands out as the best candidate.

In the present study, the electrical and optical properties of anUV-LED with Ni/Au NMs of 2 μm periodicity deposited on top of

N and UV-LED structures (a); Top view SEM images of Ni/Au NMs with

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim2 of 8)

Figure 2. Schematic image illustrating I–V measurement setup (a); I–V curves between as-fabricated planar Ni/Au contact and Ni/Au NMs on p-GaN(b); Influence of thermal annealing on the I–V characteristic of Ni/Au NM-1 (c) and NM-2 (d).

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the Mg-doped GaN top layer were investigated. The influences ofthe nanohole diameter of the Ni/Au NMs and thermal annealingconditions on the metal-semiconductor (MS) ohmic character-istic as well as light transparency were thoroughly discussed, andfurther supported by simulation results. Finally, the I–Vcharacteristic and light output power of the UV-LEDs withNi/Au NMs and conventional planar contact were compared.The results presented herein provide an important strategy inachieving high-efficiency, flexible optoelectronic devices.

2. Experimental Section

p-GaN epilayers and full LED structure were grown on 2-inchsapphire substrate via low-pressure metalorganic chemicalvapor deposition (MOCVD) system. p-GaN epitaxial layerconsists of 2 μm undoped GaN template, and 500 nmMg-dopedGaN with hole concentration of 7� 1017 cm�3. The full LEDstructure consists of a 5mmGaN buffer layer, an 1.7mm n-GaNcontact layer, 9 periods of InGaN/(Al)GaN (3 nm/8 nm) MQWs,a 30 nm p-GaN/AlInGaN electron blocking layer and a 100 nmp-GaN contact layer. Trimethylaluminum (TMA), triethylgal-lium (TEG), and ammonia (NH3) were used as precursors forAl, Ga, and N, respectively, with hydrogen (H2) and nitrogen(N2) as the carrier gases. In order to fabricate Ni/Au NMs, thesurface of the p-GaN or LED were uniformly coated withpolystyrene (PS) spheres of 2.16 μm diameter via a large-areamicro-propulsive injection (MPI) method.[33] The colloidalsolution was injected into the water to form a Langmuir-Blodgett film in a hexagonal configuration at the air/waterinterface. By draining the water, the monolayer of closely

Phys. Status Solidi A 2018, 1800684 1800684 (

packed PS spheres is transferred onto the LED surface whichwas previously immersed in the water.[34] These closely-packedPS sphere monolayer act as templates for the deposition ofmetal contact. The sizes of the PS sphere were shrinked by 5and 20%, respectively, by O2 plasma condition. Subsequently,Ni and Au were deposited into the PS sphere interspacing by e-beam evaporation with the thickness of Ni and Au being 50 nm,respectively. The PS spheres were then removed by sonicationin toluene for 5min. The surface morphology of the Ni/Aunanomesh was characterized by a Hitachi S-4800 FieldEmission Scanning Electron Microscope (FE-SEM) and VeecoDimension 3100V Atomic Force Microscopy (AFM). I–Vcharacteristic between two adjacent Ni/Au NM pads andbetween the n and p contact of the UV-LEDs were analyzed by aKeithley 4200-SCS semiconductor characterization system.Specific contact resistivities of the planar Ni/Au and NMelectrode were measured by I–V system using a Transmission-Length-Mesurement (TLM). Light transmission through theNi/Au NMs were characterized by a Perkin Elmer Lambda 950UV/Vis/NIR Spectrometer equipped with an integratingsphere. Finally, electroluminescence (EL) of the LEDs werecollected by a home-build EL setup with a two metallic probeand a spectrometer equipped with a multi-mode UV opticalfiber.

3. Results and Discussions

The schematic process flow in fabricating Ni/Au NMs on top ofp-GaN epitaxial layer was shown in Figure 1a. In order toinvestigate the influence of the morphology of NMs on their

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim3 of 8)

Figure 3. The comparison of the curves of total resistance versus contactseparation in TLM measurement for planar Ni/Au contact, NM-1, andNM-2 contact.

Figure 4. Measured transmission spectra of planar Ni/Au and Ni/AuNMs on sapphire substrate (a); Normalized transmission spectra bytaking ratios of transmittance of planar Ni/Au and Ni/Au NMs overtransmittance of p-GaN (b).

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electrical behavior and optical transparency, two differentnanohole diameters were achieved by shrinking the PS spheresvia O2 plasma etching of 30min and 60min, respectively, prior tothe metal deposition. NMs with smaller and larger metalcoverage are denoted as NM-1 and NM-2, respectively. The top-view SEM images are shown in Figure 1b,c. The uniformdistributed nanohole arrays are observed from the SEM images,confirming the closely-packed nature of nanospheres during PScoating. The filling factors of the metallic NMs are calculated tobe 18.2 and 41.2%, respectively.

Ni/Au metal stacks have been widely used as p-type contact innitride-based optoelectronic devices due to similar fermi levelsbetween Ni and p-GaN.[35,36] Therefore, the ohmic characteristicof Ni/Au NM contacts on p-GaN is of great importance. In thiswork, rounded-shape electrode pads were fabricated on top of p-GaN using a shadow mask with inter-spacing of 4mm and paddiameter of 0.5mm (Figure 2a). The I–V characteristics betweentwo adjacent contact pads with or without perforated nanoholesare shown in Figure 2b. Conventional planar contact shows alinear I–V behavior with current larger than 0.1mA at bias of10V. In contrast, S-shape non-linear curve was observed for bothNM-1 and NM-2. As the diameter of perforated holes becomelarger, current level also decreases due to larger resistance insuch network structure. Traditionally, thermal annealing isusually performed on metal contact aiming to form a heavilydoped interfacial layer with improved electrical conductivity.[37]

Therefore, rapid thermal annealing (RTA) of Ni/Au NMs in N2

ambient was carried out at 300 �C and 600 �C for 5min afternanostructure fabrication. Figure 2c,d show the I–V curves ofNM-1 and NM-2 before and after RTA. It clearly illustrates thatRTA considerably improves the electrical behavior of the M-Scontact. Especially, after 600 �C annealing, I–V curves becomelinear, and current levels reach almost similar values ofconventional Ni/Au contact pad, indicating the absence ofcontact barrier at the interface between Ni/Au and p-GaN afterthermal treatment. Therefore, 600 �C RTA was used in futurestudies for optimized device performance.

TLM characterization was also performed to characterize thespecific contact resistivity of the planar andNMNi/Au contact on

Phys. Status Solidi A 2018, 1800684 1800684 (

top of p-GaN.[38,39] TLM pad was deposited through a metalshadow mask. The length of the pad is 1mm and the distancebetween each pad ranges from 20 to 500 μm. Total resistancebetween two contact pads Rtotal versus contact separation L forplanar Ni/Au, NM-1, and NM-2 after RTA treatment is shown inFigure 3. The curve for planar Ni/Au contact shows smallstandard deviation, whereas the fitting errors of NM-1 and NM-2are relatively larger, possibly due to non-uniform metaldistribution. Specific contact resistivity Rc can be calculated byequation: Rc ¼ R2

tRs

where Rt and Rs are transfer resistance andsheet resistance, respectively. The contact resistivity of planar Ni/Au, NM-1, and NM-2 were calculated to be 0.23, 2.41, and0.90Ω cm2, respectively. The increased contact resistivity of NMcontact compared to planar Ni/Au can be explained by smallercontacting area and thus less uniform current transport.Compared to NM-1, NM-2 exhibits much reduced Rc as a resultof better metal/p-GaN contact property. It is interesting to notethat the slopes of Rtotal-L curves are different among these threesamples, which is against our expectation since the sheetresistance Rs should remain constant. This observation suggests

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that the incorporation of NM contact in the TLM configurationmay result in deviation of current paths from perpendicular tothe length of the metal pad, or existence of current crowdingeffects.

To have a better understanding on the influence of NMcontact on the optical properties of GaN-based LED, lighttransmission of Ni/Au NMs and planar Ni/Au on p-GaNsurface were collected. As shown in Figure 4a, sharp absorptionband locating at �365 nm was observed for all samples,coincides well with the band gap of 3.4 eV for wurtzite GaN.Pure p-GaN without metallic thin film or NM exhibitstransmission of 66.8% at wavelength of 400 nm. This is veryclose to the calculated transmittance of 67.8% when consider-ing an incident light passing through a thin film of finitethickness. The transmitted light can be calculated as:I ¼ I0

1�Rð Þ21�R2 , where I, I0 and R are transmitted light intensity,

incident light intensity, and vertical reflection from Fresnelequation.[40,41] Broadband enhanced transmissions wereattained for NM-1 and NM-2 from 365 to 800 nm rangecompared to p-GaN with planar Ni/Au contact, whosetransmission is almost zero. Interference fringes can beobserved in the spectra due to abrupt interface betweensapphire and GaN, as well as GaN and Ni/Au contact. The trade-off between optical transmittance and contact resistivity wasdecided by the nanohole diameter: Larger diameter in the Ni/Au NM is correlated with higher transmission due to less lightblocking effect from the metal, but suffers from larger contactresistivity, as already shown in Figure 3. Interestingly, thermal

Figure 5. Top-view SEM images of as-fabricated Ni/Au NM-2 (a) and strurespectively.

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annealing on the Ni/Au NMs even increase the lighttransmission by approximately 3%, which deserves in-depthstudy in this work. Figure 4b shows the normalized lighttransmittance of Ni/Au when the reflection from p-GaN waseliminated. The transmission curves are nearly wavelength-independent, with the highest transmittance over 75% achievedfor Ni/Au NM-1.

Figure 5 shows the top-view SEM images of as-fabricated Ni/Au NM-2 (a) and under 300 �C (b), 600 �C(c), and 900 �C(d)thermal treatment, respectively. Rounded-shape PS residuals areobserved locating in the middle of the nanohole, which may bethe consequence of polymer hardening effect under plasmaetching condition. These residuals were partially removed after300 �C annealing, and are completely gone after thermaltreatment at higher temperatures such as 600 and 900 �C.Polystyrene was reported to have a large UVabsorption,[42] whichgreatly reduces the light transmission as previously indicated inFigure 4. A closer look at the surface morphology of Ni/Au NMssuggests that, the surface of Ni/Au remains smooth after300 �C treatment, yet becomes grainy due to metal agglomera-tion when RTA temperature was raised to 600 �C. At even highertemperature of 900 �C, large Ni/Au nanoparticles start to form atthe connecting point of the NMs, degrading the contactuniformity and conductivity. Therefore, 600 �C serves as theoptimal RTA condition in this study, which not only improvedthe ohmic behavior and reduces the contact resistivity of Ni/AuNMs on p-GaN, but also provided the largest light extractionefficiency in UV-LEDs.

ctures under thermal treatment of 300 �C (b), 600 �C(c) and 900 �C(d),

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim5 of 8)

Figure 6. Simulated light transmission spectra of NM-1 and NM-2 (a) and electric field distribution in the x-z plane of the simulation domain of NM-1(b–d) and NM-2 (e–g) at wavelength of 300 nm (b,e), 400 nm (c,f), and 500 nm (d,g).

Figure 7. I–V characteristic of LED with conventional planar Ni/Auelectrode and Ni/Au NM electrode.

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To further correlate the transmittance enhancement with thestructure of the Ni/Au NMs, we conducted simulations based onFinite-Element-Method. Light transmissions of planar and Ni/Au NM coated sapphire were simulated by solving the Maxwellequation.[43] It was shown in Figure 6a that strong interferencefringes are observed in the spectra. The dashed lines are guidedby the eye, and in good agreement with the experimental data:Light transmission is around 58% for NM-2, and reaches 73%for NM-1. Inset images are 3D E-filed distributions inside thesimulation domain of sapphire substrate coating with Ni/AuNMs at 400 nm. The variation in light transmission can furtherbe confirmed by the electric field distributions in Figure 6 atthree different wavelengths of 300 nm (b,e), 400 nm (c,f), and500 nm (d,g). The cross-sectional E-field distributions from b to dare NM-1, whereas e-g denote NM-2. The E-field intensitydistributions are obtained in x-z plane at the center of y direction(y¼ 0). The interface between p-GaN and metal (Air) isillustrated by the black dashed line. Even though a plane wavewas used as the incident light source, the E-field was mostlyconcentrated in the center of the simulation domain, locatingbeneath the nanohole structure. NM-1 shows stronger E-fieldintensity compared to that of NM-2 as a consequence of lessshadowing effect from metal stack. Additionally, differentwavelengths also have great influences on the E-field distribu-tion. More evenly distributed electric field was observed atshorter wavelength, whereas relatively more concentrated E-fieldand thus more directional light transmission can be achieved atlonger wavelength. Despite of the fact that the distributions of E-fields are different at various wavelengths, light wave propaga-tion towards the center of nanoholes is always observed. Thus, itcan be concluded that the main contribution of the coupledtransmission mostly comes from the strong propagating modesthrough the nanoholes.

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In order to testify the effect of NM on the performance of UV-LEDs, A 400-nm-emission UV-LED was fabricated based onconventional lithography technique, mesa etching, and electrodedeposition. Ti/Al/Ni/Au metal stack was used as the n-typeelectrode deposited on n-GaN.[44] In the process of NMformation on top of p-GaN epitaxial layer as p-contact, PSspheres were coated on the LEDmesas where only the top p-GaNwas exposed. Followed by PS shrinking and metal deposition,residual PS and photoresist were removed by toluene andacetone, respectively. Figure 7 illustrates the I–Vcurve of our UV-LEDs fabricated with conventional planar Ni/Au electrode andNM electrode. The turn-on voltages for LED with planar and NM

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electrodes are 3.4 and 4.1 V, respectively. Additionally, UV-LEDwith NM electrode has a larger slope than that with conventionalplanar electrode. Both of them indicate an increased seriesresistance in the LED due to network p-GaN contact. Furtheroptimization on the Ni/Au NM contact is underway in order toreduce the contact resistivity and turn-on voltage of the LEDs.

Finally, the EL spectra of the LED fabricated with planarelectrode and NM electrode were collected at input currents of 8,16, 20, and 30mAwith ameasurement error of�1mA. Note thatthicker Ni/Au contact (100 nm/100 nm) was utilized in the ELmeasurement to avoid surface scratching by the probe. As theinput current varied from 8 to 30mA, the EL intensity increasedwithout obvious peak shift. Multiple modes are present in the ELspectra of LED with NM electrode, possibly due to the formationof Fabry-Perot cavity between the nitride/sapphire interface andnitride/metal interface.[45] A higher output power was clearlyobserved in LED with NM electrode than planar electrode. Insetoptical microscope images of Figure 8 show the LEDmesa under16mA current. It is clearly illustrated that planar Ni/Au electrodeblocks the UV emission, whereas light emission is uniformlydistributed in the round-shape LED mesa if NM electrode wasincorporated in the structure. Strong luminescence is clearlyobserved in the area of NMs. It is believed that the key merits of

Figure 8. EL spectra of LEDs fabricated with planar electrode (a) and Ni/Au NM electrode (b) under injection current of 8, 16, 20, and 30mA. Theinset images are the optical micrograph showing the violet light emissionacross the mesa.

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metal NMs are simple and large-scale fabrication process, largelight extraction efficiency, and potential application towardflexible optoelectronic devices.

4. Conclusion

In summary, we have presented an alternative transparent metalcontact on top of UV-LEDs by using a stack of Ni and Au thin filmsperforated with hexagonally periodic holes. The effect of Ni/Aunanomesh ohmic contact on the performance of UV-LEDs werediscussed in details. UV-LEDs with Ni/AuNMs contact demonstrateexcellent optoelectronicproperties of 75%transmittancewith specificcontact resistivity of 0.90Ω cm2. Enhanced electroluminescence intheUV-LEDwasclearlydemonstratedwhen incorporatedwithNi/AuNMs instead of planar contact. Experimental results were furthersupported by the simulated light transmission and electric fielddistribution. This work demonstrates the possibility of using Ni/AuNM contact as a potential transparent electrode in the application ofhigh efficiency UV emitters.

AcknowledgementsMS and HX contributed equally to this work. This work was partiallysupported by the National Key Research and Development Program ofChina (Grant 2016YFB0400802), and National Natural Science Founda-tion of China (Grant 61704176, 61574145 and 61874177) and ZhejiangProvincial Natural Science Foundation (LY15F040003).

Conflict of InterestThe authors declare no conflict of interest.

Keywordslight-emitting diodes, light extraction efficiency, nanomeshes, ohmicbehavior, p-GaN contact, specific contact resistivity

Received: August 28, 2018Revised: October 31, 2018

Published online:

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