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Formation of an indium tin oxide nanodot/Ag nanowire electrode as a current spreader for

near ultraviolet AlGaN-based light-emitting diodes

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2017 Nanotechnology 28 045205

(http://iopscience.iop.org/0957-4484/28/4/045205)

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Formation of anindium tin oxide nanodot/Agnanowire electrode as a current spreader fornear ultraviolet AlGaN-based light-emittingdiodes

Jae-Seong Park1, Jae-Ho Kim1, Jun-Yong Kim1, Dae-Hyun Kim2,Jin-Young Na3, Sun-Kyung Kim3, Daesung Kang2,4 and Tae-Yeon Seong1,2

1Department of Materials Science and Engineering, Korea University, Seoul 02841, Korea2Department of Nanophotonics, Korea University, Seoul 02841, Korea3Department of Applied Physics, Kyung Hee University, Gyeonggi-do 17104, Korea4 Chip development group, LG Innotek Co., Ltd, Paju, Gyeonggi-do 10842, Korea

E-mail: tyseong@korea.ac.kr

Received 5 February 2016, revised 27 July 2016Accepted for publication 19 October 2016Published 19 December 2016

AbstractIndium tin oxide (ITO) nanodots (NDs) were combined with Ag nanowires (Ag NWs) as ap-type electrode in near ultraviolet AlGaN-based light-emitting diodes (LEDs) to increase lightoutput power. The Ag NWs were 30 ± 5 nm in diameter and 25 ± 5 μm in length. Thetransmittance of 10 nm-thick ITO-only was 98% at 385 nm, while the values for ITO ND/AgNW were 83%–88%. ITO ND/Ag NW films showed lower sheet resistances (32–51Ω sq−1)than the ITO-only film (950Ω sq−1). LEDs (chip size: 300×800 μm2) fabricated usingthe ITONDs/Ag NW electrodes exhibited higher forward-bias voltages (3.52–3.75 V at 20 mA) than theLEDs with the 10 nm-thick ITO-only electrode (3.5 V). The LEDs with ITO ND/Ag NWelectrodes yielded a 24%–62% higher light output power (at 20 mA) than those with the 10 nm-thick ITO-only electrode. Furthermore, finite-difference time-domain (FDTD) simulations wereperformed to investigate the extraction efficiency. Based on the emission images and FDTDsimulations, the enhanced light output with the ITO ND/Ag NW electrodes is attributedtoimproved current spreading and better extraction efficiency.

Keywords: Ag nanowire, ITO nanodot, NUV light-emitting diode, current spreading

(Some figures may appear in colour only in the online journal)

1. Introduction

Development of AlGaN-based ultraviolet (UV) light-emittingdiodes (LEDs) is technologically important because of their usein various applications such as UV curing, water purification,and solid-state lighting [1–4]. However, because most of thephotons generated from the active region were absorbed by thep-type contacts, the external quantum efficiency (EQE) is quitelow. Thus, to improve thelight extraction efficiency andthereby to enhance the EQE, Ni/Au or transparent conductingoxides (TCOs), such as indium tin oxide (ITO) and ZnO, havebeenwidely investigated to develop p-type contacts for UV

LEDs [5–11]. For example, Chae et al [8] found that F-dopingof ITO increased the work function and the energy bandgap offilms but reduced their Schottky barrier height, and thecorresponding UV LEDs exhibited 148% higher output powerat 100mA than LEDs with undoped ITO. Tamura et al [10]investigated the effect of amolecular beam epitaxy-grown Ga-doped ZnO (ZnO:Ga) transparent electrode on the performanceof InGaN-based UV LEDs, and reported that ZnO:Ga films hadhigher transmittance in thenear UV (NUV) wavelength rangethan oxidized Ni/Au p-type electrodes. Consequently, theoptical power of LEDs fabricated using a ZnO:Ga p-typeelectrode was found to be nearly twofold compared to that of

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Nanotechnology 28 (2017) 045205 (6pp) doi:10.1088/1361-6528/28/4/045205

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LEDs with aNi/Au p-electrode. Nevertheless, the transmit-tance of aTCO-based p-type electrode in the NUV region needsto be further improved. Thus, nanostructure-based transparentconducting electrodes, including graphene, carbon nanotubesand silver nanowires (Ag NWs), have been actively investigatedbecause they have high optical transmittance and low electricalresistance [12–19]. For example, Chandramohan et al [12]found that usingAuCl3-doped graphene and Au layer/graphenesheets as acurrent spreading electrode resulted in alower for-ward voltage compared with LEDs with anITO electrode. Kimet al [13] observed that UV LEDs (368 nm) with atwo-layer-graphene electrode exhibited higher electroluminescence (EL)than LEDs with afour-layer-graphene electrode. Jeong et al[16] observed that theuse of Ag NWs as a current spreaderresulted in much higher EL intensity at 45mA compared withLEDs without Ag NWs. The enhancement was attributed to theimproved current spreading effect. Seo et al [19] reported thatAg NW-decorated graphene (ADG) electrodes exhibited atransmittance of 87.7% at 375 nm and a sheet resistance of50±5Ω sq−1. The UV LEDs (375 nm) with optimal ADGelectrode exhibited approximately four-fold higher EL intensitycompared with LEDs with abare graphene electrode. Thisenhancement was attributed to the reduced sheet resistance andcontact resistance. For the fabrication of ITO nanostructures,there are two types of methods: (1) thin film deposition; (2)

solution-based synthesis [20]. Deposition-based methods, suchas avapor evaporation process, spray pyrolysis, and pulsedlaser deposition, have poor control over the size of ITO nano-particles. Solution-based methods, such as solvo-thermalsynthesis, thePenchini method, and theco-precipitationmethod, contain many critical steps in the preparation processesof ITO nanoparticles that directly affect the tunability of particlesize. In this study, a simple wet-etching process using a diluteHCl acid solution followed by rapid-thermal annealing wasused to form ITO nanodots (NDs). This method is effective inproducing highly crystalline and size-controlled ITO NDs bycontrolling the thickness of theITO films and acid concentra-tion [20]. Here, we investigatehow hybrid ITO ND/Ag NWelectrodes affect the electrical and optical properties of NUV(385 nm) LEDs. Emission images are obtained to characterizethe output performance of LEDs with different p-type electro-des. Finite-difference time-domain (FDTD) simulations are-performed to investigate the extraction efficiency.

2. Experimental procedure

A metalorganic chemical vapor deposition system was used togrow NUV (385 nm) AlGaN/InGaN-based LED structures on(0001) sapphire substrates. The epitaxial structure consisted of

Figure 1. SEM images of ITO filmsetched in an HCl:DI water (1:2) solution for 20 s (a) before and (b) after annealing at 550 °C for 1 min inair. Note theND-like surface morphology of the etched ITO, which isreferred to here as ITO ND. SEM images of hybrid ITO ND/Ag NWspin-coated at (a) 1000, (b) 2000, and (c) 3000 rpm. The white dots and wires denote ITO NDs and Ag NWs, respectively.

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Nanotechnology 28 (2017) 045205 J-S Park et al

a 2.0 μm-thick undoped GaN layer, a 4.0 μm-thick n-typeGaN:Si (nd=9.0×1018 cm−3) layer, a 50 nm-thick AlGaN/InGaN multiple quantum well active layer, a 30 nm-thickAlGaN electron blocking layer, and a 50 nm-thick p-type GaN:Mg (na=3.0×1019 cm−3) layer. Ag NWs (diameter:30±5 nm; length: 25±5 μm) dispersed in ethanol at0.3 wt% were purchased from NANOPYXIS Inc. Prior tometal deposition, all the samples were treated with a bufferedoxide etch solution for 1 min and rinsed in deionized water. Tofabricate the LEDs (chip size=300×800 μm2), standardphoto-lithography and inductively coupled plasma reactive-ionetching processes were performed. A transparent p-type ohmiccontact was formed as follows. First, to form ND-shaped sur-face roughening (which isreferred to here as ITO ND), as-deposited (electron-beam-deposited) ITO layers (100 nm) weredipped in aHCl:deionized (DI) water (1:2) solution for 20 s,followed by annealing at 550 °C for 1 min in air [6]. Thescanning electron microscopy (SEM) micrographs of ITO NDsbefore and after annealing are shown in figures 1(a) and (b),respectively. It is evident that etching the as-deposited ITO filmresulted in the formation of ITO NDs. After that, the Cr/Ni/Au(20 nm/25 nm/50 nm) layer was deposited as both p-pad andn-pad. The ethanol Ag NW solution was subsequently spin-coated over the ITO NDs at three different coating speeds of1000, 2000, and 3000 rpm for 30 s each. The surface

morphologies of different ITO ND/Ag NW were observed bySEM, as shown in figures 1(c)–(e). The ITO NDs wereemployed to improve current spreading by connecting AgNWs with another. For comparison, LEDs with 10 nm-thickITO-only and ITO ND-only electrodes were also fabricated.The schematic structures of LEDs with ITO-only, ITO ND-only, and ITO ND/Ag NW are shown in figures 2(a)–(c),respectively. The current–voltage (I–V ) curves and light outputpower measurements were performed using a Keithley 238 anda Newport dual channel powermeter, respectively. Transmit-tances of the ITO-only, ITO ND-only, and ITO ND/Ag NWfilms on sapphire substrates were measured using a UV–visiblespectrophotometer.

3. Results and discussion

Figure 3 exhibits the transmittances of ITO ND/Ag NW filmsdeposited at different coating speeds. For comparison, thetransmittances of 10 nm-thick ITO-only and ITO ND-onlyfilms are also shown. A bare sapphire substrate was used asthe reference for all samples. Compared to the ITO-only andITO ND-only films, the ITO ND/Ag NW films exhibitedlower transmittance across the 350–450 nm region. At a

Figure 2. Schematic diagrams of LEDs with (a) ITO-only, (b) ITO ND-only, and (c) ITO ND/Ag NW.

Figure 3. Transmittances of 10 nm-thick ITO-only, ITO ND-only,and ITO ND/Ag NW films deposited at different coating speeds.

Figure 4. I–V characteristics of NUV (385 nm) LEDs fabricated with10 nm-thick ITO-only, ITO ND-only, and ITO ND/Ag NWelectrodes.

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Nanotechnology 28 (2017) 045205 J-S Park et al

wavelength of 385 nm, the 10 nm-thick ITO-only and ITOND-only films show higher transmittance than the ITO ND/Ag NW films coated at 1000, 2000, and 3000 rpm, as sum-marized in table 1. The fact that the transmittance increaseswith increasing coating speed can be attributed to the decreasein the density of Ag NWs.

The sheet resistances of the 10 nm-thick ITO-only, ITOND-only, and ITO ND/Ag NW films were characterized.Because of the highly conductive Ag NWs and ohmic con-tacts between ITO NDs and Ag NWs [21], ITO ND/Ag NWfilms coated at 1000, 2000, and 3000 rpm have much lowersheet resistances than the ITO-only film, as shown in table 1.

Figure 4 shows the I–V characteristics of NUV (385 nm)LEDs fabricated with 10 nm-thick ITO-only, ITO ND-only,and ITO ND/Ag NW electrodes. The LEDs with the ITO-only and ITO ND-only electrodes exhibit forward-bias vol-tages of 3.50 and 3.80 V at 20 mA, respectively. On the otherhand, the LEDs with ITO ND/Ag NW electrodes have higherforward voltages than the LEDs with the ITO-only electrode,as shown in table 1. In addition, the LEDs with the ITO-onlyelectrodes gave lower series resistance than those with ITOND/Ag NW electrodes (table 1). Note that the forward-biasvoltage decreases with decreasing coating speed. With the

fact that the sheet resistance decreased with coating speed, theimproved forward voltage characteristics could be ascribed tothe enhanced lateral current flow caused by the increaseddensity of Ag NWs.

Figure 5 exhibits the light output powers of NUV LEDsfabricated with various electrodes as a function of the forwardcurrent. The measurements exhibit that the LEDs with theITO ND/Ag NW electrodes yield higher output power thanthose with the ITO-only electrode, although the latter gavehigher optical transmittance and lower forward voltage, assummarized in table 1. The lower light output of the LEDswith the ITO-only electrode is believed to be related to thecurrent crowding caused by the high sheet resistance of thethin ITO film, as described previously (table 1). The fact thatthe light output of LEDs with ITO ND/Ag NW filmsdecreased with increasing coating speed could be related tothe reduced lateral current flow due to the decreased densityof Ag NWs.

To investigate the current spreading and light emissionbehavior of the NUV LEDs (chip size=300×800 μm2)fabricated with different ITO-based Ag NWs electrodes, theemission images of the chips were obatined at a forward biasof 10 mA. Figure 6 shows the plan-view emission imagesfrom the LEDs with ITO-only, ITO ND-only, and ITO ND/Ag NW electrodes. It is noted that the original emissionimages were reduced to 250 different colors using theMATLAB program. For the LEDs with ITO-only and ITOND-only electrodes (figures 6(a) and (b), respectively), thephotoemission is not uniform and issignificantly localizedaround the p-pad because of current crowding. This can beascribed to the higher sheet resistance of the electrodes. Onthe otherhand, the LEDs with ITO ND/Ag NW electrodescoated at 1000, 2000, and 3000 rpm (figures 6(c)–(e),respectively) exhibit better light emission across the chip area,although they arestill non-uniform. The better uniformity isdue to the fact that the highly conducting Ag NWs formingohmic contacts to ITO [21] played a vital role in improvingcurrent spreading, resulting in a much higher output power. Itis noted that the LED fabricated with Ag NWs coated at1000 rpm showed better light emission than those with2000and 3000 rpmcoated Ag NWs electrodes. This can beexplained by the improved sheet resistance due to theincreased density of Ag NWs. In spite of the poorer

Figure 5. Light output powers of NUV LEDs fabricated with 10 nm-thick ITO-only, ITO ND-only, and ITO ND/Ag NW electrodes as afunction of the forward current.

Table 1. Summary of the optical and electrical properties of ITO-only, ITO ND-only, and ITO ND/Ag NW films deposited at differentcoating speeds. T, R, Vf, and Rsdenote transmittance, sheet resistance, forward voltage, and series resistance, respectively. LOP indicates theimprovement of the light output of LEDs with ITO NDs and ITO ND/Ag NW electrodes against that of LEDs with anITO-only electrode.Note that theLED with anITO ND-only electrode gives a reduction of29% in the light output.

ITO-only (10 nm)(reference)

ITOND-only

ITO ND/Ag NW(1000 rpm)

ITO ND/Ag NW(2000 rpm)

ITO ND/Ag NW(3000 rpm)

T at 385 nm [%] 98 95 83 85 88R(Ω sq−1) 950 — 32 44 51Vf at 20 mA (V) 3.5 3.8 3.52 3.70 3.75Rs(Ω) 13.9 17.0 15 15.7 16.5LOP enhancement withreference to ITO-only (%)

— −29 63 38 25

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Nanotechnology 28 (2017) 045205 J-S Park et al

transmittance and forward voltages, LEDs with ITO ND/AgNW electrodes had much higher output power than those withITO-only electrodes (figure 5). This improvement can beexplained by the lower sheet resistance (and so better currentspreading) as confirmed by the emission images (figure 6).

Furthermore, to investigate whether the presence of ITONDsand Ag NWs increases the extraction efficiency, FDTDsimulations were performed on GaN/sapphire LED structureswith a planar surface, ITO NDs, and Ag NWs (figure 7(a)).The ITO NDsand Ag NWs were randomly distributed on theupper GaN layer. Fill factors (FFs) of the Ag NWs and ITOdots were measured using the MATLAB program. The ITOND/Ag NW films coated at 1000, 2000, and 3000 rpm hadFFs of 14.8%, 9.2%, and 6.6%, respectively (Ag NWs). Onthe otherhand, for the ITO NDsample, theFF was 17%(NDs). However, for simplicity, the FFs were set to be 15%,10%, and 5% (inset, figure 7(a)) (for Ag NWs) and 15% (forITO NDs). The simulations show that the presence of both AgNWs and ITO NDsenhances light extraction. For the AgNWs, the extraction efficiency increases gradually withincreasing FF (namely, with decreasing coating speed). It isnoted that the Ag NWs films with FFs of 10% and 15% givehigher extraction efficiency than ITO dots. This is because thecomplex refractive index of Ag forms a high-index-contrastpattern, thus inducing strong scattering [22]. The snapshots ofelectric field intensity reveal that the ITO NDsand the AgNWs effectively interact with the light generated from adipole source (figure 7(b)). This illustrates an enhancement inthe light extraction in the LEDs with ITO ND/Ag NW (the1000 rpm sample) compared with the others. Thus, theemission images and the FDTD simulation results indicatethat the combined effects of imrpoved current spreading andextract efficiency are responsible for the higher output powerof the LEDs with ITO ND/Ag NW films coated at 1000 rpm.

Figure 7. (A) Simulated extraction efficiencies of GaN/sapphire LEDstructures with a planar surface, ITO NDs, and Ag NWs with FFs of 5%,10%, and 15%. Insets show simulated structures. (B) Snapshots ofelectric field intensity for GaN/sapphire LED structures with a planarsurface, ITO dots, and Ag NWs with three different FFs.

Figure 6. Plan-view emission images obtained from LEDs with (a) ITO-only, (b) ITO ND-only, and ITO ND/Ag NW electrodes coated at (c)1000, (d) 2000, and (e) 3000 rpm.

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Nanotechnology 28 (2017) 045205 J-S Park et al

4. Summary and conclusion

We investigated the output performance of NUV AlGaN-based LEDs fabricated with ITO ND/Ag NW electrodes andcompared it with that of LEDs with a10 nm-thick ITO-onlyelectrode. The ITO ND/Ag NW films showed lower trans-mittance at 385 nm and lower sheet resistance than the ITO-only film. LEDs with these ITO ND/Ag NW electrodesyielded higher light output than LEDs with anITO-onlyelectrode, due to enhanced current spreading and improvedlight extraction. These results show that these hybrid ITOND/Ag NW films could be a promising electrode for thedevelopment of high-efficiency NUV LEDs.

Acknowledgments

This work was supported by the Brain Korea 21 plus programfunded by the Ministry of Science, ICT and Future Planning,Korea, and by LG Innotek Co., Ltd. S-KKwas supported bythe Basic Science Research Program through the NationalResearch Foundation of Korea (NRF), which is funded by theMinistry of Science, ICT, and Future Planning (NRF-2013R1A1A1059423).

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