High extraction efficiency GaN-based light-emitting diodes on embedded SiO 2nanorod array and nanoscale patterned sapphire substrateHung-Wen Huang, Jhi-Kai Huang, Shou-Yi Kuo, Kang-Yuan Lee, and Hao-Chung Kuo Citation: Applied Physics Letters 96, 263115 (2010); doi: 10.1063/1.3456385 View online: http://dx.doi.org/10.1063/1.3456385 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/96/26?ver=pdfcov Published by the AIP Publishing Articles you may be interested in 282-nm AlGaN-based deep ultraviolet light-emitting diodes with improved performance on nano-patternedsapphire substrates Appl. Phys. Lett. 102, 241113 (2013); 10.1063/1.4812237 Efficiency measurement of GaN-based quantum well and light-emitting diode structures grown on siliconsubstrates J. Appl. Phys. 109, 014502 (2011); 10.1063/1.3530602 The aspect ratio effects on the performances of GaN-based light-emitting diodes with nanopatterned sapphiresubstrates Appl. Phys. Lett. 97, 023111 (2010); 10.1063/1.3463471 Improved crystal quality and performance of GaN-based light-emitting diodes by decreasing the slanted angleof patterned sapphire Appl. Phys. Lett. 96, 051109 (2010); 10.1063/1.3304004 Enhanced light extraction efficiency of GaN-based light-emitting diodes with ZnO nanorod arrays grown usingaqueous solution Appl. Phys. Lett. 94, 071118 (2009); 10.1063/1.3077606

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High extraction efficiency GaN-based light-emitting diodes on embeddedSiO2 nanorod array and nanoscale patterned sapphire substrate

Hung-Wen Huang,1,a� Jhi-Kai Huang,1,2 Shou-Yi Kuo,3,a� Kang-Yuan Lee,2 andHao-Chung Kuo1

1Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30050, Taiwan2Unilite Corporation, Miaoli 350, Taiwan3Department of Electronic Engineering, Chang Gung University, Tao-Yuan 333, Taiwan

�Received 15 March 2010; accepted 25 May 2010; published online 2 July 2010�

In this paper, GaN-based LEDs with a nanoscale patterned sapphire substrate �NPSS� and a SiO2

photonic quasicrystal �PQC� structure on an n-GaN layer using nanoimprint lithography arefabricated and investigated. The light output power of LED with a NPSS and a SiO2 PQC structureon an n-GaN layer was 48% greater than that of conventional LED. Strong enhancement in outputpower is attributed to better epitaxial quality and higher reflectance resulted from NPSS and PQCstructures. Transmission electron microscopy images reveal that threading dislocations are blockedor bended in the vicinities of NPSS layer. These results provide promising potential to increaseoutput power for commercial light emitting devices. © 2010 American Institute of Physics.�doi:10.1063/1.3456385�

The high brightness GaN-based light-emitting diodes�LEDs� have made possible uses in traffic signals, backlightin liquid crystal displays, and solid state lighting.1,2 How-ever, there is still a great need to improve the internal andexternal quantum efficiency �EQE� in order to increase thelight output performance, and thus reduce the total cost ofLED modules. The further improvement in the EQE of aGaN-based LED on a patterned sapphire substrate �PSS� isrequired, and most high-quality GaN-based LEDs are affixedonto a microscale PSS.3,4 The microscale patterns serve as atemplate for the epitaxial lateral overgrowth �ELOG� of GaNand the scattering centers for the guided light. Both the epi-taxial crystal quality and the light extraction efficiency areimproved by utilizing a microscale PSS. Besides, a numberof attempts have been made to reduce the dislocation effectusing strategies such as the insertion of a microscale epitax-ial ELOG layer of SiO2 or a SixNy pattern on the GaN thinfilm.5–7 However, they are typically fabricated by photoli-thography or self-assembly. To fabricate high brightnessLEDs more easily, one candidate method is nanoimprint li-thography �NIL�.

In this paper, we utilize a nanoimprinting technique tofabricate a nanoscale patterned sapphire substrate �NPSS�and a SiO2 photonic quasicrystal �PQC� on an n-GaN layerfor mass production. Experimental results reveal that thelight output power of LED with a NPSS and a SiO2 PQCpattern on an n-GaN layer is significantly greater than that ofa conventional LED.

Figure 1 shows the schematic diagram of GaN-basedLED with a NPSS and a SiO2 PQC structure on an n-GaNlayer. Similar to the conventional LED, the LED structureconsisting of a Cr/Pt/Au p-electrode, an indium tin oxide�ITO� transparent layer, a LED epitaxial layer, a smoothp-GaN surface, and a Cr/Pt/Au n-electrode on NPSS struc-ture. Particularly, the LED epitaxial structure has inset a SiO2PQC pattern on an n-GaN layer by NIL for comparison. In

our study, three patterned LEDs are fabricated in order toinvestigate the influence of the NPSS and a SiO2 PQC on ann-GaN layer on the LED light output power and beam profileperformance. For convenience, LED with a SiO2 PQC, LEDwith a NPSS, and LED with a NPSS and a SiO2 PQC struc-ture are denoted as LED A, LED B, and LED C, respectively.

The following details the process flow of NPSS by usingNIL technique on a flat sapphire substrate. First, we spin coata 200 nm polymer layer on the sapphire substrate surface.Second, we place a patterned mold onto the dried polymerfilm. By applying a high pressure, we can heat the sapphiresubstrates to above the glass transition temperature of thepolymer. After that, the sapphire substrates and the mold arecooled down to room temperature to release the mold. Fi-nally, we use an inductively coupled plasma reactive ionetching �ICP-RIE� with BCl3 plasma to transfer the patternonto sapphire substrate and remove the polymer layer withO2 plasma etching gas in a RIE system. Figure 2�a� showsthe cross-section view of scanning electron microscope�SEM� image on a GaN epitaxial layer with a NPSS. TheSEM image reveals the NPSS exhibit characteristic ofnanolens pattern. The lattice constant �a� of NPSS structure

a�Electronic addresses: stevinhuang737672@msn.com andsykuo@mail.cgu.edu.tw.

SiO2 PQC

InGaN/GaN MQWP-AlGaN

Cr/Pt/AuITO

P-GaN

Cr/Pt/Au

U-GaN

N-GaN

Sapphire

FIG. 1. �Color online� Schematic diagram of LED with a NPSS and a SiO2

PQC structure.

APPLIED PHYSICS LETTERS 96, 263115 �2010�

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is 750 nm and the nanolens diameter �d� is 455 nm. In addi-tion, the etching depth of NPSS is approximately 182 nm.

Figure 2�b� shows a cross-section SEM image on a GaNepitaxial layer with a SiO2 PQC pattern. The NIL processflow of inserting a SiO2 layer on an n-GaN layer is similar tothe NPSS process described previously. After finishing mold-ing, we use an RIE with CF4 plasma to transfer the patternonto GaN sample and remove the polymer layer with O2plasma etching gas in RIE system.

All LED samples are grown by metal organic chemicalvapor deposition with a rotating-disk reactor �Veeco� on ac-axis sapphire �0001� substrate at a growth pressure of 200mbar. The LED structure consists of a 50 nm thick GaNnucleation layer grown at 500 °C, a 3 �m thick undopedGaN buffer layer grown at 1050 °C, a 2 �m thick Si-dopedGaN layer grown at 1050 °C, an unintentionally dopedInGaN/GaN multiple quantum wells �MQWs� active regiongrown at 770 °C, a 50 nm thick Mg-doped p-AlGaN elec-tron blocking layer grown at 1050 °C, and a 120 nm thickMg-doped p-GaN contact layer grown at 1050 °C. TheMQW active region consists of five periods of 3 nm/20 nmthick In0.18Ga0.82N /GaN quantum well layers and barrierlayers.

Different from the photonic crystals �PhCs� with highnatural lattice symmetry, PQCs appear random in that at firstglance; however, closer inspection reveals them to possesslong-range order but short-range disorder. Figure 2�c� shows

a top-view image of an atomic force microscopy �AFM� with12-fold PQC pattern based on a square-triangular lattice. Wechoose the 12-fold PQC pattern due to the better enhance-ment of surface emission.8 This is obtained from the PhCswith a dodecagonal symmetric quasicrystal lattice, as op-posed to regular PhCs with triangular lattice and eightfoldPQC.8 The recursive tiling of offspring dodecagons packedwith random ensembles of squares and triangles in dilatedparent cells forms the lattice. The lattice constant and roddiameters are 750 nm and 500 nm, respectively.

All LED samples are fabricated using the following stan-dard processes with a mesa area of 300�300 �m2. A SiO2layer with thickness of 300 nm is deposited onto the LEDsample surface by using plasma enhanced chemical vapordeposition. Photolithography is used to define the mesa pat-tern after wet etchings of SiO2 by a buffered oxide etchantsolution. The mesa etching is then performed withCl2 /BCl3 /Ar etching gas in an ICP-RIE system in order totransfer the mesa pattern onto n-GaN layer. A 270 nm thickITO layer is subsequently evaporated onto the LED samplesurface. The ITO layer has a high electrical conductivity anda high transparency ��95% at 460 nm�. Cr/Pt/Au contact issubsequently deposited onto the exposed n- and p-type GaNlayers to serve as the n- and p-type electrodes.

Figure 3�a� shows the characteristics of a typical current-voltage �I-V� measurement. It is found that the measuredforward voltages under injection current 20 mA at room tem-perature for conventional LED, LED A, LED B, and LED Care 3.16 V, 3.15 V, 3.15 V, and 3.23 V, respectively. In addi-tion, the dynamic resistance of conventional LED, LED A,LED B, and LED C are about 14.7 �, 14.8 �, 15.3 �, and15.4 �, respectively. Therefore, in terms of dynamic resis-tance, there is no influence on these type of devices by in-

GaN

Sapphire

GaN

Sapphire

(a)

1 μm

n-GaN

SiO2

ITOp-GaN

n-GaN

SiO2

ITOp-GaN

(b)

1 μm

[μm]

(c)

0

1

2

3

4

0 1 2 3 4[μm]

FIG. 2. �Color online� �a� Top-view and �b� cross-section SEM images ofsapphire surface with a NPSS and �c� top-view AFM image of an n-GaNsurface with a SiO2 PQC.

0 20 40 60 80 1000

10

20

30

40

50

60

70

80Conventional LEDLED with a SiO2 PQC (LED A)LED with a NPSS (LED B)LED with a NPSS+ a SiO2 PQC (LED C)

Lightoutputpower(mW)

Forward current (mA)

0 20 40 60 80 1000

1

2

3

4

5

6

Forwardvoltage(V)

Forward current (mA)

Conventional LEDLED with a NPSS (LED B)LED with a SiO2 PQC (LED A)LED with a NPSS+ a SiO2 PQC (LED C)

(a)

(b)

FIG. 3. �Color online� �a� Current-voltage �I-V� and �b� intensity-current�L-I� characteristics of conventional LED, LED with a NPSS, LED with aSiO2 PQC structure on an n-GaN layer, and LED with a NPSS and a SiO2

PQC structure on an n-GaN layer.

263115-2 Huang et al. Appl. Phys. Lett. 96, 263115 �2010�

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corporating a NPSS and a SiO2 PQC structure by NIL pro-cess.

The light output is detected by calibrating an integratingsphere with Si photodiode on the device with TO-can pack-age so that light emitted in all directions from the LED canbe collected. The intensity-current �L-I� characteristics ofconventional LED, LED A, LED B, and LED C are shown inFig. 3�b�. At an injection current of 20 mA and peak wave-length of 460 nm, the light output powers of conventionalLED, LED A, LED B, and LED C with TO-can package are12.8 mW, 15.4 mW, 17.3 mW, and 18.9 mW, respectively.Hence, the enhancement percentages of LED B, LED C, and

LED D are 20%, 35%, and 48%, respectively, compared tothat of conventional LED. The enhancement of output poweris due to the better crystal quality and more reflection at theinterface of GaN/sapphire. NPSS is regarded as an effectiveway to reduce threading dislocation between GaN and theunderneath sapphire substrate, and allows more light to re-flect from sapphire substrate onto the top direction. In addi-tion, the use of a 12-fold SiO2 PQC pattern also result inhigher epitaxial crystal quality which increases more lightoutput power.5–7

To confirm the speculations above, transmission electronmicroscopy �TEM� images were employed to investigate thecrystalline quality of GaN layers epitaxially grown on a flatsapphire substrate and a NPSS. As shown in Figs. 4�a� and4�b�, it is obvious that in Fig. 4�b� the threading dislocationdensity �TDD� of GaN grown on a NPSS and a SiO2 PQCstructure was drastically reduced from that grown comparedwith flat sapphire substrate �as shown in Fig. 4�a��. From Fig.4�c�, we found that a number of stacking faults often oc-curred above the nanolens patterns, where visible threadingdislocations �TDs� were rarely observed in the vicinities. It isbelieved that the presence of stacking faults could block thepropagation of TDs.7 Moreover, the TDs of the GaN layer ona NPSS and a SiO2 PQC structure mainly originated fromexposed sapphire surface, which could be bent due to thelateral growth of GaN. Figure 4�c� shows the TEM image ofLED with a NPSS and a SiO2 PQC structure is the disloca-tion bending with visible turning points.7 Accordingly, theseobservations support our assumption as well.

In conclusion, the GaN-based LEDs with a NPSS and aSiO2 PQC structure are fabricated and demonstrated by NILsystem. At a driving current of 20 mA on TO-can package,the light output power of LEDs with a NPSS and a SiO2PQC structure is enhanced by a factor of 1.48. The higheroutput power of the LED with a NPSS and a SiO2 PQCstructure is due to higher reflectance on NPSS and higherepitaxial quality on n-GaN layer using a SiO2 12-fold PQCpattern. This work demonstrate a promising way to effec-tively increase output powers of commercial light emittingdevices.

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Sapphire

GaN

turning points

stacking faults

Sapphire

GaN

turning points

stacking faults

Sapphire

GaN

SiO2

Sapphire

GaN

SiO2

Sapphire

GaN

Sapphire

GaN(a)

(b)

(c)

1 μm

1 μm

500 nm

FIG. 4. The TEM images of GaN/sapphire interface for the GaN epilayergrown on a �a� flat sapphire substrate and �b� a NPSS and a SiO2 PQCstructure on an n-GaN layer. �c� The dislocation bending phenomenon withvisible turning points on GaN epilayer with a NPSS and a SiO2 PQC struc-ture on an n-GaN layer.

263115-3 Huang et al. Appl. Phys. Lett. 96, 263115 �2010�

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