Anodic nanoclusters of GaNKeunjoo Kim, Jaeho Choi, and Tae Sung Bae Citation: Applied Physics Letters 90, 181912 (2007); doi: 10.1063/1.2734901 View online: http://dx.doi.org/10.1063/1.2734901 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Enhanced optical performance of amber emitting quantum dots incorporated InGaN/GaN light-emitting diodeswith growth on UV-enhanced electrochemically etched nanoporous GaN Appl. Phys. Lett. 98, 191906 (2011); 10.1063/1.3589969 The fabrication of GaN-based nanopillar light-emitting diodes J. Appl. Phys. 108, 074302 (2010); 10.1063/1.3488905 Emission color control from blue to red with nanocolumn diameter of InGaN/GaN nanocolumn arrays grown onsame substrate Appl. Phys. Lett. 96, 231104 (2010); 10.1063/1.3443734 Optical properties of InGaN/GaN nanopillars fabricated by postgrowth chemically assisted ion beam etching J. Appl. Phys. 107, 023522 (2010); 10.1063/1.3280032 Optical and structural properties of GaN nanopillar and nanostripe arrays with embedded In Ga N ∕ Ga N multi-quantum wells J. Appl. Phys. 100, 054314 (2006); 10.1063/1.2234812

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Anodic nanoclusters of GaNKeunjoo Kima�

Department of Mechanical Engineering, Chonbuk National University, Jeonju 561-756, Republic of Koreaand Research Center for Industrial Technology, Chonbuk National University, Jeonju 561-756,Republic of Korea

Jaeho ChoiDepartment of Mechanical Engineering, Chonbuk National University, Jeonju 561-756, Republic of Koreaand Research Center for Industrial Technology, Chonbuk National University, Jeonju 561-756,Republic of Korea

Tae Sung BaeJeonju Center, Korea Basic Science Institute, Jeonju 561-756, Republic of Korea

�Received 19 March 2007; accepted 5 April 2007; published online 1 May 2007�

The authors report an anodization of the deposited Al layer on a p-GaN surface of InGaN/GaNmultiquantum-well light-emitting-diode structures, which forms the anodic nanoclusters of GaN aswell as the disordered alumina nanopore layer. The GaN nanoclusters show the shape of the radialhemisphere similar to an orange. The formation mechanism comes from the nanofluidic channel forsupplying the electrolyte in electrochemical etching reaction. The nanorods with a diameter of about100 nm in nanocluster structures enhance the photoluminescence intensity by three times comparedto the bare sample without anodization. © 2007 American Institute of Physics.�DOI: 10.1063/1.2734901�

Anodic porous semiconductor materials have been ex-tensively investigated as key materials for the fabrication ofnanometer-scale structures in nanoelectronics andnanophotonics.1 Especially, various semiconducting materi-als have been investigated for the optoelectronic property ofporous structures. Porous structures can cause quantum sizeeffect for 1 nm sized crystallites and enhancement of thephoton extraction from 100 nm scaled crystallites at the sur-face of semiconductors. The porous Si nanostructure as a Siquantum wire array showed photoluminescence �PL� at670–700 nm1,2 and large-area photonic crystals showed pho-tonic band gaps in the midinfrared spectral range.3 The po-rous GaAs nanocrystallites showed enlarged intensities in theinfrared PL centered at 880 nm and in the green PL centeredat 540 nm.4 The nanocolumnar porous GaN has cross-sectional diameters of 1–4 nm having light emissions at 334and 358 nm compare with the light emission at 369 nm fromthe bulk GaN indicating the quantum size effect.5 And also,the nanoporous GaN with diameter of 70 nm showed en-hancement of light emission without the shift of the peakposition.6 Until now, there is no report on the anodization ofsemiconductors through a nanohole for supplying the elec-trolyte.

In this letter, we report on the formation of anodic nano-clusters of GaN on InGaN/GaN multiquantum well light-emitting-diode �LED� structure and their effect on the en-hancement of photoluminescence from the InGaN/GaNmultiquantum-well structures. After the electrochemical an-odization of the deposited Al on GaN surface, the formedalumina nanopores were used for the anodization of GaN asa nanofluidic channel for the transport of electrolyte.

The blue LED structure shown in Fig. 1 was grown bymetal-organic chemical vapor deposition in the vertical modeof the reactor. A sapphire substrate was heated to 1055 °C in

a stream of hydrogen and the substrate temperature was thenlowered to 520 °C in order to grow the GaN nucleationbuffer layer with thickness of about 350 Å. The 1-�m-thickundoped GaN film and the 2-�m-thick Si-doped n-type GaNfilm have been sequentially grown at a temperature of1055 °C and a pressure of 500 Torr. The five-periodInGaN/GaN multiquantum-well �MQW� structure with athickness of 20/100 Å was grown at a temperature of720 °C at 200 Torr. Subsequently, the Mg-doped p-typeGaN film with a thickness of 0.25 �m was grown at a tem-perature of 1020 °C and a pressure of 200 Torr. The LED

a�Electronic mail: kimk@chonbuk.ac.kr

FIG. 1. �Color online� Schematic diagrams of anodic process of GaN nano-clusters of InGaN/GaN MQW LED structure with the image of AAO layerand the energy band diagram at the interface between p-GaN and the elec-trolyte without electric bias.

APPLIED PHYSICS LETTERS 90, 181912 �2007�

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wafer was processed by a rapid thermal annealing at 550 °Cin order to achieve the p-type activation of Ohmic contact.

A 0.9-�m-thick Al layer was deposited on a p-GaN sur-face of InGaN/GaN MQW blue LED structure usingelectron-beam evaporation. The clean Al layer was achievedby the pretreatment of electropolishing in a mixed solution ofperchloric acid and ethanol �1:4 in volume� for 60 s. The firstanodization was performed by applying a dc voltage of 40 Vin a 0.3M oxalic acid �H2C2O4� solution at 3 °C for 20 min.7

From the anodization the anodized aluminum oxide �AAO�layer was grown on the hexagonal texture, as shown in Fig.1. After removing the porous alumina layer by wet chemicaletching in phosphoric acid �5 wt % � at the temperature of30 °C for 20 min, the second anodization was carried out inthe same condition, and in certain nanopores where the holeswere open to the GaN surface, the anodization process ofGaN happened. The further plasma etching process in theBCl3 and Cl2 mixed ambient has been processed and finally,the remaining AAO and Al layers were removed again bywet chemical etching.

Figure 2�a� shows a field-emission scanning electron mi-croscope �FE-SEM� image of a surface view of the randomlyoriented anodic nanoclusters of GaN. After removing theAAO and Al layers, the surface of GaN shows the formationof the nanoclusters. The diameters of GaN nanoclusters were1–3 �m. The anodic electrodes have contacted at multiplepoints and formed radial-directed nanopores. Furthermore,there exists an empty and concave hemisphere of nanoclus-ters where nanorods were resolved out, as shown by the ar-row. Figure 2�b� shows the enlarged surface FE-SEM imageof GaN nanoclusters. The surface image shows a nanodotlikeembossing shape and radial nanorods in a circular shape ofnanocluster. Figure 2�c� shows a hemispherical nanoclusterwith a diameter of 100 nm similar to the shape of orange.This implies that the electric field focused from epitaxiallayers to a point at the surface. The depth of nanoclusterincludes the region of InGaN/GaN MQW active layer.

The formation mechanism of nanoclusters can be a sur-face corrosion through the nanofluidic channels and the holetransport at the interface between the p-GaN and the electro-lyte can be explained in the energy band diagram shown inFig. 1. A two electrode cell consisting of a GaN work elec-trode and a Pt counterelectrode is used and the reaction hap-pened at a constant current.8 It is supposed that the initiationof the porous structure occurs at localized regions on theGaN surface where random defects create minute concavedepressions.9 The surface corrosion of p-type GaN requires asupply of valence band holes at the surface, thus, p-type GaNcould be etched in the dark by supplying holes. Once thesurface corrosion is initiated, pores propagate through thematerial under the combined influence of the electric field,the hole concentration, and the preference of certain crystal-lographic planes similar to the porous Si formation.10

The nanoporous GaN structure is formed by anodic etch-ing of p-type GaN with an acceptor density of 2�1017 cm3,corresponding to the Fermi level of Ef =0.13 eV above theedge of the valence band.11 The p-type GaN surface wasexposed to the 0.3M C2H2O4 electrolyte. During the electro-chemical reaction process, the electrochemical potential ofthe electrolyte of Ef ,r is similar to the Fermi level in thesemiconductor Ef =0.13 eV. The energy barrier Ehb of bandbending for the metal/p-GaN interface has been reported torange from 0.49 to 2.47 eV, which gives an estimation for

the electrolyte/p-GaN interface.12 The corresponding spacecharge layer LSC of depletion width for the hole concentra-tion of 2.6�1017 cm−3 is about 44–100 nm. For the electro-lyte of 0.3M oxalic acid, the Helmholtz layer LH and thediffuse layer LD are about 3–10 nm and 0.3–0.8 mm,respectively.13,14

Application of a strong positive potential of 40 V versusnormal hydrogen electrode leads to severe band bending atthe p-GaN/electrolyte interface and the hole transport viananofluidic channels of nanopores was strongly enhanced forelectrochemical etching reaction. Since the band bendingrepresents a barrier for charge transfer with the electrolyte,interband tunneling of electrons from the valence band orfrom band gap states to the conduction band takes place,

FIG. 2. FE-SEM images of anodic nanoclusters of GaN. The single oroverlapped nanocusters have radial nanorods in the hemispherical shapesimilar to oranges. The arrow shows a shell without nanorods.

181912-2 Kim, Choi, and Bae Appl. Phys. Lett. 90, 181912 �2007�

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generating holes at the surface, which are consumed in an-odic dissolution of GaN. The lower the barrier is the fasterthe etching. The electric field also enhances at the pore tips,which attracts and concentrates valence band holes in thisregion. For the sample of GaN thin film deposited on sap-phire insulating substrate, the electric field forms to the ra-dial direction for a point charge providing the path of thepore direction. The dimension of the pores was determinedby the depletion of charge for the tunneling of the barrier,and the corrosion reaction terminates. The nanorodlike shapecan affect the optical property in PL measurement.

Figure 3 shows room temperature PL spectra of theInGaN/GaN multiquantum-well active layers for the opticalpumping of He–Cd laser with an excitation wavelength of325 nm. The blue light emission from the bare sample with-out the anodic GaN nanoclusters was observed at the PLpeak of 474 nm. The oscillating PL intensity in bare sampleindicates that the epitaxial thin film forms the optical reflec-tance in mirrorlike surface. The peak positions between thesamples with and without nanoclusters were similar to474 nm. The PL intensity was strongly enhanced, and eventripled by introducing anodic etching process compared tobare sample without the anodization. With the further plasmaetching on the anodic GaN nanoclusters, the sample shows alittle change in both the light enhancement and the peak shift�470 nm�. The nanoclusters with micrometer-sized diametersspanned to the InGaN/GaN active quantum well layers. Theplasma etching could not reduce the diameters of nanorods to1 nm size in the active layers to show the quantum confine-ment of the electron carriers. Furthermore, no Raman shiftfrom nanoclusters of GaN was measured.

For the roughened surface of p-GaN through the photo-electrochemical �PEC� process, the PL intensity showed en-hancement up to 1.42 times at 300 K and the peak at 464 nmfor the bare sample is blueshifted to 456 nm, indicating thepartial reduction of the piezoelectric field. However, for theroughened surface of n-GaN through the combined laser lift-

off and PEC etching process,15 the electroluminescence in-tensity was strongly enhanced and even tripled at 300 K andthere was no peak shift. The critical angle for a light escapecone from Snell’s law is about 23° for GaN �n=2.5� and thelight extraction ratio from the top surface is around 4%, in-dicating that the increase of the external quantum efficiencyby introducing the surface roughening is more importantthan that of internal quantum efficiency. Therefore, the for-mation of nanoclusters on the surface can also effectivelyenhance the light extraction due to the surface roughnesseffect.

In summary, we have fabricated anodic nanoclusters ofp-GaN layer of blue LEDs. We deposited Al on the p-GaNlayer and anodized Al to form the nanofluidic channel ofalumina nanopore layer the electrolyte. The anodic dissolu-tion of the p-GaN layer through the nanochannel of nanop-ores for the transport of electrolyte of the oxalic acid solutionwas carried out. For the sample with anodic nanoclusters ofGaN compared to the bare sample without the anodization,the photoluminescence property was strongly enhanced. Thenanorods with the dimension of 100 nm diameter provide nopeak shift in photoluminescence indicating no quantum sizeeffect but the enhancement of light extraction due to thesurface roughness effect on multiple-photon scattering pro-cess.

This work was supported through the Korean ResearchFoundation Grant No. �KRF-2004-041-D00296� and theChonbuk National University funds for overseas research�CBNU-11-2007�. One of the authors �K.K.� is visiting forresearch works at the Department of Electrical Engineeringand Computer Science and Solid State Electronics Labora-tory, The University of Michigan, Ann Arbor, Michigan48109, U.S.A.

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FIG. 3. �Color online� PL spectra of InGaN/GaN MQW LED samples withnanoclusters of GaN compared to the bare sample. The plasma etching onthe anodic GaN nanoclusters shows a little change in both the light enhance-ment and the peak shift.

181912-3 Kim, Choi, and Bae Appl. Phys. Lett. 90, 181912 �2007�

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