Journal of Colloid and Interface Science 363 (2011) 446–449

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Journal of Colloid and Interface Science

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Fabrication of 2D photonic crystals using block copolymer patternson as grown LEDs

Md. Mahbub Alam, Jin-Yeol Kim, Woo-Gwang Jung ⇑Department of Advanced Materials Engineering, Graduate School of Kookmin University, Seoul 136-702, Republic of Korea

a r t i c l e i n f o

Article history:Received 11 April 2011Accepted 26 July 2011Available online 2 August 2011

Keywords:Block copolymerNanopatternLight emitting diodeReactive ion etchingPhoto enhanced chemical etchingPhotoluminescenceElectroluminescence

0021-9797/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.jcis.2011.07.084

⇑ Corresponding author. Fax: +82 2 910 4320.E-mail address: wgjung@kookmin.ac.kr (W.-G. Jun

a b s t r a c t

Di-block copolymer polystyrene-block-polymethyl methacrylate (PS-b-PMMA) was used to make pat-terns over a large area of as grown LEDs. The polymer patterns on LEDs surface could be transferred tothe underlying p-GaN, the topmost layer of as grown LEDs by both reactive ion etching (RIE) andphoto-enhanced chemical (PEC) etching. Removal of remaining polymer chains results in patterned LEDswhich shows higher light extraction efficiency. In our experiment, much higher intensity for patternedLEDs in both photoluminescence (PL) and electroluminescence (EL) data plot were found. Similarimprovements were found in I–V and L–I curves for patterned LEDs.

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1. Introduction

Due to the limitations of conventional lithographic techniques,alternative methods for patterning the substrates on a nanoscalelevel have attracted considerable interest. Among those methods,block copolymer lithography appears very promising for creatingregular periodic patterns. Successful nanopatterning has beenreported using block copolymers with spherical [1–4] and cylindri-cal [3–9] morphologies. Self-assembly of block copolymers is a ver-satile way to prepare nanoparticles and to control their size, shape,and location [10–12]. One commonly studied diblock copolymer ispolystyrene-block-methyl-methacrylate (PS-b-PMMA) where theblocks PS and PMMA are chemically and physically distinct.

Gallium nitride (GaN) has become a prominent material in theoptoelectronics market, with numerous evolutionary products thathave been commercialized such as light-emitting diodes (LEDs)and laser diodes [13]. Recently, as the brightness of GaN-basedLEDs has increased, applications such as displays, traffic signals,backlights for cell phones, exterior automotive lighting, and print-ers have become possible. But due to the large difference in refrac-tive indices between GaN materials and outer ambient air, only alimited fraction of extracted photons can escape from the LEDs tothe air. Research into improving the light extraction efficiency(external quantum efficiency) and brightness in the LEDs has beenintense. In particular, rapid progresses have been reported in the

ll rights reserved.

g).

past several years to increase the light extraction from the LEDby roughening or incorporating the photonic crystal structures intothe emitting surface with the help of nanoscale patterning technol-ogy such as porous anodic alumina [14], laser interface lithography(LIL) [15–18], e-beam lithography (EBL) [19], nanoimprint lithogra-phy [20] and colloidal lithography [21]. In this paper, an approachis presented to fabricate photonic crystal structure using blockcopolymer self-assembly on as grown LEDs, resulting much higherlight extraction efficiency.

2. Materials and methods

Diblock copolymer PS-b-PMMA with Mn: PS (46,100), PMMA(21,000) and Mw/Mn: 1.09, having a PS volume fraction of 0.69(Polymer Source, Inc.) was used as polymer materials. One wt.%polymer solution in toluene was spin-coated onto as grown LEDssurface to form a thin polymer film. The samples were then bakedfor 48 h with a continuous flow of Ar gas at approximately 200 �C,which is well above the glass transition temperatures (Tg) of bothPS (100 �C) and PMMA (115 �C). During baking, the polymer com-ponents self-assemble into a phase separated layer on the sub-strate surface. Being negative photoresist, PMMA chains getdegraded upon exposure to UV irradiation and could be easilyremoved by rinsing in acetic acid followed by wishing with DIwater to get PS cylindrical patterns. A certain depth of the top layerof as grown LEDs, p-GaN was etched along the gaps of PS cylindersby means of both reactive ion etching and photo enhanced chem-ical etching. Cl2/Ar gas was used for reactive ion etching and the

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samples were stained with ruthenium tetroxide (RuO4) vapor priorto the etching process [22]. For photo enhanced chemical etching,the samples were kept on 0.04 M KOH solution and exposed to UVexposure. Finally the remaining PS cylinders were removed usingacetone which results in patterned LEDs.

The surface morphology was examined by field emission scan-ning electron microscopy (FE-SEM, JEOL JSM-7401F). The opticalproperty of patterned LED was measured by photoluminescencespectroscopy (PL, Accent RPM 2000) with Nd-YAG laser as the exci-tation light source with wavelength of 266 nm at room tempera-ture. The performance of patterned LED was made byelectroluminescence measurement (Optel-Precision OPI-150) atthe injected current of 20 mA.

3. Results and discussion

Diblock copolymers are macromolecular surfactants those arecapable of spontaneous self-organization into several orderedmicrophases, depending principally on molecular composition, ifthe constituent (A and B) sequences are sufficiently incompatible[23,24]. With care, the spin coating method can give films withlow surface roughness over areas of square of millimeters. The filmthickness can be controlled through the spin speed, the concentra-tion of the block copolymer solution or the volatility of the solvent,which also influences the surface roughness [25]. Polystyrene (PS)and poly methylmethacrylate (PMMA) have significantly differentphotodegradation properties. PMMA is known to be a negativephoto resist, i.e., with UV or electron beam irradiation, the polymer

Fig. 1. (a) High magnification and (b) low magnification images of the PS patternson as grown LEDs.

chains get degraded via chain scission. The chemical processes tak-ing place in PS upon exposure to deep UV radiation, on the otherhand are less well defined, with cross linking, chain scission, andoxidation taking place [8]. Deep UV exposure of ordered PS-b-PMMA should, therefore, lead to a degradation of the PMMA block,whereas the PS matrix becomes insoluble. The degradation prod-ucts from PMMA can then be rinsed away, leaving a patternedsurface.

The PS cylindrical patterns after removing PMMA can be seen inthe FE-SEM images (Fig. 1). In our experiment, to ensure the re-moval of unreacted polymer chains the samples were sonicatedin toluene after baking and to ensure the complete removal ofPMMA chains after UV exposure, the samples were sonicated inacetic acid for small times. The patterns could be made throughoutthe entire surface. Fig. 1a shows the high magnification SEM imageof the surface patterns of PS cylinders and Fig. 1b is the low mag-nification SEM image where it shows the uniformity of the patternsover the whole surface.

The PS pattern on as grown LED could be transferred to under-lying p-GaN BY reactive ion etching (RIE). Before so doing, the sam-ples were exposed to ruthenium tetroxide vapor, which yields PScylinders containing Ru atoms and therefore exhibits improvedchemical and thermal stability. In our experiment, the RIE has beendone with Cl2/Ar gas having flow rates 10/25 sccm using an induc-tively coupled plasma (ICP) reactor. ICP/bias power = 300/100 Wwas used with a chamber pressure of 5 mTorr.

After etching, the PS polymer chains were removed using ace-tone. The removal of PMMA leaves the LEDs with p-GaN patternson its surface. The patterns of p-GaN on LEDs surface can be seen

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Fig. 2. (a) The patterned surface of LEDs after RIE and removing PMMA and (b) thecomparison of PL intensity of as grown and patterned LEDs.

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Fig. 3. (a) SEM image of patterned LEDs surface found by wet etching and (b)comparison of PL intensity of as grown and patterned LEDs.

448 Md.M. Alam et al. / Journal of Colloid and Interface Science 363 (2011) 446–449

in the SEM image shown in Fig. 2a and b shows the comparison ofphotoluminescence (PL) intensities of as grown and patternedLEDs.

The patterned LEDs show much higher PL intensity than that ofas grown LED. The reason for the higher photoluminescence inten-sity is due to the variety of angles it offers for the light extraction asa result of surface roughening. Moreover the surface area of thepatterned LEDs is much higher than as grown LEDs and the largersurface area could be another reason for the better PL efficiency.During RIE high voltage is needed to apply which may lead tothe damage of PS masks. So, it is very difficult to get smooth etchedsidewall as can be seen in Fig. 2a. To avoid this problem, we haveetched p-GaN by photo-enhanced chemical (PEC) etching which isone of the common wet etching techniques for GaN. The sampleswere kept in 0.04 M KOH solution in DI and exposed to UV irradi-ation at wavelength 254 nm for 15 min and no electrodes or otherinstruments were used. The remaining PS domains were then re-moved by acetone. The SEM image of patterned LEDs found bywet etching can be seen in Fig. 3a.

The wet etched sample has the p-GaN patterns on the surface ofas grown LEDs similar to those as the PS patterns. Fig. 3a shows thesmooth and undamaged patterned LEDs surfaces which show sig-nificant improvement in light extraction and gives much higherintensity in PL data plot (Fig. 3b) in comparison to those foundby RIE. The origins of the optical enhancement through the pho-tonic crystal can be explained from various points of views. Thepresence of air-holes converts the otherwise smooth surface intoroughened surface, which promotes light extraction via geometri-cal effects. The air-holes onto the surface also offer a large surfacearea for photon outcoupling via a wider range of emission angles.

For electroluminescence data measurement, the etching wasperformed under an inductively coupled Cl2/CH4/H2/Ar (30/8/8/16 sccm) plasma in order to expose n-GaN surface. The N2 plasmatreatment at 30 W of RF power (dc bias of�1 V) on the etched LEDs

as grown LED patterned LED

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Fig. 4. Comparison of (a) electroluminescence data, (b) I–V data and (c) L–I data of as grown and patterned LEDs.

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was performed in the PECVD via a load-lock chamber immediatelyafter the etching process, thus ensuring that the samples were notexposed to air. Thus electrodes in n-GaN (Ti/Al) and p-GaN (Ni/Au)were deposited and annealed at 550 �C for 5 min in a rapid thermalannealing (RTA) system.

Electroluminescence (EL) of the devices was demonstrated atroom temperature in probe measurement geometry and the corre-sponding optical spectra were collected via an optical fiber. Fig. 4ashows the EL spectra for both as grown LEDs and patterned LEDs atthe injected current of 20 mA. The EL intensity for the patternedLEDs was significantly higher than that of as grown LEDs, henceproves the higher light extraction rate for the incorporation of 2Dphotonic crystal structure in an as grown LED.

Similar improvements were found for current–voltage (I–V) andluminescence–current (L–I) data plots. The patterned LEDs showmuch higher slopes for both I–V curve and L–I curve than thoseof as grown LEDs shown in Fig. 4b and c. Larger surface area dueto the patterning on as grown LED could be the reason for suchimprovements in I–V and L–I data for patterned LED samples.

PS patterns could be made on the surface of as grown LEDs over alarge area using block copolymer PS-b-PMMA. The etching of p-GaNalong the holes and subsequent removal of PS resulted in patternedLEDs which shows significantly better optical efficiency as found inphotoluminescence and electroluminescence intensities. Photo en-hanced chemical etching is an effective way to avoid the damageof PS masks in case of reactive ion etching. The patterned LEDs showbetter performance in current–voltage data and luminescence–cur-rent data too. Although it was not clear that the improvement camefrom the photonic crystal effect or from just random scattering dueto surface roughening.

4. Summary

High density arrays of nanostructures over large area can beformed by self-assembly of block copolymers on GaN substrate.The PS patterns on as grown LED could be used as masks to etchp-GaN to obtain 2D photonic crystal structure. The pattern ofPS-b-PMMA block copolymer was transferred to the surface p-GaN of as grown LEDs by the reactive ion etching (RIE) andphoto-enhanced chemical (PEC) etching. The patterned LED showsthe significant improvement in light extraction and gives muchhigher intensity in PL and EL data plots. This procedure can be ap-plied to enhancing the efficiency of LEDs and as well as a widerange of substrates, which would allow the fabrication of uniquetools for nanometer scale electrical devices.

Acknowledgments

This study was supported by the ERC (Center for Materials andProcesses of Self Assembly) Program of MOST/KOSEF (R11-2005-048-00000-0) and the Basic Science Research Program throughthe National Research Foundation of Korea (NRF) funded by theMinistry of Education, Science and Technology (KRF-2008-313-D00521).

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