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Scripta Materialia 77 (2014) 13–16

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Laser lift-off transfer printing of patterned GaN light-emittingdiodes from sapphire to flexible substrates using a Cr/Au laser

blocking layer

Jaeyi Chun,a Youngkyu Hwang,b Yong-Seok Choi,b Jae-Joon Kim,a Tak Jeong,c

Jong Hyeob Baek,c Heung Cho Kob and Seong-Ju Parka,b,⇑aDepartment of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of KoreabSchool of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea

cLED Device Research Center, Korea Photonics Technology Institute, Gwangju 500-779, Republic of Korea

Received 20 December 2013; revised 3 January 2014; accepted 3 January 2014Available online 9 January 2014

We develop a method to directly transfer the array of GaN-based light-emitting diodes (LEDs) from sapphire onto flexiblesubstrates by a laser lift-off (LLO) process. Cr/Au layers are employed as a laser blocking layer to protect the supporting polymerlayers from the laser beam and sharply separate the LEDs from the sapphire during the LLO process. This method dramaticallyincreases the transfer yield of patterned LEDs up to 95% by decreasing the laser-induced damage in the supporting polymer layersand LEDs.� 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: GaN-based light-emitting diodes; Transfer printing; Laser lift-off; Flexible display

GaN-based light-emitting diodes (LEDs) havebeen widely used for high-performance solid-state light-ing systems due to their high internal and external quan-tum efficiencies, low power consumption and long-termstability [1–3]. Recent research has demonstrated amethodology to fabricate flexible LEDs by patterningthe GaN layer grown on a silicon or sapphire substrateinto printable formats and transferring the patterns ontounconventional flexible polymer substrates [4–9]. Thistechnology enables the realization of next-generationdeformable displays and biomedical/implantable opto-electronic systems [10–12].

Maintaining the spatial alignment and orientation ofthe GaN LED array is crucial in the transfer printingprocess. In particular, high-quality GaN LEDs grownepitaxially on a sapphire substrate require careful trans-fer onto flexible target substrates without misalignmentbecause the GaN LEDs should be repetitively trans-ferred to and from dissimilar supporting substrates after

1359-6462/$ - see front matter � 2014 Acta Materialia Inc. Published by Elhttp://dx.doi.org/10.1016/j.scriptamat.2014.01.005

⇑Corresponding author at: School of Materials Science and Engi-neering, Gwangju Institute of Science and Technology, Gwangju500-712, Republic of Korea. Tel.: +82 62 715 2309; fax: +82 62 7152304; e-mail: sjpark@gist.ac.kr

the array of GaN LEDs is separated from the sapphiresubstrate by the laser lift-off (LLO) process [6–8]. How-ever, direct transfer printing by adhering a flexible sub-strate onto the GaN LEDs and then removing thesapphire substrate can provide an effective pathway toreduce complex process steps and preserve the originallayout of the GaN LED array. Furthermore, directtransfer printing could be developed into a cost-effectivemass production method for flexible devices with extre-mely small spacing or with a complicated layout. To di-rectly transfer an array of GaN LEDs fabricated onsapphire to a flexible substrate, polymer-based adhesivesshould be used to adhere the GaN LEDs onto the flex-ible target substrate. However, employing a polymer-based supporting layer or adhesive introduces anotherproblem associated with the LLO process. The intenselaser beam penetrating between the GaN LED chipscould induce considerable stress on the polymer-basedlayers, thereby generating unwanted damage, such ascracks in the polymer-based supporting or adhesive lay-ers. Therefore, a new strategy should be developed toprotect the polymer layers or glue from the intense laserbeam and ensure a high yield of transfer printing ofarrays of GaN LEDs.

sevier Ltd. All rights reserved.

14 J. Chun et al. / Scripta Materialia 77 (2014) 13–16

Here, we report on a method to directly transfer GaNLEDs from sapphire substrates to flexible substrateswith polymer glue and supporting layers. We introducea laser blocking layer (LBL) to prevent unwanted pene-tration of the laser beam in the region between the GaNLED chips and enhance the sharp separation of theLEDs and supporting polymer layers from the sapphiresubstrate. To demonstrate the efficacy of this method,we show the successful transfer of 22 � 23 GaN LED ar-rays onto a flexible poly(ethylene terephthalate) (PET)film with an adhesive of conductive epoxy and the elec-troluminescence (EL) emission from the GaN LEDs indeformed configurations.

Figure 1 presents a schematic illustration of the fab-rication of printable formats from a GaN LED layergrown on a sapphire substrate and the transfer printingof GaN LEDs onto a PET substrate by the LLO pro-cess. The GaN LED film, with a total thickness of3.5 lm on the sapphire substrate consisted of n-GaN(3.2 lm), five pairs of InGaN/GaN (3 nm/7 nm) multi-ple quantum wells (MQWs) and p-GaN (250 nm). Theprocess began with patterning an Ni masking layer onthe top of the GaN LED layer and etching the unpro-tected GaN region by inductively coupled plasma reac-tive ion etching to produce patterned LED pixels(lateral size of each pixel: 300 lm � 300 lm) (Fig. 1a).A 1 lm thick SiO2 film deposited by plasma-enhancedchemical vapor deposition was used to passivate the en-tire area of the GaN LEDs, including the sidewalls ofthe GaN LED chips. After the patterning of photoresist(PR) by conventional photolithography, etching of theunprotected SiO2 region in a buffered oxide etchant(BOE) generated the via-holes for metal contacts. Elec-tron-beam (e-beam) evaporation of Ni/Au (5 nm/10 nm) and lifting off of the residual PR followed byannealing of the sample at 500 �C for 1 min provide p-ohmic contacts (Fig. 1b). Next, a Cr/Au (50 nm/

Figure 1. (a–f) Schematic illustration of the fabrication steps totransfer GaN LEDs onto a PET film.

150 nm) layer was deposited as an LBL over the entirearea by e-beam evaporation (Fig. 1c). The LBL shouldprevent the penetration of laser beams, thus protectingthe upper organic epoxy layers from unwanted laserirradiation, and could be ablated by the excimer laserduring the LLO process. Next, the sidewalls of theGaN LEDs were coated and patterned with SU-8 tosmooth and improve the adhesion of a glue layer(Fig. 1d). After contacting a PET film coated with con-ductive epoxy (ITW Chemtronics, CW2400) to the sam-ple for bonding, it was cured at 90 �C for 90 min(Fig. 1e). The back side of the sapphire substrate wasthen scanned with a KrF excimer laser beam (kmax:248 nm, 750 mJ cm�2, beam spot: 400 lm � 400 lm) toseparate the GaN LED patterns and transfer them ontothe PET substrate (Fig. 1f). Finally, the sample wasdipped in diluted HCl, Cr/Au etchants and BOE to re-move any residual Ga, Cr, Au and SiO2 on the surfaceof the separated GaN LED chips.

Figure 2 illustrates the advantage of using an LBL forthe direct transfer printing of patterned GaN LEDs onthe PET substrate. During the LLO process, irradiationof the GaN LED region with the excimer laser inducesphotothermal decomposition of GaN into Ga dropletsand gaseous N2, thus causing interfacial fracture be-tween the patterned GaN LEDs and the sapphire sub-strate (Region A of Fig. 2a) [13–15]. On the otherhand, the intense laser beam is prevented from passingthrough the GaN-free region between the GaN LEDchips (Region B of Fig. 2a) by the LBL (yellow layerin Fig. 2a) due to the very low transmittance of theCr/Au layer at a laser wavelength of 248 nm (Fig. 2b).

Figure 2. (a) Schematic illustration of the LLO process to transferGaN LEDs onto a PET film by employing a Cr/Au LBL. (b) Opticaltransmittance spectra of sapphire, Cr (50 nm)/sapphire and Cr(50 nm)/Au (150 nm)/sapphire. The dashed line is the wavelength ofthe KrF excimer laser, which is 248 nm. (c) Cross-sectional SEM imageof the GaN LED transferred onto a silicon substrate. The siliconsubstrate was used to cleave the sample easily. (d) Magnified image ofthe boxed area of (c). (e) Schematic illustration of the LLO process totransfer GaN LEDs onto a PET film without using an LBL. (f)Transfer yield and representative photographs of the GaN LEDstransferred onto the PET film by using the LLO process with andwithout an LBL.

J. Chun et al. / Scripta Materialia 77 (2014) 13–16 15

Figure 2c and d shows that the conductive epoxy isprotected by an LBL and that the LEDs are separatedfrom the sapphire substrate. The inner polymer layerand SiO2 passivation layer are also detached from thesapphire substrate after the LLO process. The effectiveprotection of the inner polymer layers from the intenselaser beam by the LBL can provide more freedom inchoosing the flexible organic materials, such as conduc-tive epoxy and PET, for various device applications. Inaddition, the ablation of the LBL metal layer by the la-ser beam would also enhance the sharp separation evenin Region B, to give a high yield of transfer printingonto the PET substrate. In contrast, without the LBL,as shown in Figure 2e, the intense laser beam damagedthe conductive epoxy layer and even caused the GaNLEDs to fall apart from both the sapphire substrateand the PET film. As a result, there is a large differencein the transfer yield of GaN LEDs with and without anLBL, as presented in Figure 2f. Introducing an LBL en-ables the GaN LED pixels and PET film to remainstrongly adhered, resulting in a high transfer yield upto 95% with the correct arrangement (see the uppertwo photographs of the inset in Fig. 2f). Otherwise,the transfer yield is very low because a large numberof GaN LED pixels are lost from the PET substrate(see the lower photograph of the inset in Fig. 2f).

To understand the microscopic process of the sharpseparation of the GaN-free region upon exposure to alaser beam, the surface of the separated sapphire sub-strate after the LLO process was examined by opticalmicroscopy (OM), scanning electron microscopy(SEM) and energy-dispersive X-ray spectrometry(EDX). The contrasts in the OM image (Fig. 3a) indi-cate the regions corresponding to the GaN LED squarepatterns that had already been lifted off and the remain-ing area of the GaN-free region. Next, EDX peaks of Al(1.48 keV), Si (1.74 keV) and Au (2.12 keV) which comefrom the sapphire, SiO2 passivation layer and LBL,respectively, were measured from the GaN LED region(Zone-A), the GaN-free region (Zone-B) and the edge of

Figure 3. (a) OM image of the sapphire donor substrate after the LLOprocess. (b) SEM image of the selected area of (a). (c) The relativeatomic proportions of Au, Si and Al from EDX data in the selectedarea of (b). (d) Magnified SEM image of Zone-B in (b). (e) The relativeatomic proportion of Au, Si and Al from EDX data in the selectedarea of (d).

the laser beam spot in the GaN-free region (Zone-C) inFigure 3b. The atomic composition of 98.3% Al and1.7% Si in Zone-A, as shown in Figure 3c, indicates thatthe separation of the GaN LEDs mainly occurs at theinterface between the GaN and the sapphire substrate.The increased atomic composition of Au in Zone-Ccompared with that in Zone-B is presumably related tothe faster resolidification caused by an edge effect inthe laser-exposed area [16,17]. The droplet-like surfacemorphology in Zone-B, shown in Figure 3d, originatesfrom the vaporization of the Cr/Au LBL metals by thelaser beam, which has also been observed by othergroups after laser ablation of a Cr or Au surface[16,17]. As shown in Figure 3e, the different atomic ratioof Al/Si/Au of 0%/11.6%/88.4% in the droplet region(Zone-B1 in Fig. 3d) compared with 2.3%/94.6%/3.1%in the remainder (Zone-B2 in Fig. 3d) also indicates thatthe droplets are formed from the LBL metal by the in-tense laser beam. The EDX analysis and SEM imagein Figure 2c strongly indicate that the LBL provides asharp delamination plane without significant damageon the inner polymer layers during the LLO process.

Figure 4a shows 22 � 23 arrays of LEDs (dark gray)fabricated by LBL-assisted LLO transfer printing onto aPET film coated with conductive epoxy (silver). Thetransfer yield of GaN LEDs from the sapphire substrateto the PET film was 95%. The EL image obtained at4 mA from a randomly selected LED pixel indicates thatlaser-induced damage was not introduced in the deviceduring the LLO process (inset of Fig. 4a). The EL emis-sion from the LEDs upon bending up to 13 mm in ten-sile mode and 26 mm in compressive mode confirms themechanical flexibility of the device under the variousbending configurations (Fig. 4b and c). In particular,the successful EL emission upon folding the GaN-freeregion by 90�, as shown in Figure 4d, also indicates thatthe LBL protects the inner layers, such as the conductiveepoxy layer, during the LLO process. We also measuredthe EL spectra of the GaN LEDs before and after trans-fer printing, as shown in Figure 4e. The GaN LEDs on asapphire substrate show an EL peak at 463 nm withweak shoulders at an injection current of 4 mA. Onthe other hand, the GaN LEDs on a PET substrateshowed an EL peak wavelength of 469 nm with moreinterference fringes. The strong interference fringes areassociated with enhanced Fabry–Perot interference bya reflector of LBL (yellow layer in the inset of Fig. 4e)under the GaN LED chips [18]. The 6 nm red shift ofthe EL peak is attributed to the increased junction tem-perature in the transferred GaN LEDs caused by thepoor heat dissipation due to the relatively low thermalconductivities of the epoxy and polymer substrate com-pared to the sapphire [19,20]. This result is similar to theprevious reports on GaN LEDs transferred onto flexiblepolymer substrates [6,7]. It is believed that the EL peakshift due to low heat dissipation can be improved byintroducing nanoparticles to the polymer materials forhigh thermal conductivity or designing micro/nanosizetextured reflectors as efficient heat sinks for LEDs[21,22]. Figure 4f shows the light output power–cur-rent–voltage (L–I–V) characteristics of GaN LEDs ona PET film. The device shows a typical rectifying I–Vcurve and the linear increase in the light output power

Figure 4. (a) Photograph of flexible 22 � 23 GaN LEDs on a PET filmthrough LLO transfer printing using a Cr/Au LBL. The inset of (a)shows a zoomed optical microscope image of EL from a randomlyselected pixel of the GaN LEDs at an injection current of 4 mA. (b–d)Photographs of EL from a randomly selected pixel of the GaN LEDsat an injection current of 4 mA upon bending up to 13 mm in tensilemode (b) and 26 mm in compressive mode (c), and upon folding 90�against the edge of acryl cube (d). The insets of (b–d) show cross-sectional photographs of the deformed configurations of the device. (e)Representative normalized EL spectra at 4 mA for GaN LEDs beforeand after transfer printing. The inset of (e) illustrates the configurationof GaN LEDs on a PET film. (f) Representative light output L–I–V

characteristics of GaN LEDs on a PET film.

16 J. Chun et al. / Scripta Materialia 77 (2014) 13–16

with increasing injection current, indicating that theGaN LEDs are not significantly damaged by the LLOprocess during the LBL-assisted transfer printingprocess.

In conclusion, the synergetic effects of an LBL in pro-tecting inner polymer layers from an intense laser beamand inducing sharp separation of GaN LEDs and innerpolymer layers from a sapphire substrate enable the di-rect and precise transfer of GaN LED arrays by theLLO process. The successful transfer printing of pat-terned GaN LEDs onto a PET film with a high transferyield of 95% and their EL emission upon bending inboth tensile and compressive modes demonstrate theversatility of this method for various applications,including next-generation deformable displays and otheroptoelectronic systems.

This work was supported by the National Re-search Foundation of Korea (NRF) funded by the Min-istry of Science, ICT & Future Planning (Grant No.2008-0062606, CELA-NCRC) and the Industrial Strate-gic technology development program (10041878), Devel-opment of WPE 75% LED device process and standardevaluation technology funded by the Ministry of Trade,Industry & Energy (MOTIE/KEIT).

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