Light-emitting diodes fabricated on an electrical conducting flexible substrate

Solid-State Electronics 127 (2017) 57–60

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Light-emitting diodes fabricated on an electrical conducting flexible substrate Won-Sik Choi a,b, Wan Jae Kim a, Si-Hyun Park a,⇑, Sung Oh Cho b,c, June Key Lee c, Jun Beom Park b,d, Jun-Seok Ha d, Tae Hoon Chung b, Tak Jeong b,⇑ a

Department of Electronic Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan-si, Gyeongsangbuk-do 38541, Republic of Korea LED Device Research Center, Korea Photonics Technology Institute, Gwangju 500-779, Republic of Korea Department of Materials Science and Engineering, and Optoelectronics Convergence Research Center, Chonnam National University, Buk-gu, Gwangju 500-757, Republic of Korea d School of Applied Chemical Engineering, Chonnam National University, Buk-gu, Gwangju 500-757, Republic of Korea b c

a r t i c l e

i n f o

Article history: Received 27 April 2016 Received in revised form 27 October 2016 Accepted 28 October 2016 Available online 31 October 2016 The review of this paper was arranged by Prof. E. Calleja

a b s t r a c t An array of InGaN-based flexible light-emitting diodes (FLEDs) was fabricated on a Ni-embedded electrical conducting flexible fabric with a full-scale 2-in. size. The FLED chip operation under current injection was realized using a single current probe as the negative electrode on the n-GaN surface; the conducting substrate was used as the positive electrode. The stability of the output power in the FLEDs was improved dramatically on the Ni-embedded conducting flexible fabric compared to that on the conventional polyimide flexible substrate. The former showed linear operation up to an input current 950 mA with no wavelength shift, whereas the latter exhibited rolling-over behavior after an input current of 200 mA. Ó 2016 Elsevier Ltd. All rights reserved.

Keywords: InGaN Flexible Light-emitting diodes Conducting fabric

1. Introduction Light-emitting diodes (LEDs) with flexible device feasibility have attracted considerable attention for emerging applications in biomedical and displays. Although organic material-based LEDs are inherently congruous to flexible devices, inorganic materialbased flexible LEDs (FLED) are also expected because of their basic advantage over organic LEDs, such as high brightness, high efficiency and high stability. A range of novel applications with InGaN-based FLEDs have been actively sought, including smart textile, deformable displays, and implantable biomedical electronics [1–5]. Although some advances have been being made in the field of InGaN-based FLEDs [6–13], there are still challenges that need to be addressed. One of them is the transfer of high-quality epitaxial layers onto flexible substrates in a wafer level, which was first successfully demonstrated on a 2 in. wafer level by Gossler et al. [13]. Although an individual InGaN-based chip or an array of chips have been transferred to flexible substrates, multiple transfer processes and/or the accompanying complex proce⇑ Corresponding authors. E-mail (T. Jeong).

addresses: [email protected] (S.-H. Park),

http://dx.doi.org/10.1016/j.sse.2016.10.040 0038-1101/Ó 2016 Elsevier Ltd. All rights reserved.

[email protected]

dures might hinder the wafer scale production for cost-effective mass production. The other is the problems of the operating quality in the output power of FLED, which may be due to the poor electrical and thermal properties of the conventionally-used polymer-based flexible substrate. Recently, we reported the fabrication of InGaN-based FLEDs at a full-scale 2-in. wafer level [14,15]. Using a simple direct-transfer process based on a laser lift-off (LLO) technique, an array of InGaN-based FLED chips were transferred to a full-scale 2-in. polyimide substrate and showed FLED operation under current injection. Although the possibility of the mass production of high-quality InGaN-based FLEDs with high-current operation was demonstrated, there is still room for improving the output power quality of FLEDs. As one of the ways to improve the output power quality of FLEDs, it may reasonable to replace the conventional polymerbased flexible substrate with an electrically conducting flexible substrate. Through this, improved output quality from the better thermal conductivity as well as current injection operation only with single wiring from the conducting substrate as the other electrode are expected. Based on this consideration, in this study, Ni-embedded electrical conducting flexible fabric was used as the flexible substrate of

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FLEDs. The simple direct-transfer process based on the LLO technique was applied to transfer an array of InGaN-based FLED chips on a full-scale 2-in. conducting flexible substrate. The operation properties were compared with those of FLEDs with a conventional polymer-based flexible substrate.

2. Experiment Arrays of FLEDs were fabricated using a simple direct transfer process developed previously [14]. An InGaN/GaN epitaxial layer for LEDs was grown on 2 in. sapphire substrates. The patterning

(a)

of SiO2 open windows and p-Ohmic metal deposition inside the open window were performed on the epitaxial layer. Cu was electroplated onto the p-Ohmic layer for improving the thermal stability of the LEDs. The prepared sample was then bonded with a Ni-embedded electrical conducting flexible fabric. The electrical conducting flexible fabric we used in this research is a conducting tape from WINNOVA Co. Ltd. [16]. The tape actually consists of conductive fabric and conductive adhesive in addition to release paper as shown in Fig. 1(a) which is the schematic diagram for the cross section of it. The thickness of the conductive fabric plus the conductive adhesive is 0.12 mm and that of the release paper is 0.130 mm. From scanning electron microscopy (SEM) images

Release paper Conductive adhesive Conductive fabrics

(c)

(d) carbon

12.25 µm

(b)

Ni

Fig. 1. (a) Schematic diagram for the cross section of the conducting tape, and SEM images of (b) the conductive fabric in global view, (c) and (d) the constituent single string of the fabric.

(a)

(b)

Pulsed laser

Sapphire n-GaN p-GaN SiO2

P-Ohmic metal

Cu n-GaN p-GaN

(Bonding) SiO2

P-Ohmic metal

Cu

(c)

Fig. 2. Schematic diagram for the FLED fabrication process (a) bonding process, (b) LLO process and (c) the final FLED chip on a full-scale 2-in. size.

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4.0

(a)

Output Intensity (a.u.)

3.5 3.0 2.5 2.0 1.5 1.0

FLEDs on conductive fabric FLEDs on polyimide

0.5 0.0

0

200

400

600

800

1000

Current (mA) 1.8

Electroluminescence Intensity (a.u.)

1.6

(b)

1.4 1.2 1.0 FLEDs on conductive fabric

0.8

500 mA 300 mA 100 mA 50 mA

0.6 0.4 0.2 0.0 -0.2

400

450

500

550

600

Wavelength (nm) Fig. 4. (a) Output light vs. input current for FLEDs with a conducting fabric and with polyimide, and (b) spectrum of output light for FLED with the conducting fabric.

Fig. 3. (a) SEM image of the cross section of FLED chip, (b) full-scale 2-in. FLED in the bent state, and (c) light-emitting operation under the current injection.

of the conductive fabric in (b)–(d), a constituent single string of the fabric consists of carbon core with 12.25 lm diameter and Ni foil with 0.5 lm thickness. The conductive adhesive consists of Si- based adhesive and Ni powder. The conductivity and peel adhesive are 0.1 (Ohm 1 m 1) and 900 (g/peel speed 300 mm/min), respectively. As shown in Fig. 2(a), we bonded the conducting fabric to Cu plated-surface of the sample using the conducting adhesive by means of simply rubbing it with hands, being careful not to leave the air bubbles or gaps between the bonded surfaces. After bonding of the conducting fabric with the sample, a dummy sapphire substrate was bonded to the other side of the conductive fabric, for the purpose of the sample handing, using the conventional paraffin wax by means of pressing them under the condition of the pressure of 5 M Pa and the time duration of 10 min. A LLO process by the KrF laser was then used to remove the sapphire substrate from the InGaN/GaN epitaxial layer as shown in (b). Then chip isolation and n-electrode deposition were processed and the dummy sapphire substrate was removed using acetone treatment. The FLEDs on the electrical conducting flexible fabric were finally completed with the individual chip size of 1300  1300 lm2 on a full-scale 2-in. size as shown in (c)

wafer, all FLED chips were fabricated with no loss. In (c), the electrical operation of the device was observed in the bent state. Here, only single current probe for the negative electrode in the exposed n-GaN was used and the current was passed through a conducting substrate as the other electrode. In other words, the conductive fabric substrate in our structure connects all the p-contacts while the n-contact is individually connected as shown in Fig. 2(c). Then an LED array can be individually addressed. This operating structure clearly originated from the conducting substrate. With conventional insulating substrates, such as a polymer-based flexible substrate, however, paired electrodes or wires are essential. Therefore, this type of chip structure with only single wiring has a large advantage in its commercial products. FLEDs were also prepared with a polyimide substrate using the same processes to allow a comparison of the LED operation. Fig. 4 (a) shows that the FLED with the conducting fabric exhibits stable linear operation in the output light until an input current of 950 mA, whereas the FLED with the polyimide substrate starts rolling-over only after an input current level of 200 mA. This is due basically to the improved thermal conductivity of the Ni-embedded fabric. In addition, the elevated junction temperature during LED operation generally results in a red shift in the output spectrum [14,17]. In the output spectrum of FLEDs with the conducting fabric, as shown in (b), however, there is no wavelength shift with the increased input current at least until a high current of 500 mA, which is also due to the high thermal properties of the Ni-embedded fabric. It was observed that there is no difference between two chips in I-V curves.

3. Results and discussion 4. Conclusion Fig. 3(a) shows a cross section SEM image of the fabricated FLED chip. The GaN epitaxial layer was well bonded to the conducting fabric with the adhesive. In (b), an array of FLEDs in a full-scale 2-in. size was observed under a bent state. Over the entire 2-in.

InGaN-based FLEDs were fabricated successfully on a Ni-embedded electrical conducting flexible fabric on a full-scale 2-in. size, making use of both conductivity and flexibility in the

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substrate. In addition to the expected normal FLED operation in the bent state, the conducting substrate makes it possible to use only a single current probe for current injection, not paired-probes, providing an advantage from the simple structure in future commercial products. The conducting substrate also allows better thermal conductivity than the conventionally-used polymerbased flexible substrates, showing improved operation properties, such as a linear operation up to an input current 1000 mA, and no wavelength shift in the high input current. Acknowledgments This work was supported in part by the R&D Program of the Ministry of Trade, Industry and Energy, Korea, under grant 10053149 and 10062128. References [1] Cherenack K, Zysset C, Kinkeld T, Münzenrieder N, Tröster G. Woven electronic fibers with sensing and display functions for smart textiles. Adv Mater 2010;22:5178–82. [2] Park SI, Xiong Y, Kim RH, Elvikis P, Meitl M, Kim DH, et al. Printed assemblies of inorganic light-emitting diodes for deformable and semitransparent displays. Science 2009;325:977–81. [3] Koo M, Park SY, Lee KJ. Biointegrated flexible inorganic light emitting diodes. Nanobiosensors Disease Diag. 2012;1:5–15. [4] Lee SY, Park KI, Huh C, Koo M, Yoo HG, Kim S, et al. Water-resistant flexible GaN LED on a liquid crystal polymer substrate for implantable biomedical applications. Nano Energy 2012;1:145–51.

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