gan-based light-emitting diodes on graphene-coated flexible substrates

GaN-based light-emitting diodes on graphene-coated flexible substrates

Gwangseok Yang,1 Younghun Jung,1 Camilo Vélez Cuervo,2 Fan Ren,2 Stephen J. Pearton,3 and Jihyun Kim1,*

1 Department of Chemical and Biological Engineering, Korea University, Seoul 136-701, South Korea 2 Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, USA

3 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, USA *[email protected]

Abstract: We demonstrate GaN-based thin light-emitting diodes (LEDs) on flexible polymer and paper substrates covered with chemical vapor deposited graphene as a transparent-conductive layer. Thin LEDs were fabricated by lifting the sapphire substrate off by Excimer laser heating, followed by transfer of the LEDs to the flexible substrates. These substrates were coated with tri-layer graphene by a wet transfer method. Optical and electrical properties of thin laser lift-offed LEDs on the flexible substrates were characterized under both relaxed and strained conditions. The graphene on paper substrates remained conducting when the graphene/paper structure was folded. The high transmittance, low sheet resistance and high failure strain of the graphene make it an ideal candidate as the transparent and conductive layer in flexible optoelectronics.

©2014 Optical Society of America

OCIS codes: (230.0230) Optical devices; (230.3670) Light-emitting diodes; (230.4000) Microstructure fabrication.

References and links 1. J. G. McCall, T. I. Kim, G. Shin, X. Huang, Y. H. Jung, R. Al-Hasani, F. G. Omenetto, M. R. Bruchas, and J. A.

Rogers, “Fabrication and application of flexible, multimodal light-emitting devices for wireless optogenetics,” Nat. Protoc. 8(12), 2413–2428 (2013).

2. H. S. Kim, E. Brueckner, J. Song, Y. Li, S. Kim, C. Lu, J. Sulkin, K. Choquette, Y. Huang, R. G. Nuzzo, and J. A. Rogers, “Unusual strategies for using indium gallium nitride grown on silicon (111) for solid-state lighting,” Proc. Natl. Acad. Sci. U.S.A. 108(25), 10072–10077 (2011).

3. M. Pagliaro, R. Ciriminna, and G. Palmisano, “Flexible solar cells,” ChemSusChem 1(11), 880–891 (2008). 4. Y. Jung, X. Wang, S. H. Kim, F. Ren, J. Kim, and S. J. Pearton, “A facile method for flexible GaN-based light-

emitting diodes,” Phys. Status Solidi 6, 421 (2012). 5. Y. Sun, S. Trieu, T. Yu, Z. Chen, S. Qi, P. Tian, J. Deng, X. Jin, and G. Zhang, “GaN-based LEDs with a high

light extraction composite surface structure fabricated by a modified YAG laser lift-off technology and the patterned sapphire substrates,” Semicond. Sci. Technol. 26(8), 085008 (2011).

6. K. J. Lee, M. A. Meitl, J.-H. Ahn, J. A. Rogers, R. G. Nuzzo, V. Kumar, and I. Adesida, “Bendable GaN high electron mobility transistors on plastic substrates,” J. Appl. Phys. 100(12), 124507 (2006).

7. J.-H. Ahn, H.-S. Kim, E. Menard, K. J. Lee, Z. Zhu, D.-H. Kim, R. G. Nuzzo, J. A. Rogers, I. Amlani, V. Kushner, S. G. Thomas, and T. Duenas, “Bendable integrated circuits on plastic substrates by use of printed ribbons of single-crystalline silicon,” Appl. Phys. Lett. 90(21), 213501 (2007).

8. X. Lu and Y. Xia, “Electronic materials: buckling down for flexible electronics,” Nat. Nanotechnol. 1(3), 163–164 (2006).

9. J. A. Rogers, T. Someya, and Y. Huang, “Materials and mechanics for stretchable electronics,” Science 327(5973), 1603–1607 (2010).

10. G. Cocco, E. Cadelano, and L. Colombo, “Gap opening in graphene by shear strain,” Phys. Rev. B 81(24), 241412 (2010).

11. G. Jo, M. Choe, S. Lee, W. Park, Y. H. Kahng, and T. Lee, “The application of graphene as electrodes in electrical and optical devices,” Nanotechnology 23(11), 112001 (2012).

12. C. M. Weber, D. M. Eisele, J. P. Rabe, Y. Liang, X. Feng, L. Zhi, K. Müllen, J. L. Lyon, R. Williams, D. A. Vanden Bout, and K. J. Stevenson, “Graphene-based optically transparent electrodes for spectroelectrochemistry in the UV-Vis region,” Small 6(2), 184–189 (2010).

13. B.-J. Kim, G. Yang, H.-Y. Kim, K. H. Baik, M. A. Mastro, J. K. Hite, C. R. Eddy, Jr., F. Ren, S. J. Pearton, and J. Kim, “GaN-based ultraviolet light-emitting diodes with AuCl3-doped graphene electrodes,” Opt. Express 21(23), 29025 (2013).

#203914 - $15.00 USD Received 30 Dec 2013; revised 27 Mar 2014; accepted 28 Mar 2014; published 7 Apr 2014(C) 2014 OSA 5 May 2014 | Vol. 22, No. S3 | DOI:10.1364/OE.22.00A812 | OPTICS EXPRESS A812

14. Y. Jung, X. Wang, J. Kim, S. H. Kim, F. Ren, S. J. Pearton, and J. Kim, “GaN-based light-emitting diodes on origami substrates,” Appl. Phys. Lett. 100(23), 231113 (2012).

15. A. C. Siegel, S. T. Phillips, M. D. Dickey, N. Lu, Z. Suo, and G. M. Whitesides, “Foldable printed circuit boards on paper substrates,” Adv. Funct. Mater. 20(1), 28–35 (2010).

16. R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, and A. K. Geim, “Fine structure constant defines visual transparency of graphene,” Science 320(5881), 1308 (2008).

17. X. Li, Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, R. D. Piner, L. Colombo, and R. S. Ruoff, “Transfer of large-area graphene films for high-performance transparent conductive electrodes,” Nano Lett. 9(12), 4359–4363 (2009).

18. Y.-H. Lee and Y.-J. Kim, “Electrical and lattice vibrational behaviors of graphene devices on flexible substrate under small mechanical strain,” Appl. Phys. Lett. 101(8), 083102 (2012).

19. G. Yang, C. Lee, J. Kim, F. Ren, and S. J. Pearton, “Flexible graphene-based chemical sensors on paper substrates,” Phys. Chem. Chem. Phys. 15(6), 1798–1801 (2013).

20. Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, and H.-M. Cheng, “Three-dimensional flexible and conductive interconnected graphene networks grown by chemical vapour deposition,” Nat. Mater. 10(6), 424–428 (2011).

1. Introduction

Flexible optoelectronic devices are fascinating due to their potential applications in display, optogenetics, lighting, military, and photovoltaics [1–3]. Thin epitaxial layers of the inorganic semiconductors have been commonly grown on thick and inelastic substrates, which make them non-bendable and non-stretchable. In case of light-emitting diodes (LEDs), the flexibility of the LED itself can be achieved by removing the underlying stiff substrate, separating the active layer. A lot of effort has been made on approaches such as laser lift-off (LLO) and chemical lift-off processes because the remaining epitaxial layers are very thin and compliant [4–6]. Recently, these flexible lift-off LEDs have been implanted to study the neural systems of animals [1]. Although active components such as transistors and diodes have been successfully made bendable [6, 7], robust deformable interconnections and electrodes are still an issue [8]. Indium-tin-oxide (ITO) which is the conventional transparent-conductive layer in LEDs and solar cells due to its high transmittance and low sheet resistance unfortunately suffers from low failure strain. Therefore, a lot of effort has been concentrated on fabricating flexible structures without ITO-based electrodes or interconnections. Rogers et al. reported stretchable electronics with buckled, arc-shaped interconnections [9]. These structures can hold applied strains up to 100%. However, the fabrication processes are very complicated.

Graphene’s two-dimensional structure is considered to be an attractive option because it has higher failure strain (>10%) than ITO (2~3%) [10, 11]. In addition, graphene is highly conductive and transparent from the ultra-violet to infra-red spectral ranges [12]. In addition, its thermal conductivity is exceptionally high which is advantageous to dissipate the heat generated in the electronic devices effectively. Although the sheet resistance of pristine mono-layer graphene is relatively high (>1000 Ω/ ), its electrical conductivity can be controlled by varying the total thickness of the graphene via multiple transfer processes or doping the graphene chemically. The chemical doping method of using various dopants has been used to lower the sheet resistance of the graphene and the doped graphene was reported to have a comparable resistance to ITO after doping with AuCl3 [11, 13].

Polymer substrates including polyethylene terephthalate (PET) and PEN (Polyethylene Naphthalate) have been the conventional substrates for flexible devices [1–4, 6–8]. Paper substrates are also favored because they are cheap, environmentally-friendly and light-weight with low Young’s modulus. Jung et al. demonstrated blue LED devices mounted on the origami substrates after the sapphire substrate was removed [14]. Siegel et al. reported paper-based foldable circuit boards [15]. In this work, we incorporated thin InGaN/GaN LED with compliant substrates covered with tri-layer graphene, where the thin LED devices were obtained by removing the sapphire substrate by LLO process.


Fig. 1. Schematic fabrication process of LLO-LED / carbon-tape / graphene / flexible substrate

2. Experimental details

Commercial GaN LED wafers with InGaN/GaN multi-quantum-wells were grown on sapphire substrates by the metal-organic chemical vapor deposition technique. Standard LED fabrication processes were employed including photo-lithography, chlorine-based inductively coupled plasma etching, electron-beam evaporation, and thermal annealing. For the LLO process, an ArF excimer laser system with a wavelength of 193 nm (JPSATM IX-260) was used to lift the sapphire substrate off, with an irradiation dose of approximately 1.1 J/cm2 with pulse duration of 25 ns. The details of the LLO process and the structure of LLO-LED can be found elsewhere [14].

Fig. 2. (a, b) photograph under relaxed condition (a) before and (b) under applying the forward bias (c, d) photograph under 0.5% strained condition (c) before and (d) under applying the forward bias. Note that Figs. 2(a) and 2(c) and Figs. 2(b) and 2(d) were obtained under the room light and with the light-off, respectively.

Figure 1 summarizes the fabrication process. Graphene was grown on both sides of copper foil (300 μm thick, Alfa Aesar) at 1000°C for 30 min by chemical vapor deposition method.


The back-side of the copper foil was reactively ion etched while the front-side was protected with spin-coated poly(methyl methacrylate) (PMMA). Then the PMMA / graphene / copper foil was held in 1 wt. % (NH4)2S2O8 solution for 8 hours to remove the copper foil, followed by mounting PMMA/graphene on compliant (PET or paper) substrates, which was fabricated with Ag paste or carbon tape as electrical contact-pads. The thickness of PET and paper were 0.2 mm and 0.1 mm, respectively. The sheet resistance of the tri-layer graphene on PET was obtained by four-point probe measurements (Desk 205, MS TECH). In our experiment, the paper substrate was cut from commercial cleanroom notebooks because it was robust and chemically-resistant. A probe-station connected to semiconductor parameter analyzer (4155C, Agilent) was used to obtain current-voltage (I-V) characteristics when the substrate was either relaxed or strained.

Fig. 3. (a) Transmittance of the tri-layer graphene on PET substrate (inset: photograph of the graphene on PET) (b) I-V characteristics when relaxed and 0.5% strained conditions, respectively. (inset: ELs from the LLO-LEDs)

3. Results and discussion

Figure 2 shows the tri-layer graphene used as a conductive, flexible and transparent layer. Figures 2(a) and 2(c) are photographs under room light before the forward bias was applied to the LLO-LED. Figures 2(b) and 2(d) are photographs under forward bias condition when the room lights were off. Figure 2(d) shows that the graphene acts as a transparent conductive layer at a tensile strain of 0.5%. The insets of Figs. 2(b) and 2(d) are EL images when the PET substrate was relaxed and strained by 0.5%, respectively. Since the probe can damage the graphene, carbon tape was used as a contact pad to the tri-layer graphene, as shown in Fig. 1. The transmittance data of our graphene/PET structure in Fig. 3(a) are consistent with the previous result [16]. Inset of Fig. 3(a) shows the tri-layer graphene transferred on PET substrate, where the graphene can be found inside the shaded area. The average sheet resistance of our tri-layer graphene was approximately ~400 Ω/ , which is in good agreement with the results from other groups [17]. The sheet resistance of the graphene can be controlled by varying the number of the graphene layers or introducing the chemical dopants such as AuCl3 and HNO3. However, both methods can deteriorate the optical transmittance because there is a trade-off between the transmittance and sheet resistance. Therefore, optimization will still be needed for the implementation of the graphene as transparent conductive layer. The peak position of the EL spectra did not change at a tensile strain of 0.5%, compared with the relaxed condition (Fig. 3(b)). However, the currents decreased by 13% at a forward bias of 6 V when the tensile strain (0.5%) was applied as shown in Fig. 3(b), which can be attributed to the increase in the sheet resistance of the graphene under the tensile strain because there is nominal change in the turn-on voltage and the non-optimized contacts between graphene / carbon-tape and LLO-LED. It has been reported that the electrical properties of the graphene depends on the mechanical strain, where the resistance increased under tensile strain conditions [18, 19]. Lower forward currents compared with the conventional InGaN/GaN LEDs on sapphire substrate are attributed to the resistive n-GaN


buffer layer which was exposed after the LLO process, the longer current path and the un-annealed contact.

Fig. 4. (a) LLO-LED/carbon-tape/graphene/flat paper under the forward bias condition (inset: EL image from the LLO-LED device) (b, c) LLO-LED/carbon-tape/graphene/folded paper under the forward bias condition (inset: EL image from the LLO-LED device) (d) I-V characteristics of LLO-LEDs on (flat or folded) paper substrate

Figures 4(a) and 4(b) are photographs before and after the paper substrate covered with the tri-layer graphene was folded, respectively. The grey square with a size of 1 cm × 1 cm is where the tri-layer graphene was deposited by a chemical transfer method. The lines in the paper substrate were ruled by 0.65 cm spacing. In this experiment, one of the contacts was formed by the Ag paste. The tri-layer graphene conducted current even when it was folded, as shown in Fig. 4(b) and 4(c). The electrical properties under the forward bias condition were greatly degraded although the leakage currents did not change (Fig. 4(d)). We believe that the robustness of the graphene can be improved by employing three-dimensional graphene structures [20]. Conventional ITO films that require high vacuum process cannot survive this type of folding due to its low failure strain (2~3%). We observed a high failure strain for the graphene, which can be deposited on compliant substrates with small Young’s modulus by wet-transfer method without using high vacuum processes such as sputtering and evaporation. The applications of flexible LEDs integrated with these graphene-based transparent conductive layers include bio-medical sensors, personal displays and decoration lightings. Therefore, we believe that these graphene-based transparent conductive layers can play an important role in the next generation of flexible electronics.

4. Conclusion

PET and paper substrates covered with tri-layer pristine graphene were used as flexible substrates. The graphene layer was transparent, flexible and conductive (sheet resistance: ~400 Ω/ ). The forward currents of LLO-LED/graphene/PET structures at 6 V dropped by 13% when the structure was placed under a tensile strain of 0.5%. Bright EL was observed even after the graphene/paper structure was folded, although I-V characteristics were degraded. These graphene layers exhibit a high failure strain and show a great potential for high performance transparent and conductive layer in flexible optoelectronics.


Acknowledgments

The research at Korea University was supported by LG Innotek-Korea University Nano-Photonics Program, the Center for Inorganic Photovoltaic Materials (No. 2012-0001171) Grant funded by the Korea government (MEST), a Korea University Grant and the Human Resources Development program (No. 20124030200120) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. The work at UF was partially supported by NSF 1159682 (J. M. Zavada)


gan-based light-emitting diodes on graphene-coated flexible substrates

Documents