Enhanced light output power of near UV light emitting diodes with ...

Enhanced light output power of near UV light emitting diodes with graphene / indium tin oxide nanodot nodes for transparent and current spreading electrode Tae Hoon Seo,1 Kang Jea Lee,1 Ah Hyun Park,1 Chang-Hee Hong,1 Eun-Kyung Suh,1,* Seung Jin Chae, 2 Young Hee Lee,2 Tran Viet Cuong,3 Viet Hung Pham,3 Jin Suk Chung,3 Eui Jung Kim, 3 and Seong-Ran Jeon 4 1

School of Semiconductor and Chemical Engineering & Semiconductor Physics Research Center, Chonbuk National University, Jeonju 561-756, Korea, 2 BK21 Physics Division, WCU department of Energy Science, Sungkyunkwan Advanced Institute of Nanotechnology, Center for Nanotubes and Nanostructured Composites, Sungkyungkyun University, Suwon 440-746, Korea, 3 School of Chemical Engineering and Bioengineering, University of Ulsan, Ulsan 680-749, Korea 4 Korea Photonics Technology Institute (KOPTI), Gwangju 500-460, Korea * [email protected]

Abstract: We report GaN-based near ultraviolet (UV) light emitting diode (LED) that combines indium tin oxide (ITO) nanodot nodes with twodimensional graphene film as a UV-transparent current spreading electrode (TCSE) to give rise to excellent UV emission efficiency. The light output power of 380 nm emitting UV-LEDs with graphene film on ITO nanodot nodes as TCSE was enhanced remarkably compared to conventional TCSE. The increase of the light output power is attributed to high UV transmittance of graphene, effective current spreading and injection, and texturing effect by ITO nanodots. ©2011 Optical Society of America OCIS codes: (230.0230) Optical device; (160.6000) Semiconductor materials; (230.3670) Light emitting diode.

References and links A. Khan, K. Balakrishnan, and T. Katona, “Ultraviolet light-emitting diodes based on group three nitrides,” Nat. Photonics 2(2), 77–84 (2008). 2. N. F. Gardner, G. O. Muller, Y. C. Shen, G. Chen, S. Watanabe, W. Gotz, and M. R. Krames, “Blue emitting InGaN–GaN double-heterostructure light-emitting diodes reaching maximum quantum efficiency above 200 A/cm2,” Appl. Phys. Lett. 91(24), 243506-1–243506-3 (2007). 3. W. S. Wong, T. Sands, N. W. Cheung, M. Kneissl, D. P. Bour, P. Mei, L. T. Romano, and N. M. Johnson, “Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off,” Appl. Phys. Lett. 75(10), 1360–1362 (1999). 4. T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. DenBaars, and S. Nakamura, “Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening,” Appl. Phys. Lett. 84(6), 855–857 (2004). 5. D. W. Kim, H. Y. Lee, G. Y. Yeom, and Y. J. Sung, “A study of transparent contact to vertical GaN-based lightemitting diodes,” J. Appl. Phys. 98(5), 0531021–0531024 (2005). 6. T. H. Seo, T. S. Oh, T. S. Lee, H. Jeong, J. D. Kim, H. Kim, A. H. Park, K. J. Lee, C.-H. Hong, and E.-K. Suh, “Enhanced Light Extraction in GaN-Based Light Emitting Diodes with Holographically Fabricated Concave Hemisphere-Shaped Patterning on Indium-Tin-Oxide Layer,” Jpn. J. Appl. Phys. 49, 092101-1–092101-3 (2010). 7. J.-K. Sheu, M.-L. Lee, Y. S. Lu, and K. W. Shu, “Ga-Doped ZnO Transparent Conductive Oxide Films Applied to GaN-Based Light-Emitting Diodes for Improving Light Extraction Efficiency,” IEEE J. Quantum Electron. 44(12), 1211–1218 (2008). 8. T.-Y. Park, Y.-S. Choi, J.-W. Kang, J.-H. Jeong, S.-J. Park, D. M. Jeon, J. W. Kim, and Y. C. Kim, “Enhanced optical power and low forward voltage of GaN-based light-emitting diodes with Ga-doped ZnO transparent conducting layer,” Appl. Phys. Lett. 96(5), 051124-1–051124-3 (2010). 9. S. H. Tu, C. J. Lan, S. H. Wang, M. L. Lee, K. H. Chang, R. M. Lin, J. Y. Chang, and J. K. Sheu, “InGaN gallium nitride light-emitting diodes with reflective electrode pads and textured gallium-doped ZnO contact layer,” Appl. Phys. Lett. 96(13), 133504-1–133504-3 (2010). 10. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, “Electric field effect in atomically thin carbon films,” Science 306(5696), 666–669 (2004). 1.

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Received 25 Aug 2011; revised 6 Oct 2011; accepted 13 Oct 2011; published 31 Oct 2011 7 November 2011 / Vol. 19, No. 23 / OPTICS EXPRESS 23111

11. F. Bonaccorso, Z. Sun, T. Hasan, and A. C. Ferrari, “Graphene photonics and optoelectronics,” Nat. Photonics 4(9), 611–622 (2010). 12. G. H. Jo, M. H. Choe, C. Y. Cho, J. H. Kim, W. J. Park, S. C. Lee, W. K. Hong, T. W. Kim, S. J. Park, B. H. Hong, Y. H. Kahng, and T. H. Lee, “Large-scale patterned multi-layer graphene films as transparent conducting electrodes for GaN light-emitting diodes,” Nanotechnology 21, 175201-1–175201-6 (2010). 13. T. H. Seo, K. J. Lee, T. S. Oh, Y. S. Lee, H. Jeong, A. H. Park, H. Kim, Y. R. Choi, E.-K. Suh, T. V. Cuong, V. H. Pham, J. S. Chung, and E. J. Kim, “Graphene network on indium tin oxide nanodot nodes for transparent and current spreading electrode in InGaN/GaN light emitting diode,” Appl. Phys. Lett. 98(25), 251114-1–251114-3 (2011). 14. X. Wang, L. Zhi, and K. Mullen, “Transparent, conductive graphene electrodes for dye-sensitized solar cells,” Nano Lett. 8(1), 323–327 (2008). 15. T. V. Cuong, V. H. Pham, J. S. Chung, E. W. Shin, D. H. Yoo, S. H. Hahn, J. S. Huh, G. H. Rue, E. J. Kim, and S. H. Hur, G. S. Rue, E. J. Kim, S. H. Hur, and P. A. Kohl, “Solution-processed ZnO-chemically converted graphene gas sensor,” Mater. Lett. 64(22), 2479–2482 (2010). 16. Y. Zhu, S. Murali, W. Cai, X. Li, J. W. Suk, J. R. Potts, and R. S. Ruoff, “Graphene and graphene oxide: synthesis, properties, and applications,” Adv. Mater. (Deerfield Beach Fla.) 22(35), 3906–3924 (2010). 17. W. S. Hummers and R. E. Offeman, “Preparation of Graphitic Oxide,” J. Am. Chem. Soc. 80(6), 1339–1339 (1958). 18. F. Guneş, H.-J. Shin, C. Biswas, G. H. Han, E. S. Kim, S. J. Chae, J. Y. Choi, and Y. H. Lee, “Layer-by-layer doping of few-layer graphene film,” ACS Nano 4(8), 4595–4600 (2010). 19. T. V. Cuong, V. H. Pham, Q. T. Tran, S. H. Hahn, J. S. Chung, E. W. Shin, and E. J. Kim, “Photoluminescence and Raman studies of graphene thin films prepared by reduction of graphene oxide,” Mater. Lett. 64(3), 399–401 (2010). 20. V. H. Pham, T. V. Cuong, S. H. Hur, E. W. Shin, J. S. Kim, J. S. Chung, and E. J. Kim, “Fast and simple fabrication of a large transparent chemically-converted graphene film by spray-coating,” Carbon 48(7), 1945– 1951 (2010). 21. Z. Luo, Y. Lu, L. A. Somers, and A. T. C. Johnson, “High yield preparation of macroscopic graphene oxide membranes,” J. Am. Chem. Soc. 131(3), 898–899 (2009). 22. D. Li, M. B. Müller, S. Gilje, R. B. Kaner, and G. G. Wallace, “Processable aqueous dispersions of graphene nanosheets,” Nat. Nanotechnol. 3(2), 101–105 (2008). 23. X. S. Li, W. W. Cai, J. H. An, S. Kim, J. Nah, D. X. Yang, R. D. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo, and R. S. Ruoff, “Large-area synthesis of high-quality and uniform graphene films on copper foils,” Science 324(5932), 1312–1314 (2009). 24. H. Ago, T. Kugler, F. Cacialli, K. Petritsch, R. H. Friend, W. R. Salaneck, Y. Ono, T. Yamabe, and K. Tanaka, “Work function of purified and oxidised carbon nanotubes,” Synth. Met. 103(1-3), 2494–2495 (1999). 25. P. Guéret, P. Buchmann, K. Daetwyler, and P. Vettiger, “Resistance of very small area ohmic contacts on GaAs,” Appl. Phys. Lett. 55(17), 1735–1737 (1989). 26. H. W. Choi, C. W. Jeon, M. D. Dawson, P. R. Edwards, R. W. Martin, and S. Tripathy, “Mechanism of enhanced light output efficiency in InGaN-based microlight emitting diodes,” J. Appl. Phys. 93(10), 5978–5982 (2003).

1. Introduction A solid-state UV light source is of special interest for their use in germicidal instrumentation, biological agent identification, chemical sensing, fluorescence excitation, and optical data storage [1]. In order to improve LED efficiency, most of current researches focus on two technological issues, namely, the optimization of the internal quantum efficiency including the mitigation of the IQE droop [2], and improvement of light extraction efficiency(LEE). Various designs have been proposed to extract generated photons from the active semiconductor layers [3,4]. However, during GaN-based LED fabrication, elementary Ohmic contact and injection current distribution problems may be encountered. The p-type electrode requires both low contact resistance with p-type GaN and high transmittance for the extraction of photons from active layers. Typical p-type thin GaN layers with high lateral sheet resistance resulted in severe current crowding under the vertical direction of the electrode and low current spreading through the full emitting area. For these reasons, indium tin oxide (ITO) [5,6], gallium-doped ZnO (GZO) [7–9] etc. are widely used as transparent conductive oxide (TCO) electrodes and current spreading layers in LEDs. Nevertheless, the TCO materials show relatively high absorption in the UV region, and hence, an alternative transparent electrode is required with optical and electrical performances similar to or better than those of TCO materials but without having drawbacks in the UV region. Graphene has attracted much attention owing to its fascinating properties such as high optical transmittance of 97.7% over the visible and UV region, high thermal conductivity of ~5000 W/m·K, and high intrinsic carrier mobility of over 21,000 cm2/Vs at room temperature [10,11]. Several pioneering works have been reported in the application of graphene-based #153419 - $15.00 USD (C) 2011 OSA


films as transparent electrodes in LEDs [12,13], solar cells [14], sensors [15] and electronic devices [16]. Though graphene has a high mobility and transmittance, some difficulties arise when applied for LEDs; a direct contact of graphene to the p-GaN layers leads to high potential barriers that frustrate reliable LED operation, resulting in a high forward operating voltage and low light output power. In this work, we prepared two types of graphenes, a chemically converted graphene (CCG) from graphene oxide (GO) [17] and large-area graphene synthesized by chemical vapor deposition (CVD) method [18]. A prototype current spreading electrode for near UV LED was constructed by combining ITO nanodots with graphene layers of each type. 2. Experimental The AlInGaN-based UV LEDs were grown on sapphire substrate by metal-organic chemical vapor deposition. A 30 nm thick GaN buffer layer was deposited on sapphire substrate at 550°C, before the growth of an un-doped GaN layer with a thickness of 1.5 μm and a Si doped n-GaN layer with a thickness of 2.0 μm at 1050°C. Then five pairs of In0.04Ga0.96N QWs with 2 nm thickness and Al0.08Ga0.92N barrier layers with 12 nm thickness were grown at 800°C. Finally, Mg-doped p-Al0.25Ga0.75N electron blocking layer with a thickness of 25 nm and 100 nm thick p-GaN contact layer were grown at 1040°C. In order to form ITO nanodots on the p-GaN surface of LED wafer, a 100 nm ITO layer was deposited onto the p-GaN surface of LED wafer using amorphous ITO powders as the vaporizing target source. The asdeposited ITO layers were opaque and dark black in color. For the crystallization of ITO grains, these ITO films were annealed in N2 and O2 mixed ambient at 600 °C for 60 s in a rapid thermal annealing (RTA) chamber. During this process, the ITO films were converted to partially transparent films. When the ITO films were etched in dilute HCl:3H 2O acid for about 5 s, ITO nanodots were formed on the p-GaN surface [13] as shown in Fig. 1.

Fig. 1. The SEM image of ITO nanodots on the p-GaN surface of UV LED wafer.

A fast, low-cost, and simple method to produce scalable CCG films through the spray deposition of graphene dispersion solution has been reported in previous work [19]. Through spray deposition on a preheated LED template covered by ITO nanodot nodes, thin graphene layers were formed at preheating temperature of 240 °C [20]. The prepared CCG film has a 1 sheet resistance of 2.2×103 and 85% transmittance at a reference wavelength of 550 nm. On the other hand, large scale graphene layers were synthesized on ~70 µm thick Cu-foil by CVD method. Details can be found in Ref. 18. In order to make electrode, the CVD-grown graphene film was transferred using poly methyl methacrylate (PMMA). Graphene was transferred twice onto the ITO nanodot nodes on p-GaN surface. The sheet resistance was 290 1 with a transmittance of 95% in the 400~800 nm wavelength region. The sample was annealed under a H2/Ar (90/10 sccm) gas mixture for 30 min at 500 °C to prevent the oxidation of UV-LED device and to remove the PMMA. To fabricate discrete UV-LED devices of size of 315×315 µm2, the graphene transferred UV-LED template were etched by inductively coupled plasma etching process using Cl2/BCl3/H2/Ar source gases until the n-type GaN layer was exposed for an n-type ohmic

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contact. Finally, Cr (50 nm)/Au (250 nm) metals for the p- and the n-electrodes were deposited onto both graphene films and the n-GaN layer using electron beam evaporator. 3. Results and Discussion Figure 2 (a) shows the optical transmittance of various TCO materials and graphene as a function of wavelength. The photoluminescence (PL) spectrum of ~380 nm of our near-UV LED device is also shown by the shaded curve. The as-deposited ITO film exhibited only less than 20% transmittance in the visible to UV wavelength range. After RTA, the transmittance of the ITO film increased as the film structure altered from amorphous to crystalline state. In spite of the enhanced transmittance in the visible wavelength range, the crystallized ITO and GZO film deposited by radio frequency sputter revealed strong absorption band in near UV region. On the other hand, the transmittance of ITO nanodot template was greater than 90% at UV and visible wavelength ranges. Unlike the crystallized ITO and GZO films, the transmittance of graphene revealed more than 90% throughout the entire range of wavelength including UV region. Figure 2 (b) shows the UV to visible absorbance spectra of prepared graphene samples. The GO sample showed the dominant absorbance peak around 233 nm, which is ascribed to ππ* transition of aromatic C–C bonds, and a shoulder at 300 nm, associated with n π* transition of C═O bonds [19]. The shoulder around ~300 nm disappeared after chemically converted by hydrazine treatment, most likely due to the decrease in the concentration of carboxyl groups [21], and main peak was red-shifted to 270 nm as the electronic conjugation was restored [19,22]. In contrast, the CVD-grown monolayer graphene shows no sp3-like characters.

Fig. 2. (a) The optical transmittance of graphene, as-deposit ITO, annealed ITO, ITO nanodots, and Ga-doped ZnO films, respectively, as a function of wavelength. The photoluminescence spectrum of UV-LED structure is shown with the shaded peak. (b) Absorbance spectra in UV to visible wavelength region of (i) graphene-oxide, (ii) chemically converted graphene (CCG), and (iii) multi-layer graphene (MLG) films synthesized by CVD. Absorbance includes the effect of sapphire substrate.

Figure 3 shows optical microscopy image and the atomic force microscope (AFM) image of CVD-grown graphene. The graphene synthesis on metal substrates by CVD has advantages of obtaining large-area graphene transferrable to other substrates [23]. Figure 3(a) shows optical microscopy image of monolayer graphene exhibiting very clean without residues of Cu catalysis particles. AFM image shows that the synthesized graphene film was uniformly deposited and wrinkles were formed for thermal stress minimization as shown in Fig. 3(b). On the other hand, the AFM image of CCG revealed that CCG flakes with sizes of a few microns were randomly overlapped with one another to create a continuous thin film in the form of mosaic picture with broad wrinkles or folds as shown in our previous work [13]. Additionally, small white dots on the surface were observed on the spray-coated CCG films, which are identified as carbon oxide complexes, originated from gaseous byproducts during GO reduction process.

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Fig. 3. (a) Optical microscopy image and (b) AFM image of graphene layers, prepared by CVD.

Figure 4(a) shows I-V curves for UV-LEDs with CCG and CVD-grown graphene films on ITO nanodot nodes, and compares them with UV-LEDs either with CVD-grown bare graphene layer or with conventional 250 nm thick ITO films. The forward voltages (Vf) at an input current of 20 mA were found to be 5.9, 4.9, 4.42, and 4.35 V for the LED of current spreading electrodes with bare CVD-grown film, CVD-grown graphene film on ITO nanodot nodes, CCG on ITO nanodot nodes, and conventional ITO layer, respectively. The Vf value for UV-LED with CCG network on ITO nanodot nodes was close to the conventional LED with planar ITO conducting layer. The Vf of LED with graphene film only was higher than that of the LED with graphene film on ITO nodes due to less mature ohmic contact and the difference of work function between p-GaN and graphene film [24]. The combination of graphene and ITO layer yielded a lower contact resistance of ohmic junction. Because the inplane current is more favorable than inter-plane current, the current can be spread over the graphene layer, followed by current migration through ITO nanodot nodes from the top graphene film to the p-GaN epilayer, as schematically shown in Fig. 4(b). These results show that graphene film as TCSEs can provide efficient current diffusion pathways for ITO nanodot nodes which then deliver current to the active junctions of LED through p-GaN layer. Despite the fact that the current sinking area between p-GaN surface and ITO nanodots is very low, below 30%, due to the low coverage of ITO nanodots as shown in the SEM image in Fig. 1, the Vf values of UV-LED with CCG on ITO nanodot nodes as TCSEs revealed very reasonable values. Also, the light emission was uniform over the whole surface of UV LED. It appeared that, even though the CVD-grown graphene films have higher structural and electrical qualities compared with CCG films, the I-V characteristics of CCG films appears to be better. This can be attributed to the fact that some remaining oxygen-related functional groups in CCG film facilitate proximate Ohmic contact between CCG film and ITO nanodots due to similarity of oxygenation.

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Fig. 4. (a) The I-V curves, (b) schematic current flow diagram for UV-LEDs with graphene network on ITO nanodot nodes.

The EL spectra and light output powers of InGaN/AlGaN MQW UV-LEDs with various TCSEs examined in Fig. 4 are shown in Fig. 5(a) and 5(b), respectively. All UV-LEDs were well operated at 380 nm emission wavelength with graphene TCSEs and ITO electrodes at an injection current of 100 mA. The emission power of UV-LED with graphene TCSEs was significantly enhanced, as can be seen in Fig. 5(b); about 150% enhancement for the CCG and 60% for the CVD-grown graphene film with ITO nanodots compared to conventional planar ITO at an operation current of 100 mA. The enhancement of output power of graphene film on ITO nanodot structure is remarkable. We suggest the mechanism of the light output power enhancement associated with graphene layer on ITO nanodots is caused by the small area of ohmic contact [25], the quasi nano-pixellated emission source effects [26], and the texturing effect of ITO nanodots. The dispersed ITO nanodots on p-GaN surface and graphene layers effectively supply the injection current with rather uniform current densities into the whole emission area. The small ITO nanodot current sink sources do not disperse the current laterally on p-GaN layers but directly supply the current to pn active junction. Additionally, the ITO nanodots are excellent light texturing centers for light extraction path, reducing total internal reflection of generated photons at the surface. Also, the reduced Ohmic contact area by small ITO nanodots reduces the photon absorption center at the interface between ITO and p-GaN layer at which some alloying components are generated during thermal annealing. As a consequence, EL spectra and the light output powers of InGaN/AlGaN MQW UV LEDs with graphene film on ITO nanodot nodes are greatly improved.

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Fig. 5. (a) EL spectra at an applied current of 100 mA and (b) the light output powers as a function of injection current of InGaN/AlGaN MQW UV-LEDs with various conducting layer structures.

4. Conclusion In conclusion, we propose graphene films on ITO nanodot nodes as transparent and current spreading electrode in InGaN/AlGaN MQW UV-LEDs which demonstrated significant enhancement of light output characteristics at 380 nm wavelength LEDs. The light output power is remarkably enhanced without electrical drawbacks. Acknowledgments This work was supported by Basic science Research Laboratory (BRL: 2010-0019694) Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology. One of us (YHL) acknowledges the WCU (World Class University) program through the NRF funded by the MEST (R31-2008-10029).

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