Enhanced Light Output Power of Near-Ultraviolet Light-Emitting Diodes with Au-Doped Graphene for Transparent and Current-Spreading Electrode Tae Hoon Seo 1 , Seung Jin Chae 2 , Bo Kyoung Kim 1 , GangU Shin 1 , Young Hee Lee 2 , and Eun-Kyung Suh 1 1 School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center (SPRC), Chonbuk National University, Jeonju 561-756, Korea 2 BK21 Physics Division, Department of Energy Science, Sungkyunkwan Advanced Institute of Nanotechnology, Center for Nanotubes and Nanostructured Composites, Sungkyungkyun University, Suwon, Gyeonggi 440-746, Korea Received July 17, 2012; accepted October 3, 2012; published online October 23, 2012 We report the implementation of Au-doped graphene film as a transparent and current-spreading electrode (TCSE) in GaN-based near-ultraviolet (UV) light-emitting diode (LED) to achieve good UV emission efficiency. The TCSE effects of Au-doped graphene film were clearly seen in both the electroluminescence (EL) and current–voltage (I –V ) characteristics. The EL output power of 380-nm wavelength UV-LEDs with Au-doped graphene film was enhanced by about 20% at an injection current of 20 mA compared with that of conventional UV-LEDs. The increase of the light output power is attributed to the high UV transmittance of graphene, effective current spreading, and injection. # 2012 The Japan Society of Applied Physics G aN-based ultraviolet (UV) light-emitting diodes (LEDs) are of special interest for their use in ger- micidal instrumentation, biological agent identifica- tion, chemical sensing, fluorescence excitation, and optical data storage. 1) In particular, GaN-based near-UV LEDs of wavelengths around 380–400 nm are being widely used as efficient pumping sources for organic and inorganic lumi- nescent materials for white-light generation. 2) However, UV LEDs have suffered from low external quantum efficiency, mainly due to high threading dislocation density, high activa- tion energy for Mg-doped p-GaN, and high optical absorption in the p-GaN clad/contact layers. In particular, typical p-type thin GaN layers with high lateral sheet resistance and low carrier mobility resulted in severe current crowding under the vertical direction of the electrode and low current spreading through the full emitting area. The p-type electrode requires both low contact resistance with p-type GaN and high trans- mittance for the extraction of photons from active layers. Indium tin oxide (ITO) has been widely used as transparent conducting electrodes (TCEs) in solar cells and LEDs. How- ever, the ITO layer has appeared to be problematic due to the absorption in the UV region. 3,4) Therefore, an alternative transparent electrode is required with both optical and elec- trical performances similar to or better than those of ITO but without having drawbacks in the UV region. Graphene, a monolayer planar sheet of sp 2 -bonded hexagonal carbon atoms, has been attracting much attention owing to its fascinating physical properties such as quantum electronic transport characteristics, high intrinsic mobility, thermal conductivity, and high optical transmittance. 5–9) The graphene with superb optical and electrical merits has drawn attention for applications as a TCE material in optoelectronic devices. Several pioneering works have reported applica- tions of graphene films as TCE in GaN-based LEDs. 10,11) Though graphene has a high mobility and transmittance, some difficulties arise when applied to GaN-based LEDs due to the difference in work function between bare graphene and p-GaN layer and high sheet resistance of graphene layer, resulting in a high forward operating voltage and low light output power. To realize the practical graphene electrode, a method to resolve those disadvantages of graphene should be developed. Chemical charge transfer doping has been shown to increase the work function of graphene as a result of the modifica- tion of the Fermi level. 12,13) Notably, doping can reduce the sheet resistance and thus improve the conductivity. 12) Chandramohan et al. 14) introduced work-function-tuned multi layer graphene films by aqueous gold chloride (AuCl 3 ) chemical doping as a TCE for the GaN-based blue LEDs. In this work, we fabricated near-UV LED with an emis- sion wavelength of 380 nm with a pristine graphene film as a transparent and current-spreading electrode (TCSE). To enhance the optoelectrical performance of the near-UV LED, the Au-doped graphene film was transferred onto the near UV LED as a TCSE. The AlInGaN-based UV LEDs were grown on a sapphire substrate by metal organic chemical vapor deposition. A 30-nm-thick GaN buffer layer was deposited on the sapphire substrate at 550  C, before the growth of an undoped GaN layer with a thickness of 1.5 m and a Si doped n-GaN layer with a thickness of 2.0 m at 1040  C. Then, five pairs of In 0:04 Ga 0:96 N QWs and Al 0:08 Ga 0:92 N barrier layers with thicknesses of 2 and 12 nm, respectively, were grown at 800  C. Finally, a 25-nm-thick Mg-doped p-Al 0:25 Ga 0:75 N electron blocking layer and a 100-nm-thick p-GaN contact layer were grown at 1040  C. After the growth, the mesa region was defined by an inductively coupled plasma etcher using Cl 2 and BCl 3 gases until n-GaN layer for the n-elec- trode contact was exposed. Large-scale graphene layers were synthesized on a 70-m-thick Cu foil by chemical vapor depostion. Details can be found in ref. 15. Then, AuCl 3 in nitromethane was spin coated (speed at 2000 rpm) for 60 s on the graphene-transferred UV LED. The sheet resistance and transmittance of graphene are strongly dependent on AuCl 3 concentration. 12,14) Typical sheet resistance and transmit- tance of the doped graphene film under optimum condition were about 90 /Ã and 93%, respectively, obtained at a AuCl 3 concentration of 10 mM. 12) In order to fabricate the electrode, the as-grown and doped graphene films were transferred twice onto the pre patterned LED epilayers with a mesa of dimension of 315  315 m 2 . Hence, in overall, bi- layer graphene film was applied as TCSE in this work and the thickness of the bi-layer graphene can be approximated as the interlayer distance of the bilayer, 0.38 nm, as confirmed by transmission electron microscopy. 15) To prevent oxidation of GaN-LED and strengthen the adhesion between p-GaN and  E-mail address: eksuh@jbnu.ac.kr Applied Physics Express 5 (2012) 115101 115101-1 # 2012 The Japan Society of Applied Physics http://dx.doi.org/10.1143/APEX.5.115101