- Email: [email protected]
GaN LEDs with a NiOx inter-layer
Solid-State Electronics 109 (2015) 47–51
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Letter
Enhanced opto-electrical properties of graphene electrode InGaN/GaN LEDs with a NiOx inter-layer Caichuan Wu 1, Fengyuan Liu 1, Bin Liu ⇑, Zhe Zhuang, Jiangping Dai, Tao Tao, Guogang Zhang, Zili Xie, Xinran Wang, Rong Zhang ⇑ Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, PR China Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, PR China
a r t i c l e
i n f o
Article history: Received 15 November 2014 Accepted 3 March 2015 Available online 31 March 2015 Keywords: Light-emitting diode Gallium nitride Graphene NiOx
a b s t r a c t We report the fabrication of gallium nitride (GaN)-based light-emitting diode (LED) with uniform and monolayer graphene as transparent current spreading layer. Two-dimensional graphene successfully provides efficient current spreading and hole injection into the active layers of the LEDs for light emission. To further reduce the ohmic contact resistance between p-GaN and graphene film, ultrathin NiOx inter-layer is introduced in the device, improving its electrical and optical performance. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Gallium nitride (GaN)-based light-emitting diodes (LEDs) have aroused widespread interest from academia and industry because of their advantages such as long lifetime, high efficiency and low energy consumption. For GaN-based LED device, the p-type GaN layers own relatively high resistivity resulting in the inhomogeneous distribution of current density [1]. Obviously, transparent current spreading layers (TCSLs) are crucial for the homogeneous hole injection into the active regions of the LEDs. Until now, indium tin oxide (ITO) has been predominantly adopted as TCSL in the fabrication of GaN-based LEDs. However, ITO has several drawbacks limiting its application in high-power LEDs, such as high and rising price because of the scarcity of indium and poor transparency at wavelengths below 450 nm [2,3]. Recently, graphene (Gr) is considered as an alternative transparent conductive material to the traditionally used ITO in optoelectronic devices because of its high electrical conductivity and excellent transparency to visible light (97% for monolayer graphene) [2,4]. Several pioneering works have reported the application of graphene film as TCSLs in GaN-based LEDs [1,2,5–7]. However, it is still difficult to form good ohmic contact between ⇑ Corresponding authors at: Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, PR China. E-mail addresses: [email protected] (B. Liu), [email protected] (R. Zhang). 1 These two authors contributed equally to this work. http://dx.doi.org/10.1016/j.sse.2015.03.005 0038-1101/Ó 2015 Elsevier Ltd. All rights reserved.
graphene and p-GaN due to the mismatch of work function, which causes high operating voltage and great power loss. In this work, chemical vapor deposition (CVD) synthesized graphene mounts directly on GaN-based LED in place of ITO as TCSL. Experiments show that graphene can provide efficient current spreading and hole injection into the active layers of the LED for light emission. To further reduce the ohmic contact resistance between p-GaN and graphene film, a 3 nm-thick NiOx inter-layer is introduced in the device, whose electrical and optical performance is greatly improved and very close to that of the conventional ITO-electrode LED.
2. Experiment details Fig. 1 illustrates the fabrication procedures of GaN-based Gr-electrode LED without and with NiOx inter-layer. Graphene film is grown on 25 lm-thick copper (Cu) catalytic substrate using CVD system. Briefly, 25 lm-thick Cu foil (purchased from Alfa Aesar) is loaded in a CVD tube and then the tube is evacuated and heated to 1000 °C in a gas mixture of hydrogen and argon (25 sccm:500 sccm). To obtain a smoother surface, the Cu foil is annealed at 1000 °C for 1 h. A mediate flow of methane (25 sccm) is then introduced into the tube for a 30 min’s growth to obtain a complete monolayer graphene film. After growth, one layer of polymethyl methacrylate (PMMA) is spun onto the Cu foil and dried naturally at room temperature [Fig. 1(a)]. The removal of the Cu substrate is performed by immersing it into ferric chloride
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Fig. 1. Schematics of the fabrication of Gr-electrode LEDs without and with NiOx inter-layer.
solution (FeCl3) [Fig. 1(b)]. The resultant film with PMMA and graphene is transferred to the pre-patterned LED epi-wafer and then PMMA is removed using acetone. For the LEDs with NiOx inter-layer, NiOx inter-layer is made by annealing at 500 °C in air for 10 min after deposition of a 3 nm-thick Ni film on the pre-patterned LED epi-wafer. The LEDs all have a mesa of dimension 350 lm 350 lm. The epitaxial structures, grown on c-plane sapphire substrate by metal organic chemical vapor deposition, include 15 pairs of InGaN (3 nm)/GaN (12 nm) MQWs sandwiched in 400 nm-thick p-GaN and n-GaN. The LED epi-wafer is etched by an inductively coupled plasma etching process using Cl2/BCl3 source gases with a 200 nm-thick SiO2 layer as a mesa etching mask. Graphene film on the n-GaN layer is completely etched using reactive ion etching processes with O2 gas only [Fig. 1(c) and (d)]. The p-electrode and n-electrode composed of Cr/Au (50 nm/200 nm) are evaporated onto graphene layer and n-GaN layer by e-beam evaporating [Fig. 1(e) and (f)). Besides, the LEDs with 200 nm-thick ITO as transparent electrodes are also fabricated and tested.
3. Results and discussion According to the top-view scanning electron microscope (SEM) images of Gr-electrode LEDs with and without NiOx inter-layer [Fig. 2(a) and (b)], it is obvious that the graphene film is nearly continuous and uniform, and closely contact with the p-GaN layer. The inset of Fig. 2(a), enlarged SEM image of the region marked by the dotted line, exhibits that NiOx inter-layer is composed of randomly distributed nanoparticles with a diameter of 20–30 nm. The visible (514 nm) Raman spectra of graphene film transferred to the LEDs with and without NiOx inter-layer are separately presented in Fig. 2(c) and (d). Two prominent peaks in the Raman spectra of graphene corresponding to the G peak (1583 cm 1) and the 2D peak (2705 cm 1) are observed. According to the intensity ratio of G/2D peaks (<1) and symmetric 2D band with a full width at half
maximum of 35 cm 1 in Fig. 2(c) and (d), we conclude that the used graphene is monolayer [8]. The weaker intensity of disorder-induced Raman D-band located at 1343 cm 1, compared with that of the G-band, indicates the low density of defects, local disordered carbons, wrinkles or edges in graphene film [8]. Notably, the D peak of graphene with NiOx inter-layer is slightly enhanced compared with that of graphene without NiOx interlayer, which is probably resulting from the higher density of defects or wrinkles induced by rougher surface of the p-GaN layer due to the NiOx nanoparticles. By collecting the transmission spectra of graphene, NiOx and NiOx/graphene at visible wavelength range [Fig. 2(e)], it is discovered that NiOx inter-layer and graphene film both exhibit a transparency of nearly 90% to visible light, and NiOx/graphene hybrid structure acceptably exhibits a transparency above 81% to visible light. Besides, the transparency of NiOx/graphene to blue and ultraviolet light is still higher than that of a 200 nm ITO layer [2], suggesting its application potential in transparent conducting electrodes for blue or ultraviolet LEDs. To characterize the NiOx/graphene contact with the underlying p-GaN layer, the current–voltage (I–V) curves of different TCSLs are collected. As illustrated in Fig. 2(f), the resistances Rc of graphene, NiOx/graphene and NiOx are estimated from the slopes of the I–V curves as 260 X, 75 X and 2000 X. By introducing NiOx inter-layer, the I–V curve becomes more linear compared with the sample without NiOx inter-layer, which indicates that the ohmic contact between graphene film and p-GaN is greatly improved. Notably, the I–V curve of the sample having only NiOx exhibits a smaller slope because NiOx inter-layer is composed of randomly distributed non-coalescent nanoparticles, causing the difficulty of current spreading [5,7]. Fig. 3(a) shows the schematic of the individual Gr-electrode LED. The structure includes the graphene anode, the active luminescent layer (p-GaN/MQW/n-GaN), Cr/Au cathode, undoped GaN, and the sapphire substrate. Fig. 3(c) and (d) shows the electroluminescence (EL) photographs of the individual Gr-electrode
C. Wu et al. / Solid-State Electronics 109 (2015) 47–51
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Fig. 2. Plan-view SEM images of Gr-electrode LED with (a) and without (b) NiOx inter-layer, Raman spectra of transferred graphene (c and d). (e) Optical transmittances of different TCSLs. (f) I–V curves of different contacts on p-GaN. The inset of (f) is the schematics of a typical test structure.
Fig. 3. (a) Side-view illustration of Gr-electrode LED. EL photographs of LEDs with different TCSLs operated at the injection current of 100 lA, (b) NiOx, (c) graphene, and (d) NiOx/graphene.
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C. Wu et al. / Solid-State Electronics 109 (2015) 47–51
LED with graphene alone and NiOx/graphene as TCSLs operated at injection current of 100 lA, respectively. Regardless of the introduction of NiOx inter-layer, blue–purple light emission and uniform emission area are clearly observed even at a low input current of 100 lA. However, the EL photograph of the LED with only NiOx nanoparticles as TCSL exhibits that the emission area is mainly concentrated around p-pad [Fig. 3(b)] [7]. These results suggest that graphene successfully provides efficient current spreading and hole injection into the active layers of the LEDs for light emission. Moreover, the LED with NiOx/graphene is much brighter at the same input current, suggesting the smaller power loss due to the smaller ohmic contact resistance between p-GaN and graphene film. Therefore, it can be deduced that NiOx interlayer plays a crucial role in improving the ohmic contact between graphene film and p-GaN and fails to effectively spread the injection current owing to the fact that NiOx inter-layer is composed of non-coalescent nanoparticles. These results are corroborated with the above analysis [Fig. 2(f)] [7]. Fig. 4(a) and (b) compares the I–V curves and EL spectra of the LEDs with graphene, NiOx/graphene and ITO as TCSLs. When the input current is defined at 20 mA, the forward voltage is found to be 6.4 V for Gr-electrode LED without NiOx inter-layer, much higher than that of the ITO-electrode LED (3.9 V).
This is due to the high ohmic contact resistance between p-GaN and graphene deriving from the mismatch of work function (p-GaN: 5.5–5.9 eV and graphene: 4.4–4.65 eV) [9]. However, the corresponding forward voltage significantly reduces to 4.5 V, slightly higher than that of ITO-electrode LED [2,9], when the NiOx inter-layer is introduced in the device. This is ascribed to the large work function (>5 eV) and relative high hole concentration of NiOx inter-layer [10], lowering the Schottky barrier and facilitating ohmic contact formation with a small loss of the overall transparency. Hence, the LED performance is significantly improved. Fig. 4(c) and (d) severally present the EL spectra of Gr-electrode LED with and without NiOx inter-layer at different injection currents. Evidently, the EL intensity increases with the rise of the injection current. The integrated EL intensity as a function of the injection currents is shown in Fig. 4(e). It is observed that the integrated EL intensity of Gr-electrode LED with NiOx inter-layer is 85% of that of ITO-electrode LED and 150% of that of Gr-electrode LED without NiOx inter-layer under the injection current of 20 mA. The EL peak position firstly blue-shifts slightly and then red-shifts as the injection current increases (Fig. 4(f)), indicating that graphene film acts as a p-type electrode in GaN-based LEDs in a suitable manner. The blue shift is due to the combined effects of the band-filling phenomena of the localized energy states formed
Fig. 4. I–V curves (a) and EL spectra (b) of the fabricated LEDs with different TCSLs (measured at 20 mA). The EL intensity of the Gr-electrode LEDs with (c) and without (d) NiOx inter-layer. The integrated EL intensity (e) and peek shift (f) of the Gr-electrode LEDs as a function of the injection current.
C. Wu et al. / Solid-State Electronics 109 (2015) 47–51
by potential fluctuations in the MQWs and the screening effect of the polarization induced electric field produced by carriers [11]. The red shift is due to the rise of junction temperature caused by heat effect. In addition, the brightness variation of the light emission from Gr-electrode LED is clearly observed at a wide range of input currents (0.1–100 mA), indicating that the graphene electrode successfully operates as the TCSL over the current range. 4. Conclusions This letter reports the fabrication of Gr-electrode LEDs with and without NiOx inter-layer. Our experiments have shown that graphene can succeed in providing efficient current spreading and hole injection into the active layers of the LED for light emission. Moreover, the EL performance of the device is enhanced by 1.5 times by introducing a 3 nm-thick NiOx inter-layer between graphene and p-GaN and its overall performance can compare favourably with that of the conventional ITO-electrode LEDs. The enhancement is due to the reduced ohmic contact resistance between graphene and p-GaN deriving from the impact of the NiOx inter-layer with a large work function and high hole concentration. These achievements show the promising prospects of graphene electrode LEDs. Acknowledgements This work is supported by Special Funds for Major State Basic Research Project (2011CB301900), National Nature Science Foundation of China (61422401, 61274003, 61176063), Program
51
for New Century Excellent Talents in University, China (NCET-110229), Nature Science Foundation of Jiangsu Province (BK2011010, BY2013077), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] Chandramohan S, Ko Kang Bok, Yang Jong Han. Performance evaluation of GaN light-emitting diodes using transferred graphene as current spreading layer. J Appl Phys 2014;115:054503. [2] Jo Gunho, Choe Minhyeok, Cho Chu-Young. Large-scale patterned multi-layer graphene films as transparent conducting electrodes for GaN light-emitting diodes. Nanotechnology 2010;21:175201. [3] Youn Doo-Hyeb, Yu Young-Jun, Choi Hong Kyw. Graphene transparent electrode for enhanced optical power and thermal stability in GaN lightemitting diodes. Nanotechnology 2013;24:075202. [4] Gao Weilu, Shu Jie, Qiu Ciyuan. Excitation of plasmonic waves in graphene by guided-mode resonances. ACS Nano 2012;6:7806–13. [5] Zhang Yiyun, Li Xiao, Wang Liancheng. Enhanced light emission of GaN-based diodes with a NiOx/graphene hybrid electrode. Nanoscale 2012;21:5852–5. [6] Lee Jung Min, Choung Jae Woong, Yi Jaeseok. Vertical pillar-superlattice array and graphene hybrid light emitting diodes. Nano Lett 2010;10:2783–8. [7] Chandramohan S, Kang Ji Hye, Ryu Beo Deul. Impact of interlayer processing conditions on the performance of GaN light-emitting diode with specific NiOx/graphene electrode. ACS Appl Mater Interfaces 2013;5:958–64. [8] Malard LM, Pimenta MA, Dresselhaus G. Raman spectroscopy in graphene. Phys Rep 2009;473:51–87. [9] Seo Tae Hoon, Oh Tae Su, Chae Seung Jin. Jpn enhanced light output power of GaN light-emitting diodes with graphene film as a transparent conducting electrode. Mechanism investigation of NiOx in Au/Ni/p-type GaN ohmic contacts annealed in air. J Appl Phys 2011;50:125103. [10] Lee Ching-ting, Lin Yow-jon, Lee Tsung-hsin. J Electron Mater 2002;32:341–5. [11] Kuokstis E, Yang JW, Simin G. Two mechanisms of blueshift of edge emission in InGaN-based epilayers and multiple quantum wells. Appl Phys Lett 2002;80: 977–9.
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Letter
Enhanced opto-electrical properties of graphene electrode InGaN/GaN LEDs with a NiOx inter-layer Caichuan Wu 1, Fengyuan Liu 1, Bin Liu ⇑, Zhe Zhuang, Jiangping Dai, Tao Tao, Guogang Zhang, Zili Xie, Xinran Wang, Rong Zhang ⇑ Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, PR China Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, PR China
a r t i c l e
i n f o
Article history: Received 15 November 2014 Accepted 3 March 2015 Available online 31 March 2015 Keywords: Light-emitting diode Gallium nitride Graphene NiOx
a b s t r a c t We report the fabrication of gallium nitride (GaN)-based light-emitting diode (LED) with uniform and monolayer graphene as transparent current spreading layer. Two-dimensional graphene successfully provides efficient current spreading and hole injection into the active layers of the LEDs for light emission. To further reduce the ohmic contact resistance between p-GaN and graphene film, ultrathin NiOx inter-layer is introduced in the device, improving its electrical and optical performance. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Gallium nitride (GaN)-based light-emitting diodes (LEDs) have aroused widespread interest from academia and industry because of their advantages such as long lifetime, high efficiency and low energy consumption. For GaN-based LED device, the p-type GaN layers own relatively high resistivity resulting in the inhomogeneous distribution of current density [1]. Obviously, transparent current spreading layers (TCSLs) are crucial for the homogeneous hole injection into the active regions of the LEDs. Until now, indium tin oxide (ITO) has been predominantly adopted as TCSL in the fabrication of GaN-based LEDs. However, ITO has several drawbacks limiting its application in high-power LEDs, such as high and rising price because of the scarcity of indium and poor transparency at wavelengths below 450 nm [2,3]. Recently, graphene (Gr) is considered as an alternative transparent conductive material to the traditionally used ITO in optoelectronic devices because of its high electrical conductivity and excellent transparency to visible light (97% for monolayer graphene) [2,4]. Several pioneering works have reported the application of graphene film as TCSLs in GaN-based LEDs [1,2,5–7]. However, it is still difficult to form good ohmic contact between ⇑ Corresponding authors at: Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, PR China. E-mail addresses: [email protected] (B. Liu), [email protected] (R. Zhang). 1 These two authors contributed equally to this work. http://dx.doi.org/10.1016/j.sse.2015.03.005 0038-1101/Ó 2015 Elsevier Ltd. All rights reserved.
graphene and p-GaN due to the mismatch of work function, which causes high operating voltage and great power loss. In this work, chemical vapor deposition (CVD) synthesized graphene mounts directly on GaN-based LED in place of ITO as TCSL. Experiments show that graphene can provide efficient current spreading and hole injection into the active layers of the LED for light emission. To further reduce the ohmic contact resistance between p-GaN and graphene film, a 3 nm-thick NiOx inter-layer is introduced in the device, whose electrical and optical performance is greatly improved and very close to that of the conventional ITO-electrode LED.
2. Experiment details Fig. 1 illustrates the fabrication procedures of GaN-based Gr-electrode LED without and with NiOx inter-layer. Graphene film is grown on 25 lm-thick copper (Cu) catalytic substrate using CVD system. Briefly, 25 lm-thick Cu foil (purchased from Alfa Aesar) is loaded in a CVD tube and then the tube is evacuated and heated to 1000 °C in a gas mixture of hydrogen and argon (25 sccm:500 sccm). To obtain a smoother surface, the Cu foil is annealed at 1000 °C for 1 h. A mediate flow of methane (25 sccm) is then introduced into the tube for a 30 min’s growth to obtain a complete monolayer graphene film. After growth, one layer of polymethyl methacrylate (PMMA) is spun onto the Cu foil and dried naturally at room temperature [Fig. 1(a)]. The removal of the Cu substrate is performed by immersing it into ferric chloride
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C. Wu et al. / Solid-State Electronics 109 (2015) 47–51
Fig. 1. Schematics of the fabrication of Gr-electrode LEDs without and with NiOx inter-layer.
solution (FeCl3) [Fig. 1(b)]. The resultant film with PMMA and graphene is transferred to the pre-patterned LED epi-wafer and then PMMA is removed using acetone. For the LEDs with NiOx inter-layer, NiOx inter-layer is made by annealing at 500 °C in air for 10 min after deposition of a 3 nm-thick Ni film on the pre-patterned LED epi-wafer. The LEDs all have a mesa of dimension 350 lm 350 lm. The epitaxial structures, grown on c-plane sapphire substrate by metal organic chemical vapor deposition, include 15 pairs of InGaN (3 nm)/GaN (12 nm) MQWs sandwiched in 400 nm-thick p-GaN and n-GaN. The LED epi-wafer is etched by an inductively coupled plasma etching process using Cl2/BCl3 source gases with a 200 nm-thick SiO2 layer as a mesa etching mask. Graphene film on the n-GaN layer is completely etched using reactive ion etching processes with O2 gas only [Fig. 1(c) and (d)]. The p-electrode and n-electrode composed of Cr/Au (50 nm/200 nm) are evaporated onto graphene layer and n-GaN layer by e-beam evaporating [Fig. 1(e) and (f)). Besides, the LEDs with 200 nm-thick ITO as transparent electrodes are also fabricated and tested.
3. Results and discussion According to the top-view scanning electron microscope (SEM) images of Gr-electrode LEDs with and without NiOx inter-layer [Fig. 2(a) and (b)], it is obvious that the graphene film is nearly continuous and uniform, and closely contact with the p-GaN layer. The inset of Fig. 2(a), enlarged SEM image of the region marked by the dotted line, exhibits that NiOx inter-layer is composed of randomly distributed nanoparticles with a diameter of 20–30 nm. The visible (514 nm) Raman spectra of graphene film transferred to the LEDs with and without NiOx inter-layer are separately presented in Fig. 2(c) and (d). Two prominent peaks in the Raman spectra of graphene corresponding to the G peak (1583 cm 1) and the 2D peak (2705 cm 1) are observed. According to the intensity ratio of G/2D peaks (<1) and symmetric 2D band with a full width at half
maximum of 35 cm 1 in Fig. 2(c) and (d), we conclude that the used graphene is monolayer [8]. The weaker intensity of disorder-induced Raman D-band located at 1343 cm 1, compared with that of the G-band, indicates the low density of defects, local disordered carbons, wrinkles or edges in graphene film [8]. Notably, the D peak of graphene with NiOx inter-layer is slightly enhanced compared with that of graphene without NiOx interlayer, which is probably resulting from the higher density of defects or wrinkles induced by rougher surface of the p-GaN layer due to the NiOx nanoparticles. By collecting the transmission spectra of graphene, NiOx and NiOx/graphene at visible wavelength range [Fig. 2(e)], it is discovered that NiOx inter-layer and graphene film both exhibit a transparency of nearly 90% to visible light, and NiOx/graphene hybrid structure acceptably exhibits a transparency above 81% to visible light. Besides, the transparency of NiOx/graphene to blue and ultraviolet light is still higher than that of a 200 nm ITO layer [2], suggesting its application potential in transparent conducting electrodes for blue or ultraviolet LEDs. To characterize the NiOx/graphene contact with the underlying p-GaN layer, the current–voltage (I–V) curves of different TCSLs are collected. As illustrated in Fig. 2(f), the resistances Rc of graphene, NiOx/graphene and NiOx are estimated from the slopes of the I–V curves as 260 X, 75 X and 2000 X. By introducing NiOx inter-layer, the I–V curve becomes more linear compared with the sample without NiOx inter-layer, which indicates that the ohmic contact between graphene film and p-GaN is greatly improved. Notably, the I–V curve of the sample having only NiOx exhibits a smaller slope because NiOx inter-layer is composed of randomly distributed non-coalescent nanoparticles, causing the difficulty of current spreading [5,7]. Fig. 3(a) shows the schematic of the individual Gr-electrode LED. The structure includes the graphene anode, the active luminescent layer (p-GaN/MQW/n-GaN), Cr/Au cathode, undoped GaN, and the sapphire substrate. Fig. 3(c) and (d) shows the electroluminescence (EL) photographs of the individual Gr-electrode
C. Wu et al. / Solid-State Electronics 109 (2015) 47–51
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Fig. 2. Plan-view SEM images of Gr-electrode LED with (a) and without (b) NiOx inter-layer, Raman spectra of transferred graphene (c and d). (e) Optical transmittances of different TCSLs. (f) I–V curves of different contacts on p-GaN. The inset of (f) is the schematics of a typical test structure.
Fig. 3. (a) Side-view illustration of Gr-electrode LED. EL photographs of LEDs with different TCSLs operated at the injection current of 100 lA, (b) NiOx, (c) graphene, and (d) NiOx/graphene.
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LED with graphene alone and NiOx/graphene as TCSLs operated at injection current of 100 lA, respectively. Regardless of the introduction of NiOx inter-layer, blue–purple light emission and uniform emission area are clearly observed even at a low input current of 100 lA. However, the EL photograph of the LED with only NiOx nanoparticles as TCSL exhibits that the emission area is mainly concentrated around p-pad [Fig. 3(b)] [7]. These results suggest that graphene successfully provides efficient current spreading and hole injection into the active layers of the LEDs for light emission. Moreover, the LED with NiOx/graphene is much brighter at the same input current, suggesting the smaller power loss due to the smaller ohmic contact resistance between p-GaN and graphene film. Therefore, it can be deduced that NiOx interlayer plays a crucial role in improving the ohmic contact between graphene film and p-GaN and fails to effectively spread the injection current owing to the fact that NiOx inter-layer is composed of non-coalescent nanoparticles. These results are corroborated with the above analysis [Fig. 2(f)] [7]. Fig. 4(a) and (b) compares the I–V curves and EL spectra of the LEDs with graphene, NiOx/graphene and ITO as TCSLs. When the input current is defined at 20 mA, the forward voltage is found to be 6.4 V for Gr-electrode LED without NiOx inter-layer, much higher than that of the ITO-electrode LED (3.9 V).
This is due to the high ohmic contact resistance between p-GaN and graphene deriving from the mismatch of work function (p-GaN: 5.5–5.9 eV and graphene: 4.4–4.65 eV) [9]. However, the corresponding forward voltage significantly reduces to 4.5 V, slightly higher than that of ITO-electrode LED [2,9], when the NiOx inter-layer is introduced in the device. This is ascribed to the large work function (>5 eV) and relative high hole concentration of NiOx inter-layer [10], lowering the Schottky barrier and facilitating ohmic contact formation with a small loss of the overall transparency. Hence, the LED performance is significantly improved. Fig. 4(c) and (d) severally present the EL spectra of Gr-electrode LED with and without NiOx inter-layer at different injection currents. Evidently, the EL intensity increases with the rise of the injection current. The integrated EL intensity as a function of the injection currents is shown in Fig. 4(e). It is observed that the integrated EL intensity of Gr-electrode LED with NiOx inter-layer is 85% of that of ITO-electrode LED and 150% of that of Gr-electrode LED without NiOx inter-layer under the injection current of 20 mA. The EL peak position firstly blue-shifts slightly and then red-shifts as the injection current increases (Fig. 4(f)), indicating that graphene film acts as a p-type electrode in GaN-based LEDs in a suitable manner. The blue shift is due to the combined effects of the band-filling phenomena of the localized energy states formed
Fig. 4. I–V curves (a) and EL spectra (b) of the fabricated LEDs with different TCSLs (measured at 20 mA). The EL intensity of the Gr-electrode LEDs with (c) and without (d) NiOx inter-layer. The integrated EL intensity (e) and peek shift (f) of the Gr-electrode LEDs as a function of the injection current.
C. Wu et al. / Solid-State Electronics 109 (2015) 47–51
by potential fluctuations in the MQWs and the screening effect of the polarization induced electric field produced by carriers [11]. The red shift is due to the rise of junction temperature caused by heat effect. In addition, the brightness variation of the light emission from Gr-electrode LED is clearly observed at a wide range of input currents (0.1–100 mA), indicating that the graphene electrode successfully operates as the TCSL over the current range. 4. Conclusions This letter reports the fabrication of Gr-electrode LEDs with and without NiOx inter-layer. Our experiments have shown that graphene can succeed in providing efficient current spreading and hole injection into the active layers of the LED for light emission. Moreover, the EL performance of the device is enhanced by 1.5 times by introducing a 3 nm-thick NiOx inter-layer between graphene and p-GaN and its overall performance can compare favourably with that of the conventional ITO-electrode LEDs. The enhancement is due to the reduced ohmic contact resistance between graphene and p-GaN deriving from the impact of the NiOx inter-layer with a large work function and high hole concentration. These achievements show the promising prospects of graphene electrode LEDs. Acknowledgements This work is supported by Special Funds for Major State Basic Research Project (2011CB301900), National Nature Science Foundation of China (61422401, 61274003, 61176063), Program
51
for New Century Excellent Talents in University, China (NCET-110229), Nature Science Foundation of Jiangsu Province (BK2011010, BY2013077), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] Chandramohan S, Ko Kang Bok, Yang Jong Han. Performance evaluation of GaN light-emitting diodes using transferred graphene as current spreading layer. J Appl Phys 2014;115:054503. [2] Jo Gunho, Choe Minhyeok, Cho Chu-Young. Large-scale patterned multi-layer graphene films as transparent conducting electrodes for GaN light-emitting diodes. Nanotechnology 2010;21:175201. [3] Youn Doo-Hyeb, Yu Young-Jun, Choi Hong Kyw. Graphene transparent electrode for enhanced optical power and thermal stability in GaN lightemitting diodes. Nanotechnology 2013;24:075202. [4] Gao Weilu, Shu Jie, Qiu Ciyuan. Excitation of plasmonic waves in graphene by guided-mode resonances. ACS Nano 2012;6:7806–13. [5] Zhang Yiyun, Li Xiao, Wang Liancheng. Enhanced light emission of GaN-based diodes with a NiOx/graphene hybrid electrode. Nanoscale 2012;21:5852–5. [6] Lee Jung Min, Choung Jae Woong, Yi Jaeseok. Vertical pillar-superlattice array and graphene hybrid light emitting diodes. Nano Lett 2010;10:2783–8. [7] Chandramohan S, Kang Ji Hye, Ryu Beo Deul. Impact of interlayer processing conditions on the performance of GaN light-emitting diode with specific NiOx/graphene electrode. ACS Appl Mater Interfaces 2013;5:958–64. [8] Malard LM, Pimenta MA, Dresselhaus G. Raman spectroscopy in graphene. Phys Rep 2009;473:51–87. [9] Seo Tae Hoon, Oh Tae Su, Chae Seung Jin. Jpn enhanced light output power of GaN light-emitting diodes with graphene film as a transparent conducting electrode. Mechanism investigation of NiOx in Au/Ni/p-type GaN ohmic contacts annealed in air. J Appl Phys 2011;50:125103. [10] Lee Ching-ting, Lin Yow-jon, Lee Tsung-hsin. J Electron Mater 2002;32:341–5. [11] Kuokstis E, Yang JW, Simin G. Two mechanisms of blueshift of edge emission in InGaN-based epilayers and multiple quantum wells. Appl Phys Lett 2002;80: 977–9.