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Japanese Journal of Applied Physics 51 (2012) 020204 DOI: 10.1143/JJAP.51.020204
Enhancement in the Light Output Power of GaN-Based Light-Emitting Diodes with Nanotextured Indium Tin Oxide Layer Using Self-Assembled Cesium Chloride Nanospheres Yiyun Zhang, Jing Li, Tongbo Wei, Jing Liu1 , Xiaoyan Yi, Guohong Wang, and Futing Yi1 Semiconductor Lighting Technology Research and Development Center, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China 1 Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China Received September 13, 2011; accepted November 30, 2011; published online January 30, 2012 In this study, enhanced light output power in GaN-based light-emitting diodes (LEDs) with a nanotextured indium tin oxide (ITO) transparent conductive layer was observed. Wafer-scale self-assembled cesium chloride nanospheres were formed on the ITO transparent conductive layer and served as the mask in a dry etching process. After the inductively coupled plasma (ICP) etching process, nanoscale islands were fabricated on the ITO layer. Compared with LEDs with a planar ITO layer, the light output power of LEDs with a nanotextured ITO layer was improved by 23.4%. Optoelectronic measurement showed that the performance of the fabricated LEDs was greatly enhanced. # 2012 The Japan Society of Applied Physics
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aN-based light-emitting diodes (LEDs) have attracted considerable attention for their various applications including traffic lights, outdoor displays, liquid crystal display backlights, and automobile headlights, and have great potential to replace conventional fluorescent lamps and incandescent bulbs.1,2) So far, various techniques have been proposed to improve LED light extraction efficiency, such as the use of a patterned sapphire substrate,3,4) surface roughening,5–7) and the use of a twodimensional photonic crystal (2D-PC) structure.8–12) Among these methods, it has been proved that surface roughening is a very effective way to improve LED light extraction efficiency. The enhanced light output power (LOP) is caused by the higher probability of photons escaping from the textured surface.5) Recently, extensive research has focused on the use of a roughened indium tin oxide (ITO) surface to improve the performance of LEDs.13–17) However, some of these techniques need either nanoimprinting equipment or an electronic beam lithography (EBL) system, which are considered to be too complex and costly to be applied in commercial production. In addition, some methods cannot provide a wafer-level uniform rough surface.16) Moreover, some of the proposed techniques may deteriorate the electrical properties of conventional LEDs.17) In ref. 7, we first reported the use of self-assembled cesium chloride nanospheres as inductively coupled plasma (ICP) etching masks for p-GaN roughening. However, in this method, the cesium chloride nanospheres needed to be carefully chosen so as not to deteriorate the electrical properties of the nanotextured LEDs after the ICP etching process. In this study, we fabricated a nanotextured ITO surface using self-assembled cesium chloride nanospheres. The current–voltage (I–V ) characteristics of the fabricated LEDs with a nanotextured ITO layer showed that the electrical properties did not deteriorate. Compared with other methods of obtaining a textured ITO surface, the fabricated cesium chloride nanospheres have several advantages, such as simple fabrication, low cost, and a whole-wafer-level process. Furthermore, the cesium chloride nanospheres are water-soluble, making them very easy to remove from the ITO layer without deteriorating the integrity of the ITO layer
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and the electrical properties. By virtue of these superior properties of cesium chloride, the performance of massproduced LEDs can be greatly improved using this method. A conventional GaN LED structure was grown on 2-in. sapphire substrates by metal–organic chemical vapor deposition (MOCVD). The epitaxial LED structure consisted of a 30-nm-thick low-temperature-grown GaN buffer layer, a 2-m-thick undoped GaN layer, and a 2-m-thick heavily doped n-type GaN layer, followed by 8 pairs of InGaN (3 nm)/GaN (12 nm) multiple quantum wells (MQWs) with a total thickness of 0.15 m and a 0.2-m-thick p-type GaN layer. After a 300-nm-thick ITO transparent conductive layer was deposited on the wafer by an e-beam system, cesium chloride thin films with different thicknesses were thermally evaporated onto the ITO layer of different wafers at room temperature. Then the samples were exposed to water vapor, resulting in the self-assembly of cesium chloride nanospheres and their random distribution on the ITO layer. The average diameter of the cesium chloride nanospheres can be controlled by varying the cesium chloride film thickness, relative humidity, and development time.7) After that, an ICP etching process was implemented. Then the wafers were placed in deionized water to remove the residual cesium chloride particles. Next, an LED mesa (1 1 mm2 ) was obtained by standard lithography patterning followed by applying an ICP etcher using Cl2 and BCl3 to expose the n-type GaN layer. Finally, p- and n-electrodes composed of Cr/Pt/Au (50/50/1500 nm) were evaporated onto the ITO transparent conductive layer and the n-type GaN layer by an e-beam evaporator. Schematics of the fabrication process are illustrated in Fig. 1. By varying the cesium chloride film growth conditions, such as film thickness, relative humidity, and development time, different cesium chloride nanospheres can be formed on the ITO transparent conductive layer.7) In this paper, the thicknesses of the deposited cesium chloride films were 50, 100, and 150 nm. The corresponding sizes of the selfassembled cesium chloride nanoparticles were 450, 700, and 950 nm. Meanwhile, the average densities of the fabricated nanoparticles were 1:6 109 , 4:8 108 , and 1:3 108 cm 2 , respectively. Figures 2(a) and 2(b) show atomic force microscopy (AFM) images of the 700 nm self-assembled cesium chloride nanospheres on an ITO layer. The height
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# 2012 The Japan Society of Applied Physics
Jpn. J. Appl. Phys. 51 (2012) 020204
Y. Zhang et al.
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Fig. 1. (Color online) Schematics of the fabrication process: (a) cesium chloride film deposition, (b) ICP etching, (c) nanoislands on the ITO layer, and (d) metal pads fabrication.
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Fig. 3. (Color online) AFM images of surface morphology of the ITO transparent layer (a) before etching and after the ICP etching process using (b) 450 nm, (c) 700 nm, and (d) 950 nm cesium chloride nanospheres.
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Fig. 2. (Color online) Atomic force microscopy (AFM) images of the 700 nm self-assembled cesium chloride nanospheres on the ITO layer: (a) the shape and (b) height and width of the fabricated cesium chloride nanospheres.
and width of the nanoparticles were 300 and 700 nm, respectively, which showed that the fabricated cesium chloride nanospheres had an approximately hemispherical shape. After ICP etching for 400 s, nanoscale truncatedcone-shaped islands were formed on the ITO layer. Figure 3 shows AFM images of the surface morphology of the etched ITO layers after the dry-etching process using different sizes of cesium chloride nanospheres. Furthermore, in order to investigate the impact of nanoislands on the light output of LED devices, the same sizes of cesium chloride nanospheres were used as ICP etching masks with etching for 300, 400, 500, or 600 s. Figure 4(a) shows the average height of the nanoislands on the ITO layer and the RMS roughness of the etched surface using different sizes of cesium chloride nanospheres. When the size of the cesium chloride nanospheres was decreased from 950 to 700 nm, the RMS roughness of the etched ITO surface increased from 24.9 to 28.4 nm. This is due to the fact that the fabricated nanoislands were more densely distributed while their height remained almost unchanged when the size of the cesium chloride nanospheres decreased. However, as the size of the cesium chloride nanospheres was further decreased to 450 nm, the RMS roughness of the etched ITO surface sharply decreased to 14.6 nm and the height also significantly decreased from 80
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Fig. 4. (Color online) Average height of the nanoislands on the ITO layer and the RMS roughness of the etched surface (a) using different cesium chloride nanospheres and (b) for various ICP etching times.
to 30 nm. This phenomenon can be explained by the fact that the 450 nm cesium chloride nanospheres were too small to fully cover the ITO layer during the ICP etching process. Figure 4(b) shows the average height of the nanoislands on the ITO layer and the RMS roughness of the etched surface
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Jpn. J. Appl. Phys. 51 (2012) 020204
Y. Zhang et al.
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Fig. 5. (Color online) (a) LOP–I–V curves of LEDs using different cesium chloride nanospheres and (b) far-field radiation patterns of the nanotextured LEDs with 700 nm cesium chloride nanospheres after ICP etching for 300, 400, 500, and 600 s.
after ICP etching for 300, 400, 500, and 600 s. Here, we used 700 nm cesium chloride nanospheres as ICP etching masks. When the ICP etching time was increased from 300 to 500 s, the average height of the ITO nanoislands increased from 55.7 to 95.3 nm, and the RMS roughness of the etched surface was also increased from 20.1 to 31.2 nm. However, both the height of the ITO nanoislands and the RMS roughness decreased when the ICP etching time was further increased to 600 s. This is because the cesium chloride nanospheres on the ITO layer were etched off in the ICP etching process. The morphology of the ITO surface was strongly affected by the total exposure time to the plasma. Moreover, our optoelectronic measurements showed that all these changes had a great impact on the LOP of the LED devices. LOP–I–V curves of LEDs using different cesium chloride nanospheres are shown in Fig. 5(a). At an injection current of 350 mA, the LOP of the nanotextured LEDs was improved by 6.8% (450 nm), 13.1% (950 nm), and 23.4% (700 nm), compared with that of an LED with an unetched planar ITO layer. This LOP enhancement is attributed to the fact that more of the photons emitted from the MQWs escape from the GaN into the air because of the roughened ITO surface.5) Obviously, there is an optimal size of the cesium chloride nanospheres. When the cesium chloride nanospheres are too large, the light scattering occurring at the interface between the ITO layer and the air will be reduced because of the fewer nanoislands fabricated on the ITO layer. On the other hand, too small cesium chloride
nanospheres cannot entirely cover the ITO layer, which strongly affects the surface morphology of the etched ITO layer. Our results showed that when the cesium chloride nanospheres were 700 nm, we obtained the maximum LOP from the LED devices. Moreover, the LOP of the nanotextured LEDs also depends on the nanoislands on the ITO layer. Figure 5(b) shows far-field radiation patterns of nanotextured LEDs with 700 nm cesium chloride nanospheres after ICP etching for 300, 400, 500, and 600 s. We obtained maximum overall light intensity when the ICP etching time was 500 s, and the LED light intensity decreased when the ICP etching time was further increased to 600 s. This is consistent with the measurement of the textured ITO surface morphology in Fig. 4(b). In addition, according to the measured I–V characteristics, there were no significant differences in the electrical properties of these LEDs. It is concluded that the electrical properties of the fabricated LEDs are not damaged or degraded by the ICP roughness of the ITO layer. Furthermore, it is worth noting that the voltage of the LEDs using 700 nm cesium chloride nanospheres is slightly (about 0.1 V) higher than that of other LEDs over the whole measured current range. This slight increase in voltage may be due to the fact that the surface morphology of these LEDs is too rough to form good ohmic contacts on the ITO transparent conductive layer.18) In this paper, we proposed a method to fabricate an ITO transparent conductive layer with a nanotextured surface for use in GaN-based LEDs using self-assembled cesium chloride nanospheres. The sizes of the cesium chloride nanospheres and the nanoislands on the ITO layer were optimized and their impact on the LOP of LEDs was also investigated. The LOP of LEDs with a textured ITO layer was enhanced by 23.4% compared with that of an LED with a planar ITO transparent conductive layer at an injection current of 350 mA. Furthermore, there were no significant differences in the electrical properties among these LEDs. The proposed method provides a simple, feasible, and economical way to improve LED performance. Acknowledgements This work was partly supported by the National High Technology Program of China under Grant Nos. 2008AA03A197 and 2011AA03A103, National Natural Sciences Foundation of China under Grant No. 60806001, and National Basic Research Program of China under Grant No. 2011CB301904.
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