Enhanced Light Output of Dipole Source in GaN-Based Nanorod Light-Emitting Diodes by Silver Localized Surface Plasmon

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2014, Article ID 180765, 8 pages http://dx.doi.org/10.1155/2014/180765 Research Article Enhanced Light Output of Dipole Source in GaN-Based Nanorod Light-Emitting Diodes by Silver Localized Surface Plasmon Huamao Huang,1 Haiying Hu,2 Hong Wang,1 and Kuiwei Geng3 1 Engineering Research Center for Optoelectronics of Guangdong Province, School of Physics and Optoelectronics, South China University of Technology, Guangzhou, Guangdong 510640, China 2 School of Civil and Transportation Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China 3 School of Electronics and Information Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China Correspondence should be addressed to Hong Wang; Received 20 July 2014; Accepted 6 August 2014; Published 20 August 2014 Academic Editor: Xijin Xu Copyright © 2014 Huamao Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The light output of dipole source in three types of light-emitting diodes (LEDs), including the conventional planar LED, the nanorod LED, and the localized surface plasmon (LSP) assisted LED by inserting silver nanoparticles in the gaps between nanorods, was studied by use of two-dimensional finite difference time domain method. The height of nanorod and the size of silver nanoparticles were variables for discussion. Simulation results show that a large height of nanorod induces strong wavelength selectivity, which can be significantly enhanced by LSP. On condition that the height of nanorod is 400 nm, the diameter of silver nanoparticle is 100 nm, and the wavelength is 402.7 nm, the light-output efficiency for LSP assisted LED is enhanced by 190% or 541% as compared to the nanorod counterpart or the planar counterpart, respectively. The space distribution of Poynting vector was present to demonstrate the significant enhancement of light output at the resonant wavelength of LSP. 1. Introduction High-efficiency GaN-based light-emitting diode (LED) has tremendous potential for general lighting. However, in conventional planar epilayers, the InGaN/GaN multiple quantum well (MQW) contains large strain, which would induce a high dislocation density and piezoelectric field, due to the mismatches in lattice constant and thermal expansion between heteroepitaxial layers. To mitigate the strain in MQWs, nanorod LEDs were proposed [1]. The active layer is composed of nanoscale rod array in nanorod LEDs instead of planar thin-film in conventional counterparts. A straightforward fabrication method for nanorod LEDs is to etch the planar epilayers with nanoscale patterned mask [2–10]. In these published literatures, most researchers focused on the strain relaxation processes, and few studies were involved in light-output enhancement. To improve the light output, the nanorod LED was annealed in a mixture of N2 and NH3 gases [2]. Moreover, the size of the nanorods [6] and the spatial occupation factor of the nanorod sidewall [7], which was defined by the sidewall length over the unetched area of planar epilayer, should be carefully selected. As for the light extraction efficiency, after the alumina powders were spin-coated on the p-GaN layer as mask for etching of nanorod array, the residual alumina particles on the top of nanorods benefit the light extraction efficiency [10]. Localized surface plasmon (LSP) has attracted much attention for the enhancement of light output in LEDs [11– 20]. The LSP provides a fast energy transfer channel by coupling the excited dipole energy of MQWs into surface plasmon modes of noble metal particles and consequently enhances the spontaneous emission rate of MQWs, thereby improving the light output of LEDs. However, due to the exponential decay of the LSP evanescent field, the penetration depth of the LSP field into the GaN material is limited to be several tens of nanometers [21]. On the other hand, the p-GaN is generally thicker than 100 nm to maintain p-n junction. In order to place the metal particles within 2 Journal of Nanomaterials PML Monitor p-GaN SiO2 MQW Dipole n-GaN Sapphire PEC (a) (b) wg SiO2 Ag hNR s0 wNR dAg s0 (c) s0 (d) Figure 1: Schematic structure of (a) planar LED, (b) nanorod LED, and (c) LSP assisted LED. (d) is an enlarged view of the vicinity of dipole source in (c). The red circle is the dipole source. the fringing field of MQWs for effective MQW-LSP coupling, the metal particles were embedded into the n-GaN [11–13] or p-GaN [14–17] layers. However, the epitaxial growth process must be interrupted for the fabrication of metal particles and the epilayers following the metal particles may exhibit poor crystal quality. Alternately, after the epitaxial wafer was completely finished, the p-GaN layer was partially etched; if the etching part is thinner enough, the LSP assisted light emission was significantly enhanced [18–20]. For the case of nanorod LEDs, the noble metal particles can be placed in the gaps between the nanorods without additional etching process. In this paper, the nanorod LEDs with the assistance of silver LSP are proposed, and the light output of dipole source in the planar LED, the nanorod LED, and the LSP assisted LED is studied by two-dimensional finite difference time domain (2D FDTD) method. 2. Materials and Methods In order to clarify the effects of the nanorod array and the LSP, three types of LED chips shown in Figure 1 were simulated by 2D FDTD method. The first type is the conventional planar LED. The second type is the nanorod LED, in which the nanorod array was achieved by etching part of planar epilayers and filling SiO2 in the gaps for passivation. The third one is the LSP assisted LED by inserting silver (Ag) nanoparticles in the gaps between nanorods. The width of the nanorods was set to be 𝑤NR = 100 nm and the height, ℎNR , was chosen as a variable. The spacing between the Ag nanoparticle and the surrounding SiO2 sidewall was set to be 𝑠0 = 10 nm, as shown in Figure 1(d). This can be realized by employing core-shell Ag/SiO2 nanoparticles. The widths of gaps were set to be 𝑤𝑔 = 𝑑Ag + 2 × 𝑠0 , where 𝑑Ag is the diameter of Ag nanoparticles. In order to reduce the computation resource [22], our model is only composed of four layers, including a 0.2 𝜇m thick p-GaN layer, a 2 𝜇m thick n-GaN layer, a 1 𝜇m thick sapphire substrate, and a perfect electrical conductor (PEC) layer. The MQW layer was simplified as the interface between the two types of GaN layers, and the electric point dipole was chosen as the source for spontaneous emission. The dipole source was placed at the middle of the horizontal axis of the chip, of which the width was set to be 𝑤chip = 5.22 𝜇m and the nanorods were in symmetric distribution with regard to the dipole source. (...truncated)