Enhanced optical output power of blue light-emitting diodes with quasi-aligned gold nanoparticles
© Jin et al.; licensee Springer. 2014
Received: 12 October 2013
Accepted: 5 December 2013
Published: 6 January 2014
The output power of the light from GaN-based light-emitting diodes (LEDs) was enhanced by fabricating gold (Au) nanoparticles on the surface of p-GaN. Quasi-aligned Au nanoparticle arrays were prepared by depositing Au thin film on an aligned suspended carbon nanotube thin film surface and then putting the Au-CNT system on the surface of p-GaN and thermally annealing the sample. The size and position of the Au nanoparticles were confined by the carbon nanotube framework, and no other additional residual Au was distributed on the surface of the p-GaN substrate. The output power of the light from the LEDs with Au nanoparticles was enhanced by 55.3% for an injected current of 100 mA with the electrical property unchanged compared with the conventional planar LEDs. The enhancement may originate from the surface plasmon effect and scattering effect of the Au nanoparticles.
Much research has been devoted towards gallium nitride (GaN)-based semiconductor devices, especially in terms of applications for light-emitting diodes (LEDs), which operate in the blue and green wavelength regions. GaN-based LEDs have attracted interest for use in full-color display panels and solid-state lighting because they have advantages such as low energy consumption, long lifetimes, and relatively small sizes. However, in conventional planar LEDs, the light extraction efficiency is limited by several factors including the high refractive index of p-GaN (approximately 2.52), leading to a low total internal reflection (TIR) angle. To enhance the output light power, various approaches, such as surface texturing[3, 4], photonic crystals[5–7], and metal oxide nanoparticles[8–11], have been studied.
Surface plasmons (excitations on a rough metallic surface by the interaction between light and the metal nanoparticles) were suggested as a way to significantly enhance the light emission efficiency in LEDs. Several methods have been suggested to fabricate metal nanoparticles (NPs) on LEDs to improve their efficiency. These include thermal annealing process[13–16], chemical synthesis, and gas etching technique. For noble metal nanoparticles on GaN surfaces, the collective oscillations of the electrons are localized surface plasmons (LSPs)[18, 19]. Under the resonance condition, this LSP effect enables the metallic nanoparticles to capture the trapped light in the p-GaN layer of the LEDs, enhancing the extracted efficiency of the light. However, the LSP resonance effect is affected by the geometry and separation of the nanoparticles. When noble metal nanoparticles are fabricated with a thermal annealing process, it is important to control the distribution and size of the nanoparticles. Furthermore, the residual metal after the annealing process has a negative influence on the device performance.
We report a simple method for making quasi-aligned Au nanoparticle arrays on p-GaN surfaces using superaligned multiwall carbon nanotubes (CNTs). The LED devices containing quasi-aligned Au particles exhibited efficient electrical properties, and the optical output power was significantly increased compared with devices without Au particles. By eliminating any chemical or etching processes, this method has potential for excellent integration with semiconductor technologies. Furthermore, we observed that the quasi-aligned Au nanoparticle arrays also had an effect on the polarization performance of the LEDs.
After fabricating the Au nanoparticles, the GaN wafers were used to fabricate LEDs using standard procedures with a mesa area of 1 mm2. A transparent conducting layer (TCL) of Ni (2 nm)/Au (5 nm) was deposited on the p-GaN surface. Ni (5 nm)/Au (100 nm) electrodes were then deposited by photolithography exposure and electron-beam evaporation on the n-GaN layer and the TCL as n- and p-pads, respectively. For comparison, a standard LED device was fabricated with a TCL deposited directly on the p-GaN surface with all other fabrication processes kept the same as those used for the Au nanoparticle LEDs.
Results and discussion
In conclusion, the optical output power of the LEDs was enhanced by employing Au nanoparticles fabricated from an Au-CNT system. The enhancement was mainly originated from the surface plasmon effect and surface scattering effect from the Au nanoparticles. The optical output power of these LEDs was enhanced up to 55.3% for an input current of 100 mA. The Au nanoparticle arrays also affected the polarization to a certain degree. Compared with the traditional metal annealing process, Au nanoparticles with a more regular distribution and a controllable size in the subwavelength region could be made using this CNT-based annealing process. This method is simple, cheap, and suitable for mass production in the semiconductor industry.
This work was financially supported by the National Basic Research Program of China (2012CB932301) and National Natural Science Foundation of China (90921012).
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