| Light emmiting diodes (LEDs) have been widely used in lighting, display, optical communication and other fields for its excellent optical and electrical performance. It is universally believed that GaN-based LEDs for white lighting is replacing the fluorescent lamps in the near future. However, the emission efficiency of GaN-based LEDs has to be further improved for solid state lighting. Therefore, it is of great importance to improve the LEDs efficiency and thus, to realize high efficiency bright LEDs. A promising way to enhance LEDs light output is the use of surface plasmons (SPs) induced by metal/dielectric nanostructures.In this paper, a dielectric layer of silicon nitride (SixN) or alumina (Al2O3) was prepared on the surface of GaN quantum wells (QWs) by PECVD or ALD. A layer of silver nanoparticles with different coverage was then fabricated on the spacer layer by means of gas-phase cluster beam deposition. The enhancement of internal quantum efficiency (IQE) and light extraction efficiency (LEE) through effective SPs coupling, as well as the controlling of SPs properties induced by silver nanoparticle arrays on dielectric spacer layer has been investigated.We studied the surface plasmon resonance (SPR) of Ag nanoparticle arrays on SixN and Al2O3spacer layers. By selecting the silicon nitride dielectric layer and annealing process, we realized the modulation of SPs of Ag nanoparticles for the match between SPR and the luminescent wavelength of GaN-based LEDs (450nm).We also studied the influence of SPs in GaN quantum wells by the method of photo luminescence (PL) spectra measurements. A unique combinational configuration of measurements is designed to achieve quantitative analysis of LEDs emission enhancement factor, by taking into account the role of reflection, scattering and extinction of the Ag nanostructures.A5nm-thick dielectric spacer layer of silicon nitride was fabricated between the GaN quantum wells and the silver nanoparticle layer, to study the internal quantum efficiency enhancement due to the surface plasmons. The results show that, within an appropriate coverage, the IQE is increasing with the coverage of silver nanoparticles. A3-fold photoluminescence (PL) enhancement has been achieved. Time-resolved photoluminescence (TRPL) analysis provides further evidence that higher coverage of Ag nanoparticles can result in shorter lifetime and faster rate of radiative recombination, leading to the enhancement of IQE. Since SPR of silver nanoparticles on SixN layer peaks at about450nm, it results in high density of states near the QWs, which accelerates the spontaneous emission, In addition, the coupling of spontaneous emission from QWs into the SP modes can also improve the IQE of LEDs.A500nm-thick silicon nitride dielectric layer was fabricated to study the light extraction efficiency enhancement owing to surface plasmons. We proposed an approach to scatter the evanescent wave accompanying the total internal reflection (TIR) by nanoparticle arrays in order to improve LEDs light extraction efficiency. We also performed a quantitative measurement on an analogous system by fabricating silver nanoparticle arrays on a spectro-prism surface. The results show that the LEE is increasing with the coverage of silver nanoparticles at low coverage, and decreasing at high coverage as well. Therefore, the LEDs quantum efficiency can be improved by a suitable coverage tailoring of Ag nanoparticles.Our results indicate that the surface plasmons induced by metal nanoparticles can be precisely controlled, choosing a suitable dielectric spacer layer and metal material, along with tailoring the size, morphology and coverage of nanoparticles. Then high efficiency bright LEDs can be realized by improve its internal quantum efficiency and light extraction efficiency through surface plasmons. |
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