Plasmonic nanostructures for enhanced LED efficiency

Due to the important role that they play in solid-state illumination, LEDs represent a promising candidate for next-generation lighting technology. However, confinement—which occurs due to the large difference in refractive index between the semiconductor and the ambient media—leads to severe total internal reflection at the interface, lowering the light extraction efficiency (LEE) of III-nitride LEDs. The light trapped inside of the LED device is eventually reabsorbed, thereby decreasing its efficiency. To achieve high light excitation and output performance, LEE enhancement is crucial.¹

By modifying the chip shape or its surface morphology, the LEE can be improved. Several approaches have been proposed for the fabrication of different structures, either inside of the substrate or on its surface (e.g., photonic crystals, nanopyramids, a patterned substrate, surface roughness, and reflectors). The typical scale of these structures ranges from a few hundred nanometers to a few micrometers, depending on the resolution limit of the optical lithographic technology used. The texturing process still has several disadvantages, however, including non-uniformity, high cost, material degradation, and limited efficiency enhancement.

Due to its high optical transparency and low resistivity, a transparent conductive oxide layer (TCL) can be employed on the LED surface as an effective current-spreading layer and graded refractive index material.^{2, 3} We have developed a novel TCL embedded with a plasmonic nanostructure to enhance the effective light extraction in LEDs.⁴

Surface plasmons (collective charge oscillations, SPs), which are excited by the interaction between light and metal surfaces, can enhance light absorption in molecules and increase Raman scattering intensities. Fluctuations of the electromagnetic fields (surface plasmon polaritons, SPPs) accompany the charge fluctuations that arise due to SP oscillation. If the metal/semiconductor surface is flat, extracting light from the SPP mode is difficult, and because the SP is non-radiative, the SPP energy thermally dissipates. However, if roughness or nanostructures are introduced on the surface of the metal layer, the SPP energy can be extracted as light due to scattering of high-momentum SPPs. This scattering causes them to lose momentum and couple, forming radiative light.⁵

To fabricate the TCL, we embedded silver-based nanoparticles into indium tin oxide (ITO). We grew these nanoparticles on the positively-doped gallium arsenide (p-GaN) layer of blue LED wafers and analyzed them via transmission electron microscopy. Several nanoparticles, with an average size of 25±10nm, were distributed randomly in the ITO film (see Figure 1). These metallic nanoparticles are able to scatter the propagating light and couple the localized surface plasmons (SPs confined to areas on the material surface, LSPs) to the light that is internally trapped in the ITO/p-GaN interface and the ITO layer. This light subsequently radiates outside, enhancing the LEE of the LEDs.

Figure 1. Cross-sectional high-resolution transmission electron microscope images of an indium tin oxide (ITO) film embedded with silver-based nanoparticles. Ag₂O: Silver oxide. d: The distance between adjacent planes.⁴

To embed the nanoparticles, we employed spin coating using a nanosilver solution (particle diameter ∼20nm). The silver layer was subsequently coated uniformly on the p-GaN top layer, leading to the formation of a nanoparticle morphology during the baking process. Because the optical response of the nanoparticles strongly depends on their size, shape, and density, we controlled the nanostructures by modulating the concentration of the nanosilver solution with the appropriate spin coating rate. Deposition of the ITO layer onto the nanoparticles via electron-beam evaporation resulted in the formation of a plasmonic nanostructure embedded within the TCL.

We measured device performance according to the optical output power as a function of the injection current (see Figure 2). An 88.10% enhancement in output power was achieved at an injection current of 350mA due to the excellent electrical characteristics and LSP resonance (LSPR) effects. As Figure 3 illustrates, the normal direction and large-angle far-field pattern of the LEDs were also subject to greater enhancement than those of conventional LEDs. These results indicate that the silver-based nanoparticles efficiently increase light extraction, even at large incident angles, and enhance light diffusion.

Figure 2. Light output power as a function of injection current for LEDs with a novel transparent conductive oxide layer (TCL), embedded with (blue line) and without (black line) a plasmonic nanostructure (silver nanoparticles, Ag). The inset shows the current-voltage characteristics of the devices.⁴

Figure 3. Far-field emission patterns of LEDs with (red) and without (black) silver nanoparticles at an injection current of 350mA.⁴

The transmission spectra of the silver nanoparticles on the glass substrate show an absorption peak at 437nm. This occurs due to the LSPR generated at the plane interface between the nanoparticles and glass under blue light (see Figure 4).⁶ The effect of LSP-light coupling on the high-energy side of the electroluminescence (EL) spectrum therefore leads to a blueshift of 2nm from its emission peak wavelength with respect to that of a conventional LED (see Figure 4). This spectral blueshift supports the existence of LSP-light coupling, resulting in an effective emission with a 70% increase in EL peak intensity.

Figure 4. Electroluminescence (EL) spectra of the LED samples measured at 20mA. The inset shows the transmission spectra of silver nanoparticles on the glass substrate.⁴ a.u.: Arbitrary units.

In summary, the excellent performance of our device indicates that the incorporation of a metallic solution via spin coating could enable fabrication of blue LEDs suitable for high-power solid-state lighting. If the resonance wavelength of the LSPs is closer to the LED emission wavelength, it should result in more efficient coupling to LSPs. We are now planning to precisely control the resonance wavelength by incorporating metallic nanoparticles with various sizes, shapes, and periodic distances. Our next challenge is to demonstrate fabrication of ultraviolet LEDs with an LEE lower than that achieved in visible-wavelength LEDs.