Improved UV LED performance using transparent conductive films embedded with plasmonic structures

Abstract

Recently, near-ultraviolet light-emitting diodes (NUV-LEDs) have been used in many applications such as light sources for ultraviolet curing, environmental cleaning, biomedical instrumentation, counterfeit bill detection and phosphor-based white LEDs. However, it is difficult to fabricate NUV-LEDs with high emission efficiency. As the wavelength of NUVLEDs decreases, the most dominant emission will be photons with transverse-magnetic (TM) polarization. For LED structures grown on a c-plane substrate, TM-light propagates mainly in the lateral direction, and it suffers strong effects of total internal reflection (TIR) due to the large incident angle on the interface. Therefore, light extraction efficiency (LEE) of NUV-LEDs is still lower than that of visible LEDs. In this study, a spin coating process in which the grating structure comprises the metallic nanoparticle layer coated on a p-GaN top layer was developed. Various sizes of metallic nanoparticles forming a suspended nanoparticle layer (SNL) embedded in a transparent conductive layer were clearly observed after the deposition of indium tin oxide (ITO). The SNL enhanced the light extraction efficiency of NUVLEDs. Light output power was 1.4 times the magnitude of that of conventional NUV-LEDs operating at 350 mA, but retained nearly the same current-voltage characteristic. Unlike in previous research on surface-plasmon-enhanced LEDs, the metallic nanoparticles were consistently distributed over the surface area. Device performance can be improved substantially by using the three-dimensional distribution of metallic nanoparticles in the SNL, which scatters the propagating light randomly and is coupled between the localized surface plasmon and incident light internally trapped in the LED structure through TIR.Keywords: Surface plasmon, nanoparticle, light-emitting diode (LED), internal quantum efficiency (IQE), external quantum efficiency (EQE), light extraction efficiency (LEE), electroluminescence spectrum, metal-organic chemical vapor deposition (MOCVD), localized surface plasmons (LSPs), transparent conductive layer.