Enhanced light extraction from GaN based light-emitting diodes using a hemispherical NiCoO lens.
We report on the improvement of light extraction efficiency from GaN-based light-emitting diodes (LEDs) using Ni(1-x)Co(x)O nanoparticles (NPs) formed on the p-GaN layer. After formation of Ni(1-x)Co(x)O hemispherical lens arrays on the blue LEDs by drop-casting colloidal NPs, electroluminescence (EL) and photoluminescence (PL) measurements are conducted to investigate the electrical and optical properties. The PL and EL intensities from the blue LEDs with the Ni(1-x)Co(x)O NPs are 1.74 and 1.61 times greater, respectively, than a conventional LED. Finally, a hybrid approach using ZnO nanorod arrays grown on the NiCoO hemispherical lens shows further increase of light extraction by 3.8 and 6.2 times compared to LEDs with bare NiCoO NPs and without any NPs, respectively. Finite-difference time-domain (FDTD) simulation results also agree well with the experimental results. The enhancement of light extraction from LEDs with ZnO nanorods and NiCoO NPs can be attributed to an enlarged escape angle cone and increased probability of light scattering.