Enhancement of light extraction by ZnO nanostructures on vertical GaN light-emitting diodes

Controllable growth of ZnO nanostructures were realized on the surface patterned vertical-type GaN light emitting diodes (LEDs) by chemical solution method at low temperature. Aqueous ammonia has been added to manipulate the pH values of the growth solution to optimize the morphology of the ZnO nanostructures and light extraction efficiency from GaN LED surface. It is revealed that ZnO nanostructures were mainly grown on the tips of the GaN surface pyramids at pH value of 8.5. With decreasing pH value, ZnO nanostructures were observed grown both on tip and bottom of the GaN pyramids. On the contrary, the growth of ZnO nanostructures was greatly inhibited by increasing pH value above 10. The electroluminescence (EL) spectra intensity was enhanced by 40% for GaN LEDs topped with ZnO nanostructures at pH value of 8.5. This enhancement is attributed to the ZnO nanostructures tip-grown on vertical GaN LEDs effectively reducing the total internal reflection and minimizing Fresnel loss.