Optical Characterizations of Light-emitting Diodes with Bendable Structures and Ga-doped ZnO Nanoneedles

Through optical characterizations, the emission behaviors of six light-emitting diodes (LEDs) of different substrate structures, including a lateral LED grown on sapphire substrate, a vertical LED wafer-bonded onto Si (111) substrate, a bendable LED Ag-epoxied onto a flat iron plate, and another three bendable LEDs Ag-epoxied onto circular iron rods of different radii of curvature are compared. The results of Raman scattering measurements show that a stronger tensile strain is generated in a bendable LED for reducing the QCSE and hence increasing emission efficiency when the epitaxial layer is not tightly bonded onto a hard substrate. Such a behavior is particularly stronger when the bending radius of curvature is smaller. Also, through optical characterizations, the combined effects of a few mechanisms for emission efficiency enhancement generated in the overgrowth of a transparent conductor layer of GaZnO on a surface Ag-nanoparticle (NP) coated LED, including surface plasmon (SP) coupling, current spreading, light extraction, and contact resistivity reduction are demonstrated. With a relatively higher GaZnO growth temperature (350 oC), Ag NPs can be used as catalyst for forming GaZnO nanoneedles (NNs) such that light extraction efficiency can be increased. In this situation, certain residual Ag NPs are buried in the simultaneously grown GaZnO layer for inducing SP coupling. With a relatively lower GaZnO growth temperature (250 oC), all the Ag NPs are preserved for generating a stronger SP coupling effect. By using a thin annealed GaZnO interlayer on p-GaN before Ag NP fabrication, the contact resistivity at the GaZnO/p-GaN interface and hence the overall device resistance can be reduced.