High-power GaN-based Vertical-structure LEDs With Periodic Surface Microstructure

GaN-based vertical-structure light-emitting diodes(VS-LEDs) have many advantages such as large emission area, well heat dissipation, and uniform current distribution while compared with traditional-structure LEDs. Hence, VS-LEDs are appropriate for high-power solid-strate lighting, and become an important direction for development of LED application in general lighting. However, the fabrication of VS-LEDs is much difficult. In this paper, to achieve high-power VS-LEDs, the fabrication techniques and chip structure are investaged in detail.Photon loss in the VS-LEDs caused by Fresnel reflection and the total reflection at air/GaN interface was ascribed for lower light extraction efficiency. To solve this problem, we optimized the surface structure of N-face n-GaN with traditional plan surface and periodic surface microstructure in VS-LEDs, fabricated from flat sapphire and patterned sapphire substrate, respectively. It is well known that the N-face GaN is easy to be etched in KOH solution. With an etching concentration of 6 mol/L, etching temperature of 80℃ and etching time of 8 min, the light output power of the VS-LED increased by 175% at 100 mA, as compared to the VS-LEDs with unetched plan N-face GaN surface. For N-face GaN with periodic surface microstructure, an enhancement of the light output power by 23.08% at 100 mA was achieved, after the etching treatment with an etching concentration of 2 mol/L, etching temperature of 95℃ and etching time of 8 min.Current crowd under the n electrode of VS-LEDs happened when operated at high injection current. To solve this problem, the size and spacing of n-type electrode were firstly analyzed by two-dimension circuit modeling, and annulospiral electrode structure was proposed for fabrication of the VS-LEDs. Then, finite element analysis model in Comsol Multiphysics was built to study the current density distribution in the active layer of VS-LEDs with the annulospiral electrodes. It is found that the current density distribution became more uniform when the electrode spacing reduced. Finally, VS-LEDs with various annulospiral electrodes were fabricated using high-reflectivity p-type ohmic electrode, copper substrate electroplating and laser lift-off techniques. The wall plug efficiency of VS-LEDs with the electrode spacing of 146.25 μm was 26.8% at 350 mA, which was higher than that of the VS-LEDs with other electrode spacings.