Enhanced performance of vertical GaN-based LEDs with a highly reflective ohmic contact and a nano-roughened indium-zinc oxide transparent conduction layer

The use of Polystyrene Spheres (PSs) to realize nano-roughened Indium-Zinc Oxide (IZO) surface and a high reflective ohmic p-contact to improve the optoelectronic properties of larger-area (1×1 mm²) vertical metallic-substrate GaN-based light-emitting diodes (VLEDs) were proposed and investigated. A metal system consisting of annealed- Pt/Al/Pt was employed to serve as a reflector and ohmic contact to p-GaN, which exhibits a good ohmic contact (1.84×10^-3 Ωcm²) and high reflectivity (88% at 465 nm). After the removal of sapphire using laser lift-off process (LLO) and etching of u-GaN by ICP, Ti/IZO film was then deposited to serve as a transparent conduction layer (TCL). After that, the polystyrene spheres (PSs) were dispersed on the IZO surface, followed by second sputtering-deposition of IZO film to fill the space between neighboring PSs. The PSs were then removed to form a nano-roughened IZO top-layer. Compared to regular VLEDs with Ni/Au ohmic contact and Ti/Al/Ti/Au as reflector layer, the fabricated VLED shows a typical increase in light output power (i.e., ΔLop/Lop) by 72.2% at 350 mA and a decrease in forward voltage (Vf) from 3.43 V down to 3.33 V. It is expected that the proposed PSs nano-roughening technology and high reflection annealed- Pt/Al/Pt metal system for ohmic contact to p-GaN would be a potential candidate for the fabrication of high power GaNbased LEDs for solid-state lighting in the near future.