Use of Highly Reflective Ohmic Contact and Surface KrF Laser Roughening to Improve Light Output of Vertical GaN-Based Light-Emitting Diodes

A highly reflective ohmic contact and surface roughening by KrF excimer laser technique to improve the optoelectronic properties of high-power vertical metallic-substrate GaN-based light-emitting diodes (VM-LEDs) were proposed and investigated. A metal system comprising of Ni/Ag/Ni was employed to serve as a reflector and ohmic contact to p-GaN, which exhibits a good ohmic contact (3.05times10^-4 Omega cm²) and high reflectivity (89% at the wavelength of 465 nm) after thermal annealing at 500degC in N₂ ambient for 10 min. After the removal of sapphire using laser lift-off process (LLO), KrF excimer laser irradiation was adopted to etch the u-GaN layer and then roughen the surface of the exposed n-GaN layer. As compared to regular lateral-structure GaN-based LEDs with ITO transparent conduction layer (TCL), the fabricated VM-LEDs with a chip size of 1000 mumtimes1000 mum demonstrated a typical increase in light output power (Lop) (i. e., DeltaLop/Lop) by 325.5% at 350 mA with a decrease in forward voltage (Vf) from 3.67 V down to 3.41 V. As compared to the VM-LEDs with u-GaN etching employing inductively coupled plasma, about 125% enhancement in Lop has been obtained at 350 mA from the proposed samples.