GaN-Based Light-Emitting Diodes With Step Graded-Refractive Index $({\hbox {ZnO}}) _{x}({\hbox {SiO}}_{2})_{1-x}$ Micropillar Array

Step graded-refractive index (SGRI) (ZnO)_x(SiO₂)_1-x micropillar multilayers have been introduced and demonstrated on GaN-based LEDs combined with the mesh ITO. The SGRI (ZnO)_x(SiO₂)_1-x micropillars were produced by controlling the Zn/Zn+Si ratio of co-sputtered ZnO and SiO₂. The introduced three-layered SGRI (ZnO)_x(SiO₂)_1-x micropillars improved both critical angle inside GaN LEDs and Fresnel transmittance coefficient (η_Fr) as well as had better light coupling into the micropillars. Moreover, a high number of layers of the SGRI micropillars would aid the light coupled in the pillars to escape from the side wall of the pillar. LEDs with three-layered SGRI (ZnO)_x(SiO₂)_1-x micropillars exhibited output power enhancements of 12.2% with a 20 mA Vf of 3.19 V. The output power of the mesh ITO LEDs with SGRI (ZnO)_x(SiO₂)_1-x micropillars was further enhanced to 15.3% by improving the current spreading.