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) $({\hbox{ZnO}}) _{x}({\hbox{SiO}}_{2})_{1-{x}}$ micropillar multilayers have been introduced and demonstrated on GaN-based LEDs combined with the mesh ITO. The SGRI $({\hbox{ZnO}}) _{x}({\hbox{SiO}}_{2})_{1-{x}}$ micropillars were produced by controlling the ${\hbox{Zn}}/{\hbox{Zn}}+{\hbox{Si}}$ ratio of co-sputtered ZnO and ${\hbox{SiO}}_{2}$ . The introduced three-layered SGRI $({\hbox{ZnO}})_{x}({\hbox{SiO}}_{2})_{1-x}$ micropillars improved both critical angle inside GaN LEDs and Fresnel transmittance coefficient ( $\eta_{\rm Fr}$ ) 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 $({\hbox{ZnO}})_{x}({\hbox{SiO}}_{2})_{1-{x}}$ micropillars exhibited output power enhancements of 12.2% with a 20 mA ${V} _{f}$ of 3.19 V. The output power of the mesh ITO LEDs with SGRI $({\hbox{ZnO}})_{x}({\hbox{SiO}}_{2})_{1-x}$ micropillars was further enhanced to 15.3% by improving the current spreading.