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.