Design of ITO/SiO2/TiO2 distributed Bragg reflectors as a p-type electrode in GaN-based flip-chip light emitting diodes

Two different SiO₂/TiO₂ distributed Bragg reflectors (DBR) with ITO ohmic contacts were investigated as p-type reflective electrodes in InGaN/GaN flip-chip light-emitting diodes (FC-LEDs). The DBR structures were designed to have reflectance over 95% at wavelengths of 400–520 nm (DBR 1) and 400–720 nm (DBR 2). These ranges are wider than those of the electroluminescence spectrum of blue FC-LEDs. The FC-LEDs with DBR 1 showed an output power of 114 mW at 200 mA, while those with DBR 2 presented higher output power of 135 mW. A ray tracing simulation showed that the anomalously low output power of the FC-LEDs with DBR 1 can be attributed to the low incidence angle to the DBR due to the diffraction of the light at the patterned sapphire substrate. A finite-difference time-domain simulation and angle-dependent reflectance measurement of the DBRs were also carried out. The results showed that DBR 2 yielded high reflectance even at low incidence angles, while DBR 1 presented low reflectance at incidence angles lower than 30°, followed by a reduction of the output power.