High-reflectance of hybrid reflector with distributed Bragg reflector and metal mirror for flip-chip ultra-violet light-emitting diodes

Three types of reflectors, including the distributed Bragg reflectors (DBRs), the first hybrid reflectors composed of DBR and Al mirror (DBR-Al), and the second hybrid reflectors composed of DBR, an additional low-refractive-index layer, and Al mirror (DBR-L-Al), were investigated by use of thin-film theory at the central wavelength of 300 nm for flip-chip ultra-violet light-emitting diodes (UV-LEDs). The number of DBR pairs and various high-refractive-index materials were studied. It is shown that the lossless materials with high refractive-index contrast should be selected for DBRs, and the DBR-Al hybrid reflectors provides higher reflectance comparing to DBRs. However, the Al mirror causes a sharp drop near the central wavelength and a blue shift of the peak position. These drawbacks can be suppressed by additional low-refractive-index layer attached on the Al mirror. In addition, the DBR-L-Al reflector leads to higher reflectivity and larger FWHM as compared to DBR-Al reflectors. By use of Monte Carlo ray tracing method, the light-extraction efficiency (LEE) for flip-chip UV-LEDs with (SiO2/ZrO2)(3)-SiO2-Al hybrid reflector or perfect mirror were simulated. The calculated LEE for the (SiO2/ZrO2)(3)-SiO2-Al hybrid reflector is 97% of that for the perfect mirror. Moreover, the sharp drop in the angular reflectance spectrum of the (SiO2/ZrO2)(3)-SiO2-Al hybrid reflector induces a slightly reduction of light intensity as compared to the perfect mirror.