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Understanding omni-directional reflectors and nominating more dielectric materials for deep ultraviolet light-emitting diodes with inclined sidewalls

In this work, unique properties and tremendous advantages for an omni-directional reflector (ODR) on an inclined sidewall for flip-chip AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) are systematically investigated via a three-dimensional finite-difference time-domain method. It is found that although the reflectivity of Ag is far lower than that of Al, the light extraction efficiency for the inclined sidewall DUV LED with the Ag-based ODR is not always lower than that with Al-based one within certain incident angles. The reason is that the total internal reflection (TIR) of the ODR on the inclined sidewall plays an important role in extracting light. We also find that the reflectivity for TIR can be strongly suppressed if surface plasmon polaritons (SPPs) resonance absorption occurs. To avoid SPP resonance absorption, our studies in this work propose the design strategy for ODR on the inclined sidewall as follows: if the ODR structure of the DUV LED with an inclined sidewall adopts thin dielectric layers, the high-index material should be selected as the dielectric layer; if a thick dielectric layer is adopted for the ODR structure, the low-index material should be selected as the dielectric layer.