Effects of the substrate thickness and chip area on the light extraction efficiency of chip-scale liquid cup encapsulated AlGaN-based deep-ultraviolet light-emitting diodes

The low light extraction efficiency (LEE) of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) has hindered the explosive growth of the application market. In this study, theoretical calculations reveal that the fluorinated liquid encapsulation can significantly alleviate the total internal reflection (TIR) at both the surface and sidewalls of DUV LED chips. Among various liquid encapsulation schemes, the chip-scale liquid cup shows the largest enhancement in the LEE of DUV LEDs because the inclined sidewall of the liquid cup can redirect light towards the center of the quartz hemisphere. Furthermore, the LEE of DUV LEDs increases with the increased sapphire substrate thickness and the decreased chip area. The trend is attributed to fewer internal reflections and less light absorption before photons reach the chip sidewalls. With the optimized chip encapsulation strategy, a maximum wall-plug efficiency (WPE) of 15% at 2 mA and a fitted LEE of 18.38% were obtained at an emission wavelength of 275 nm.