Boosting external quantum efficiency of green flip-chip mini-LEDs by nanoimprinting hemisphere-shaped microstructure on sapphire substrate

High-efficiency GaN-based green miniaturized light-emitting diodes (mini-LEDs) are of great significance to the development of full-color high-resolution displays. Here, a double-sided hemisphere-shaped patterned sapphire substrate (PSS) was fabricated for improving the external quantum efficiency (EQE) of green flip-chip mini-LEDs using nanoimprint lithography (NIL). We find that the hemisphere-shaped microstructures are more effective at extracting light into the air compared to the frustum-shaped and cone-shaped microstructures. Furthermore, the green flip-chip mini-LED with double-sided hemisphere-shaped PSS has a more uniform distribution of light-emitting intensity than that with single-sided hemisphere-shaped PSS. The EQEs of the green flip-chip mini-LEDs with single-sided hemisphere-shaped PSS and double-sided hemisphere-shaped PSS were 27.02% and 33.09% at 5 mA, respectively. Finite-difference time-domain simulations show that the double-sided hemisphere-shaped microstructures can increase the total light extraction efficiency (LEE) of green flip-chip mini-LED by 17.39% compared to the single-sided hemisphere-shaped microstructures.