Investigation of light extraction efficiency comparison of AlGaN-based deep- and mid-ultraviolet flip-chip light-emitting diodes with patterned sapphire substrate

Abstract

Nitride-based ultraviolet (UV) emitters have attracted substantial attentions for various applications due to compact sizes and higher energy efficiencies. Despite the remarkable improvement in external quantum efficiency (ηEQE) for near- UV light-emitting diodes (LEDs), typical ηEQE for planar mid- and deep-UV LEDs is still low (<10%). One of the primary reasons for such low ηEQE is the strong anisotropic emission due to dominant transverse-magnetic (TM)- polarized output in high Al-content AlGaN quantum wells (QWs) while conventional planar LED structure favors extracting light traveling along c-axis. Here, we investigated the polarization dependence light extraction efficiency (ηextraction) of AlGaN-based flip-chip UV LEDs emitting at 230 nm and 280 nm with microdome-shaped patterning on sapphire based on three-dimensional (3D) finite-difference time-domain (FDTD) simulations. Our results show that microdome-shaped patterning on sapphire substrate is predominantly beneficial in enhancing TM-polarized output where up to 6.1-times and 2.4-times enhancement in TM-polarized ηextraction can be achieved for 230 nm and 280 nm flipchip UV LEDs with microdome-shaped patterned sapphire substrates respectively as compared to conventional flip-chip UV LEDs with flat sapphire substrates. In contrast, very minimal transverse-electric (TE)-polarized ηextraction enhancement (<1%) can be obtained from both 230 nm and 280 nm flip-chip UV LEDs with microdome-shaped patterned sapphire substrates. In particular, microdomes with diameter ≥ 200 nm for the investigated 280 nm UV LEDs are acting as reflector that severely limit light extraction through sapphire substrate. It is expected that this study will shed light on further optimizations of flip-chip UV LED designs for both deep-UV and mid-UV regimes.