Simulations of light extraction and light propagation properties of light emitting diodes featuring silicon carbide substrates

Abstract Although silicon carbide (SiC) possesses excellent material properties and is a good candidate to replace sapphire as the substrate material for high-power light emitting diodes (HP-LEDs), the light extraction behavior of SiC substrate-based gallium nitride (GaN) LEDs had not been reported in detail previously. In this study, we measured the optical properties of SiC substrates and determined their effects on SiC substrate-based LEDs. Because the optical constants of SiC are very different from those of sapphire substrates, the light propagating and light extracting behavior of SiC substrate-based LEDs differs dramatically from that of traditional sapphire substrate-based LEDs. Herein, we used rigorous coupled-wave analysis and finite-difference time domain methods to systematically analyze the light propagating behavior of flip-chip and conventional-type SiC substrate-based LEDs. Our measured and simulated data revealed that the absorption of the SiC substrate would seriously affect the light extraction efficiency of flip-chip SiC LEDs. For the conventional-type LEDs, on the other hand, because the refractive index of SiC is greater than that of GaN, light readily propagates through the GaN–SiC interface into the SiC substrate to form the SiC substrate guided mode, which cannot be extracted. Therefore, maximizing the first direct extraction light from conventional-type SiC LEDs would be the most effective approach toward increasing the light extraction efficiency. Accordingly, we analyzed some periodic structures that enhanced the light extraction efficiency of SiC substrate-based LEDs.