Novel patterned sapphire substrates for enhancing the efficiency of GaN-based light-emitting diodes.

In this study, a novel patterned sapphire substrate (PSS) was used to obtain mesa-type light-emitting diodes (LED), which can efficiently reduce the threading dislocation densities. Silicon nitride (Si 3 N 4 ) was used as a barrier to form the PSS, replacing the commonly used silicon dioxide (SiO 2 ). The refractive index of Si 3 N 4 is 2.02, which falls between those of sapphire (1.78) and GaN (2.4), so it can be used as a gradient refractive index (GRI) material, enhancing the light extraction efficiency (LEE) of light-emitting diodes. The simulation and experimental results obtained indicate that the LEE is enhanced compared with the conventional PSS-LED. After re-growing, we observed that an air void exists on the top of the textured Si 3 N 4 layer due to GaN epitaxial lateral overgrowth (ELOG). Temperature-dependent PL was used to estimate the internal quantum efficiency (IQE) of the PSS-LED and that of the PSS-LED with the Si 3 N 4 embedded air void (PSA-LED). The IQE of the PSA-LED is 4.56 times higher than that of the PSS-LED. Then, a TracePro optical simulation was used to prove that the air voids will affect the final luminous efficiency. The luminous efficiency of the four different structures considered is ranked as Si 3 N 4 (PSN-LED) > PSA-LED > PSS-LED with SiO 2 (PSO-LED) > PSS-LED. Finally, we fabricated LED devices with different thickness of the Si 3 N 4 barrier. The device shows the best luminance-current-voltage (LIV) performance when the Si 3 N 4 thickness is 220 nm.