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

Abstract

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 (Si₃N₄) was used as a barrier to form the PSS, replacing the commonly used silicon dioxide (SiO₂). The refractive index of Si₃N₄ 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 Si₃N₄ 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 Si₃N₄ 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 Si₃N₄ (PSN-LED) > PSA-LED > PSS-LED with SiO₂ (PSO-LED) > PSS-LED. Finally, we fabricated LED devices with different thickness of the Si₃N₄ barrier. The device shows the best luminance–current–voltage (LIV) performance when the Si₃N₄ thickness is 220 nm.