Recently, there are growing interest in micro-scale light-emitting diodes (LEDs) that can be utilized as light sources of deformable displays, optogenetics, and visible light communications. In this work, we perform... [ view full abstract ]
Recently, there are growing interest in micro-scale light-emitting diodes (LEDs) that can be utilized as light sources of deformable displays, optogenetics, and visible light communications. In this work, we perform three-dimensional (3-D) finite-difference time-domain (FDTD) simulations to investigate the light extraction efficiency (LEE) of flip-chip micro-LED structures. A whole micro-LED structure is included inside the FDTD computational domain, and the dependence of dipole source positions and polarizations on LEE is investigated.
Figure 1(a) shows a schematic cross-sectional view of the FDTD computational domain for a micro-LED structure. It is basically composed of an n-GaN layer, InGaN/GaN multiple-quantum-well active region, and a p-GaN layer on a high-reflectance Ag mirror. The micro-LED chip is assumed to have cylindrical shape with a diameter of 20 mm, and enclosed with an epoxy encapsulant. The thickness of the n-GaN layer and the active region was set at 3000 and 50 nm, respectively. In the source spectrum, center wavelength and full-width at half maximum of the spectrum are chosen to be 460 and 20 nm, respectively.
Figure 2(a) shows simulated results of LEE as a function of the source position in the horizontal direction, x. The LEE decreases as the source position increases especially for the y-polarization. As x increases, the coupling of the dipole source with whispering gallery (WGM) modes becomes stronger. The light coupled to the WGM cannot radiate easily out of the micro-LED structure, resulting in the decrease of LEE. Fig. 2(b) shows the averaged LEE as a function of the p-GaN thickness. A strong dependence of the averaged LEE on the p-GaN thickness is observed. In this case, the p-GaN thickness of 100 nm is the best choice for achieving a high LEE of ~0.6.
In this work, we numerically investigated the LEE of blue micro-LED structures with 20-mm diameter using 3-D FDTD simulations. It was found the LEE of micro-LEDs depends strongly on dipole source positions, dipole polarizations, and the p-GaN thickness. For a properly chosen p-GaN thickness, the LEE of ~60% was obtained. More optimization of the micro-LED structure is expected to lead to even higher LEE.
