Light Extraction Improvement of GaN LEDs using Nano-scale Top Transmission Gratings

In this paper, we use a Finite-Difference Time-Domain GaN LED model to study constant wave (CW) average power of extracted light. The structure simulated comprises of a 200nm-thick p-GaN substrate, 50nm-thick MQW, 400nm-thick n-GaN substrate, and a 200nm n-GaN two-dimensional Photonic Crystal(2PhC) grating. We focus on optimizing three design parameters: grating period (A), grating height (d), and fill factor (FF). In the primary set of simulations, we fix the fill factor at 50% and simulate ten different grating periods (100 to 1000nm in steps of 100nm) and four different grating heights (50 to 200nm in steps of 50nm), and calculate the average power output of the device. The results from these simulations show that for both conical and cylindrical gratings, the maxmium light extraction improvement occurs when A =100nm. In the second set of simulations, we maintain a constant grating period A = 100nm and sweep the fill factor from 25 to 75%. The results of these simulations show that the fill factor affects clyindrical and conical gratings differently. As a whole, we see that the nano-structure grating mostly depends on period, but also depends on height and fill factor. The grating structure improves light extraction in some cases, but not all