The Analysis of Light Extraction Efficiency of InGaN LEDs with Paraboloidal Microlens Array

1. The Analysis of Light Extraction Efficiency of InGaN LEDs with Paraboloidal Microlens Array Jeng-Feng Lin, Chin-Chieh Kang, Pei-Chiang Kao Department of Electro-Optical Engineering, Southern Taiwan University of Science and Technology, Tainan, Taiwan E-mail: jengfeng@stust.edu.tw Abstract We analyzed the absorption and upper limit of light extraction efficiency of InGaN LEDs. Simulation results indicate that the active layer dominates the material absorption and addition of the microstructure increases the light extraction from the top surface. In addition, the upper limits of light extraction efficiency corresponding to two considered setups are 61.0% and 58.6%, respectively. Device structure We assume a square InGaN LED die with size of 100100 mm2. Its structure is shown in Fig. 1. A reflective Al layer with reflectance of 0.95 is under the sapphire. The InGaN layer is actually the active layer with multiple quantum structure. To enhance the light extraction, a microstructure consisting of paraboloidal microlens array, as shown in Fig. 2, can be added on the top surface of the LED.Assume the material of the microlens array is p-GaN. The thickness, refractive index, and absorption coefficient of each layer are listed in Table 1. Simulation results The structure of the InGaN LED was built in the ASAP. The light emission was modeled as an isometric emission layer in the middle of InGaN layer; one million rays were emitted in the simulation. Assume the die was in the air without packaging. Rays escaped from the semiconductor can emerge from the top surface or sidewall of the LED. Photon recycling was not considered. First we analyzed the effect of radius of curvature on light extraction efficiency. ratio of flux absorbed inside each layer to flux emitted from the emission layer of the LED during the process of ray’s escaping from the semiconductor. Two scenarios were considered: without and with microstructure, as shown in Fig. 2, on the top surface. Assume the microstructure was formed by semi-ellipsoids with conic constant of -0.25 and radius of curvature of -1 mm. The simulated results are listed in Table 2. The results indicate that the active layer dominates the material absorption and addition of the microstructure increases the light extraction from the top surface. Next we tried to evaluate the upper limit of light extraction efficiency. Two setups as shown in Fig. 3 were considered. In Fig. 3(a) the amount of light reached the top surface and sidewalls was calculated; in Fig. 3(b) the amount of light reached the top surface and leaved sidewalls was calculated. In both figures the total flux absorbed by the absorptive surfaces to the flux emitted from the emission layer of the LED is the upper limit of the light extraction efficiency. The difference between these two setups is Fig. 3(b) considers some of the reflected light from sidewalls could be reabsorbed by the device. Therefore, Fig. 3(b) represents a tighter upper limit. The simulated results are listed in Table 3. The upper limits of light extraction efficiency corresponding to Fig. 3(a) and 3(b) are 61.0% and 58.6%, respectively. • Conclusion • We analyzed the absorption and upper limit of light extraction efficiency of InGaN LEDs. Simulation results indicate that the active layer dominates the material absorption and addition of the microstructure increases the light extraction from the top surface. In addition, the upper limits of light extraction efficiency corresponding to Fig. 3(a) and 3(b) are 61.0% and 58.6%, respectively. This research was sponsored by the Ministry of Education. • References • 1. C. Huh, K. S. Lee, E. J. Kang, andS. J. Park, “Improved light-output and electrical performance of InGaN-based light-emitting diode by microroughening of the p-GaN surface,” J. Appl. Phys. 93(11), 9383–9385 (2003). • 2. T. Kim, A. J. Danner, and K. D. Choquette, “Enhancement in external quantum efficiency of blue light-emitting diode by photonic crystal surface grating,” Electron. Lett. 41(20), 1138–1139 (2005). • 3. S. J. Lee, J. Lee, S. Kim, and H. Jeon, “Fabrication of reflective GaN mesa sidewalls for the application to high extraction efficiency LEDs,” Phys. Stat. Solidi (c) 4, 2625- 2628 (2007). • 4. E. H. Park, J. Jang, S. Gupta, I. Ferguson, C. H. Kim, S. K. Jeon, and J. S. Park, “Air-voids embedded high efficiency InGaN-light emitting diode,” Appl. Phys. Lett. 93(19), 191103 (2008).