1. The Analysis of Light Absorption and Extraction of InGaN LEDs 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. Introduction Light-emitting diode (LED) has become the choice for the future lighting. Today one of the critical issues about LEDs is how to increase its light extraction efficiency. Various techniques have been proposed, such as roughened top surface [1], photonic crystal on the top surface [2], oblique mesa sidewall [3], and so on. Currently InGaN LEDs added with phosphor are the most popular way to produce white light LEDs. In this paper the analysis of the light extraction efficiency of InGaN LEDs is proposed. We used the professional optical simulation software, ASAP from Breault Research Organization, to simulate light propagation. Simulation studies were conducted using Monte Carlo ray tracings. Note that wave-optics based modeling should provide improved accuracy in the simulation of the structure. However, Monte Carlo ray tracing is sufficient for simulating our current device structure [4]. Device structure We assume a square InGaN LED die with size of 300  300 mm 2. 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 conic 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. p-GaN InGaN n-GaN sapphire Al RbRb Fig. 1 Schematic diagram for the structure of the InGaN LED. Fig. 2 Conic mirolens array Table 1 Thickness, refractive index, and absorption coefficient of each layer of the LED. 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 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. Table 2 Absorption of each layer for the LED with and without microstructure LED layerAbsorption without microstructure(%)Absorption with microstructure (%) P-GaN1.611.2 InGaN60.2750.58 N-GaN10.4514.87 Sapphire15.3411.47 Total87.6778.12 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. Fig. 3. Table 3 Flux absorbed by the absorptive surfaces Arrangement of inner absorptive surfaces Ratio of flux absorbed by the top absorptive surfaces (%) Ratio of flux absorbed by the absorptive surfaces at sidewalls (%) Upper limit(%) top surface and sidewalls 48.5112.51 61.0 top surface56.532.1 58.6 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, and S. 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. 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