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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: [email protected]
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 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 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
Rb
sapphire
Al
Fig. 2 Conic mirolens array
Fig. 1 Schematic diagram for the structure of the InGaN LED.
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.
LED layer
P-GaN
InGaN
N-GaN
Sapphire
Total
Table 2 Absorption of each layer for the LED with and without microstructure
Absorption without microstructure(%)
Absorption with microstructure (%)
1.61
1.2
60.27
50.58
10.45
14.87
15.34
11.47
87.67
78.12
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
top surface and
sidewalls
top surface
Ratio of flux absorbed by the top
Ratio of flux absorbed by the
absorptive surfaces (%)
absorptive surfaces at sidewalls (%)
48.51
12.51
56.53
2.1
Upper limit(%)
61.0
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
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2625- 2628 (2007).
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