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The light extraction efficiency of GaN-based LEDs as a function of the position of the light source over the active layer is studied. Several parameters, including chip dimensions, absorption coefficients and package are analyzed on the basis of a Monte-Carlo ray tracing simulation. The light extraction efficiency of a Thin-GaN LED is studied with respect to a sapphire-based LED, including the surface texture.
©2005 Optical Society of America
1. Introduction
Light-emitting diodes (LEDs) for solid-state lighting have been extensively studied. One of the most important topics of study includes GaN LEDs, which can be applied to various applications, including traffic signals, back lighting in liquid-crystal display (LCD) and outdoor lighting by white light LEDs. However, the property such as the energy efficiency of current GaN LEDs is not yet sufficient to satisfy consumer demands. Therefore, obtaining more energy efficient GaN LEDs is a goal attracting a great deal of attention. Generally, there are two main approaches in improving the energy efficiency. The first is to increase the internal quantum efficiency, which is determined by the material quality, epitaxial layer structure and thermal dissipation. [1,2] The second is to increase the light extraction efficiency (simplified LEE) on the chip. Many methods to improve the LEE have been proposed, including altering surface texture, chip shaping and other approaches [1–8].
In this Letter, we present the LEE analysis of GaN LEDs as a function of the position of the point source in the active layer as affected by several parameters, including chip dimensions, absorption coefficients and package. These analyses are helpful in the design of high-brightness GaN LEDs.
2. Analysis of LEE by the Monte-Carlo ray tracing method
The LEE of the LED is defined by the ratio of rays escaping from the LED chip to the total number of rays generated by the active layer, and is limited by critical angle loss, Fresnel loss, current spreading conditions and absorption of the materials. In a conventional GaN LED, the critical angle of the light escape cone is about 23°, if the refractive index is 2.5 as for GaN. Generally, the LEE is less than 20% so how to increase the LEE is important.
To figure out the LEE under various conditions and parameters, we can use a Monte-Carlo ray tracing simulation to obtain the light distribution across the whole volume of the chip [8,9]. In the following simulations, we assume that the current spreading is uniform and the photon recycling effect is negligible.
3. Position-dependent light extraction of GaN-based LEDs
The structure of the simulated GaN LEDs is described in Table 1. The chip area is 300×300 µm2 and the central wavelength of the emitting light is assumed to be 400 nm. First at all, we analyze the LEE for different single point source positions on the active layer. The position-dependent LEE as a function of the absorption coefficient is shown in Fig. 1. We find that the LEE is larger when the point source is near the chip boundary. The simulation results give no surprise that the total LEE decreases when the absorption coefficient increases. Concerning the chip dimensions issue, Fig. 2 shows the LEE vs. the chip dimensions for the point source at different position on the active layer without considering absorption. We find that the light emitted from the central portion of the active layer has more difficulty escaping from the larger chip than from a smaller one. In addition, the LEE from the four side-faces decreases when the chip dimensions increases and the absorption coefficient increases, as shown in Fig. 3.
Table 1. Parameters of each layer in the simulated LED.
Fig. 1. LEE vs. absorption coefficient of the active layer for the point sources in three different positions.
4. GaN-based LEDs vs. Thin-GaN LEDs
Next we apply the simulation to study the LEE of a Thin-GaN LED. The Thin-GaN LED was obtained using a removal process from the sapphire substrate, and with chip bonding [10]. In comparison with a conventional sapphire-based LED, the Thin-GaN LED has a more uniform current distribution and excellent heat dissipation. Figure 4 shows the LEE simulations for both sapphire-based and Thin-GaN LEDs with a reflection layer on the bottom face. We find that the LEE of a Thin-GaN LED is not larger than that of a sapphire-based one; however most light escaped from the top surface causing a larger photometric intensity. This is why a Thin-GaN LED may look brighter than a sapphire-based one. It should be noted that the LEE at a Thin-GaN LED is more sensitive to the absorption coefficient of the active layer than that of a sapphire-based one. The reason is that the dimensions of the four side-faces in a Thin-GaN LED are relatively small. Light has difficultly escaping from the four faces, since multiple reflections from the top and bottom faces cause most of the light to be absorbed in the active layer. Such a situation can be improved by the surface texture because most light can escape from the top surface, if it is roughened. In order to determine the effect of surface texture, we utilized the surface texture with pyramid pattern. The sample was 5 µm in both width and height [8]. The four cases are shown in Fig. 5. Figure 6 illustrates a comparison of the LEE without considering the absorption among the LEDs. We find that the LEE of the Thin-GaN LEDs is a little smaller than that of sapphire-based LEDs. The texture of the surface is more useful for this than that inside the LEDs.
Fig. 4. A comparison of the LEEs of sapphire-based and Thin-GaN LEDs with different absorption coefficients. The blue and orange bars indicate the LEE from the top surface and other surfaces, respectively. Both of the LEDs are coated with reflective film with a reflectivity of A, 100%; B, 85%; and C, 75%.
Fig. 5. Four types of LEDs with different surface textures, the upper ones represent sapphire-based LEDs and the lower ones are Thin-GaN LEDs.
5. Summary
In summary, based on a Monte Carlo simulation, we first conclude that the LEE of the corner point source is higher than that of the center point source. This is helpful in the design of the current spreading region to obtain high LEE. Second, the LEE decreases as the absorption coefficient of the active layer and chip size increases. In the case of Thin-GaN LED, the simulation results show that most of the light is escaped from the top surface so it may look brighter. However, this LEE is not larger than the sapphire-based one and is more sensitive to the absorption coefficient of the active layer. The LEE can be greatly improved by the surface texture. This study shows that a pyramid patterns induced on the surface rather than that inside the LED greatly enhances the LEE. This stipulation holds for both sapphire-based and Thin-GaN LEDs.
Acknowledgments
This study was sponsored by the National Science Council of the Republic of China with the contract no. NSC 93-2622-E-008-011-CC3.
References and links
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