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Illumination & Displays

Brighter and smarter LEDs via graded refractive indexes

Micro-patterned coatings offer highly efficient LEDs with a controllable emission pattern.

7 June 2013, SPIE Newsroom. DOI: 10.1117/2.1201305.004900

LEDs are the next-generation lighting source. However, their efficiency and the uniformity of the illumination they produce is limited by the large contrast between the refractive index (n) of the LED semiconductor (n = 2.5–3.5) and air (n = 1). The resulting total internal reflection (TIR) gives planar-surface LEDs a very low light-extraction efficiency (LEE). In addition, the emission pattern has a peak emission intensity along the LED surface normal with decreasing intensity away from the surface normal, which provides uneven illumination.

Until now, these two problems have been solved separately. The LEE of GaN-based LEDs, for instance, is enhanced by surface roughening at the nitrogen-face GaN surface using substrate lift-off followed by crystallographic wet chemical etching.1 The emission pattern of LEDs, on the other hand, is controlled by secondary optics such as freeform lenses.2 However, since both problems originate from the large refractive-index contrast between the LED semiconductor and air, a complete solution for both problems requires advanced and detailed control of the refractive index and its spatial distribution (i.e., surface structure) at the LED semiconductor surface, which current techniques do not address. In response, we developed so-called GRIN LEDs with patterned graded-refractive-index (GRIN) coatings, which provide full control of refractive index and surface structure at the LED surface.3,4

GRIN LEDs enable full control of the refractive index and surface structure at the LED surface by using GRIN coatings in layers and patterned into arrays of micropillars (see Figure 1). We use five dielectric layers made of (TiO2)x(SiO2)1−x with various composition and refractive index. The bottom layer (adjacent to the semiconductor) has the highest refractive index, and the top layer (adjacent to air) has the lowest refractive index. These are shaped as micropillars with a surface structure, including their planar geometric shape and size, organized for maximum LEE and smarter emission-pattern control.

Figure 1. Schematic diagrams of (a) graded-refractive-index (GRIN) LED coated with an array of five-layer GRIN micropillars shaped as four-pointed stars and (b) the cross-section of a GRIN micropillar. n: Refractive index. SiO2: Silicon dioxide. TiO2: Titanium dioxide. GaN: Gallium nitride.

We designed a GRIN LED with an array of five-layer GRIN micropillars shaped like four-pointed stars: see Figure 1(a). The refractive index of each layer is shown in Figure 1(b). The refractive index and the width-to-thickness ratio of each layer is designed so that light rays entering the GRIN micropillars will either be extracted from the pillar sidewalls of the current layer or enter the layer above until they reach the top layer and are extracted. We optimized the five-layer design in order to extract light incident at all angles. Ray-tracing simulations show that the four-pointed-star shape is very well suited to promote light extraction.

Patterned GRIN coatings can make the surface of an LED chip optically functional. That is, they have a dual function. They can eliminate TIR inside the LED semiconductor (and hence enhance the LEE of the LED), and they also can increase the light emission in desired directions (and thus control the emission pattern of the LED). The GRIN LED with an array of five-layer four-pointed-star-shaped GRIN micropillars shows a 155% enhancement in light output power (LOP) over the planar reference LED (see Figure 2). Our analysis also reveals that the GRIN LED has a bidirectional emission pattern with intensity peaks at off-surface-normal directions (see Figure 2). This confirms strong light extraction through the pillar sidewalls.

Figure 2. Measured emission intensity of a planar reference LED and a GRIN LED patterned with an array of five-layer four-pointed-star-shaped GRIN micropillars. λ: Wavelength. T: Temperature. I: Current.

Figure 3 shows the emission patterns of GRIN LEDs with an angle of peak emission intensity (θp) at ±20° to ±50° from the surface normal. The emission pattern of GRIN LEDs could be optimized for specific applications, such as street lights or backlights, by controlling the GRIN micropillars' planar geometric shapes, sizes, and spacing.

Figure 3. Emission pattern of GRIN LEDs with an angle of peak emission intensity (θp) at (a) ±20°, (b) ±30°, (c) ±40°, and (d) ±50° from the surface normal.

In summary, we designed and demonstrated micro-patterned GRIN coatings that enhance LED LEE and enable smart control of the LED emission pattern. GaInN LEDs patterned with an array of five-layer four-pointed-star-shaped GRIN micropillars show a 155% enhancement in LOP over an uncoated planar reference LED. In addition, the angle of peak emission intensity of the GRIN LEDs is shown to be controllable from ±20° to ±50° off the surface normal, thereby demonstrating GRIN LEDs with high LEE and emission-pattern control. We are now studying GRIN micropillars with tapered sidewalls to further enhance light extraction.

Ming Ma, Jaehee Cho, E. Fred Schubert
Rensselaer Polytechnic Institute
Troy, NY
Gi Bum Kim, Chelsoo Sone
Samsung Electronics Company Ltd.
Yongin, South Korea

1. D. A. Stocker, E. F. Schubert, J. M. Redwing, Crystallographic wet chemical etching of GaN, Appl. Phys. Lett. 73, p. 2654, 1998.
2. K. Wang, S. Liu, F. Chen, Z. Qin, Z. Liu, X. Luo, Freeform LED lens for rectangularly prescribed illumination, J. Opt. A: Pure Appl. Opt. 11, p. 105501, 2009.
3. M. Ma, J. Cho, E. F. Schubert, G. B. Kim, C. Sone, Optically functional structures on GaN-based light-emitting diodes for emission pattern control and light-extraction efficiency enhancement, Nat. Commun. (Under review.)
4. M. Ma, A. Noeumaun, J. Cho, E. F. Schubert, G. B. Kim, C. Sone, Emission pattern control and polarized light emission through patterned graded-refractive-index coatings on GaInN light-emitting diodes, Opt. Express 20, p. 16677, 2012.