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Illumination & Displays

Light-emitting diode design allows precise control of colors and intensity

Polychromatic light-emitting diodes that incorporate 3D faceted microstructures provide good control of color, including pure white emission, without depending on phosphors.
29 April 2008, SPIE Newsroom. DOI: 10.1117/2.1200804.1109

The color spectra required for illumination strongly depend on the application. For example, general lighting should approximate the chromaticity of blackbody radiators, while lighting for specific tasks such as surgery or endoscopy has different requirements. For these applications, the lighting should highlight the subtle difference in the red color of living organs, which is determined by the degree of oxygen supersaturation of hemoglobin. Therefore, illumination composed of red (600–780nm) and its complementary color of blue-green (480–500nm) is more suitable. In order to emphasize the desired colors of objects, spectral synthesis of the light source is indispensable.

In a related lighting issue, there is strong demand to improve the energy efficiency of lighting because ∼20% of generated electricity is currently consumed in lighting. Solid-state lighting (SSL) based on semiconductor LEDs—which features high emission quantum efficiencies, long lifetime, and harmless constituent atoms—could be an attractive option to replace traditional fluorescent lamps and incandescent bulbs.1 Therefore, realizing advanced LED-based SSL with controllable color emission would be the ultimate goal of lighting technology.

Figure 1. Solid-state lighting schemes need to offer good control of both the color (wavelength) and intensity.

Several LED-based designs have been presented for SSL, as shown in Figure 1. Note that to achieve spectral synthesis, multi-wavelength emissions must be mixed with an arbitrary relative intensity. The most successful proposal is white LEDs,2–5 which consist of an indium gallium nitride (InGaN) blue LED pumping a yellow phosphor. Considerable efforts have been devoted to improve both their light extraction efficiency5,6 and their internal quantum efficiency.3,4 As a result, the luminous efficiency has recently reached 169lm/W, which is far better than the efficiency of fluorescent lamps.4 Once the production cost is reduced, the displacement of traditional lighting by this revolutionary lighting technology will be tremendously accelerated. However, the performance in terms of color controllability is insufficient to meet specific requirements, mainly because of the broad phosphor emission. Furthermore, some energy is lost due to the Stokes shift when the phosphor absorbs higher-energy blue light and emits lower-energy yellow light. No matter how efficient the phosphor, Stokes energy loss is unavoidable.

Another design, which uses stacked quantum well (QW) structures, includes multiple InGaN QWs emitting different colors sandwiched in a GaN-based pn junction diode.7,8 This design enables more control of the emission colors. However, each QW in this device is in series in terms of the current flow, and hence, the relative emission intensities of the QWs are primarily determined by their intrinsic optical properties. This difficulty in controlling the relative intensity leads to poor apparent color tunability. To date, the device configuration that offers the best control of the emission color uses three individual LEDs, one blue, one green, and one red. This device configuration has two drawbacks: it requires outer optics to thoroughly mix the outputs from three LEDs, and increases the difficulty of device assembly.

Herein, we demonstrate a new class of monolithic, polychromatic InGaN/GaN LEDs in which InGaN QWs emitting different colors are electrically connected in parallel in order to achieve good color controllability. Figure 2 schematically explains the device concept. We selected InGaN QWs as the light-emitting layer, because by changing the In composition we can tune the bandgap energy of InGaN from 0.6eV (InN)9,10 to 3.4eV (GaN). This covers the entire visible region.

As shown in Figure 2(a), conventional LEDs are grown on sapphire (0001) substrates and have planar structures. We noted reports that regrowth of GaN on patterned silicon dioxide (SiO2) masks by metalorganic vapor phase epitaxy (MOVPE) creates 3D microstructures, which consist of several facets on planes including (0001), , and .11,12 These results inspired us to fabricate InGaN/GaN LEDs on microfacets, as shown in Figure 2(b).

Figure 2. (a) Conventional indium gallium nitride (InGaN) LED structure is planar, unlike (b) the newly developed microfacet InGaN LEDs, with facets A and B. SEM: Scanning electron microscopy. QW: Quantum well. SiO2: Silicon dioxide.

We investigated MOVPE growth conditions and their optical properties,13–16 and found that they exhibit facet-dependent emission colors due to the facet-dependent InGaN well thickness and the In composition. Furthermore, varying the SiO2 mask geometry alters the microfacet structures, which provides another opportunity to control the emission color. In our microfacet LEDs, two different structures, A and B, were constructed within one LED chip to strengthen our ability to control the emission color. (The new period—composed of m periods of A and n periods of B—is denoted as A : B = m : n.) Consequently, the wavelengths and relative intensities of the emission bands can be tuned separately,12 which may lead to a much greater control of emission color than vertically stacked QWs or conventional white LEDs. Figure 2(b) displays a cross-sectional scanning electron microscopy (SEM) image of a microfacet LED before the device process. In this particular case, A : B = 1 : 1. In this study, the mask openings for A and B were 5 and 15μm wide, respectively, and the mask was 5μm wide for both.

Recently, we optimized the MOVPE growth conditions particularly for p-GaN and the device processes to make our 3D structures, which led to the successful fabrication of current-driven LEDs. The electroluminent properties were characterized at room temperature under the direct current operation. Figure 3 shows typical results.

Figure 3. Electroluminescence spectra and photographs of microfacet LEDs grown on different mask patterns composed of microstructures A and B with ratios indicated in the figure. RT: Room temperature.

Although those LEDs were fabricated in the same growth run, the pattern mixture resulted in emission colors that varied from green to blue. More interestingly, the emission colors obtained with the mixed patterns were rather whitish, as indicated by the photograph in Figure 3, due to the polychromatic emissions. This was confirmed by plotting the spectra on the Commission Internationale de l'Éclairage (CIE) 1931 chromaticity diagram. The emissions from LED with A : B = 1 : 2, 1 : 1, and 2 : 1 were located at (0.185, 0.535), (0.214, 0.463), and (0.200, 0.345), respectively, in the diagram, all of which are far away from the values for pure colors. Such pastel colors cannot be realized by conventional LEDs because the bandgap of the light-emitting layer dictates the emission color. Consequently, the color from conventional LEDs must be monochromatic. In contrast, arbitrary colors may be extracted from nitride-based LEDs using the proposed microstructures, even though phosphors are not used as color converters. The most extreme example is white LEDs. Figure 4 displays a microfacet white LED with A : B = 1 : 1 that is emitting at a color temperature of 5000K.

Figure 4. A microfacet white LED (A : B = 1 : 1) emits at a color temperature of 5000K.

To summarize, we demonstrated proof-of-concept LEDs composed of GaN-based microstructures. The microfacet LEDs feature polychromatic emissions and good color controllability, neither of which can be realized by conventional LEDs. In addition, the proposed LEDs are free of phosphors that cause Stokes energy loss. Therefore, our monolithic, polychromatic LEDs will be a key device for next-generation lighting.

Yoichi Kawakami, Mitsuru Funato 
Department of Electronic Science and Engineering
Kyoto University
Kyoto, Japan

Yoichi Kawakami is a professor. He was educated at Osaka University, and has been at Kyoto University since 1989. He has studied the optical properties of wide-bandgap II-VI and III-nitride semiconductors. His current interests are in the investigation of light-matter coupling by spectroscopy and its application to optical devices.

Mitsuru Funato is an associate professor. He has studied crystal growth and optical/structural properties of wide-bandgap semiconductors.