10.1117/2.1200804.1109
Light-emitting diode design
allows precise control of colors
and intensity
Yoichi Kawakami and Mitsuru Funato
Polychromatic light-emitting diodes that incorporate 3D faceted
microstructures provide good control of color, including pure white
emission, without depending on phosphors.
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.
Several LED-based designs have been presented for SSL, as
shown in Figure 1. Note that to achieve spectral synthesis, multiwavelength 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
Figure 1. Solid-state lighting schemes need to offer good control of both
the color (wavelength) and intensity.
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
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10.1117/2.1200804.1109 Page 2/4
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 lightemitting 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), {112̄2}, and {112̄0}.11, 12 These results inspired
us to fabricate InGaN/GaN LEDs on microfacets, as shown in
Figure 2(b).
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 currentdriven LEDs. The electroluminent properties were characterized
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.
at room temperature under the direct current operation. Figure 3
shows typical results.
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.
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10.1117/2.1200804.1109 Page 3/4
energy loss. Therefore, our monolithic, polychromatic LEDs will
be a key device for next-generation lighting.
Author Information
Yoichi Kawakami and Mitsuru Funato
Department of Electronic Science and Engineering
Kyoto University
Kyoto, Japan
http://www.optomater.kuee.kyoto-u.ac.jp/index-e.html
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 IIInitride semiconductors. His current interests are in the investigation of light-matter coupling by spectroscopy and its application
to optical devices.
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.
Mitsuru Funato is an associate professor. He has studied crystal
growth and optical/structural properties of wide-bandgap semiconductors.
References
Figure 4. A microfacet white LED (A : B = 1 : 1) emits at a color
temperature of 5000K.
Figure 4 displays a microfacet white LED with A : B = 1 : 1 that
is emitting 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
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c 2008 SPIE