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Efficiency Gains Boost High-power LED Performance

Designers are exploiting increasingly sophisticated strategies, such as surface roughening and incorporating photonic crystals, to boost efficiency and enhance light output. Susan Curtis reports.

Until recently, market penetration of high-power LEDs - defined as devices operating at 1 W or more - has been limited by performance and reliability concerns, as well as high deployment costs. But continuing improvements in light output and the efficiency of light emission are now yielding power LEDs that are starting to challenge other lighting solutions in a range of applications.

Today s power LEDs deliver 50-60 lm, enabling just 50 LEDs to produce the same light output as a 3000 lm fluorescent tube. At the same time, luminous efficacy has risen to around 60 lm/W, which far surpasses the performance of incandescent bulbs and is fast approaching the energy efficiency of fluorescent lighting.

As a result, high-power LED manufacturers are now working to develop products addressing the general illumination market, which is currently valued at around $12 bn. However, real success in the mainstream lighting market will require manufacturers to reduce the price of LEDs, while continuing to deliver steady improvements in the device s luminous efficacy and total lumen output.Increasing light output

In theory, it should be possible to produce more light by simply driving bigger chips, typically 1 × 1 mm, with higher currents. The Luxeon III, for example, delivers up to 190 lm at red-orange wavelengths by drawing 1.4 A, compared with a flux of about 25 lm for typical devices running at 350 mA.

However, most high-power LEDs convert only about 15% of the input power into light, with the rest being lost as heat. And yet LEDs must be protected from overheating, as sustained operation at high junction temperatures reduces the device s lumen output and shifts its emission wavelength. Improving luminous efficacy is therefore essential for producing more light from the same input power.

Continual advances in materials, fabrication techniques and device structures have already produced LEDs with high internal efficiencies. The best red-emitting devices based on AlGaInP deliver internal quantum efficiencies (IQEs) approaching 100%, while GaN-based green and blue light emitters deliver IQEs of 50%, despite the large numbers of dislocations formed during epitaxial growth on a foreign substrate.

Although efforts are continuing to improve the IQE of GaN devices, larger gains in luminous efficacy are expected to come from increasing the device s extraction efficiency. In traditional LED chips, only a small proportion of the photons generated at the p-n junction leave the device. Since the semiconducting materials used to produce LEDs have a large refractive index, only those photons emitted within typically 17° of the normal direction can exit the chip s front surface. The rest are trapped by total internal reflection and eventually absorbed by the material, leading to heat generation rather than light extraction.

One particular problem for high-power LEDs is that as the die area is increased, light extraction efficiency falls. Data from Lumileds show that the external quantum efficiency (EQE) of both AlGaInP and AlInGaN chips falls by about 20% as the die area is increased from 0.3 to 1.5 mm>sup>2 (figure 1). This is partly because light emission from the chip s sidewalls is less effective for large-area die, as photons have to travel much further before reaching a surface, which increases internal absorption. This means that it is essential to improve the light emission efficiency from the chip s top surface.

Established techniques to increase the extraction efficiency include the use of a current-spreading layer, also known as a window layer, which ensures that light emission occurs over the full area of the p-n junction. Many leading GaN-based LED manufacturers also exploit a flip-chip geometry, with the device mounted face-down, which allows light to be extracted through the substrate rather than from the device s top surface. As well as offering higher extraction efficiencies, flip-chip assembly enables better thermal management, high-speed electrical interconnection, smaller module sizes and higher reliability.

For AlGaInP chips, the GaAs substrate absorbs all visible light, so either a reflecting structure has to be inserted between the active region and the substrate, or, better still, the GaAs substrate has to be replaced with a transparent GaP carrier. The use of transparent substrates, first pioneered by Hewlett-Packard (later Agilent) and now adopted by Lumileds, can deliver at least twice the luminous flux of equivalent devices grown on absorbing substrates, but requires complex and expensive processing technology to achieve wafer bonding with precise crystallographic alignment. Texture and shape

Some chip manufacturers, notably Cree and Lumileds, also enhance their LEDs external efficiency by shaping the die to reduce reflections and increase light extraction. Lumileds truncated-inverted-pyramid (TIP) geometry can yield extraction efficiencies approaching 60% for AlGaInP chips, and R&D at the company indicates that TIP-LED chips measuring 1 × 1 mm can offer efficacies of 100 lm/W at 605 nm. However, TIP-LED structures are not suited to high-volume manufacturing, and although some of the light is extracted from the sidewalls, this emission is less effective for larger chip areas.

Several other approaches have been developed to combine high extraction efficiencies with more scalable processing technologies. One of the most successful is the use of surface roughening, which reduces internal reflections and scatters the light outwards. Shuji Nakamura and colleagues at the University of California, Santa Barbara, have combined laser lift-off with photoelectrochemical etching to produce regular cone-like features on the surface of a flip-chip GaN-based LED (figure 2). The power output from these LEDs increases by a factor of 1.9 after a 2 min etch, and 2.3 after a 10 min etch, compared with flat-surface devices.

Osram Opto Semiconductors has incorporated this concept into its latest generation of thin-film LED chips. Thin-film technology, which involves bonding the LED to a metallized carrier substrate before the original substrate is removed, offers a simpler, lower-cost alternative to transparent substrates, but on its own it does not deliver a high EQE. In the Osram approach, inclined microreflectors are formed in the top part of the epitaxial layer, before the structured surface is covered with mirror layers and bonded to the carrier. The bonding interface need not be optically transparent, which allows the use of metal-to-metal bonding.

Tests indicate that the luminous flux from Osram s buried-microreflector LEDs is up to 70% higher than that from thin-film LEDs produced without microreflectors. The company s thin-film technology is already being exploited in its latest product release, the high-power Ostar LED. This 5 W device incorporates four thin-film chips measuring 1 × 1 mm2, and the RGB version emits more than 120 lm from a package measuring 3 × 1 cm2, with the light source consisting of one red, one blue and two green thin-film chips.

More sophisticated approaches to improving LED extraction efficiency include the use of a 2D photonic crystal, a regular array of 100-250 nm diameter holes formed in the current-spreading layer to guide light to the device s surface. This approach has been investigated by Sandia National Laboratories researchers in collaboration with Lumileds to improve the efficiency of blue GaN-based LEDs - in small-area LEDs (∼0.036 mm2) they have reported a two-fold increase in brightness.

However, these small devices suffer from edge effects that limit the improvements that are theoretically possible with photonic crystals, while fabricating large-scale photonic structures remains a challenge. Current devices rely on electron-beam lithography, while research is continuing on new processing techniques that will enable the rapid patterning of large areas.

For example, Steven Brueck at the University of New Mexico has been developing a lithography technique that exploits interference between coherent optical beams to create a periodic pattern in a single large-area exposure that takes just a few seconds. A 1 mm2 GaN-based LED produced by this method has been shown to deliver uniform light emission, but efforts are still underway to evaluate the effect of the photonic structure on the quantum efficiency of the device. Work is also continuing to produce a better engineered, full-wafer patterning tool.Microcavity LEDs

Increases in light output can also be produced with microcavity LEDs, which are expected to yield high extraction efficiencies from planar, rather than shaped, LED devices. Planar devices are particularly important for telecommunications applications, in which the light emitted from the LED must be coupled into an optical fiber with a typical core diameter of 100 μm. However, the EQE of standard planar LEDs that do not contain a microcavity is typically limited to a few percent, even when encapsulated in an epoxy dome.

Microcavity LEDs exploit optical confinement between two mirrors to modify the angular pattern of light emission, causing a large fraction of the light to be emitted into a resonant mode almost entirely contained in the extraction cone (figure 3). For example, researchers at Infineon Technologies in Germany and the Ecole Polytechnique Fédérale de Lausanne in Switzerland have fabricated a microcavity LED in a GaAs/AlGaAs device by forming distributed Bragg reflectors (DBRs) either side of the active region. This device s EQE was 14% into air and 20.6 % with encapsulation into a lens-shaped epoxy dome.

Scientists at University of California, Santa Barbara, and the Ecole Polytechnique in Palaiseau, France, have calculated that the extraction efficiency from planar microcavity LEDs could reach 40%, provided that the structure s thickness and composition is controlled with the required precision. Since most of the non-extracted light is trapped in lateral guided modes, the researchers believe that incorporating photonic-crystal structures into microcavity LEDs could further improve the EQE by scattering light towards the device s surface.

A group at Sandia National Laboratories are also investigating this type of device in a two-year project that began at the end of 2004. The researchers plan to fabricate and test InGaN LEDs that combine a 400-460 nm photonic structure with a planar microcavity formed by conductive GaN/InGaN DBRs with the ultimate aim of doubling the device s EQE.

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