LED makers reveal performance records and high-power products

Keller detailed white LEDs with a color temperature of 6000 K, a record efficacy of 82 lm/W at 20 mA, and a flux of 5.2 lm. At the lower color temperature of 4600 K, efficacy and flux are improved (92 lm/W and 5.8 lm, respectively). These developments have resulted in a record quantum efficiency of 50% - not far short of that exhibited by today s best red AlGaInP LEDs. Cree has demonstrated equally impressive results with white LEDs fabricated using its much larger XB900 chips (900 x 900 x 250 μm), including a flux of 67 lm at 350 mA and an associated efficacy of 57 lm/W. At a higher current of 1 A, the efficacy drops to 34 lm/W but the total lumen output from a single chip is 142 lm.

Keller believes that further improvements will be possible through optimizing the light extraction from the chip and the epitaxial quality of its various layers. He cited chips with high efficiencies at high current densities as the key to future progress. Providing an insight into Cree s evaluation methods, Keller described the reliability tests on its 7090 series XLamp products. Although these devices are not hermetically sealed they can withstand conditions of 85 °C and 85% humidity.

One of Cree s intentions is to tailor its line-up of LED chips to meet the needs of different applications. To this end, the company has developed a 500 x 500 μm chip (XB500) designed to run at 125 mA with a radiant flux of 90 mW. This will bridge the gap between the XB900 (200 mW at 350 mA) and the XT270 (64 mW at 20 mA).

According to Nichia s Drake Stalions, the Japanese company agrees with Cree s philosophy and also provides a choice for its customers by placing its white LEDs into four categories: 5 mm; surface mounted; mid-flux (named Rigel); and high-flux (Jupiter). While the first two have a flux of around 3 lm, the Rigel packages provide 15 lm for channel letters and task lights and the Jupiter LEDs provide 42 lm with a range of available optics.

Despite the 5 mm and surface mounted white LEDs having a low flux, they exhibit a high efficacy of around 50 lm/W. Stalions revealed that the efficacy of 5 mm LEDs has doubled since 2002 and that targets for 2007 and 2010 are 70 and 100 lm/W, respectively.

Klaus Streubel from Osram Opto Semiconductors described progress with the company s thin-film technology, which resulted in very high brightness LEDs, emitting in the red (AlGaInP), infrared (AlGaAs) and blue/green (InGaN) regions of the spectrum.

The fabrication process for its AlGaInP thin-film LEDs starts with epitaxy of the LED structure on a 4 inch GaAs wafer. A gold/tin alloy is deposited over this structure and a 4 inch germanium carrier wafer. These two wafers are soldered together before a wet chemical etch process removes the GaAs substrate. This allows the metal layer at the bond interface to act as a highly reflective mirror and an electrical contact.

Buried prismatic structures, which have been used in Osram s commercial LEDs for a few years, are inserted to improve extraction efficiency. According to Streubel, these features have dimensions of about 10 μm. Osram has also investigated smaller structures with dimensions similar to the wavelength of visible light, although these "photonic structures" did not improve the device efficiency.

Osram has reported 614 nm LEDs with an efficacy of 108 lm/W. Streubel said that at longer wavelengths its LEDs are more efficient, but their efficacy drops because of the eye s sensitivity curve. Efficiency at shorter wavelengths is lower due to reduced carrier confinement arising from the smaller band offsets between the active and cladding regions of the structure.

For larger-area (1 mm2) AlGaInP devices operating at 627 nm, Streubel reported a flux of 96 lm with a current of 1 A, and a wall-plug efficiency of 40% at around 100 mA. He added: "There s no difference in yield between different chip sizes." Encouragingly, the larger chips are also slightly more efficient, but Streubel said this can be attributed to a more fortunate distribution of the buried prisms.

Similarly, high-performance AlGaAs thin-film devices with infrared emission at 850 nm were reported to have a flux of 500 mW for a single, 1 mm2 chip operating at 1 A.

Thin-film InGaN emitters have also been produced with a process similar to that used to fabricate AlGaInP LEDs. Device structures were grown on 2 inch sapphire substrates with the substrate removed via laser lift-off. Osram s green LEDs have a low forward voltage of 3.1 V, and, according to Streubel, when housed in a lensed Golden Dragon package, produce output powers of 55 lm at 350 mA and 110 lm at 1.4 A. By comparison, white LEDs in a Golden Dragon package without a lens produced a flux of 48 lm at 350 mA and 3.1 V (equivalent to an efficacy of 44 lm/W), and 110 lm at 1.4 A (25 lm/W).

Japan s Rohm Electronics exhibited a series of high-power LED packages based on chips from Cree and other suppliers. Although little information was available on the products, which were also demonstrated at CEATEC in Japan, the lumen output was listed as follows: red: 35 lm; orange: 40 lm; yellow: 30 lm; green: 25 lm; blue: 15 lm; and white: 35 lm.

Rohm also displayed side-emitting white LEDs with a very high brightness of 600 mcd at 20 mA. The surface-mount package has a thickness of 0.8 mm, making it suitable for thin, full-color liquid-crystal-display designs.

An alternative approach to LED fabrication, namely replacing either the emitter or color-converting phosphor with nanocrystals, was outlined by Clinton Ballinger, CEO of US-based Evident Technologies. Ballinger explained that nanocrystals may be thought of as quantum dots formed in solution by a method akin to liquid-phase epitaxy, instead of being grown on a substrate. As with quantum dots, a nanocrystal s emission wavelength depends on its size. "If you want to make a larger nanocrystal, heat it for longer," revealed Ballinger. At Evident, online monitoring allows control of the emission properties of nanocrystals to within a few nanometers.

Evident s first nanocrystals were made from CdSe, but devices containing cadmium will soon be banned in Europe and Japan, so the firm is now working with InP-based nanocrystals. According to Ballinger, these nanocrystals - expected to be released commercially during 2005 - are better suited to LED applications. This superiority arises from the larger difference in wavelength between emission and absorption of the InP-based material, which reduces re-absorption of emitted light, thereby increasing device output.

Ballinger expects Evident to start impacting on the LED market in 2005, initially with a purple LED. This color is produced by mixing the emission from a blue emitter with the red light from down-converting nanocrystals. The white LED market is seen as a mid-term project, two years from revenue generation.

Evident has also made LEDs with an active region that contains nanocrystals with conducting ligands embedded in a conducting polymer matrix. The devices will probably not rely on expensive epitaxial techniques.