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21 Dec 2006
A Japanese-US collaboration headed up by University of California, Santa Barbara researchers says that it has set a new world best for the efficiency of LEDs based on non-polar and semi-polar GaN.
An effort to develop high-brightness LEDs based on non-polar and semi-polar GaN has realized record quantum efficiencies for electro-optic conversion in the materials, researchers have claimed.
The team involves world-leading GaN specialists working at the University of California, Santa Barbara, and under a Japan Science and Technology Agency program.
In laboratory work that has not yet been published by a peer-reviewed journal, the collaboration says that its latest efforts have delivered 300 µm x 300 µm non-polar LEDs with an external quantum efficiency of 41 percent, and a radiant power of 25 mW when driven at 20 mA.
Similar devices based on semi-polar GaN showed a 30 percent efficiency and output power of 18 mW. Details of the particular wavelengths emitted have not been released by the team.
To be used in general lighting applications, as the researchers believe will be the end result for these materials, much larger, higher-power chips emitting more light are likely to be needed.
However, the latest claims are way in excess of the output powers detailed by team leader Shuji Nakamura earlier this year at the International Symposium on Blue Lasers and LEDs (ISBLLED) held in Montpellier, France.
Those initial devices, emitting blue or green light, produced only 1 mW when driven at 20 mA.
LEDs based on non-polar GaN are expected by some leading researchers to be a key technology in the development of solid-state lighting suitable for widespread commercial deployment.
GaN-based LEDs are usually grown on c-plane sapphire, which results in a highly polarized structure because of the very different electro-negativity of gallium and nitrogen atoms (see magazine feature for more details).
This creates a strong electric field in the crystal lattice and, according to Nakamura, puts a fundamental limit on device performance. "Such fields interfere with the proper recombination of electrons and holes to make light," said the GaN pioneer.
Using different planes of sapphire material, which are harder to produce than conventional c-plane material, allows deposition of a non-polar or semi-polar crystal, which emits polarized light and, in theory, leads to much better device characteristics for LEDs, lasers and transistors.
The collaboration also claims to have produced very highly efficient emitters based on the conventional c-plane approach, and a white LED-based lamp with a luminous efficacy of 116 lm/W.
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