Philips Lumileds prepares the most photoefficient crystal LEDs to date

The triangular lattice directly etched into the GaN LED is close to the encapsulant used to improve light extraction efficiency while avoiding common defects. Philips Lumileds has produced a photonic crystal LED that claims to be the most efficient light source to date, and its brightness is twice that of a packaged chip.

In previous experiments by Philips Lumileds, the length of the side LEDs was often optimized to be around 200 μm. Image Source: Nature Photonics / Philips Lumileds In a paper published in the March issue of Nature Photonics, a Lumileds team revealed that their best photonic crystal devices emit 73% of the light. In contrast, typical LEDs whose surfaces are randomly roughened have a light extraction efficiency of 80% because they are encapsulated with silica gel or a light-transmissive material. An unsealed chip whose surface is roughened has a light-emitting efficiency of only 65%; on the contrary, in terms of brightness, the sealant can be protected from its refractive index to improve light escape.

In optimizing the structure of photonic crystals, researchers at Philips Lumileds tested films with thicknesses between 0.5 and 1 μm, lattice constants between 0.2 and 1 μm, lattice depths of 0.1 to 0.3 μm, and triangular, honeycomb, and Archimedean A7 and A13 type. Image credit: Nature Photonics / Philips Lumileds A researcher at the company, Aurelien David, explains that the brightness is inversely proportional to the square of the reflectivity of the sealant, typically 1.5. He said that if an LED chip has the same light extraction as a sealed LED, the brightness of the latter is reduced by about 2.25 times. Lumileds studied various lattice patterns and film thicknesses of 450nm InGaN/GaN LEDs, and the original sapphire substrate of the LED was replaced by a mirror. They found that the best performance GaN film caused the light that the chip would capture and interact with the photonic crystal. The GaN layer of the best device is 700 nm thick with a 250 nm patterned and dry etched photonic crystal to refract light within the LED. The photonic crystal uses a triangular lattice called "Archimedean A13" with a lattice constant of 455 nm, and each unit cell includes 13 holes. The photonic crystal LEDs provided by Lumileds have an external quantum efficiency of 47% at 2 mA and 36% at 20 mA. The internal quantum efficiency is between 60-69%, and the light-emitting efficiency of the combined LED is between 68-78%, and the final average index is 73%. In addition to affecting the brightness of the device, the encapsulant also degrades at the maximum density stream. Therefore, for higher brightness LEDs, a method can be found that achieves high extraction efficiency without the use of a sealant.

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