08 Apr 2009
Philips Lumileds has produced what it says is the highest-extraction-efficiency photonic-crystal LED to date.
Triangular lattices etched directly into thin GaN LEDs come close to matching encapsulants for light extraction, while avoiding their disadvantages. Writing in the March issue of Nature Photonics, a Lumileds team estimates that its best photonic-crystal design extracts 73% of the light produced by its device.
This compares to 80% for LEDs with a randomly roughened surface that are typically encapsulated in silicone or similar optically clear material.
An unencapsulated rough-surface die would reach just 65% extraction-efficiency, while using an encapsulant trades off elevated light extraction against the impact of its refractive index on brightness.
Brightness is inversely proportional to the square of the encapsulant's refractive index, which is typically 1.5, explains Aurilien David, a researcher at the San Jose, California, company.
"If a bare and an encapsulated LED have the same light extraction, the encapsulated one's brightness is decreased by a factor of approximately 2.25," David told compoundsemiconductor.net.
The Lumileds team investigated a variety of lattice patterns and film thicknesses, on a 450 nm-emitting InGaN/GaN LED with the sapphire substrate removed and replaced by a silver mirror.
Thin GaN films provided the best performance, they found, allowing light that would otherwise have been trapped in the die to interact with the photonic-crystal.
The best device had a 700 nm deep GaN layer, with a patterned and dry-etched photonic-crystal penetrating 250 nm into it to diffract light out of the LED.
The optimal photonic-crystal used was an "Archimedean A13" lattice – a triangular pattern, where the unit cell features 13 holes – with a 455 nm lattice constant.
Lumileds' champion photonic-crystal LED boasted 47% external quantum efficiency at 2 mA and 36% at 20 mA.
Its internal internal quantum efficiency was measured in the range 60–69%. Deriving the extraction-efficiency from these results gave a range of 68–78%, eventually giving an average figure of 73%.
As well as impacting on device brightness, encapsulants also tend to degrade at higher flux densities. Therefore, finding ways to reach high extraction-efficiencies without them holds promise for higher-brightness LEDs.
• This article first appeared on our sister website compoundsemiconductor.net.
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