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Voids boost LED light emission

Higher light extraction efficiency results from a novel architecture formed by laser drilling and photochemical etching
A partnership between researchers in the US and China has increased LED output power by more than one-fifth via the creation of air gaps within the device.To deliver the increase in extraction efficiency, the researchers from National Chung Hsing University and Yale University drilled holes with a laser, before etching a heavily doped GaN layer beneath the LED structure.

Fabrication of these novel LEDs began with growth of an epistructure on sapphire that had a low-temperature buffer, an 1.2µm-thick undoped GaN layer, a heavily doped 0.2µm -thick GaN layer and a 0.2µm-thick undoped GaN layer. On top of this they deposited a relatively conventional device structure, which was capped with a 250 nm-thick layer of the transparent conductive material indium tin oxide.

Engineers employed a 355 nm laser to drill 10µm -diameter holes down to the heavily doped n-type layer, and used an electrochemical wet-etching process to create disk-shaped voids with a diameter of 40µm.

Driven at 20 mA, the etched LED delivered a light output power that was 22 percent higher than that of the control. However, this gain in the output came at the expense of a small increase in operating voltage from 3.12 V to 3.19 V, due to the slightly reduced emission area and the increased resistance of the treated LED structure.

Ref. C. -F. Lin et. al. Appl. Phys. Express 7 076501 (2014)

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