Abstract
The luminous efficiency of light-emitting diodes (LEDs) improves and approaches that of fluorescent lamps. However, a significant portion of the light generated in the InGaN luminous layer of the LED chip is lost by total internal reflection and thus cannot be used for lighting purposes. We have developed a method of improving the light extraction efficiency from LED chips by strongly reducing these losses. The sapphire substrate is lifted off from the LED chip and a relief structure is created on the surface of the InGaN layer by direct ablation using a femto-second excimer laser with a wavelength of 248 nm and a pulse duration of 500 fs. This laser enables precise sub-μm structuring of the semiconductor layer with minimum thermal damage. As a result, light from the luminous layer diffracts at the solid / air interface and is not reflected back inside the film. In this way, the light extraction efficiency could be improved by a factor of two. After covering the structured InGaN surface with a protective resin, the light extraction efficiency could further be improved by an additional factor of 1.65 resulting in a total enhancement of more than a factor of three.
