IR LEDs have traditionally been low-powered devices used primarily for IR data transmission and various sensing applications. State-of-the-art LEDs require a drive current of about 100 mA to generate just 40 mW of output power, for example. Meanwhile, illumination applications such as covert security and night vision require more than 10 times the output power of traditional 5-mm radial LEDs. To address this demand, we developed an IR LED capable of 25% higher efficiency using a technology platform that can be scaled to produce even greater output powers.
In a conventional volume-emitting LED, approximately 50% of the light generated from the active or epitaxial layer is absorbed by, and propagates through, the substrate (see figure 1). As a result, these structures emit light from the top as well as from the four sides of the die and require good reflectors to collect the light from the sides and project it to the top. We have developed an alternative light-extraction process - our "thin-film" design emits more than 95% of the light from a thin layer on the top of the die.
Figure 1. Volume-emitting LEDs (left) emit light from the top and sides of the die. The thin-film die (right) emits light from the top only.
The thin-film die manufacturing process is similar to that of the flip chip. We start by growing the gallium-arsenide-based epitaxial structure on a base substrate. After epitaxy, we metallize the upper surface of the die to create a highly reflective surface. The die is then flipped over and attached to a carrier substrate. We remove the base substrate on top of the die by etching, which also roughens the newly exposed surface. The resulting thin-film die consists of a substrate-less, thin active layer centered between the rough surface on the top and a highly reflective metal mirror on the bottom that is then attached to a carrier substrate (see figure 2).1 A 320-µm X 320-µm thin-film die can produce 50 mW at 100 mA and has a wall-plug efficiency of 33%.
Figure 2. The thinness of the active layer minimizes internal absorption, the metallized layer minimizes substrate absorption, and the roughened top layer lessens total internal reflection. The combination results in increased efficiency.
Several design features increase efficiency. The metallized reflector at the bottom of the active layer reflects most of the downward-emitted light through the top surface and prevents the absorption of light by the carrier substrate, which occurs in conventional volume emitters. The thin active layer increases light-extraction efficiency and mini-mizes internal absorption. The rough, or tapered, surface randomizes the angles of total internal reflection, reducing light loss caused by internal absorption of the light reflected from the interface.
The light generated in the die is emitted entirely through the randomized surface. The devices are therefore nearly pure surface emitters. The design eliminates the need for additional reflectors.
We can scale the technology to produce large-size chips. Because the die is primarily a surface emitter, the output power scales with die area without sacrificing efficiency. We have achieved output powers as high as 500 mW for a 1-A drive current with 1-mm X 1-mm thin-film die operating at 850 nm. Moreover, the surface-emitting property of thin-film technology allows us to assemble arrays of large-area dies to produce very-high-power light sources that offer efficiency levels as good as, or better than, small-volume emitters. oe
1. G. Kuhn et al., SID Proceedings #58-3 (2005).
Sevugan Nagappan, Michael Schwind
Sevugan Nagappan is product marketing manager and Michael Schwind is applications engineering director at Osram Opto Semiconductors, Northville, MI.