Ultra-high efficiency light-emitting-diode arrays

High efficiency light-emitting-diodes (LED's) are desired for many applications such as displays, printers, short-haul communication, and opto-electronic computer interconnects. However, there is an enormous gap between the theoretical efficiency of LED's and their actual efficiency. The internal quantum yield, η, of good quality double heterostructures can exceed 99%, as we have demonstrated recently. On the other hand run-of-the-mill commercial LED's are usually only a few percent efficient. The reason for this long-standing shortfall is the difficulty for light to escape from high refractive index semiconductor. A mere 2% of the internally generated light is coupled into free space through the 16° escape cone, the rest suffering total internal reflection and risking re-absorption. The present commercial state-of-the-art, ~20% external efficiency in AlGaAs-based LED's, is achieved by growing a thick transparent semiconductor superstrate, and total substrate removal in a particularly clean, low-loss, optical design which can add greatly to the cost.