5 August 2008—A new method of growing light-emitting diodes (LEDs) on bases of silicon could pave the way for cheaper LED lighting, researchers at Purdue University say. Timothy Sands, a professor of materials engineering and electrical and computer engineering, and his colleagues say they’ve come up with a better way to grow LEDs that are based on gallium nitride (GaN)—essential to white lighting—on silicon instead of on sapphire or silicon carbide, as is often done today. The researchers described the technique in a recent issue of Applied Physics Letters .
Sands’s team were working on a technique called nanoheteroepitaxy, designed to cut down on defects caused by growing one kind of crystal—in this case, GaN—on another. Initially, they placed a thin layer of silicon nitride on top of the sapphire and made tiny holes in it, so the GaN grew only on the holes, reducing the odds of a defect spreading from the sapphire to the GaN. But then they realized that the technique would reduce defects with a different substrate. ”We thought of silicon,” Sands says. ”That’s the logical choice, mainly because you can get it in much larger wafers and it’s cheaper, but it also has better thermal conductivity [than sapphire].” The improved thermal conductivity will allow an LED to be driven at higher voltages, thus producing more light.
One problem with silicon is that it doesn’t reflect visible light well, so a percentage of the photons generated in an LED would be wasted. The team overcame this by inserting a layer of reflective zirconium nitride between the GaN and the silicon. Under normal processing conditions, the zirconium nitride would mix with the silicon, so they also added a layer of aluminum nitride to keep the two separate.