Researchers trying to coax light from semiconductors have a case of the blues, but they couldn't be happier. A team has found a better way to build blue light-emitting diodes (LEDs) on a base of silicon, the cheap and ubiquitous substrate for microelectronics. The success, described in the 17 January issue of Applied Physics Letters, brings the technology closer to a world of possible applications, including a chip-sized replacement for the light bulb.
The garden-variety light bulb, little changed over the last century, costs just pennies to produce but is expensive to run. It pushes electricity through a tungsten filament, turning it white hot and producing soft white light--and a lot of wasted heat. Newer compact fluorescent lights do better, but LEDs have the potential to put them all in the shade. Because semiconducter chips made of gallium nitride generate far less heat, they could be almost 17 times as efficient as light bulbs. For the past several years researchers have managed to cajole semiconductor devices into emitting blue light when pumped with electricity. But blue semiconductor lights are still too expensive for general lighting, in part because they're grown on expensive substrates such as sapphire. So teams have searched for a way to make blue LEDs work on cheap silicon.
Early success came in the last couple of years from researchers at IBM and Emcore Corp., who used different methods to deposit their light emitters. But these devices still suffered flaws that limited their efficiency. Now, Asif Khan of the University of South Carolina, Columbia, and his colleagues report better results by combining the earlier techniques. They first laid down a buffer of aluminum nitride and then grew the gallium nitride on top. They also were able to place the gallium nitride only where they wanted it--a patterning technique that opens up the possibility of making full-color gallium nitride LED displays.
The new LEDs atop silicon aren't yet as bright as those grown on sapphire. Still, "it's a good development," says Fred Schubert, an electrical engineer at Boston University in Massachusetts. "Silicon substrates are cheap and big. So if a silicon LED technology succeeds, it would mean the technology could be very cheap."