Tuesday, July 29, 2008

Cheaper White-Light LEDs

Replacing sapphire with silicon could lead to more-affordable LEDs.

By Lauren Rugani

Lighting the way: Purdue professor Timothy Sands (left) watches with graduate student Mark Oliver as a layer of gallium nitride semiconductor is deposited onto a silicon wafer. Sands’s group has created a silicon-based LED that could lower the cost of making white-light LEDs. Credit: David Umberger

Light-emitting diodes (LEDs) are better than compact fluorescent bulbs--LEDs use less energy, last longer, and contain no toxic mercury--but for general white-light illumination, they're still far too expensive for mass adoption. Now, researchers at Purdue University have taken a step toward making white LEDs with cheaper materials.

LEDs are semiconductors that emit photons when a charge is applied. To make white LEDs, you need an LED that emits blue light (the blue light is then either filtered or combined with red and green LEDs, to make white). Today's commercial LEDs generate blue light from a gallium nitride semiconductor expensively fabricated on a sapphire substrate. The Purdue group found a way to build blue gallium nitride LEDs by stacking metallic layers on silicon.

The benefits are several. Silicon is cheaper and available in larger diameters than sapphire is. Silicon also carries away heat effectively, meaning that it can remain in a finished LED device and allow the lamp to last longer and stay bright. Since sapphire is a poor thermal conductor, LEDs grown on the substrate have to be removed and transferred onto a different surface--costly extra steps, says Timothy Sands, director of the Birck Nanotechnology Center at Purdue, who led the research.

Although Sands says that he can't identify an exact cost savings until the devices are manufactured, the cheapness and scalability of silicon are reason enough for him to be "confident that it will be much cheaper [than sapphire]." Other improvements, such as better heat dissipation and reflectivity, also boost performance. The researchers have yet to report how the device's efficiency and total light output compare to that of current LEDs. "What's exciting is the impact this could have on energy savings" that would be derived from cheaper white-light LEDs replacing conventional lighting, Sands says.

The group achieved the trick using metallic layers. Since silicon absorbs light--a negative when the goal is to release it--Sands's group coated the silicon with a layer of zirconium nitride that reflects the downward-traveling light back to the top. To solve a second problem--that zirconium nitride reacts with silicon in a way that degraded the LED--they put a layer of aluminum nitride in between, which prevented these reactions. The last step is the addition of a layer of gallium nitride.

"This is an interesting materials innovation," says Steven DenBaars, the codirector of the Solid-State Lighting Center at the University of California, Santa Barbara, a leading center of white-light LED research. "However, it has a long way to go."

One major problem left to solve is that the Purdue devices crack as they cool after the fabrication process. This is because silicon doesn't shrink as fast as the gallium nitride layer. But Sands believes that this can be easily addressed and that the new substrates are "ready for industry to develop further." He says that he hopes to license the technology to a company that can perfect the technique. He adds that LEDs based on the technology could be commercially available within the next few years.

Many companies are trying to improve white-light LEDs. General Electric and Philips, for example, are among companies already selling white-light LEDs for high-end architectural and commercial applications.

While compact fluorescent bulbs (CFLs) use about 25 percent of the energy of incandescents, current generations of LEDs are already more efficient than CFLs, and are expected to achieve additional efficiency gains from research advances. Some LEDs are already ubiquitous in niche products such as flashlights and traffic lights.