Method increases green LED light output

Compiled by Photonics Spectra staff

In a step toward the development of LED televisions and displays, researchers have devised a method for manufacturing green-colored LEDs with greatly enhanced light extraction, internal efficiency and light output.

Scientists at Rensselaer Polytechnic Institute (RPI) achieved these results by etching a nanoscale pattern at the interface between an LED’s sapphire base and the layer of gallium nitride that gives the LED its green color.

Green LEDs have proved to be much more difficult to create than the industry realized. “Every computer monitor and television produces its picture by using red, blue and green,” said Christian Wetzel, professor of physics and the Wellfleet Constellation Professor of Future Chips at RPI.

Researchers have developed a new method for manufacturing green LEDs with greatly enhanced light output. They etched a nanoscale pattern at the interface between the LED’s sapphire base and the layer of gallium nitride that gives the LED its green color. The new technique results in significantly enhanced light extraction, internal efficiency and light output. Courtesy of RPI/Robbins.

“We already have powerful, inexpensive red and blue LEDs. Once we develop a similar green LED, it should lead to a new generation of high-performance, energy-efficient display and illumination devices.”

The new findings are a step in the right direction, he said. As one of the least expensive and widely used substrate materials employed in manufacturing LEDs, sapphire could have important implications for the rapidly growing, fast-changing LED industry. Wetzel said the new method should also increase the light output of red and blue LEDs.

The color of light produced by LEDs depends on the type of semiconductor material it contains. The first LEDs were red, and then came orange. Years later blue LEDs were developed and are frequently used today as blue light sources in mobile phones, CD players and laptop computers.

But the holy grail of solid-state lighting is a true white LED, Wetzel noted. Commonly used in novelty lighting applications – such as key chains, auto headlights and grocery freezers – white LEDs are actually blue LEDs coated with yellow phosphorus, which adds a step to the manufacturing process and also results in a faux-white illumination with a noticeable bluish tint.

To get true white, it’s all about the green, Wetzel said. High-performance red and blue LEDs exist; combining each of those with a comparable green LED should produce every color visible to the human eye – including the true white LED. Today’s computer monitors and televisions produce their pictures by using red, blue and green, meaning that a high-performance green LED could lead to a new generation of energy-efficient display devices.

While the problem still remains that green LEDs are difficult to create, the team is investigating how to “close the green gap” and develop a green LED that is comparable to its red and blue counterparts.

Findings appeared online April 11, 2011, in Applied Physics Letters (doi: 10.1063/1.3579255).