Zeroing in on the Elusive Green LED

In a critical step toward the development of LED televisions and displays, researchers at Rensselaer Polytechnic Institute have developed a new method for manufacturing green-colored LEDs with greatly enhanced light output.

The team, led by Christian Wetzel, professor of physics and the Wellfleet Constellation Professor of Future Chips at Rensselaer, 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. Overall, the new technique results in green LEDs with significant enhancements in light extraction, internal efficiency and light output.

Researchers at RPI have developed a new method for manufacturing green LEDs with greatly enhanced light output. Christian Wetzel and his research team 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. Overall, the new technique results in green LEDs with significant enhancements in light extraction, internal efficiency and light output. (Images: Rensselaer/Robbins)

"Green LEDs are proving much more challenging to create than academia and industry ever imagined," Wetzel said. "Every computer monitor and television produces its picture by using red, blue and green. 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. This new research finding is an important step in the right direction."

Sapphire is one of the least expensive and widely used substrate materials employed in manufacturing LEDs, so Wetzel's discovery could have important implications for the rapidly growing, fast-changing LED industry. He said this new method should also be able to 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; not long thereafter, researchers tweaked their formula and developed some that produced orange light. Years later came blue LEDs, which are frequently used today as blue light sources in mobile phones, CD players, laptop computers and other electronic devices.

The holy grail of solid-state lighting, however, is a true white LED, Wetzel said. The white LEDs commonly used in novelty lighting applications — such as key chains, auto headlights and grocery freezers — 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.

The key to true white LEDs, Wetzel said, is all about green. High-performance red LEDs and blue LEDs exist. Pairing them with a comparable green LED should allow devices to produce every color visible to the human eye – including true white, he said. Today's computer monitors and televisions produce their picture by using red, blue and green. This means that developing a high-performance green LED could likely lead to a new generation of high-performance, energy-efficient display devices.

The problem, however, is that green LEDs are much more difficult to create than anyone anticipated. Wetzel and his research team are investigating how to "close the green gap" and develop green LEDs that are as powerful as their red or blue counterparts.

For more information, visit: www.rpi.edu