Next stop for blue LED is white light


Ted Moustakas with the Epitaxy machine used in his patented process
Ted Moustakas with the Epitaxy machine used in his patented process

A cell phone’s display might not seem to have much in common with a traffic light and the laser in a Playstation 3, but all three are applications of blue light-emitting diodes, or LEDs, that are based on Professor Ted Moustakas’ pioneering work at Boston University.

Until the last decade, the only LEDs bright enough to be useful were red and green, ubiquitous in clock radios and Christmas lights. But Moustakas’ research completed the primary spectrum. Now, his lab is focusing on creating the full spectrum of white light.

The big push for white, or full-spectrum, LED is coming from the Department of Energy. Incandescent light has the efficiency of about 5 percent, or 20 lumens per watt. In other words, for one watt of electricity and you get 20 lumens of light. “Ninety-five percent of the energy is going towards making heat,” said Moustakas. For fluorescent light, the efficiency improves to approximately 80 lumens per watt, which is also not very high. LED lights can be 100 percent efficient, he said.

Making white light “follows the same principle that you use when you paint,” said Moustakas. “You mix a blue LED, a red LED and a green LED. Our research is aiming to learn how to make this more efficient how to make it more like natural light.”

Nineteen years ago, Moustakas started researching semiconductors at BU. His work focused on the material gallium nitride, which belongs to a class of semiconductors, called wide bandgap semiconductors, which can emit light.

Historically, blue LEDs were made with silicon carbide. Using the process developed at BU, companies like Cree, Inc. of North Carolina and Nichia in Japan were able to produce a brighter blue light which was superior to the earlier, dimmer blue LEDs. Doing so required growing a crystal layer of gallium nitride over a sapphire substrate, a process made difficult by the two materials’ differing atomic arrangements, or lattice-spacing.

Moustakas solved the problem in 1991, when he developed a two-step process involving a buffer layer of gallium nitride. This patented process remains the only one used to make blue LEDs.

Gallium Nitride has significant applications in optical devices like LEDs and lasers, electronic devices like transistors as well as microelectromechanical devices, or MEMS. “You can make these three big classes of applications which you cannot with regular semiconductors,” he explained.

In transistors, a big advantage of using gallium nitride is that even at temperatures of 1,000 degrees Celsius, the device “still behaves like a transistor because of their wide bandgap,” said Moustakas. This means that sensors could be directly placed in the engine block of a car, for instance.

The shorter wavelength of blue light also means that more information can be packed into less space, explained Moustakas. Using blue laser to write a CD means that six times the conventional data can be recorded on it.

Moustakas and his team are working to make the production of blue and white LEDs more efficient and less expensive. 

The promise of white LED-based lighting is such that the Department of Energy is hoping by the year 2025, efficiency will reach 200 lumens per watt, Moustakas said. “The DOE has shown that the US alone will be saving in the neighborhood of $30 billion a year,” if it switches to using LED lights in homes, offices and factories. 

Moustakas’ contribution to the field of optical electronics was recently cited in the 2006 edition of Technology Transfer Works: 100 Cases From Research to Realization, published by The Association of University Technology Managers as part of the Better World Project.