For better LEDs, two V’s could mean victory

Hank Hogan, hank.hogan@photonics.com, Contributing Editor

For light-emitting diodes, a brighter, more efficient and groovier future may be at hand. The result could be billions of dollars of annual energy savings, say researchers from the National Institute of Advanced Industrial Science and Technology (AIST).

Senior scientist Xue-Lun Wang and colleagues at AIST recently announced a technique that boosts LED efficiency by 50 percent or more. They achieved this by fabricating V-shaped ridges on the LED surface and covering them with a thin layer of silicon dioxide. Adding the oxide layer doesn’t require significant changes to current LED manufacturing, but creating the small ridges is a bit more difficult, Wang said.

Small changes lead to a more efficient LED. A ridge fabricated in an LED and coated with silicon dioxide leads to multiple evanescent waves. These combine and cause more light to be emitted from the semiconductor, making it more efficient. Courtesy of Xue-Lun Wang, AIST.

The researchers demonstrated the efficiency boost by means of a photoluminescence study using near-infrared emitting gallium arsenide (GaAs) LEDs. They’re already working on the next step.

“We are just in the process of developing high-efficiency visible LEDs using this technique,” Wang said. “We just succeeded in fabricating fine ridge structures on the chip surface of the AlGaInP [aluminum gallium indium phosphide] device.”

These devices emit in the red. Another LED material to which the technique could be applied is gallium nitride (GaN), which emits in the green.

No matter what the material, in a conventional LED, the air-semiconductor interface reduces efficiency significantly. The refractive index of air is approximately 1, while for semiconductors, that parameter sits at 3 or so. This large difference means that much of the light trying to emerge from an LED undergoes total internal reflection and never makes it out. In the case of LEDs deposited on a flat surface, the efficiency of light extraction for GaAs is only about 2 percent and for GaN, only 4 percent.

Upping that number has been the subject of considerable research effort, in part because of the possible energy savings. Widespread use of more efficient LEDs, researchers estimate, could cut worldwide energy consumption by 10 percent or more. The savings could run to more than $10 billion annually.

The solutions for better light extraction demonstrated so far have not been suitable for mass production. They either cost too much or don’t improve efficiency enough, the AIST researchers say.

Their approach, in contrast, doesn’t suffer from these problems. It builds upon previous work done at AIST that demonstrated higher emission efficiency.

In the latest project, researchers grew an aluminum gallium arsenide nanostructure on top of a GaAs substrate. They fabricated V-shaped ridges adjacent to each other on the LED’s surface, with a flat spot about 0.5 µm wide at the point where the two V’s met, then deposited a 150-nm-thick layer of silicon dioxide on top of everything.

That layer helped boost the device’s light extraction efficiency, which also was increased by the device’s geometry. Its shape helped because light undergoing total internal reflection at the sidewalls of the ridge created two evanescent waves, which existed only near the semiconductor interface. These waves interacted and produced light that propagated out from the flat area between the two V’s.

A key to achieving greater extraction efficiency was that the width of the flat spot was less than the emission wavelength. Simulations by the researchers showed that the silicon dioxide layer also played an important role, in part because it doubled the number of interfaces and evanescent waves. Their device had both a semiconductor-silicon dioxide and a silicon dioxide-air interface. The important point here is that the coating layer had a refractive index between that of air and the semiconductor.

In tests, the researchers showed that the extraction efficiency of their device was at least 1.5 times that of an uncoated sample. These results, they note, were better than any that can be achieved with conventional techniques.

Their announcement has attracted commercial interest. Wang said, however, that device manufacturers are waiting for preliminary results to be available for LEDs that emit in the visible. The group is working on that now and could have results soon.

Volume low-cost manufacturing, however, will need a simple method of producing high-density small ridges on a product’s surface. The group is investigating how best to do this, Wang said. “We are trying to use techniques such as nanoimprint lithography.”