New LED Design Drops the Droop

A new manufacturing design that changes the orientation of the crystal structure in semiconductor films has yielded blue and green LEDs with high efficiency and extremely low droop.

Much as coffee enthusiasts struggle to get a buzz after their third cup of morning joe, LEDs reach a point where more electricity no longer imparts the same kick, and productivity levels off.

To overcome this dramatic drop in efficiency at high currents, known as droop, scientists from the University of California, Santa Barbara, have designed a new way to manufacture LEDs. The researchers altered the orientation of a semiconductor film’s crystal structure to achieve energy-efficient LEDs with very low droop.

Droop is one of the main problems limiting the growth of the solid-state lighting market, and blue and green LEDs are the biggest culprit. These two LED colors provide the essential hues necessary for the familiar white light we expect from household bulbs, but they are the “droopiest.”

Schematic of the transparent LED packaging design (left). A working blue LED using the design (right). (Image: Yuji Zhao, UCSB)

“We believe this technology could be a big breakthrough and has the potential to change the future of lighting,” said Yuji Zhao, a graduate student at the Solid State Lighting and Energy Center at UCSB.

Although LEDs are many times more energy efficient than incandescent bulbs, droop causes them to lose a significant fraction of their efficiency at the high current levels required for household lighting. No one is exactly sure what causes droop, and research groups have offered competing explanations.

Typical LED chips are made from layers of doped semiconductors sandwiched together. When a voltage is applied across the layers, electrons and holes migrate toward an area of the LED called the active layer, where they combine, begetting a photon in the process. In most commercially available blue LEDs, the crystals that make up the semiconductor layer are grown in a flat orientation, called a c-plane. This traditional orientation may create electrical fields that interfere with the reunion of electrons and holes.

Now, the UCSB scientists have devised a nontraditional, tilted crystal orientation that lessens the effect of the field and exhibits some of the lowest reported measures of droop. Using this new approach, the team fabricated LED chips that are smaller than standard commercial LEDs, which could reduce manufacturing costs.

Further work needs to be done.

“The biggest problem right now is the relatively high cost of the gallium nitride bulk substrates,” Zhao said. “At UCSB, we are also developing methods to mass-produce high-quality GaN bulk substrates. I have confidence in this, and I think it’s just a matter of time before [the cost of GaN] will no longer be an obstacle.”

The research will be presented May 10 at the Conference on Lasers and Electro-Optics (CLEO) in San Jose, Calif.

For more information, visit: www.ucsb.edu