New Nanoscale LED Topples the “Efficiency Droop”—And Turns Into a Laser, Too

August 28, 2020 by Luke James

Although the researchers intended to create an LED for a lab-on-a-chip, they coincidentally overcame the "efficiency droop" and turned the LED into a laser.

Scientists at NIST have built a “better” nanoscale LED that overcomes an efficiency problem that has plagued LED applications for years—and it can turn into a laser, too. 

Initially, the joint research team from the National Institute of Science and Technology (NIST), the University of Maryland, Rensselaer Polytechnic Institute, and the IBM Thomas J. Watson Research intended to create a light-emitting diode (LED) for use in extremely small applications, such as the so-called lab-on-a-chip technology.

And although they succeeded in doing so, the research team also devised a solution to the long-standing challenge of "efficiency droop" often associated with LEDs.


Overcoming the Efficiency Droop

Efficiency droop describes the situation in which an LED shines more brightly when fed with more electricity but begins to slowly dim as the brightness drops off. This makes the LED highly inefficient and prevents it from being used in a variety of promising applications, such as certain communications technology. 



The LED includes the glowing zinc oxide fin (purple), an isolating dielectric material (green), and a metal contact (yellow). Image used courtesy of B. Nikoobakht/N. Hanacek, NIST 


However, the NIST scientists believe that they’ve devised a way to overcome this efficiency limitation. The concept, which the research team demonstrated in the laboratory with microscopic LEDs, reportedly achieves a “dramatic” increase in brightness, as well as the ability to create laser light—potentially paving the way for a range of valuable applications both at scale and in a miniaturized format. 

“It’s a new architecture for making LEDs,” said NIST’s Babak Nikoobakht, who came up with the new design. “We use the same materials as in conventional LEDs. But ours uses a different shape.”


A Significant Increase in Brightness

The research paper, published in Science Advances, explains how the team found inspiration in the shape of fins; they speculated that such an elongated shape with large facets on the side would, in theory, receive more electrical current.

To their surprise, the researchers observed that the LED simply continued getting brighter with an increased current rather than burning out. Overall, the team’s LED device exhibited an increase in brightness of 100 to 1,000 times over conventional submicron LEDs and emitted wavelengths on the border between violet and ultraviolet. 

Unlike the flat, planar design that’s used in conventional LEDs, the long, thin zinc oxide strands that the researchers call fins are each around 5 micrometers long and stretch around one-tenth of the way across an average human hair’s breadth.


Diagram depicting the layout and production steps of n-ZnO fin LED on p-GaN


Diagram depicting the layout and production steps of n-ZnO fin LED on p-GaN. Image used courtesy of Science Advances


“It’s one of the most efficient solutions I have seen,” said Grigory Simin, a professor of electrical engineering at the University of South Carolina, who wasn’t involved with the project. 


From LED to Laser: an Unprecedented Outcome

The team also made another discovery as they increased the current: the LED’s wavelength eventually narrowed to two wavelengths of intense violet color. Effectively, the tiny LED had become a laser. 

Converting an LED into a laser is no easy feat. It typically requires coupling an LED to a resonance cavity that allows the light to bounce around to make a laser. However, the research study claims that the fin’s design facilitates this on its own without the addition of another cavity. 

For chip-scale applications such as next-gen electronics devices and handheld communications products, a tiny laser would be critical, says the research team. They believe that it holds a lot of potential as “an important building block” due to the absence of an efficiency droop.