New design enables brighter LEDs

Thursday, 27 August, 2020

New design enables brighter LEDs

A new design for light-emitting diodes (LEDs), developed by US scientists led by the National Institute of Standards and Technology (NIST), may hold the key to overcoming a longstanding limitation in the light sources’ efficiency.

The concept, demonstrated with microscopic LEDs in the lab, achieves a dramatic increase in brightness as well as the ability to create laser light — all characteristics that could make it valuable in a range of large-scale and miniaturised applications. It has been described in the journal Science Advances.

LEDs have existed for decades, but even modern LEDs have a limitation that frustrates their designers. Up to a point, feeding an LED more electricity makes it shine more brightly — but soon the brightness drops off, making the LED highly inefficient. Called an ‘efficiency droop’ by the industry, the issue stands in the way of LEDs being used in a number of promising applications, from communications technology to killing viruses.

The NIST team, which included scientists from the University of Maryland, Rensselaer Polytechnic Institute and the IBM Thomas J. Watson Research Center, did not initially set out to solve the problem of the efficiency droop. Their main goal was to create a microscopic LED for use in very small applications, such as lab-on-a-chip technology.

The team experimented with a whole new design for the part of the LED that shines, using familiar materials but changing their shape. Unlike the flat, planar design used in conventional LEDs, the researchers built a light source out of zinc oxide strands they refer to as fins (each fin is only about 5 µm in length, stretching about a tenth of the way across an average human hair’s breadth). Their fin array looks like a tiny comb that can extend to areas as large as 1 cm or more.

“We saw an opportunity in fins, as I thought their elongated shape and large side facets might be able to receive more electrical current,” said NIST’s Babak Nikoobakht, who conceived the new design. “At first we just wanted to measure how much the new design could take. We started increasing the current and figured we’d drive it until it burned out, but it just kept getting brighter.”

The novel design shone brilliantly in wavelengths straddling the border between violet and ultraviolet, generating about 100 to 1000 times as much power as typical tiny LEDs do. Nikoobakht characterises the result as a significant fundamental discovery.

“A typical LED of less than a square micrometre in area shines with about 22 nW of power, but this one can produce up to 20 µW,” he said. “It suggests the design can overcome efficiency droop in LEDs for making brighter light sources.”

The team made another surprising discovery as they increased the current. While the LED shone in a range of wavelengths at first, its comparatively broad emission eventually narrowed to two wavelengths of intense violet colour. The explanation grew clear: the tiny LED had become a tiny laser.

“Converting an LED into a laser takes a large effort,” Nikoobakht said. “It usually requires coupling an LED to a resonance cavity that lets the light bounce around to make a laser. It appears that the fin design can do the whole job on its own, without needing to add another cavity.”

A tiny laser would be critical for chip-scale applications not only for chemical sensing, but also in next-generation handheld communications products, high-definition displays and disinfection.

“It’s got a lot of potential for being an important building block,” Nikoobakht said. “While this isn’t the smallest laser people have made, it’s a very bright one. The absence of efficiency droop could make it useful.”

Image caption: The fin LED pixel design includes the glowing zinc oxide fin (purple), isolating dielectric material (green) and metal contact (yellow atop green). The microscopic fins, which the research team arranged into comb-like arrays, show an increase in brightness of 100 to 1000 times over conventional submicron-sized LED designs. Image credit: B. Nikoobakht, N. Hanacek/NIST.

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