Micro-LED lights the way to faster, greener data centres

A collaboration between the University of Illinois Urbana-Champaign’s Grainger College of Engineering and the University of Notre Dame has increased the optical switching speed in quantum well LEDs.

The novel gallium nitride micro-LED device has an optical modulation bandwidth of 2.75 gigahertz while consuming just 25 milliwatts of power. Its design was optimised to enhance the speed and efficiency of communication links in data centres.

As reported in IEEE Electron Device Letters, this development is the result of an extensive program of theoretical modelling and empirical testing, providing a roadmap to scaling down LED devices for optical communications. The researchers optimised the design of the device to a size of five micrometres. The team hopes that their design will prove to be a vast improvement in speed and efficiency over conventional wire interconnects and even laser-based optical links for short-range communication.

“90% of communications in data centres is short range, meaning less than 10 metres,” said Abdelghany Abouelnagga, Illinois Grainger Engineering graduate student and the study’s lead author. “It is standard to use lasers for the kind of high-frequency communications needed in this context, but lasers are power hogs. If we want to manage energy use in data centres, we need to consider lower-power optical communications such as LEDs, and we show exactly how to do so.”

To increase the speed with which data centres operate, it has been necessary to use optical communication links that transmit information using light pulses. Unlike copper wires, optical links can transmit at very high rates, or frequencies, without experiencing significant loss. It is necessary to use a tightly controlled light source to achieve high frequencies without sacrificing accuracy.

Until now, that has meant using lasers, but they require large amounts of power and generate large amounts of heat. The alternative is to use another tightly controlled light source such as LEDs. To create a device that is not only fast but optimised for use in data centres, the researchers systematically studied how the optical modulation bandwidth — a measure of how fast the micro-LED can switch on and off before the light loses brightness — and power consumption vary with device size.

“We systematically shrank our micro-LEDs down from 100 micrometres to 2 micrometres in diameter,” Abouelnagga said. “We found that, contrary to popular belief, smaller is not always better. Our work gives a development roadmap to study the trade-offs between size, bandwidth and power consumption, which shows that the best device is actually the 5-micrometre one.”

To fabricate their optimised micro-LED devices, the researchers needed to have a precise understanding of the material imperfections leading to so-called parasitic effects in device performance. Abouelnagga recalls that it was necessary to model the materials and electrical properties in detail beforehand to ensure an optimal design.

The researchers believe their 5-micrometre quantum well design is much better suited to data centre usage not only for its optical power and bandwidth, but for its low energy consumption and material platform scalability.

“To put the speed and power efficiency in perspective, our micro-LED responds to electrical signals on a gigahertz scale, comparable to computer processors, but consumes just 25 milliwatts of electrical power,” Abouelnagga said. “Of course, processors and optical transmitters perform different functions, but the comparison illustrates how fast this tiny light source can respond with a small power budget. Its 5-micrometre diameter also opens the possibility of arranging many emitters into compact, two-dimensional arrays with each carrying a separate stream of data.”

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