Engineers invent vertical, full-color microscopic LEDs | MIT News

Engineers invent vertical, full-color microscopic LEDs | MIT News

Disassemble your laptop screen and at its heart you’ll find a pixel-patterned plate of red, green and blue LEDs, arranged end-to-end like a meticulous Lite Brite display. When electrically powered, LEDs can produce all shades of the rainbow together to generate full color displays. Over the years, the size of individual pixels has decreased, allowing many more of them to be incorporated into devices to produce sharper, higher resolution digital displays.

But just like computer transistors, LEDs reach a limit to their size while still being efficient. This limitation is particularly noticeable in short throw displays such as augmented and virtual reality devices, where limited pixel density results in a “screen door effect” such that users perceive scratches in the space between screens. pixels.

Now engineers at MIT have developed a new way to create sharper, flawless screens. Instead of replacing red, green and blue LEDs side by side in a horizontal patchwork, the team invented a way to stack the diodes to create multi-colored vertical pixels.

Each stacked pixel can generate the full commercial gamut of colors and is approximately 4 microns wide. Microscopic pixels, or “micro-LEDs”, can be packed at a density of 5,000 pixels per inch.

“This is the smallest micro-LED pixel and the highest pixel density reported in reviews,” says Jeehwan Kim, associate professor of mechanical engineering at MIT. “We show that vertical pixelation is the way forward for higher resolution displays in a smaller footprint.”

“For virtual reality, there is currently a limit to how real they look,” adds Jiho Shin, a postdoctoral fellow in Kim’s research group. “With our vertical micro-LEDs, you could have a completely immersive experience and couldn’t tell virtual from real.”

The team’s findings are published today in the journal Nature. Kim and Shin’s co-authors include members of Kim’s lab, MIT researchers, and collaborators from Georgia Tech Europe, Sejong University, and several universities in the United States, France, and Korea.

Place pixels

Today’s digital screens are illuminated by organic light-emitting diodes (OLEDs) – plastic diodes that emit light in response to an electrical current. OLEDs are the primary digital display technology, but diodes can degrade over time, causing permanent burn-in effects on displays. The technology is also reaching a limit to the size of diodes that can be shrunk, limiting their sharpness and resolution.

For next-generation display technology, researchers are exploring inorganic micro-LEDs — diodes that are one-hundredth the size of conventional LEDs and are made from inorganic single-crystal semiconductor materials. Micro-LEDs could perform better, require less power and last longer than OLEDs.

But manufacturing micro-LEDs requires pinpoint precision, because the microscopic pixels of red, green, and blue must first be grown separately on wafers, then placed precisely on a plate, in exact alignment with each other. others in order to reflect correctly and produce different colors. and shades. Achieving such microscopic precision is a difficult task, and entire devices must be scrapped if pixels are found to be out of place.

“This pick-and-place fabrication is very susceptible to pixel misalignment at very small scales,” Kim says. “If you have misalignment, you need to discard this material, otherwise it could ruin a display.”

stack of colors

The MIT team found a potentially less expensive way to make micro-LEDs that don’t require precise pixel-by-pixel alignment. The technique is an entirely different vertical LED approach, unlike the conventional horizontal pixel arrangement.

Kim’s group specializes in developing techniques to fabricate pure, ultra-thin, high-performance membranes to design smaller, thinner, more flexible and functional electronics. The team previously developed a method to grow and peel perfect two-dimensional single-crystal material from silicon wafers and other surfaces – an approach they call 2D Material-Based Layer Transfer, or 2DLT.

In the current study, the researchers used this same approach to develop ultra-thin membranes of red, green and blue LEDs. They then peeled off the entire LED membranes from their base slices and stacked them to create a layer cake of red, green and blue membranes. They could then cut the cake into patterns of tiny vertical pixels, each as small as 4 microns wide.

“In conventional displays, each R, G, and B pixel is laid out laterally, which limits the size of each pixel,” Shin notes. “Because we stack the three pixels vertically, in theory we could reduce the pixel area by a third.”

As a demonstration, the team fabricated a vertical LED pixel and showed that by changing the voltage applied to each of the pixel’s red, green, and blue membranes, they could produce different colors in a single pixel.

“If you have a higher current towards red and a lower towards blue, the pixel will appear pink, and so on,” Shin explains. “We are able to create all colors mixed together, and our screen can cover nearly the available commercial color space.”

The team plans to improve how vertical pixels work. So far they have shown that they can stimulate an individual structure to produce the full color spectrum. They will work on creating an array of many vertical micro-LED pixels.

“You need a system to control 25 million LEDs separately,” Shin says. “Here we have only partially demonstrated this. How the active matrix works is something we will need to develop further.

“So far, we’ve shown the community that we can grow, peel, and stack ultra-thin LEDs,” Kim says. “It’s the ultimate solution for small screens like smartwatches and VR devices, where you want highly densified pixels to create vivid, vibrant images.”

This research was supported, in part, by the US National Science Foundation, US Defense Advanced Research Projects Agency (DARPA), US Air Force Research Laboratory, US Department of Energy, LG Electronics, Rohm Semiconductor, National Agency French research. , and the National Research Foundation in Korea.

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