Stacking light-emitting diodes instead of placing them together can enable fully immersive virtual reality displays and high-resolution digital screens. — Science Daily

Take apart your laptop’s screen, and at its heart you’ll find a plate patterned with red, green, and blue LED pixels, arranged like an intricate light-bright display. . When electrically powered, LEDs can combine to produce every shade in the rainbow to create full-color displays. Over the years, the size of individual pixels has shrunk, allowing many more of them to be packed into devices to create sharper, higher-resolution digital displays.

But like computer transistors, LEDs are reaching the limits of how small they can be while still performing efficiently. This limitation is particularly noticeable in close-range displays such as augmented and virtual reality devices, where the limited pixel density results in a “screen door effect” that causes users to see bands in the space between pixels.

Now, MIT engineers have developed a new way to create sharp, defect-free displays. Instead of switching red, green, and blue light-emitting diodes side by side in a horizontal patchwork, the team invented a way to stack the diodes to create vertical, multi-colored pixels.

Each stacked pixel can produce the full commercial range of colors and measures approximately 4 microns in width. Microscopic pixels, or “micro-LEDs,” can be packed in densities of up to 5,000 pixels per inch.

“This is the smallest microLED pixel, and the highest pixel density reported in the journal,” says Jihwan Kim, associate professor of mechanical engineering at MIT. “We show that vertical pixelation is the way to go for high-resolution displays in small maps.”

“For virtual reality, there’s still a limit to how real they can look,” says Jiho Shin, a postdoc in Kim’s research group. “With our vertical microLEDs, you can have a fully immersive experience and you won’t be able to separate the virtual from the real.”

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

Placing pixels

Today’s digital displays are lit by organic light-emitting diodes (OLEDs) — plastic diodes that emit light in response to an electric current. OLEDs are the leading digital display technology, but diodes can degrade over time, resulting in permanent burn-in effects on screens. Technology is also getting to the point where diodes can be shrunk, limiting their sharpness and resolution.

For next-generation display technology, researchers are exploring inorganic microLEDs — diodes that are one-hundredth the size of conventional LEDs and made from inorganic, single-crystalline semiconductor materials. MicroLEDs can perform better, require less energy, and last longer than OLEDs.

But microLED fabrication requires extreme precision, as the microscopic red, green and blue pixels must first be grown on separate wafers, then precisely aligned with each other. Different colors need to be reflected and produced. And color is a difficult task to achieve with such microscopic accuracy, and entire devices need to be scrapped if pixels are found out of place.

“This picking and placing is very likely to incorrectly replace pixels at very small scales,” says Kim. “If you have an error, you have to throw that material away, otherwise it can damage the display.”

Color stack

An MIT team has come up with a potentially less wasteful way to make microLEDs that don’t require precise, pixel-by-pixel alignment. This technique is a completely different, vertical LED approach as opposed to the traditional, horizontal pixel arrangement.

Kim’s group specializes in developing techniques to create pure, ultra-thin, high-performance membranes with an eye toward engineering smaller, thinner, more flexible and functional electronics. The team previously developed a method to grow and peel perfect, two-dimensional, single-crystal materials from silicon wafers and other surfaces — a method they call 2D material-based layer transfer, or 2DLT.

In the current study, the researchers used the same approach to grow ultrathin membranes of red, green and blue LEDs. They then peeled the entire LED membranes away from their base wafers, and stacked them together to create a layer cake of red, green, and blue membranes. They can then slice the cake into patterns of tiny, vertical pixels, each 4 microns wide.

“In a traditional display, each R, G, and B pixel is arranged one after the other, which limits how small you can make each pixel,” notes Shin. “Because we’re stacking all three pixels vertically, in theory we can reduce the pixel area by a third.”

As a demonstration, the team built 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 more current than red, and less current than blue, the pixel will look pink and so on,” Shin says. “We are able to create all mixed colors, and our display can cover close to the available commercial color space.”

The team plans to improve the operation of vertical pixels. So far, they have shown that they can activate individual structures to produce a full spectrum of colors. They will work toward creating an array of many vertical microLED pixels.

“You need a system to control 25 million LEDs separately,” says Shin. “Here, we’ve only partially demonstrated that. Active matrix operation is something we will need to develop further.”

“For now, we’ve shown the community that we can grow, peel and stack ultrathin LEDs,” Kim says. “It’s the ultimate solution for small displays like smartwatches and virtual reality devices, where you want high-density pixels to create lifelike, vivid images.”

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

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