Take apart your laptop screen, and at its center you’ll find a pixel-patterned plate of red, green, and blue LEDs, arranged end-to-end like a meticulous Lite Brite screen. When powered electrically, LEDs together can produce all the hues of the rainbow for full color displays. Over the years, the size of individual pixels has shrunk, making it possible to pack many more into devices to produce sharper, higher-resolution digital displays.
But just like computer transistors, LEDs are reaching the limits of how small they can be while still working effectively. This limit is especially noticeable on short-throw displays, such as virtual and augmented reality devices, where the limited pixel density results in a “screen gate effect” in which users perceive streaks in the space between pixels.
Now MIT engineers have developed a new way to make screens sharper and flawless. Instead of replacing red, green, and blue light-emitting diodes next to each other in a horizontal tile, the team has invented a way to stack the diodes to create vertical multicolored pixels.
Each stacked pixel can generate the full commercial gamut of colors and is about 4 microns wide. Microscopic pixels, or “micro-LEDs,” can be packed down to a density of 5,000 pixels per inch.
“This is the smallest micro-LED pixel and the highest pixel density reported in magazines,” says Jeehwan Kim, an associate professor of mechanical engineering at MIT. “We show that vertical pixelation is the way to go for higher resolution displays in a smaller space.”
“For virtual reality, right now there’s a limit to how real they can look,” adds Jiho Shin, a postdoc in Kim’s research group. “With our vertical micro-LEDs, you could have a completely immersive experience and you wouldn’t be able to tell virtual from reality.”
The team results are posted today In the diary Nature. Kim and Shin’s co-authors include members of Kim’s lab, researchers at MIT, and collaborators from Georgia Tech Europe, Sejong University, and several universities in the US, France, and Korea.
Today’s digital displays are illuminated by organic light-emitting diodes (OLEDs), plastic diodes that emit light in response to an electrical current. OLEDs are the leading digital display technology, but diodes can degrade over time, resulting in permanent burn-in effects on displays. Technology is also reaching a limit to the size diodes 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 made of inorganic single-crystalline semiconductor materials. Micro-LEDs could perform better, require less power, and last longer than OLEDs.
But micro-LED fabrication requires extreme precision, as microscopic red, green, and blue pixels must first be grown separately on wafers and then precisely positioned on a plate, in exact alignment with one another to correctly reflect and produce various colors. and shadows 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 likely to misalign the pixels on a very small scale,” says Kim. “If you have a misalignment, you need to throw that material away, otherwise you could ruin a screen.”
stack of colors
The MIT team has devised a potentially less wasteful way to make micro-LEDs that doesn’t require precise pixel-by-pixel alignment. The technique is a completely different vertical LED approach, in contrast to the conventional horizontal pixel arrangement.
Kim’s group specializes in developing techniques to fabricate high-performance, ultra-thin, pure membranes with a view to designing smaller, thinner, more flexible and functional electronic devices. The team previously developed a method for growing and detaching perfect, two-dimensional, single-crystalline 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 grow ultrathin membranes of red, green, and blue LEDs. Then, they peeled all the LED membranes off their wafer bases and stacked them to form a layer cake of red, green, and blue membranes. They could then carve the cake into patterns of tiny vertical pixels, each as small as 4 microns across.
“In conventional displays, each R, G, and B pixel is arranged laterally, which limits the size of each pixel,” Shin notes. “Because we’re stacking all three pixels vertically, we could theoretically reduce the pixel area by a third.”
As a demonstration, the team fabricated a vertical LED pixel and showed that by altering the voltage applied to each of the pixel’s red, green, and blue membranes, they could produce multiple colors in a single pixel.
“If you have a higher current toward red and a weaker current toward blue, the pixel will appear pink, and so on,” Shin says. “We can create all the mixed colors and our screen can cover almost the commercial color space that is available.”
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 spectrum of colors. They will work to create an array of many vertical micro-LED pixels.
“You need a system to control 25 million separate LEDs,” says Shin. “Here, we have only partially demonstrated it. The active matrix operation is something we will need to develop further.”
“For now, we’ve shown the community that we can grow, strip, and stack ultra-thin LEDs,” Kim says. “This is the ultimate solution for small screens like smartwatches and VR headsets, where you would want highly densified pixels to create vivid, vivid images.”
This research was funded, in part, by the US National Science Foundation, US Defense Advanced Research Projects Agency (DARPA), US Air Force Research Laboratory ., the US Department of Energy, LG Electronics, Rohm Semiconductor, the French National Research Agency, and the Korean National Research Foundation.
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