An international team of researchers and engineers has developed an approach to make displays, such as those for augmented and virtual reality (AR/VR), sharper and defect-free. The researchers devised an architecture in which they stacked red, green, and blue LEDs in a formation to create vertical, multicolored pixels as opposed to placing the diodes side-by-side in a horizontal patchwork. Each stacked pixel generated the full range of commercial colors. The microscopic pixels, or micro-LEDs (µLEDs), are each about 4 μm wide. The pixels can be packed to a density of more than 5000 pixels per inch. The development of smaller pixel sizes has allowed more pixels to be packed into devices to increase the quality of the display. However, there is a limit to how small LEDs can be made without affecting performance, and, much like computer transistors, LEDs are reaching a limit to how small they can be while also performing effectively. In close-range displays, such as those found in AR/VR devices, limited pixel density can degrade the display. MIT engineers and their colleagues developed a way to make sharper, defect-free displays. Instead of patterning red, green, and blue diodes side-by-side in a horizontal patchwork, the team has invented a way to stack the diodes to create vertical, multicolored pixels. Courtesy of Younghee Lee. Inorganic µLEDs show potential to perform better, require less energy, and last longer than the organic LEDs (OLEDs) that are currently the leading display technology. But µLED fabrication requires that microscopic pixels of red, green, and blue be grown on separate wafers and then precisely placed on a plate, in exact alignment with each other. “This pick-and-place fabrication is very likely to misalign pixels in a very small scale,” said MIT professor Jeehwan Kim. Further, if any pixels are found to be out of place, the entire device must be scrapped to avoid ruining the full display. The researchers developed a potentially less wasteful way to fabricate µLEDs that does not require precise, pixel-by-pixel alignment. The approach is based on a previously developed method to grow and peel away perfect, 2D, single-crystalline material from silicon wafers and other surfaces. The 2D, materials-based layer transfer technique allows the growth of RGB LEDs of near-submicron thickness on 2D, material-coated substrates via remote or van der Waals epitaxy and mechanical release and stacking of the LEDs, followed by top-down fabrication. Using this approach, which is called 2D material-based layer transfer (2DLT), the researchers grew ultrathin membranes of red, green, and blue LEDs. They peeled the LED membranes away from their base wafers and stacked them, layering red, green, and blue membranes. The researchers then divided the membrane stacks into patterns of tiny, vertical pixels as small as 4 μm wide. “In conventional displays, each red, green, and blue pixel is arranged laterally, which limits how small you can create each pixel,” researcher Jiho Shin said. “Because we are stacking all three pixels vertically, in theory we could reduce the pixel area by a third.” To demonstrate their approach, the researchers fabricated a vertical LED pixel and showed that they could produce various colors in a single pixel by altering the voltage applied to each of the pixel’s red, green, and blue membranes. “If you have a higher current to red, and weaker to blue, the pixel would appear pink, and so on,” Shin said. “We’re able to create all the mixed colors, and our display can cover close to the commercial color space that’s available.” So far, the team has shown that it can stimulate an individual structure to produce the full spectrum of colors. The next step will be to work toward making an array of many vertical µLED pixels. “You need a system to control 25 million LEDs separately,” Shin said. “Here, we’ve only partially demonstrated that.” According to Kim, the team’s work to date represents the ultimate solution for small displays like smartwatches and virtual reality devices that necessitate highly densified pixels to create vivid images. To the researchers’ knowledge, the vertically stacked µLEDs have achieved the highest array density and the smallest size reported to date. The smallest stack height of around 9 µm is the key enabler for the record-high µLED array density, they said. The research was published in Nature (www.doi.org/10.1038/s41586-022-05612-1).
