Stacked LED for fully immersive virtual reality display

Engineers at the Massachusetts Institute of Technology have developed a new method to create clearer, defect free displays. The team did not replace red, green, and blue light-emitting diodes side by side in horizontal patchwork, but invented a method of stacking diodes to create vertical, multi-color pixels, which can achieve fully immersive virtual reality displays and higher resolution digital screens.

Opening your laptop screen, you will find a board with red, green, and blue LED pixel patterns in its center, connected end-to-end, resembling a detailed Lite Brite display screen. When powered on, the LEDs together can generate each shadow in the rainbow, resulting in a full color display. Over the years, the size of individual pixels has shrunk, allowing more pixels to be encapsulated in devices to produce clearer and higher resolution digital displays.

However, like computer transistors, LEDs not only achieve effective performance, but also reach the limit of their small size.

This limitation is particularly evident in close range displays (such as enhanced and virtual reality devices), where limited pixel density can lead to the "screen gate effect", allowing users to perceive the stripes between pixels.

Now, engineers at the Massachusetts Institute of Technology have developed a new method to create clearer, defect free displays. The team did not replace red, green, and blue LEDs side by side in horizontal patchwork, but instead invented a method of stacking diodes to create vertical, multi-color pixels.

Each stacked pixel can generate a complete commercial color range, with a width of approximately 4 microns. Micro pixels or "micro LEDs" can be packaged to a density of 5000 pixels per inch.

This is the smallest micro LED pixel and the highest pixel density reported in the journal, "said Jeehwan Kim, associate professor of mechanical engineering at the Massachusetts Institute of Technology. We have found that vertical pixelization is a way to achieve higher resolution display in a smaller space

For virtual reality, there is currently a limit to how realistic they appear, "added Jiho Shin, a postdoctoral fellow at Kim's research group. With our vertical micro LED, you can achieve a completely immersive experience without distinguishing between virtual and real

The team's results were published in the journal Nature. Kim and Shin's co authors include members of Kim's laboratory, researchers from the Massachusetts Institute of Technology, and collaborators from Georgia Institute of Technology's European campus, Sejong University, and multiple universities in the United States, France, and South Korea.

Currently, digital displays are illuminated by organic light emitting diodes (OLEDs) - plastic diodes that emit light in response to current.

OLED is a leading digital display technology, but diodes can degrade over time, leading to permanent screen aging. This technology also reaches the limit of diode size reduction, thereby limiting their clarity and resolution.

For the next generation of display technology, researchers are exploring inorganic micro LEDs - diodes that are only one percent size of traditional LEDs, made of inorganic single crystal semiconductor materials. Compared to OLED, Micro LED has better performance, less energy consumption, and longer service life.

However, the manufacturing of micro LEDs requires extremely high precision, as red, green, and blue micro pixels need to first grow separately on the wafer and then be accurately placed on the board, aligned with each other accurately to reflect and generate various colors and shadows correctly. Achieving this micro precision is a challenging task, and if inappropriate pixels are found, the entire device needs to be scrapped.

This kind of picking and placement manufacturing is likely to misplace pixels within a very small range, "Kim said. If you have a misalignment, you must throw away those materials, otherwise it will destroy a display

The MIT team has come up with a possible way to reduce waste by manufacturing miniature LEDs that do not require precise pixel by pixel alignment. Compared to traditional horizontal pixel arrangement, this technology is a completely different vertical LED method.

Kim's team focuses on developing and manufacturing technologies for pure, ultra-thin, and high-performance films, with the aim of designing smaller, thinner, more flexible, and more practical electronic products. The team has previously developed a method that can grow and peel perfect two-dimensional single crystal from silicon wafers and other surfaces - they call it the layer transfer or 2DLT method based on two-dimensional materials.

In the current study, researchers used the same method to grow ultrathin films of red, green, and blue LEDs. Then, they peeled the entire LED film off its base wafer and stacked them together to create a layered cake composed of red, green, and blue films. Then they can carve the cake into tiny vertical pixel patterns, each pixel only 4 microns wide.

In traditional monitors, each R, G, and B pixel is arranged horizontally, which limits the size of each pixel you can create, "Shin pointed out. Because we stack all three pixels vertically, theoretically we can reduce the pixel area by one-third

As a demonstration, the team manufactured a vertical LED pixel and demonstrated that by changing the voltage applied to the red, green, and blue films of each pixel, they could generate various colors within a single pixel.

If you have higher red current and weaker blue current, the pixels will appear pink, and so on, "Shin said. We are able to create all mixed colors, and our monitor can cover nearly the available commercial color space

The team plans to improve the operation of vertical pixels. So far, they have demonstrated that they can stimulate a single structure to produce full spectrum colors. They will be committed to manufacturing arrays of many vertical micro LED pixels.

You need a system to control 25 million LEDs separately, "Shin said. Here, we have only partially proven this point. Active matrix operations are something we need to further develop

Currently, we have demonstrated to the community that we can grow, peel, and stack ultra-thin LEDs, "Kim said. This is the ultimate solution for small displays such as smartwatches and virtual reality devices, where you need high-density pixels to create vivid and vivid images