Researchers from the two universities collaborated to develop laminated full-color Micro LEDs

Recently, foreign media reported that Japan's City University and Saudi Arabia's King Abdullah University of Science and Technology (KAUST) jointly developed a monolithic stacked RGB gallium nitride indium (GaInN) micro LED array with a pixel density of 330 PPI.

Researchers said that the product can be used in head-mounted smart devices such as AR/VR/MR. The micron-level pixel displays required by these devices cannot be manufactured through mechanical assembly technology, so monolithic manufacturing technology is needed.

Regarding the application of gallium nitride indium materials, the researchers explained that although gallium nitride indium based LEDs encounter major efficiency problems at longer wavelengths, gallium nitride indium has made new progress for red efficiency in the visible spectrum. In addition, compared with the commonly used red LED material aluminum gallium indium phosphide, the luminous efficiency of gallium indium LEDs is less affected by the reduction of the Micro LED's size.

During the Micro LED epitaxial manufacturing stage, the research team used metal organic vapor phase epitaxy (MOVPE) technology to grow RGB Micro LED array materials on GaN substrates in two stages. Among them, the blue and green layers are grown by the Mingcheng University team, the KAUST team is responsible for the growth of the final red light stage, and the RGB layers are separated by Tunnel Junction (TJ).

Schematic cross-section of LED multi-stacked structure

In order to improve the luminous efficiency of the red layer, the red layer especially includes a superlattice epitaxial stack layer and red and blue single quantum wells (SQWs). Researchers said that inserting blue gallium nitride indium superlattice and gallium nitride indium SQW between the n-type GaN layer and the active layer in the red LED structure plays an effective role in improving the emission efficiency of red LEDs.

The researchers also pointed out that the GaN substrate used in Micro LEDs is very expensive, and for production, it is necessary to switch to cheaper materials, such as sapphire materials.

After epitaxial manufacturing is completed, the material is fabricated into RGB pixels with a density of 330 PPI (see Figure 2). The Micro LED has a size of 73m x 20m, and the device mesa size is 35m x 15m.μμμμ

Figure 2: Micro LED array process flow diagram

After testing by the researchers, data collected from the substrate side showed that the peak wavelengths of the electroluminescent (EL) spectrum of the blue-green-red Micro LED at a current density of 50 A/cm ² were 486 nm, 514 nm and 604 nm, respectively (Figure 3). The corresponding full width at half maximum (FWHM) are 29 nm, 31 nm and 52 nm. The researchers said these values were similar to those for separately manufactured LED devices.

Electroluminescent (EL) Spectra of Blue-Green-Red Micro LED

At an injection current of 10A/cm ², the optical output power (LOP) of green Micro LEDs reaches almost 0.8W, and the LOP of red and blue light is less than 0.2W and 0.1W respectively.μμμ