A team of Korean researchers has developed an LED device that emits light by directly growing a two-dimensional semiconductor material — only a few atoms thick — on a substrate. By eliminating the cumbersome process of detaching thin materials and transferring them onto a different substrate, the breakthrough significantly raises the prospects for mass production of next-generation optical devices and 2D LEDs for quantum light sources.
A research team led by Professor Jung Kun-wook of the Department of Physics at the Ulsan National Institute of Science and Technology (UNIST) announced on the 26th that it has implemented a 2D semiconductor LED device that requires no transfer process, using molybdenum disulfide (MoS2), a next-generation light-emitting material. Molybdenum disulfide, used as the light-emitting layer, emits visible light even at thicknesses of only a few atomic layers, making it a key material for next-generation optical devices and quantum light sources.
Until now, LEDs using 2D semiconductors have mostly relied on synthesizing the material separately and then transferring it onto a substrate. However, this transfer process frequently caused contamination or gaps in the material, and uneven sizes and shapes made it extremely difficult to mass-produce devices with uniform quality.
To overcome these issues, the research team optimized the process sequence and constituent materials so that the light-emitting material grows directly on the substrate, completing a high-quality device. A typical LED has a structure in which a light-emitting layer is inserted between p-type and n-type semiconductors. The researchers designed a structure in which molybdenum disulfide is directly grown on a p-type gallium nitride (GaN) substrate, with n-type zinc oxide (ZnO) nanorods then vertically aligned on top. All three materials share a hexagonal crystal structure, enabling stable layer-by-layer direct growth. In addition, by depositing gallium nitride — which requires a high-temperature process — first, the team fundamentally prevented damage to molybdenum disulfide, which is vulnerable to heat.
The newly developed device takes the form of a single-crystal stack with aligned crystal orientations, giving it structural and optical characteristics advantageous for producing devices with high uniformity. In particular, when current was injected, clear red light-emission signals at wavelengths of 630 nanometers (nm) and 705 nm were observed in the light-emitting layer. This goes beyond the role of a simple lighting LED, demonstrating the potential to be used as a quantum light source applying quantum phenomena such as "spin-orbit coupling."
"One of the reasons 2D semiconductor LEDs have remained at the level of laboratory demonstrations is the non-uniformity of the process of transferring thin films," Professor Jung Kun-wook said. "This research is a case showing that 2D semiconductor LEDs can also be made into uniform stacked structures, like conventional semiconductor processes, by growing the light-emitting layer directly on the substrate." He added, "If we further improve device efficiency and combine it with gallium nitride-based LED processes, it could be developed into red pixels or quantum light source devices."
Meanwhile, the research was sponsored by the Ministry of Science and ICT (National Research Foundation of Korea) and the Ministry of Trade, Industry and Energy (Korea Institute for Advancement of Technology). In recognition of its excellence, the work was published as a Supplementary Cover paper in the April 21 online edition of Nano Letters, a leading international journal in the nanotechnology field.
