Method Creates Atomic-Scale Semiconductors

An inexpensive material that can be “grown” in layers only one atom thick has yielded atomic-scale semiconductor thin films. The technique could be applied to make these devices wide enough to coat wafers that are 2 in. wide or larger.

“This [method] could be used to scale current semiconductor technologies down to the atomic scale — lasers, light-emitting diodes (LEDs), computer chips, anything,” said Dr. Linyou Cao, an assistant professor of materials science and engineering at North Carolina State University. “People have been talking about this concept for a long time, but it wasn’t possible. With this discovery, I think it’s possible.”

The researchers worked with molybdenum sulfide (MoS₂), an inexpensive semiconductor material with electronic and optical properties similar to materials already used in the semiconductor industry. However, the material’s ability to be grown in one-atom-thick layers is what sets it apart from other semiconductor materials.

In the new technique, sulfur and molybdenum chloride powders are placed in a furnace and raised to a temperature of 850 ºC, which vaporizes the powder to form MoS₂. This powder is deposited in a thin layer on a substrate using a “self-limiting” growth process.

The thin films developed by North Carolina State University researchers are only one atom thick, but can be made wide enough to coat wafers that are 2 in. wide or larger. The films are made of molybdenum sulfide (MoS₂), an inexpensive semiconductor material. Courtesy of Linyou Cao, North Carolina State University.

“The key to our success is the development of a new growth mechanism, a self-limiting growth,” Cao said. The researchers can precisely control the thickness of the MoS₂ layer by controlling the partial and vapor pressures in the furnace. Partial pressure is the tendency of atoms or molecules suspended in air to condense into a solid and settle onto the substrate; vapor pressure is the tendency of solid atoms or molecules on the substrate to vaporize and rise into the air.

To create a single layer of MoS₂ on the substrate, the partial pressure must be higher than the vapor pressure. The higher the partial pressure, the more layers of MoS₂ will settle to the bottom. If the partial pressure is higher than the vapor pressure of a single layer of atoms on the substrate, but not higher than the vapor pressure of two layers, the balance between the partial pressure and the vapor pressure guarantees that thin-film growth stops once the monolayer is formed — known as self-limiting growth.

Partial pressure is controlled by adjusting the amount of molybdenum chloride in the furnace — the more molybdenum is in the furnace, the higher the partial pressure.

“Using this technique, we can create wafer-scale MoS₂ monolayer thin films, one atom thick, every time,” Cao said. “We can also produce layers that are two, three or four atoms thick.”

The investigators are now working to create similar thin films in which each atomic layer is made of a different material, and plan to use the technique to create field-effect transistors and LEDs. A patent for the method has been filed.

The research, funded by the US Army Research Office, appeared in Scientific Reports (doi: 10.1038/srep01866).

For more information, visit: www.ncsu.edu