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  Date: 23/05/2013

New hot semiconductor material MoS2 researched for atomic scale chips

Though Silicon is still a most preferred material for semiconductor IC manufacturing, researchers are looking for alternate material to make devices at atomic scale and using a cheaper non-lithographic processes to make complex SoC chips. Carbon based Graphene and Molybdenum Sulfide (MoS2) are the hot material selected by researchers to make devices in nano-scale using a process different from the present IC manufacturing. Graphene is good but is more of better conductor than an insulator, so switching-on transistor made out of Graphene is easy but switching-off is difficult because of less band-gap energy. Where as Molybdenum Sulfide has band energy gap of 1.2 eV matching Silicon and making it suitable for switching device such as transistor. The advantage of Molybdenum Sulfide over Silicon is deposition of a very thin layer of Molybdenum Sulfide is easier compared to Silicon.

Researchers at North Carolina State University were able to create high-quality Molybdenum Sulfide semiconductor thin films of one atom thick. The method explained be Researchers is they place sulfur and Molybdenum Chloride powders in a furnace and gradually raise the temperature to 850 degrees Celsius, which vaporizes the powder. The two substances react at high temperatures to form MoS2. While still under high temperatures, the vapor is then deposited in a thin layer onto the substrate. The thickness of the MoS2 layer was controlled by controlling the partial pressure and vapor pressure in the furnace.

The Molybdenum Sulfide (MoS2) is also not expensive and its electronic and optical parameters match Silicon and other similar material.

The release from NCSU says "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 can ensure that thin-film growth automatically stops once the monolayer is formed. Cao calls this “self-limiting” growth."

The research team headed by Dr. Linyou Cao, an assistant professor of materials science and engineering at NC State now trying to find ways to create similar thin films in which each atomic layer is made of a different material. Dr. Cao is also working to create field-effect transistors and LEDs using this technique.

Another similar research achievement was announced by Columbia University, whose scientists have grown high-quality crystals of molybdenum disulfide (MoS2) and studied how these crystals stitch together at the atomic scale to form continuous sheets. By knowing how to stitch the semiconductor crystals, the material from the growth substrate can be removed and transferred to any arbitrary surface which helps in making flexible electronics and solar cells."

Two years back EPFL had fabricated transistor using MoS2 and Massachusetts Institute of Technology could able to make some electronic components using MoS2.
Author: Srinivasa Reddy N
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