02-11-2021 | | By Robin Mitchell
An emerging family of monolayer semiconductors now show new promise thanks to researchers who have developed efficient contacts. What challenges do monolayer semiconductors face, what did the researchers do, and could it be a viable replacement for silicon devices?
Silicon has been the de facto semiconductor for the better part of 70 years, thanks to its versatility, availability, and well-understood characteristics. However, engineers are quickly approaching the physical limits of silicon, and now other semiconductor materials are being researched to see if they can replace silicon in the long run.
Some of the characteristics that researchers look for in a material are whether it allows for physically smaller devices, if it can operate at higher power, and if it can operate at higher frequencies.
One promising technology is monolayers, atomic structures that are only one unit in height (one unit refers to the smallest crystal structure). In the case of graphene, the layer is only one carbon atom in height, and in the case of more advanced monolayers (such as MoSi2N4), the structure is approximately 6 atoms in height. These materials show promise as they often have unusual electrical characteristics such as fixed resistances (caused by the lack of impurities and only one dimension for electron transport) and the ability to make incredibly small transistors.
However, monolayers also suffer from some drawbacks, and one of these drawbacks is their electrical contact with other layers. Generally speaking, monolayers in contact with metal layers will form Schottky barriers, which require a specific voltage level to be applied before the current can flow. This required voltage not only makes using the monolayer more complex but also wastes energy.
Researchers from Nanjing University, the National University of Singapore, and Zhejiang University recently demonstrated how MoSi2N4 bonds with various contact materials to produce Schottky and ohmic contacts. The materials demonstrated by the team were titanium, scandium, and nickel, which are already used by the semiconductor industry for creating metal layers, and all these layers demonstrated ohmic properties with MoSi2N4.
The significance of an ohmic contact is that no voltage barrier needs to be overcome. As such, less energy is wasted on voltage barriers, and smaller voltages that allow for higher frequency operation can be used.
Furthermore, the new research also shows that the new semiconductor material does not exhibit Fermi level pinning, which is a semiconductor phenomenon that occurs at the surface of semiconductors regardless of the work function of the metal contact. The lack of Fermi level pinning comes from the fact that an inert layer Si-N outer layer in the semiconductor removes defects that typically cause the phenomenon to arise.
Whether monolayers will replace silicon in semiconductors is a hard question to answer and depends on the practicality of creating large-scale monolayer structures. Researchers have easily created monolayer semiconductors, created basic devices, and measured their characteristics, but there is a big difference between creating one transistor and one billion.
There is no doubt that monolayers have many advantages; they form very different contacts with other materials compared to standard crystalline structures. They have unique properties and are often free from imperfections and defects.
However, silicon as a semiconductor is an established technology. While monolayers may allow for the creation of more advanced future devices, silicon will still be the de facto technology found in everyday silicon that does not need to operate at excellent efficiency, high speed, or at nanometer scales.