An illustration of the new deposition method for fabricating molybdenum disulphide without grain boundaries. Image: FLEET.
An illustration of the new deposition method for fabricating molybdenum disulphide without grain boundaries. Image: FLEET.

Moore's law is an empirical suggestion describing how the number of transistors doubles every few years in integrated circuits (ICs). But Moore's law has begun to fail, as transistors are now so small that the current silicon-based technologies are unable to offer further opportunities for shrinking.

One possibility for overcoming Moore's law is to resort to 2D semiconductors. These 2D materials are so thin that they allow the propagation of free charge carriers – namely, the electrons and holes that carry information in transistors – along an ultra-thin plane. This confinement of charge carriers potentially allows the 2D semiconductor to easily switch between states. It also establishes directional pathways for the charge carriers, so they can move without scattering, potentially leading to transistors with infinitely small resistance.

This means that 2D materials can produce transistors that do not waste energy during their on/off switching. Theoretically, they can switch very rapidly and also switch off to absolute zero resistance values during their non-operational states. In reality, however, there are still many technological barriers that need to be overcome to create such perfect ultra-thin semiconductors. One of the barriers with current technologies is that the deposited ultra-thin films are full of grain boundaries, which cause the charge carriers to bounce back, increasing the resistive loss.

One of the most exciting ultra-thin semiconductors is molybdenum disulphide (MoS2), which has been the subject of investigations over the past two decades for its electronic properties. However, obtaining very large-scale 2D MoS2 without any grain boundaries has been proven to be a real challenge, especially with current large-scale deposition technologies.

Now, researchers at the School of Chemical Engineering in the University of New South Wales (UNSW), Australia, have developed a new deposition method involving liquid metals that can eliminate such grain boundaries. This work, reported in a paper in Advanced Functional Materials, was funded by the Australian Research Council and the ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET).

"This unique capability was achieved with the help of gallium metal in its liquid state," explained Yifang Wang, first author of the paper. "Gallium is an amazing metal with a low melting point of only 29.8 °C. It means that at a normal office temperature it is solid, while it turns into a liquid when placed in the palm of someone's hand. It is a melted metal, so its surface is atomically smooth. It is also a conventional metal, which means that its surface provides a large number of free electrons for facilitating chemical reactions."

"By bringing the sources of molybdenum and sulphur near the surface of gallium liquid metal, we were able to realize chemical reactions that form the molybdenum sulphur bonds to establish the desired MoS2," said Kourosh Kalantar-Zadeh, lead author of the paper. "The formed two-dimensional material is templated onto an atomically smooth surface of gallium, so it is naturally nucleated and grain boundary free. This means that by a second step annealing, we were able to obtain very large area MoS2 with no grain boundary. This is a very important step for scaling up this fascinating ultra-smooth semiconductor."

The researchers at UNSW are now planning to expand their method to fabricate other 2D semiconductors and dielectric materials, in order to create a number of materials that can be used as different parts of transistors.

This story is adapted from material from FLEET, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.