Improving Contact Resistance in 2D Semiconductors to Unlock Next Gen Applications

June 09, 2020 by Luke James

Driven by demand from consumers, electrical and design engineers have been toiling to find ways to create smaller, lighter, and smarter devices while retaining high performance and efficiency levels.

Making electronic devices smaller comes at a price, however. Due to quantum effects in ultracompact semiconductors, field-effect transistors (FETs) stop behaving in a way that is easy to control, and this is where device architectures such as FinFET come into play, enabling engineers to continue scaling devices down.

However, these still have limitations that will soon be reached, an inevitability that has prompted engineers to continue innovating in this space.

Now, a team of researchers from the Singapore University of Technology has reported that they have discovered a new strategy to resolve the contact resistance problem exhibited in 2D semiconductors, a challenge that has held back their development and uses in applications. 


2D Semiconductors

Two-dimensional (2D) semiconductors, measuring in at just a few atoms in thickness, have been recognized as a potential option for the next generation of ultra-compact computing electronics. This is because they facilitate electrical switching options that can be efficiently controlled without the use of sophisticated device architectures when they are made into FETs. 

When making a transistor from a 2D semiconductor, it needs to be electrically contacted by a “source” and a “drain”, two thin pieces of metal. These, however, create a contact resistance at both the source and the drain which degrades transistor performance and generates a substantial amount of wasted energy lost through heat, making it inefficient.

Given that the goals engineers are to make devices smaller while simultaneously retaining both performance and efficiency, it is easy to see why 2D semiconductors have yet to be utilized in real-world applications. To date, the search for a suitable metal that does not produce a large contact resistance is ongoing. 


A graphical representation of topological semimetal electrical contact.

A graphical representation of topological semimetal electrical contact and the reaction that leads to improving energy efficiency of 2D semiconductor transistors. Image used courtesy of Singapore University of Technology and Design


Resolving Contact Resistance

However, as reported in Physical Review Applied, the Singapore research team claim to have found a way to resolve the contact resistance problem. By performing a state-of-the-art density functional theory (DFT) computational simulation, the research team observed that an ultra-thin film of a recently discovered topological semimetal, Na3Bi, with conductive characteristics protected by crystal symmetry, can be used as a metal contact for 2D semiconductors with ultra-low contact resistance. And by ultra-thin, we mean two atomic layers thin.

“We found that the Schottky barrier height formed between Na3Bi and 2D semiconductor is one of the lowest among many metals commonly used by the industry,” said Dr. Yee Sin Ang of the Singapore-based research team. 

The Schottky barrier is a thin insulator that forms between metal and semiconductor. The height of this barrier bears great influence on the level of contact resistance observed, with a small height being desirable for low resistance.

By discovering that the Schottky barrier between Na3Bi and two commonly studied 2D semiconductors, MoS2 and WS2, is much lower than many commonly used metals, the research team uncovered the strength of topological semimetal thin films and their huge potential for designing efficient 2D semiconductor devices with minimal contact resistance. 


The Use of Na3Bi with 2D Semiconductors 

When 2D semiconductors are fused together with a contacting metal, they become metalized and lose their original electrical properties that are ideal for electronics applications.

However, the research team discovered that Na3Bi does not metalize 2D semiconductors, and thus using it as a metal contact to 2D semiconductors is highly beneficial for applications such as transistors and solar cells. 

“Our pioneering concept that synergizes 2D materials and topological materials will offer a new route towards the design of energy-efficient electronic devices, which is particularly important for reducing the energy footprint of advanced computing systems, such as internet-of-things and artificial intelligence,” concluded Ricky L. K. Ang, the team’s principal investigator.