Researchers Discover a New Family of 2D Semiconductors for Energy-Efficient Electronic Devices

Due to the increasing demand for high computational power in compact smart devices, circuits are shrinking while becoming more powerful. Therefore, the need for high device densities arises.

However, making circuit elements smaller is a challenge with various trade-offs. As the trend of miniaturization continues, engineers are trying to overcome the limitations of silicon-based technology. 

As devices such as transistors are miniaturized or scaled-down beyond a certain size, quantum tunneling effects become prominent, which leads to highly uncontrollable device behaviors. As the transistors become too small, some electrons can tunnel past the barriers even if they don't have enough energy to cross. Due to these effects, ultrasmall computer chips do not behave in a controllable way.

To continue scaling further, sophisticated device technologies, such as FinFET and Gallium Arsenide FETs, can be employed. In addition, 2D semiconductors are considered as a potential candidate for future computing electronics. These materials, being atomically thin semiconductor layers, can efficiently handle electrical switching operations.

However, for implementing 2D transistors, a 2D semiconductor needs to electrically connect by two pieces of metals that form source and drain. The contacts between metals and semiconductors form an interfacial potential barrier, known as the Schottky barrier, which can severely affect the charge injection efficiency. As a result, a stronger voltage is needed to force the flow of electricity through the Schottky barrier, wasting energy and generating heat. 

A research team from the Singapore University of Technology and Design (SUTD) is working on ohmic contacts with no Schottky barrier. They discovered a new family of 2D semiconductors, namely MoSi2N4 and WSi2N4 that form ohmic contacts with metals titanium, scandium, and nickels. Their work is described in the npj 2D Materials and Applications journal.

 

Metal contacts with MoSi2N4 monolayer. Image Courtesy of SUTD.

 

They performed a first-principle DFT (Density Functional Theory) investigation to study the metal contact properties in recently discovered 2D semiconductors. They focused on face-type contact as they are important for the development of high-performance 2D nanodevices.

"Importantly, we found that when 2D semiconductors are contacted by Na3B, the intrinsic electronic properties of the 2D semiconductor are retained," said Dr. Liemao Cao, the DFT expert from the SUTD research team. 

2D semiconductors, when combined with a contacting metal, become metalized. The metalized 2D semiconductors lose their original properties, and their electrical characteristics can no longer be controlled by external parameters. But, the researchers discovered that using Na3Bi thin films as a metal contact to 2D semiconductors does not metalize them, and can therefore be used in various applications, such as photodetectors, solar cells, and transistors.

Moreover, the researchers show that their discovered 2D monolayers can naturally suppress Fermi Level Pinning (FLP), an issue caused by defects that limit the applicability of 2D semiconductors.

"FLP is an adverse effect that happens in many metal-semiconductor contacts, and is caused by defects and complex materials interactions at the contact interface," said SUTD Assistant Professor Ang Yee Sin, who led the study. "Such an effect 'pins' the electrical properties of the contact to a narrow range regardless of the metal used in the contact."

The FLP phenomenon makes the Schottky barrier height almost completely insensitive to the material's work function as the FLP phenomenon causes some point of the bandgap to get locked to the Fermi level (the energy level occupied by electron orbits at 0 Kelvin). Therefore, design engineers are unable to tune the Schottky barrier height between the metal and the semiconductor, and as a result, are unable to flexibly design a semiconductor device.

Fortunately, the semiconductors discovered by Ang's team are naturally protected from FLP as the outer Si-N layer in both those 2D materials shield the underlying semiconductor layer from defects and other interactions. Due to this protection, the Schottky barrier height can be tuned by a design engineer for various kinds of applications.

"Our pioneering concept that synergises 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 foot-print of advanced computing systems, such as internet-of-things and artificial intelligence," said Professor Ricky L. K. Ang, the principal investigator of the research team, and the Head of the Science, Math, and Technology cluster in SUTD. 

"Some of them might be very poor in terms of electronics applications but very good for spintronics, photocatalysts, or as a building block for solar cells," he concluded. "Our next challenge is to systematically scan through all of these 2D materials and categorize them according to their potential applications."

About the Researchers

As a part of this study, the SUTD team, led by Professor Ang Yee Sin, collaborated with researchers from Nanjing University, the National University of Singapore, and Zhejiang University.