A newly discovered class of two-dimensional (2D) semiconductors could lead to the creation of high-performance and energy-efficient electronics, according to scientists from the Singapore University of Technology and Design (SUTD).
(Left Panel) Illustration of metal contacts to MoSi2N4 monolayer. A Schottky contact is formed when gold is used as an electrode material to MoSi2N4. On the other hand, an energy-efficient Ohmic contact can be achieved by using titanium electrode. (Right Panel) The “slope parameters” S of MoSi2N4 and WSi2N4 metal contacts studied in this work are among the lowest when compared to other species of 2D semiconductors, suggesting the strong potential of MoSi2N4 and WSi2N4 for electronic device applications. Image Credit: Singapore University of Technology and Design.
The results may pave the way toward the manufacture of semiconductor devices suitable for use in conventional electronics and optoelectronics — and even possibly replace silicon-based device technology completely. Details of the research have been published in npj 2D Materials and Applications.
In the mission for miniaturizing electronic devices, one renowned trend is Moore’s law, which illustrates how the parts in integrated circuits of computers double every two years.
This trend is conceivable owing to the ever-decreasing size of transistors, some of which are so tiny that millions of them can be packed onto a chip the size of a fingernail. Yet, as this trend endures, engineers are beginning to contend with the characteristic material restrictions of silicon-based device technology.
Due to the quantum tunnelling effect, shrinking a silicon-based transistor too small will lead to highly uncontrollable device behaviours. People are now looking for new materials beyond the ‘silicon era’, and 2D semiconductors are a promising candidate.
Ang Yee Sin, Study Lead and Assistant Professor, Singapore University of Technology and Design
2D semiconductor materials measure only a few atoms in thickness. Owing to their nanoscale size, such materials are robust replacement candidates for silicon in the pursuit of developing compact electronic devices. However, a number of 2D semiconductors available now are troubled by high electrical resistance when they make contact with metals.
When you form a contact between metal and semiconductor, often there will be what we call a Schottky barrier. In order to force electricity through this barrier, you need to apply a strong voltage, which wastes electricity and generates waste heat.
Ang Yee Sin, Study Lead and Assistant Professor, Singapore University of Technology and Design
This stimulated the researchers’ interest in Ohmic contacts, or metal-semiconductor contacts having no Schottky barrier.
In their research, Ang and colleagues from the National University of Singapore, Nanjing University and Zhejiang University demonstrated that a newly discovered class of 2D semiconductors, i.e., MoSi2N4 and WSi2N4, form Ohmic contacts with the metals scandium, titanium and nickel, which are extensively used in the semiconductor device sector.
Moreover, the team also showed that the new materials are not plagued by Fermi level pinning (FLP), a problem that drastically restricts the application potential of other 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. Such an effect ‘pins’ the electrical properties of the contact to a narrow range regardless of the metal used in the contact.
Ang Yee Sin, Study Lead and Assistant Professor, Singapore University of Technology and Design
Due to FLP, engineers are unable to tweak or alter the Schottky barrier between the metal and semiconductor — reducing the design flexibility of a semiconductor device.
To reduce FLP, engineers typically use approaches such as very lightly and gradually placing the metal on top of the 2D semiconductor, forming a buffer layer between the metal and semiconductor, or using a 2D metal as a contact material with the 2D semiconductor. While these approaches are achievable, they are not yet helpful and are mismatched with mass fabrication using mainstream industry methods currently available.
Remarkably, Ang’s team demonstrated that MoSi2N4 and WSi2N4 are naturally guarded against FLP because of an inert Si-N outer layer that protects the underlying semiconducting layer from flaws and material interactions at the contact interface.
Owing to this protection, the Schottky barrier is “unpinned” and can be adjusted to suit a wide range of application requirements. This enhancement in performance helps position 2D semiconductors in the running as substitutes for silicon-based technology, with key players like Samsung and TSMC already conveying an interest in 2D semiconductor electronics.
Ang anticipates that their efforts will inspire other researchers to hunt for more members of the newly discovered 2D semiconductor series for exciting properties, even those with applications beyond electronics.
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. Our next challenge is to systematically scan through all of these 2D materials and categorise them according to their potential applications.
Ang Yee Sin, Study Lead and Assistant Professor, Singapore University of Technology and Design
Journal Reference:
Wang, Q., et al. (2021) Efficient Ohmic contacts and built-in atomic sublayer protection in MoSi2N4 and WSi2N4 monolayers. npj 2D Materials and Applications. doi.org/10.1038/s41699-021-00251-y.