From sheet to stack, new nanostructures promise a leap to the next level

Tokyo, Japan – Scientists from Tokyo Metropolitan University have successfully engineered multilayered nanostructures of transition metal dichalcogenides that meet in planes to form joints. They grew layers of molybdenum disulfide structures in layers from the edges of niobium-doped molybdenum disulfide shards, creating thick, bonded planar heterostructures. They demonstrated that this could be used to create new tunnel field-effect transistors (TFETs), components in integrated circuits with very low power consumption.

The field effect transistor (FET) is an essential building block of nearly every digital circuit. They control the flow of current through them depending on the applied voltage. While metal oxide semiconductor FETs (or MOSFETs) form the bulk of the FETs in use today, the search is on for the next generation of materials to power increasingly demanding and compact devices while using less power. This is where tunneling FETs (or TFETs) come in. TFET relies on quantum tunneling, an effect in which electrons are able to pass through barriers that are normally impassable due to quantum mechanical effects. Although TFETs use less energy and have long been proposed as a promising alternative to traditional FETs, scientists have yet to find a way to implement the technology in a scalable form.

A team of scientists from Tokyo Metropolitan University led by Associate Professor Yasumitsu Miyata has worked to create nanostructures of transition metal dichalcogenides, mixtures of transition metals and group 16 elements. Transition metal dichalcogenides (TMDCs, two chalcogen atoms to one metal atom) are candidate materials. which is very good for making TFET. Their recent success has enabled them to fuse single-atom thick layers of crystalline TMDC sheets to unprecedented lengths. Now, they have turned their attention to the multi-layered structure of TMDC. Using chemical vapor deposition (CVD) techniques, they demonstrated that they could grow TMDCs that differ from the edges of stacked crystal planes fixed to a substrate. The result is a junction in a plane that is thick in layers. Most of the work on TMDC junctions uses single layers stacked on top of each other; this is because, despite the outstanding theoretical performance of in-plane junctions, previous attempts were unable to realize the high concentrations of holes and electrons required to make TFETs work.

After demonstrating the power of their technique using molybdenum disulfide grown from tungsten selenide, they turned their attention to niobium-doped molybdenum disulfide, a p-type semiconductor. By growing a multi-layered structure from an undoped molybdenum disulfide, n-type semiconductor, the team realized thick pn junctions between TMDCs with very high carrier concentrations. In addition, they found that the junctions exhibit a negative differential resistance (NDR) trend, where increasing voltage leads to less and less increase in current, a key feature of tunneling and a significant first step for these nanomaterials to enter TFET.

The method used by the team is also scalable over large areas, making it suitable for application during circuit building. It’s an exciting new development for modern electronics, with hopes it will find its way into future applications.

This work was supported by JSPS KAKENHI Grants-in-Aid, Grant Numbers JP20H02605, JP21H05232, JP21H05233, JP21H05234, JP21H05237, JP22H00280, JP22H04957, JP22H05469, JP22H05492J14549, JP2742H038, JP22H05469 JP22J14738, JP232, JP21K JP20H00316, JP20H02080, JP20K05253, JP20H05664, JP18H01822 , JP21K04826, JP22H05445 , and JP21K14498, CREST Grant Number JPMJCR16F3 and Japan Science and Technology Agency Grant Number JPMJFR213X.