From sheets to stacks: researchers develop next-gen nanostructure

Scientists from Tokyo Metropolitan University have engineered multi-layered nanostructures of transition metal dichalcogenides which meet in-plane to form junctions. They grew out layers of multi-layered structures of molybdenum disulfide from the edge of niobium doped molybdenum disulfide shards, creating a thick, bonded, planar heterostructure. The researchers demonstrated that these may be used to make new tunnel field-effect transistors (TFET), components in integrated circuits with ultra-low power consumption.

Field-effect transistors (FETs) are a crucial building block of digital circuits. They control the passage of current through circuits depending on the voltage which is put across. While metal oxide semiconductor FETs (or MOSFETs) form the majority of FETs in use today, researchers are searching for the next generation of materials to drive increasingly demanding devices using less power. This is where tunnelling FETs (or TFETs) come in.

TFETs rely on quantum tunnelling, an effect where electrons are able to pass usually impassable barriers due to quantum mechanical effects. Though TFETs use less energy and are proposed as an alternative to traditional FETs, scientists have yet to come up with a way of implementing the technology in a scalable form.

The scientists from Tokyo Metropolitan University, led by Associate Professor Yasumitsu Miyata, have been working on making nanostructures out of transition metal dichalcogenides, a mixture of transition metals and group 16 elements. Transition metal dichalcogenides (TMDCs, two chalcogen atoms to one metal atom) are a promising candidate for creating TFETs. The researchers stitched together single-atom-thick layers of crystalline TMDC sheets over “unprecedented” lengths and have now turned their attention to multi-layered structures of TMDCs.

By using a chemical vapour deposition (CVD) technique, the researchers showed that they could grow out a different TMDC from the edge of stacked crystalline planes mounted on a substrate. The result was an in-plane junction that was multiple layers thick. Much of the existing work on TMDC junctions uses monolayers stacked on top of each other; this is because, despite the strong theoretical performance of in-plane junctions, previous attempts could not realise the high hole and electron concentrations required to make a TFET work.

After demonstrating the robustness of their technique using molybdenum disulfide grown from tungsten diselenide, the researchers turned to niobium doped molybdenum disulfide, a p-type semiconductor. By growing out multi-layered structures of undoped molybdenum disulfide, an n-type semiconductor, the researchers realised a thick p-n junction between TMDCs with a high carrier concentration. Furthermore, they found that the junction showed a trend of negative differential resistance (NDR), where increases in voltage lead to less and less increased current, a key feature of tunnelling and a significant first step for these nanomaterials to make their way into TFETs.

The method employed by the team is also scalable over large areas, making it suitable for implementation during circuit fabrication. The researchers believe that this development could find its way into many applications for electronics in the future.

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