From Sheets To Stacks, New Nanostructures Promise Leap For Advanced Electronics
Scientists from Tokyo Metropolitan University have successfully 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. They 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 nearly every digital circuit. They control the passage of current through it depending on the voltage which is put across. While metal oxide semiconductor FETs (or MOSFETs) form the majority of FETs in use today, the search is on for the next generation of materials to drive increasingly demanding and compact devices using less power. This is where tunneling FETs (or TFETs) come in. TFETs rely on quantum tunneling, an effect where electrons are able to pass usually impassable barriers due to quantum mechanical effects. Though TFETs use much less energy and have long been proposed as a promising alternative to traditional FETs, scientists are yet to come up with a way of implementing the technology in a scalable form.
A team of scientists of Tokyo Metropolitan University led by Associate Professor Yasumitsu Miyata has 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 excellent candidate materials for creating TFETs. Their recent successes have allowed them to stitch together single-atom thick layers of crystalline TMDC sheets over unprecedented lengths. Now, they have turned their attention to multi-layered structures of TMDCs.
By using a chemical vapor deposition (CVD) technique, they 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 use monolayers stacked on top of each other; this is because, despite the superb theoretical performance of in-plane junctions, previous attempts could not realize 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, they turned their attention to niobium doped molybdenum disulfide, a p-type semiconductor. By growing out multi-layered structures of undoped molybdenum disulfide, an n-type semiconductor, the team realized a thick p-n junction between TMDCs with unprecedentedly 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 tunneling 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. This is an exciting new development for modern electronics, with hopes that it will find its way into applications in the future.