Next-generation semiconductors

Materials World magazine
6 Aug 2019

Producing sharp-edged nanocrystals could help bring about the next stage in semiconductors, for advanced electronics. Ceri Jones reports.

Atomically thin crystals of transition metal dichalcogenides (TMDC) have been grown continuously with sharp interfaces, with the goal of making advanced electronics for future engineering applications.

In-plane connecting of any transition metal plus a chalcogen enables them to be produced in strips or stacked in layers to form a heterostructure – a semiconductor proven to change its chemical properties when the position of the metal parts is changed. Using this principle, a normal metal can go from inert to a semi or even superconductor.

Therefore, it is desirable to create a single TMDC heterostructure with specific domains of different metals and/or patterns to produce high-performance 2D electronics. Achieving this requires perfectly defined interfaces between the materials, but existing methods often lead to contamination or alloying.

The team at Tokyo Metropolitan University, Japan, led by Dr Yu Kobayashi and Associate Professor Yasmitsu Miyata, has developed a technique that refined the production process, making significantly improved semiconductors. Their continuous feeding process creates better interfaces that can confine electrons to specific spaces on the device, enabling precision control of electron transport and resistivity. This was demonstrated with 20nm-thin strips of crystalline structures with different compositions.

High definition

The team previously developed semiconductors using vapour-phase deposition, where the precursor is a vapour, enabling deposition onto a surface of flat, crystalline layers. This approach offered little control of the crystal growth, so performance of the heterostructures was affected by the varying band gap widths between the different material domains.

By switching to liquid precursors, the researchers gained greater control of the supply feeding process and produced sharp, regular interfaces for each TMDC domain. Metal organic liquid precursors in a growth chamber were fed TMDCs one after another, so the composition structure and pattern of monolayers could be controlled, and therefore the crystal growth, and each domain linked via an atomically precise edge.

‘This versatile process can avoid air exposure during the growth process and enables the formation of in-plane heterostructures with ultraclean atomically sharp and zigzag-edge straight junctions without defects or alloy formation around the interface,’ the team said in the paper. ‘For the samples grown directly on graphite, we have investigated the local electronic density of states of atomically sharp heterointerface by scanning tunnelling microscopy and spectroscopy, together with first-principles calculations. These results demonstrate an approach to realising diverse nanostructures such as atomic layer-based quantum wires and superlattices and suggest advanced applications in the fields of electronics and optoelectronics.’

Four TMDCs – tungsten disulphide, tungsten diselenide, molybdenum sulphide and molybdenum diselenide – were grown on silicon and silicon oxide, both with and without sodium chloride. Scanning tunnelling microscopy was then used to examine the results. The team reported, ‘excellent agreement with first-principles numerical simulations of what an ideal interface should look like’. The team is aiming to achieve ‘unparalleled’ energy efficiency as well as optical and resistivity properties from further research.

Read more in the paper, Continuous heteroepitaxy of two-dimensional heterostructures based on layered chalcogenides, published in ACS Nano here: