May 20, 2014
Hybrid technology for 2D electronics
Researchers in the US have unveiled a new CMOS-compatible technology to integrate different two-dimensional materials into a single electronic device. The team, led by Tomás Palacios of the Massachusetts Institute of Technology, constructed large-scale electronic circuits based on graphene and molybdenum sulphide heterostructures grown by chemical vapour deposition where MoS2 was used as a transistor channel, and graphene as contact electrodes and circuit interconnects. The fabrication process itself might be extended to fabricate heterostructures from any type of 2D layered material with potential applications in flexible and transparent electronics, sensors, tunnelling FETs and high-electron mobility transistors.
2D materials are creating a flurry of interest in labs around the world because they have dramatically different electronic and mechanical properties from their 3D counterparts. This means that they could find use in a host of novel device applications, such as low-power electronic circuits, low-cost or flexible displays, sensors and even flexible electronics that can be coated onto a wide variety of surfaces.
The most well known 2D materials are graphene (which is a sheet of carbon just one atom thick) and the transition metal dichalcogenides. These so-called van der Waals materials have the chemical formula MX2, where M is a transition metal (such as Mo or W) and X is a chalcogen (such as S, Se and Te) and go from being indirect bandgap semiconductors in the bulk to direct bandgap semiconductors when scaled down to monolayers. The monolayers also efficiently absorb and emit light and so could be ideal for making a variety of optoelectronics devices such as light-emitting diodes and solar cells.
Separate and selective etching
Researchers have now, for the first time, combined both graphene and MoS2 heterostructures into single electronics devices and circuits thanks to a new technology that allows them to selectively and separately etch both 2D materials.
The team, which includes scientists from MIT, Harvard University and the United States Army Research Laboratory, began by growing large-area MoS2 and graphene using CVD. The researchers then patterned and etched MoS2 into an isolated channel. Next, patterned aluminium oxide was formed on this channel by low-temperature atomic layer deposition and lift-off techniques.
“The MoS2 is partially covered by the Al2O3 and this layer serves as both a dielectric layer as well as the etch-stop layer,” team members Lili Yu and Elton Santos told nanotechweb.org.
Large-area graphene sheets were then transferred onto the sample and the graphene cut by oxygen plasma into source, drain and gate electrodes while the MoS2 was protected by the Al2O3 layer.
Flexible and transparent electronics
The fabrication process could ultimately be used to construct heterostructures from any type of 2D layered material and circuits made from these heterostructures might find use in heterojunction devices, such as lasers, tunnelling FETs and high-electron mobility transistors. “And, since every component in the circuits is extremely thin, the finished devices are flexible and transparent, and so could find use in applications like wearable electronics or sensors that could be wallpapered and attached to any type of surface,” says Santos.
Yu adds that the team is now busy trying to integrate the 2D material hexagonal boron nitride into the structure it has made as a thin insulating layer. “We are also trying to create seamless graphene/MoS2 junctions,” she explained. “Some other applications for this type of hybrid junction, such as, for instance, photodetectors and memory devices are under investigation too.”
The current work is detailed in Nano Letters DOI: 10.1021/nl404795z.
About the author
Belle Dumé is a contributing editor to nanotechweb.org