Andy Pye takes a look at black phosphorus and how it could present a challenge to silicon for packing more transistors on a chip.
Silicon Valley in Northern California got its nickname from the multitude of computer chip manufacturers that sprung up in the surrounding area in the 1980s. But silicon may be facing some competition.
In 2004, physicists at the University of Manchester first isolated and explored the remarkable properties of graphene – a one-atom-thick layer of carbon. Since then, scientists have rushed to investigate a range of other two-dimensional materials. One of those is black phosphorus, otherwise known as phosphorene, 2D phosphane or simply BP. It is a form of phosphorus that is similar to graphite and can be separated easily into single atomic layers.
In July, Graphene Week 2015 was buzzing with activity throughout the University of Manchester venue, with more than 600 delegates presenting and discussing their work, networking and making plans for the future. Switching attention from graphene to other 2D materials, Yuanbo Zhang from the Institute of Physics at the Chinese Academy of Sciences spoke of his investigations into black phosphorus and the metallic compound tantalum disulphide.
Unlike graphene, which acts like a metal, black phosphorus is a natural semiconductor, which can be readily switched on and off. In terms of its electronic band gap it occupies a space between zero-gap graphene and the semiconducting 2D materials molybdenum disulphide, molybdenum diselenide and tungsten diselenide.
“Transistors work more efficiently when they are thin, with electrons moving in only two dimensions,” says Thomas Szkopek, an associate professor in McGill’s Department of Electrical and Computer Engineering.
Furthermore, the interaction with light of BP depends on the number of atomic layers used. One monolayer will emit red light, whereas a thicker sample will emit into the infrared. This variation makes it possible to manufacture a wide range of optoelectronic devices, such as lasers or detectors, in a strategic fraction of the electromagnetic spectrum.
“To lower the operating voltage of transistors, and thereby reduce the heat they generate, we have to get closer and closer to designing the transistor at the atomic level,” Szkopek says. “The toolbox of the future for transistor designers will require a variety of atomic-layered materials: an ideal semiconductor, an ideal metal, and an ideal dielectric. All three components must be optimised for a well-designed transistor. Black phosphorus fills the semiconducting-material role.”
A significant problem with phosphorene is that the material is unstable in air, with significant degradation after only a few days. It therefore needs to be shielded from the environment and electronic interactions with substrate materials.
A research team from Université de Montréal, Polytechnique Montréal and the Centre National de la Recherche Scientifique (CNRS) in France is the first to succeed in preventing two-dimensional layers of black phosphorus from oxidising.
Meanwhile, researchers working at the Institute for Basic Science (IBS) Center for Integrated Nanostructure Physics at Sungkyunkwan University (SKKU) in South Korea have created a high performance transistor using black phosphorus (BP) which has revealed some fascinating results.
While silicon has to be extrinsically doped (inserting another element into its crystal structure) to make it n-type or p-type in order for it to work in a semiconductor chip, the BP crystals can operate as both n-type and p-type or something in between, without extrinsic doping.
“The driving force in black phosphorus is the carrier mobility. Everything centres around that. The fact that the band gap changes with thickness also gives us flexibility in circuit design,” explains SKKU research fellow David Perello.
Currently, there isn’t a good method for making pure BP on a large scale. Currently, thin layers can be made only from scraping bulk crystalline BP samples. “I don’t think it can compete with silicon at the moment, but that’s a dream everybody has,” Perello says. “Silicon is cheap and plentiful and the best silicon transistors we can make have mobilities that are similar to what I am currently able to make in these BP devices.”
Two-dimensional magnetotransport in a black phosphorus naked quantum well, V. Tayari et al, published online in Nature Communications, July 7, 2015. DOI: 10.1038/ncomms8702