Graphene Makes a Comeback in Electronics
With other 2-D materials having an intrinsic band gap, graphene was losing ground in electronic applications until this year.
Two significant achievements in the application of graphene to digital electronics have marked the first quarter of 2014. In one, we have been given the most advanced realization of engineering a band gap into graphene, and in the other we have seen an avenue for realizing the use of graphene in electronics without engineering a band gap into it.
The lack of inherent band gap has been the Achilles Heel of graphene in electronic applications. A band gap gives a material the ability to start and stop the flow of electrons, which is essential to the functioning of binary digital electronics. Other 2-D materials, such as molybdenum disulfide (MoS2), do possess intrinsic band gaps, which had been giving the material an advantage in research applying graphene to logic applications over the last couple of years.
Other than the lack of a band gap (a rather large failing for a material to be used for electronics) graphene has had such attractive properties it just couldn’t be resisted. Researchers continued to investigate whether it could serve as a replacement for silicon. For electronics, graphene possessed all the benefits of carbon nanotubes (CNTs), namely its charged-carrier mobility, but it didn’t have any of the down sides, such as CNTs’ need for different processing techniques from silicon and the intrinsic difficulty of creating interconnects for CNTs.
The Quest for a Band Gap in Graphene
So almost from the moment graphene was fabricated in the lab back in 2004, researchers have been trying to engineer a band gap into it. Two electronics heavyweights, IBM and Samsung, have been the leading commercial research institutes pursuing this effort.
IBM has continued to pursue graphene in electronic applications since it engineered a band gap into the material back in 2010. In that same year, they were able to fabricate a transistor that was twice as fast as one made from silicon. And then in 2011, they were the first to successfully build an integrated circuit (IC) based on a graphene transistor.
This background has led to their latest achievement, which IBM describes as the best graphene-based IC built to date with 10,000 times better performance than anything produced to date. They even managed to get the IC to work as a wireless transmitter to send the text message “IBM”.
“This is the first time that someone has shown graphene devices and circuits to perform modern wireless communication functions comparable to silicon technology,” IBM Research director of physical sciences Supratik Guha said in a release.
IBM achieved the high performance of the IC by improving the manufacturing process, which entailed adding the graphene a little later in the process. Despite this manufacturing improvement, it seems the researchers used graphene sheets produced by mechanical cleavage (the “Scotch Tape” method in which flakes of graphene are pulled off graphite), which produces the best quality graphene but would be impossible to produce in a scaled-up method.
If less costly methods can be developed to manufacture graphene with that level of quality but with some built-in scalability, it should be interesting to see if this could really lead to a viable approach to mobile electronics.
Could Graphene Electronics Be Possible Without a Band Gap?
In more fundamental research—but perhaps no less significant for the commercial aspirations of graphene in electronic applications—an international team of researchers discovered the ballistic transport of graphene (the speeds at which electrons flow through a material at room temperature) were ten times faster than previous theoretical limits.
To achieve these record-breaking speeds, the researchers used graphene nanoribbons. Graphene nanoribbons have been investigated as a method for engineering a band gap into the material. If you reduce the dimensionality of graphene from two dimensions down to one dimension, you can confine the electrons enough that a band gap is created but not sufficient enough for electronics applications.
In this latest research, which was conducted by an international team from the Georgia Institute of Technology along with others from Leibniz Universität Hannover in Germany, the Centre National de la Recherche Scientifique (CNRS) in France and Oak Ridge National Laboratory in the United States, the aim was not to produce a band gap with graphene nanoribbons, but just to get the material down on a silicon. Their findings were published in the journal Nature (“Exceptional ballistic transport in epitaxial graphene nanoribbons”).
It was after they got the material down on the wafer and started to measure its properties that they discovered that they had shattered the previous theoretical limits of graphene’s ballistic transport.
As significant as this was and what it means to the fundamental understanding of graphene, it was comments made by Walt de Heer, a Regent's professor in the School of Physics at the Georgia Institute of Technology, that really made news with this development. He suggested that this work pointed to a new paradigm for electronics that didn’t need to base itself on how silicon operates.
"This work shows that we can control graphene electrons in very different ways because the properties are really exceptional," said de Heer, in a press release. "This could result in a new class of coherent electronic devices based on room temperature ballistic transport in graphene. Such devices would be very different from what we make today in silicon."
One of the unusual properties that they discovered in the graphene nanoribbons, beyond their ballistic transport speeds, was that if you attempted to measure the flow of electrons by applying an electrical probe to the material, you would interrupt the flow. This effectively was a way to make the electrons stop and then go, more or less how a band gap in a semiconductor like silicon is used in digital logic applications.
“This should enable a new way of doing electronics," de Heer said in the release. "We are already able to steer these electrons and we can switch them using rudimentary means. We can put a roadblock, and then open it up again. New kinds of switches for this material are now on the horizon."
Of course, this is an exciting but somewhat controversial claim and other scientists were quick to express skepticism.
In Nature, soon after the original research was published, a follow-up article appeared in which Antonio Castro Neto, director of the National University of Singapore’s Graphene Research Centre, suggested that disorder caused by imperfections in the nanoribbons would slow down the rapid conduction of the electrons. "It’s unavoidable. Unfortunately, graphene is not the material one should use for digital applications," said Neto in the Nature piece.
For the time being, Neto would appear to be right. The band gap that the IBM team developed and are using in their transistors and ICs is really only suitable for graphene optoelectronics, such as IR and THz detectors and emitters. This is evidenced by their latest use of their graphene IC as a radio frequency receiver for performing signal amplification, filtering and mixing. That’s a far cry from performing digital logic functions.
The work on graphene nanoribbons suggests a possible way forward in applying graphene to digital electronics, but the development is at such an early stage for this avenue—and with intrinsic problems already identified—it is at best a speculative approach. Nonetheless silicon is running out of breathing room as Moore’s Law continues it unstoppable march and an alternative material will need to be found. Graphene was becoming sidelined as a potential alternative, but it may just have found itself back in the game thanks to this recent research.