Feb 25, 2014
Graphene research moves towards devices
Graphene’s superlative properties have been making headlines for the past 10 years, and yet scientists and engineers are still striving to find the first real-world application to significantly benefit from graphene’s "wonder material" properties. This drive towards practical and profitable applications is now attracting serious investment, such as the EU’s one billion Euro "Graphene Flagship", which has the aim of translating graphene research from academia to society within just 10 years. The intense interest in this field has now prompted Journal of Physics D to publish a special issue entitled "Graphene: from functionalization to devices".
Guest editors Antonio Tejeda (Institut Jean Lamour/Synchrotron SOLEIL) and Patrick Soukiassian (CEA-Saclay/U.Paris Sud) in France write in their editorial for the special issue: “The year 2014 marks the first decade of the rise of graphene.” Tejeda and Soukiassian go on to highlight a number of graphene’s extraordinary attributes that hold promise for device developments, including a very high carrier mobility and diffusion length, unsurpassed mechanical strength, and an exceptional thermal conductivity scaling more than an order of magnitude greater than that of copper.
“After the first years of the graphene rush, graphene growth is now well controlled using various methods like epitaxial growth on silicon carbide substrate, chemical vapour deposition or plasma techniques on metal, insulator or semiconductor substrates,” they explain. Although fabrication challenges still remain and studies of graphene’s fundamental properties continue to suggest new opportunities, research into applications for graphene in real devices is now increasingly taking over from the initial studies on graphene production.
Graphene electronics demand integrated solutions
Electronics is one of the areas that has most animated the imaginations of scientists in applied graphene research. Here, compatibility with existing technology is key. Reporting in the special issue, Walt de Heer and colleagues at Georgia Institute of Technology in the US and CNRS-Institut Néel in France introduce a strategy for integrating graphene electronics with standard silicon CMOS technology, and in a way that requires neither graphene transfer nor patterning.
The team apply a thin monocrystalline silicon layer ready for CMOS processing on top of a layer of so-called epitaxial graphene – in other words, graphene grown on a perfect crystalline substrate, in this case silicon carbide. As they point out: “This method, inspired by the industrial development of three-dimensional hyper-integration stacking thin-film electronic devices, preserves the advantages of epitaxial graphene and enables the full spectrum of CMOS processing.”
Lateral thinking yields semiconductor success
A major challenge for graphene electronics research is that graphene doesn’t have a bandgap, which means that device elements made from graphene cannot perform the functions of basic semiconductor structures such as transistors or diodes. A number of approaches to engineering a bandgap have been explored, but these tend to compromise other characteristics of the system, such as lowering the carrier mobility or screening the gate potential. Traditional parallel graphene transistor structures also lack current saturation, which is crucial for both digital and analogue devices.
A team of scientists in Russia and Japan have instead devised a lateral design for a graphene-based transistor, which they believe addresses the shortcomings of parallel device structures. Dmitry Svintsov and colleagues at the Russian Academy of Sciences, the Moscow Institute of Physics and Technology, and Tohoku University describe in the special issue how coplanar graphene layers can serve as source and drain separated by a short tunnel gap. “The gates placed above and below the gap efficiently control both charge density in the graphene layers and the height of barrier,” they write in the special issue. Simulations of their lateral graphene-based tunnel field-effect transistor demonstrate how it combines a high carrier mobility, which allows high-frequency performance, with a superior on/off ratio and current saturation.
Graphene takes to the microwaves
Meanwhile, graphene’s high mobility makes it attractive for applications requiring classical microwave electronics, such as radio-frequency transistors. However, the semi-metallic nature of graphene limits the power gain and hence the performance of these devices. In the special issue Bernard Plaçais and colleagues at Ecole Normale Supérieure and Institut d’Electronique, de Microélectronique et de Nanotechnologie in France instead investigate the use of graphene nano-field-effect transistors for fast charge/current converters, which are not hampered by low power gain.
The researchers deposit graphene flakes onto a high-resistivity silicon oxide substrate. The flakes are produced by exfoliation, similar to the approach taken by graphene Nobel prize winners Novoselov and Geim, who used a piece of scotch tape to remove a single top layer of graphene from graphite. Plaçais and colleagues then define source and drain electrodes and a coplanar waveguide using lithography, and prepare the gate oxide by evaporating 2 nm of aluminium that is then exposed to air.
As the researchers explain in their report, their ultimate goal is on-the-fly detection of single electrons, which requires a single-gate transistor. As a result their design differs from the symmetric double-gated design typically used for radio-frequency transistors. “Our paper deals with high-speed graphene devices that can be used as sensitive and ultrafast charge detectors,” says Plaçais. “We present radio-frequency transport measurements on a nanotransistor showing a charge resolution of 180 µe/Sqrt(Hz).”
In addition to device research, the special issue covers electronic and transport properties, graphene tailoring and functionalization, as well as growth and morphology. All 17 articles in the special issue are free to read until 12 May 2014.
About the author
Anna Demming is online editor of nanotechweb.org.