Semiconductor technology has taken us a long way by making devices of ever smaller size. But eventually, fundamental physical barriers will pose a huge challenge for further shrinking of present silicon-based electronic devices, such as quantum and coherence effects (e.g., quantum tunneling of carriers through the gate insulator and through the body-to-drain junction of field-effect transistors is a highly undesirable effect), high electric fields creating avalanche dielectric breakdowns, and non-uniformity of dopant atoms and the relevance of single atom defects. The most important limitation is set by the power dissipation through various leakage mechanisms that are especially dangerous for minimal field-effect transistor (FET) dimensions and oxide thickness.
The aim of nanoelectronics is to process, transmit and store information by taking advantage of properties of matter that are distinctly different from macroscopic properties. In the context of electrical conduction, the key quantity that characterizes nanoscale systems is the current density (current pre unit area), which is typically orders of magnitude larger than those found in mesoscopic and macroscopic systems.
What will then be the form of future nanoelectronic devices? Can quantum mechanics be used to control device operation? And can they operate at reasonable temperatures? Nanoscale transistors made from graphene may provide ways to address these questions.
Graphene, a one-atom-thick crystal of carbon atoms, is an unusually simple material with startling new properties (for popular introduction see SciAm article by experimental physicists who invented it). Graphite, the common material used in most pencils, is made up of countless layers of graphene. The electrons in graphene behave as (charged) neutrinos of high energy physics, obeying an equation that resembles the relativistic Dirac equation, moving freely through barriers created by imperfections, and they show quantum effects at room temperature.
Besides offering a novel playground to test electron transport and interactions in low-dimensional systems, as well as quantum electrodynamics, the surprising discovery of graphene in 2004 has been accompanied from the outset with numerous efforts to fabricate field-effect transistors (FET). Such carbon nanoelectronic devices utilize "bulk" graphene or graphene nanoribbons (rather than usual carbon nanotubes) as a new FET channel while the gate electrodes control the transport between the source and the drain electrodes attached to such channel.
However, the bulk graphene is a zero-gap semiconductor which has made possible to reach only very small ON/OFF current ratios (∼ 10 ) in early and recent graphene FET (GFET) devices fabricated using micron-wide sheets. Thus, it has been considered that GFETs would be most useful for analogue and high-frequency circuit applications (IBM team has recently demonstrated GFET operating frequency reaching ~ 100 GHz) , as reviewed in recent Nature Nanotechnology article.
The recent experiments have finally succeded in fabrication of very narrow graphene nanoribbons (GNR) with ultrasmooth edges, demonstrating that all sub-10-nm wide GNRs are semiconducting with large band gaps. This has made possible ON/OFF current ratios up to ∼106 in top-gated GNRFETs operating at room temperature, which makes such devices suitable for digital electronics applications.
The next step is to demonstrate ability to fabricate all-graphene FETs that can be carved from a single or few graphene wafers. This direction of research requires substantial modeling that can accurately take into account unusual electronic structure of graphene offering possilibty for devices that have no analog in silicond-based electronics, quantum-chemical description of atomic scale geometry, and charge redistribution under far-from-equilibrium transport conditions.
The so-called nonequilibrium Green function method coupled with the density functional theory (NEGF-DFT) plays an important role to solve this quantum transport problem, on the provios that we can extend it to devices containing thousands of atoms and multiple graphene layers or many current, voltage, and gate electrodes.
- first-principles computation of transport properties of graphene nanoelectronic devices composed of thousands of atoms,
- molecular electronic devices with graphene nanoribbons as electrodes,
- noise in graphene devices,
- magnetism in zigzag graphene nanoribbons,
- graphene-based spintronic devices.
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D. A. Areshkin and B. K. Nikolic, Electron density and transport in top-gated graphene nanoribbon devices: First principles Green function algorithms for systems containing large number of atoms, Phys. Rev. B 81, 155450 (2010). [PDF]
D. A. Areshkin and B. K. Nikolic, I-V curve signatures of nonequilibrium-driven band gap collapse in magnetically ordered zigzag graphene nanoribbon two-terminal devices, Phys. Rev. B 79, 205430 (2009). [PDF]
R. L. Dragomirova, D. A. Areshkin, and B. K. Nikolic, Shot noise probing of magnetic ordering in zigzag graphene nanoribbons, Phys. Rev. B 79, 241401(R) (2009). [PDF]
L. P. Zarbo and B. K. Nikolic, Spatial distribution of local currents of massless Dirac fermions in quantum transport through graphene nanoribbons, Europhys. Lett. 80, 47001 (2007). [PDF]
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