Nanoelectronic devices based on graphene for high frequency applications

Project: "Nanoelectronic devices based on grapheme for high frequency applications"
Programmme: PNII IDEAS Project;
Period: 2011-2014;
Project manager: Dr. Mircea Dragoman

Graphene is a graphite monolayer with a thickness of only 0.34 nm. It is formed from carbon atoms in a sp2 hybridization state, arranged such that each carbon atom is covalently bonded to three others. So, graphene is a planar nanomaterial with a honeycomb lattice formed from two inter-penetrating triangular sub-lattices [1]. Graphene is encountered in many carbon-based materials. A simple example is graphite, which is formed by a very large number of staked graphene monolayers. Another example is the single wall carbon nanotube, which is simply a rolled-up graphene sheet. Graphene is a two-dimensional (2D) crystal, and a native 2D gas of charged particles. In many devices, graphene is deposited on a SiO2 layer with a typical thickness of 300 nm, which is grown over a doped silicon substrate. In this configuration, the Si substrate acts as a backgate, which shifts the Fermi energy level in graphene and produces a surface charge density depending directly of gate voltage i.e. .The gate-induced carriers can be seen as resulting from an electrical doping, analogous to the chemical doping typical for semiconductor devices. So, in graphene the electrons and holes are electrically induced by applying a positive or a negative voltage on a gate. After graphene deposition on the Si/SiO2 structure, electrodes are patterned on graphene to implement certain devices. Graphene was isolated for the first time in 2004 using the mechanical ex-foliation of highly-ordered pyrolytic graphite (HOPG) with an adhesive tape, followed by the release of the graphene flake on Si/SiO2 after tape removal [2]. HOPG is a stacked 3D structure consisting of vertically arranged graphene sheets. Fragments of peeled HOPG are engineered to fall directly on the Si/SiO2 interface and are immobilized on the Si/SiO2 due to Van der Waals forces. This appears to be a rudimentary way to get graphene, but even now it is successfully applied to obtain graphene flakes with dimensions up to 1 mm having a low defect density. The fabrication method described above was accompanied by another important discovery regarding the visibility of graphene [3]. Graphene is visible with an optical microscope if the incident white light is filtered by a green, blue or another colored filter depending on the thickness of SiO2 [4]. In these conditions, graphene was seen using a simple microscope, as can be seen from the figure below. Although there are, presently, a multitude of methods to visualize graphene [5], graphene monolayer and other nanostructures formed by successive layers of graphene, such as bilayer, trilayer, or multiple layers of graphene, are quite well distinguished with a microscope using optical reflection and optical contrast spectroscopy, and even their thicknesses can be determined with high accuracy [6].

Optical images of monolayer and bilayer graphene

Graphene is a 6 years old material, but applications were rapidly developed in the area graphene nanolectronics such as  transistors ,solar cells, or sensing (e.g. touching screens). [7],[8], [9].The reason why so many applications are targeted by graphene is twofold. First the fact that graphene displays impressive physical properties (carrier mobilities which are at least  an order of magnitude grater than Si and which can reach 106 cm2/Vs in suspended graphenes or graphene deposited on hexagonal BN, mean free path at room temperature grater than 0.3 mm, huge Young modulus grater than 1TPa-the strongest material ever known, bending  of 15-20% making it very attractive for flexible electronics and NEMS) and secondly the fact that processing of graphene planar and it is compatible with the usual processing techniques based on lithography and semiconductor clean room technology. Due to these properties,  here are special programs dedicated  to graphene termed also as “the wonder material” (UK, US, Germany, Spain, South Korea etc.) , big companies such as IBM, Bosch, Nokia, Thales have started to work for graphene devices and EU will launch a flagship program on graphene in 2013 with a huge budget. So, this project could lead at impressive developments regarding cooperation and international projects. Therefore, the main aim of this project is to explore graphene applications in nanoelectronics at high frequencies to connect the graphene physics to communications actual issues, an area which is very poorly treated up to now. The project will explore the graphene devices for ultrafast communications beyond 40 GHz. Similar to the famous Moore law, the Edholm law states that the need for higher bandwidths in wireless communications will double every 18 months. Today, for the wireless LANs, the carrier  frequencies are around  5 GHz and the data  rates are 110-200 Mb/s. However, according to Edholm law, wireless data rates around 1-5 Gb/s are required in few years from now. This means that the carrier frequencies for wireless communications should become higher than 60 GHz. However, in this bandwidth the devices and circuits able to form a wireless link at room temperature are very scarce. This limitation is due to relative high charge scattering rate and relative low mobilities encountered in all semiconductors at room temperature. So, in few years the ever increasing demand for ultrafast wireless communications will not be fully satisfied using the existing semiconducting technologies.  To solve this expected bottleneck , we propose a radical solution which consists in using other materials and circuit configurations to fulfill the clear tendencies indicated by Edholm law. More specifically, we intend to design, fabricate and test  novel miniaturized devices such as coplanar waveguides, multipliers and detectors which work beyond 60 GHz and forming the basic emitting/receiving key components of a communication link and all based on graphene which has electrical ,thermal and mechanical properties outperforming any known material.  Our initial results are the basis of these innovative devices.

[1]A.K Geim,K.S. Novoselov, The rise of graphene,  Nature Material. 6 , no.3, pp. 183-191, March 2007.
[2] K.S. Novoselov, A.K. Geim, S.V. Morozov, D.Jiang, V.Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306, no. 3696, pp. 666-669  (2004).
[3] P. Blake, E.W. Hill,  A.H. Castro Neto,  K.S. Novoselov, , D. Jiang, R. Yang, T.J. Booth, A.K. Geim, Making graphene visible, Appl. Phys. Lett. 91, no.6,  pp. 063124/1-3 (2007).
[4] Abergel, D.S., A. Russel, and V.I. Fal’ko, Visibility of a graphene flake on a dielectric substrate, Appl. Phys. Lett. , Vol. 91, no. 6, pp. 063125(2007).
[5] J.Kim, F.Kim, and J. Huang, “Seeing graphite-based sheets”, Materials Today, vol. 13, no.3, pp.28-38, (2010).
[6] Z.H. Ni, H.M. Wang, J. Kasim, H.M.Fan, T.Yu, Y.H. Wu, Y.P.Feng, and Z.X. Shen, Graphene thickness determination using reflection and contrast spectroscopy, Nano Letters, vol.7, pp 2758–2763 (2007).
[7] M.Dragoman, D.Dragoman, Graphene-based quantum electronics, Progr. Quantum Electronics, 33, no.6, pp.165-214, ( 2009)
[8] F.Schwierz, Graphene Transistors, Nature Nanotechnology, vol. 5,487-496 (2010).
[9] F. Bonaccorso, Z. Sun, T. Hasan, A. C. Ferrari, Graphene photonics and optoelectronics, Nature Photonics 4, 611 - 622 (2010).