Graphene has recently attracted enormous scientific interest for its promising electronic properties – comparable to those of carbon nanotubes (CNTs) - obtained from initial experiments and simulations. These include ballistic conductance at room temperature, current densities greatly exceeding those of current interconnects and carrier mobilities rivalling those of III-V devices at room temperature (~5000 cm2/Vs)
Graphene is a monolayer of carbon atoms in a dense honeycomb crystal structure. It can be considered a two-dimensional (2D) version of quasi one-dimensional (1D) carbon nanotubes and quasi zero-dimensional (0D) fullerenes. The structure of graphene with its sp² bonded carbon atoms is shown in a schematic in Fig. 1. Graphene is a semi-metal with a tiny overlap between the valence and the conduction band (zero-gap material).
- Schematic of a graphene layer
Until very recently, graphene has been known to exist only in the 1D or 0D form, or even better known in its 3D structure as graphite, which consists of graphene sheets with strong in-plane bonds and weak van der Waals-like coupling between layers. It was presumed, that a single 2D graphene sheet would be thermodynamically unstable. Novoselov et al. have now demonstrated that it is indeed possible to realize stable single and few layer graphene (FLG) sheets [1][4]. In their work, the researchers report excellent carrier mobilities between 3.000 and 10.000 cm²/Vs. They go on to describe a mean free path of L = 400 nm at room temperature which translates to ballistic transport at feature sizes typical in production today. They have further observed Shubnikov-de Haas oscillations in longitudinal direction and Hall resistivity of FLG and deduct pure 2D electronic transport. Finally, they have attributed the transport properties to a single spatially quantized subband that could be either populated by electrons with a mass of me= 0.06 m0 or by light and heavy holes with masses of mh=0.03 m0 and mh= 0.1 m0 . A further particularity arises from graphene’s linear dispersion relation: the energy is proportional to momentum rather than to the square of the momentum as in a normal system [5].
These exceptional properties make graphene an extremely attractive material candidate to overcome the limitations of conventional Si-based devices and for applications in electronics “Beyond CMOS”. A wide field of possible applications such as interconnects [3] and active devices [1][2][6][7] could be covered by graphene.
- Optical image of graphene layers
Novoselov et al. have used simple manual cleaving processes to obtain single and few layer graphene (Fig. 2), involving adhesive tape and manually rubbing graphite layers against appropriate surfaces like silicon dioxide [1][4]. This simple yet practical method has allowed them to extract the extremely promising electronic properties of graphene cited above and alert the scientific community to their findings.
Zhang et al. have recently suggested a refined approach: Here, microscale pillars of graphite have been formed using a plasma etch process [8]. These pillars have then been attached to a silicon cantilever, where they form the tip of an atomic force microscope (AFM). Finally, thin graphene layers have been sheared off onto an oxidized silicon wafer. This method is considerably more reproducible, because the downward pressure and shearing can be controlled efficiently. It is, however, based on AFM and therefore not able to deliver large areas or even wafers with single layer graphene nor a high throughput of samples.
Both of these methods involve random deposition of flakes onto target substrates. To our knowledge, no routes for accurate placement of single graphene sheets have been reported. This represents a substantial bottleneck for future applications, since selection of suitable flakes requires a substantial amount of inspection time using optical and often also scanning electron microscopy techniques. Further, this lack of suitable registration and alignment to the substrate means that only single devices can be fabricated on a given die or wafer, thus hindering statistical quantification of the effects of fabrication and/or processing conditions on device characteristics.
Berger et al. have taken an epitaxial approach to fabricate graphene made up of only three monolayers [9]. In their method, silicon carbide (6H-SiC) has been used as the basic material. Thermal desorption of silicon at temperatures between 1250°C and 1450°C has then been employed to form few layer graphene. This process can surely be classified controllable and industrially relevant [10]. A major disadvantage, however, is the extremely high cost of production grade SiC wafers and their size with a maximum available diameter of 100 mm.
AMO is working on the Graphene topic within a BMBF supported NanoFutur project ALEGRA as well as in a European research project called GRAND (Graphene-based Nanoelectronic Devices) in cooperation with leading European academic and industrial partners and is coordinating several German and European projects in the field of nano-manufacturing and nano-electronics. The main assets are AMO’s initial experience with graphene devices in conjunction with its experimental CMOS technology for nano-scale devices and novel materials, its advanced high resolution e-beam lithography (Leica EBPG5000) with direct write capabilities below 10 nm and its highly innovative UV-Nanoimprint lithography for fast cycle nanoscale research.
[1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, “Electric Field Effect in Atomically Thin Carbon Films”, Science, 306:666-669 , 22 October 2004.
[2] M.C. Lemme, T.J. Echtermeyer, M. Baus, H. Kurz, “A Graphene Field Effect Device”, IEEE Electron Device Letters, 28(4):282-284, April 2007.
[3] A. Naeemi, J.D. Meindl, “Conductance Modeling for Graphene Nanoribbon (GNR) Interconnects“, IEEE Electron Device Letters, 28(5):428-431, May 2007
[4] K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V.V. Khotkevich, S.V. Morozov, A.K. Geim, “Two-dimensional atomic crystals”, PNAS, 102(30):10451-10453, July 26 2005.
[5] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I..V. Grigorieva, S.V. Dubonos , A.A. Firsov, “Two-dimensional gas of massless Dirac fermions in graphene”, Nature 438:197-200, 10 November 2005.
[6] B. Obradovic, R. Kotlyar, F. Heinz, P. Matagne, T. Rakshit, M. D. Giles, M. A. Stettlera, D. E. Nikonov, “Analysis of graphene nanoribbons as a channel material for field-effect transistor”, Appl. Phys. Lett., 88(17), 142106, 2006.
[7] A.K. Geim, K.S. Novoselov, “The Rise of Graphene”, Nature Materials, 6, 183-191, 2007.
[8] Y. Zhang, J.P. Small, W.V. Pontius, P. Kim, “Fabrication and electric-field-dependent transport measurements of mesoscopic graphite devices”, Appl. Phys. Lett. 86, 073104, 2005.
[9] C. Berger, Z. Song, T. Li, X. Li, A.Y. Ogbazghi, R. Feng, Z. Dai, A.N. Marchenkov, E.H. Conrad, P.N. First, W.A. de Heer, “Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics”, J. Phys. Chem. B., 108:19912, 2004.
[10] W. DeHeer, C. Berger, P. First "Patterned Thin Film Graphite Devices and Method for Making Same", WO 2005/001895 A2.
