Session B28: Focus Session: Graphene II: Synthesis and Characterization

Optimizing Graphene Morphology on SiC(0001)

Many schemes to integrate graphene with microelectronics assume that reliable wafer-scale synthesis processes will be developed. One promising route to wafer-scale synthesis is to form graphene overlayers from the decomposition of SiC at high temperature. We have shown that, even at 1200 C, limited diffusion at the SiC surface leads to pit formation and a non-uniform graphene film thickness [1]. In this talk I will describe our efforts to improve both graphene domain size and thickness uniformity. One way we achieve this is by forming graphene in a background pressure of disilane, which hinders SiC decomposition. Even in rather low Si partial pressures (e.g. 1e-5 Torr), the SiC decomposition temperature can shifted several hundred degrees higher in temperature [2]. Using in situ low-energy electron microscopy (LEEM), we show that this effect can be exploited to form large graphene domains (larger than 10 um) with controlled layer thickness (e.g. 1 ML). Work performed in collaboration with R.M. Tromp. \\[4pt] [1] J.B. Hannon and R.M. Tromp, Phys. Rev. B77, 241404(R) 2008\\[0pt] [2] R.M. Tromp and J.B. Hannon, in press.   

Atmospheric pressure growth of graphene on SiC(0001)

Graphene, a single monolayer of sp$^{2}$-bonded carbon, is a very unique 2-dimensional electron gas system with electronic properties fundamentally different to other 2DEG systems [1]. Several production routes exist for graphene. Among them, the solid-state decomposition of hexagonal silicon carbide (SiC) surfaces [2] is particularly attractive for the development of graphene based electronics [3,4]. The first part of the presentation gives a brief summary of recent studies on the structural and electronic properties of graphene and few-layer graphene grown on SiC(0001) under ultra-high vacuum (UHV) conditions. The second part of the talk is devoted to recent progress in the growth of large domain graphene films on SiC(0001) in Ar atmosphere. It is shown that growth in Ar ambient leads to a significant improvement of the surface morphology and domain size as well as carrier mobility. \\[4pt] [1] A.H. Castro Neto, et al., Reviews of Modern Physics, in print (arXiv:0709.1163v2); and references therein. \\[0pt] [2] A. Charrier, et al., J. Appl. Phys. 92 (2002) 2479. \\[0pt] [3] C. Berger et al., J. Phys. Chem. B 108 (2004) 19912; C. Berger, et al., Science 312 (2006) 1191. \\[0pt] [4] A.K. Geim and K.S. Novoselov, Nature Mat. 6 (2007) 183.   

Centimeter scale pattern growth of graphene films for stretchable transparent electrodes

Large scale pattern growth of graphene is one of the most awaiting problems to be solved in order to bring this material for device application. Recently, macroscopic scale graphene films have been prepared by two-dimensional assembly of graphene sheets chemically derived from graphite crystals and graphene oxides. However, the sheet resistance of these films is found to be much larger than theoretically expected values. Here, we report the direct synthesis of centimeter-scale graphene films using chemical vapor deposition (CVD) on thin Ni layers, where the overall structures are connected by lateral electric connections. As a result, the transferred graphene films show very low sheet resistance with excellent optical transparency. At low temperatures, the single layers transferred on SiO$_{2}$ substrates show high electron mobility with the signature of quantum Hall effect, implying that the quality of CVD-grown graphene is as high as mechanically cleaved graphenes. Employing these outstanding mechanical properties of graphenes, we also demonstrate the macroscopic usage of the highly conducting and transparent electrodes for flexible/stretchable/foldable electronics.   

Fabrication and measurement of epitaxial graphene nanoribbons

Multi-layer graphene grown epitaxially on the C-terminated ($000\overline{1}$) surface of 4H-SiC in a low vacuum ($\sim 10^{-5}$ Torr), high temperature ($\sim$1420 $^{\circ}$C) induction furnace environment has been shown to be of extremely high quality and mobility. Due to its rotational stacking, the material exhibits electronic properties similar to those of isolated graphene sheets. Lithographic techniques, including electron beam lithography, are explored and sub-20 nm ribbon widths are demonstrated. Transport data for gated epitaxial graphene nanoribbons indicates quantum confinement. More than 100 nanoribbons on a single SiC chip are demonstrated, illuminating a technologically viable path towards graphene electronics.   

Self Assembly of Graphene Sheets

Chemically derived graphene sheets (GS) were found to self-assemble onto patterned gold structures via electrostatic interactions between noncovalent functional groups on GS and gold. This afforded arrays of single graphene sheets on substrates, characterized by Auger, Raman and scanning electron microscopy (SEM) imaging. Self assembly was used for the first time to produce on-substrate and fully-suspended graphene electrical devices. Molecular coatings on the GS were removed by high current ``electrical annealing,'' which recovered the high electrical conductance and Dirac point of the GS.   

Homoepitaxial Diamond Growth on Planar and Non-planar Substrates Using Carbon-13 Precursors

The growth of single crystal diamond by microwave plasma Chemical Vapor Deposition has been carried out on [100] oriented Type Ia natural diamond anvils as well as planar Type Ib synthetic diamond substrates. The effects of substrate geometry, concentrations of Carbon-13 gas precursors, nitrogen concentration in the plasma, and substrate temperatures on homoepitaxial diamond growth have been investigated. These results will be presented along with the observed changes in growth rate and surface morphology of the grown films with the variation of each parameter. We have also investigated nitrogen incorporation in diamond lattice by photoluminescence spectroscopy. Results obtained from the study on non-planar substrates have a direct impact on the growth chemistry used in the fabrication of designer diamond anvils for high pressure research.