Graphene on Carbon-face SiC{0001} Surfaces Formed in a Disilane Environment

<p>The formation of epitaxial graphene on SiC(000-1) in a disilane environment is studied. The higher graphitization temperature required, compared to formation in vacuum, results in more homogeneous thin films of graphene. Some areas of the surface display unique electron reflectivity curves not seen in vacuum-prepared samples. Using selected area diffraction, these areas are found to have a graphene/SiC interface structure with a graphene-like buffer layer [analogous to what occurs on SiC(0001) surfaces].</p

Formation of Graphene on SiC( 0001 ) Surfaces in Disilane and Neon Environments

<p>The formation of graphene on the SiC(000) surface (the <em>C-face</em> of the {0001} surfaces) has been studied, utilizing both disilane and neon environments. In both cases, the interface between the graphene and the SiC is found to be different than for graphene formation in vacuum. A complex low-energy electron diffraction pattern with √43 × √43-<em>R</em>  ± 7.6° symmetry is found to form at the interface. An interface layer consisting essentially of graphene is observed, and it is argued that the manner in which this layer covalently bonds to the underlying SiC produces the √43 × √43-R ± 7.6° structure [i.e., analogous to the 6√3 × 6√3-<em>R</em>30° “buffer layer” that forms on the SiC(0001) surface (the <em>Si-face</em>)]. Oxidation of the surface is found to modify (eliminate) the √43 × √43-<em>R</em> ± 7.6° structure, which is interpreted in the same manner as the known “decoupling” that occurs for the Si-face buffer layer.</p

Comparison of Graphene Formation on C-face and Si-face SiC {0001} Surfaces

The morphology of graphene formed on the (0001̅ ) surface (the C-face) and the (0001) surface (the Si-face) of SiC, by annealing in ultrahigh vacuum or in an argon environment, is studied by atomic force microscopy and low-energy electron microscopy. The graphene forms due to preferential sublimation of Si from the surface. In vacuum, this sublimation occurs much more rapidly for the C face than the Si face so that 150 °C lower annealing temperatures are required for the C face to obtain films of comparable thickness. The evolution of the morphology as a function of graphene thickness is examined, revealing significant differences between the C face and the Si face. For annealing near 1320 °C, graphene films of about 2 monolayers (MLs) thickness are formed on the Si face but 16 ML is found for the C face. In both cases, step bunches are formed on the surface and the films grow continuously (carpetlike) over the step bunches. For the Si face, in particular, layer-by-layer growth of the graphene is observed in areas between the step bunches. At 1170 °C, for the C face, a more three-dimensional type of growth is found. The average thickness is then about 4 ML but with a wide variation in local thickness (2–7 ML) over the surface. The spatial arrangement of constant-thickness domains are found to be correlated with step bunches on the surface, which form in a more restricted manner than at 1320 °C. It is argued that these domains are somewhat disconnected so that no strong driving force for planarization of the film exists. In a 1 atm argon environment, permitting higher growth temperatures, the graphene morphology for the Si face is found to become more layer by layerlike even for graphene thickness as low as 1 ML. However, for the C face the morphology becomes much worse, with the surface displaying markedly inhomogeneous nucleation of the graphene. It is demonstrated that these surface are unintentionally oxidized, which accounts for the inhomogeneous growth.</p

Interface structure of graphene on SiC(0001̅ )

<p>Graphene films prepared by heating the SiC(0001̅ ) surface (the C-face of the {0001} surfaces) in a vacuum or in a Si-rich environment are compared. It is found that different interface structures occur for the two situations. The former yields a well known 3 × 3 reconstructed interface, whereas the latter produces an interface with √43 × √43-R ± 7.6° symmetry. This structure is shown to contain a graphene-like layer with properties similar to the 6√3 × 6√3-R30° “buffer layer” that forms on the Si(0001) surface (the Si-face).</p

Formation of Epitaxial Graphene on SiC(0001) using Vacuum or Argon Environments

The formation of graphene on the (0001) surface of SiC (the Si-face) is studied by atomic force microscopy, low-energy electron microscopy, and scanning tunneling microscopy/spectroscopy. The graphene forms due to preferential sublimation of Si from the surface at high temperature, and the formation has been studied in both high-vacuum and 1-atm-argon environments. In vacuum, a few monolayers of graphene forms at temperatures around 1400°C, whereas in argon a temperature of about 1600°C is required in order to obtain a single graphene monolayer. In both cases considerable step motion on the surface is observed, with the resulting formation of step bunches separated laterally by >10 µm. Between the step bunches, layer-by-layer growth of the graphene is found. The presence of a disordered, secondary graphitic phase on the surface of the graphene is also identified.</p

The influence of the band structure of epitaxial graphene on SiC on the transistor characteristics

We fabricated high-mobility field-effect transistors based on epitaxial graphene synthesized by vacuum graphitization of both the Si- and C-faces of SiC. Room-temperature field-effect mobilities >4000 cm2/V s for both electrons and holes were achieved, although with wide distributions. By using a high-k gate dielectric, we were able to measure the transistor characteristics in a wide carrier density range, where the mobility is seen to decrease as the carrier density increases. We formulate a simple semiclassical model of electrical transport in graphene, and explain the sublinear dependence of conductivity on carrier density from the view point of the few-layer graphene energy band structure. Our analysis reveals important differences between the few-layer graphene energy dispersions on the SiC Si- and C-faces, providing the first evidence based on electrical device characteristics for the theoretically proposed energy dispersion difference between graphene synthesized on these two faces of SiC.</p

Thickness monitoring of graphene on SiC using low-energy electron diffraction

The formation of epitaxial graphene on SiC is monitored in-situ using low-energy electron diffraction (LEED). The possibility of using LEED as an in-situ thickness monitor of the graphene is examined. The ratio of primary diffraction spot intensities for graphene compared to SiC is measured for a series of samples of known graphene thickness (determined using low-energy electron microscopy). It is found that this ratio is effective for determining graphene thicknesses in the range 1 to 3 monolayers. Effects of a distribution of graphene thicknesses on this method of thickness determination are considered.</p

Morphology of Graphene on SiC(0001) Surfaces

Graphene is formed on SiC(0001) surfaces (the so-called C-face of the crystal) by annealing in vacuum, with the resulting films characterized by atomic force microscopy, Auger electron spectroscopy, scanning Auger microscopy and Raman spectroscopy. Morphology of these films is compared with the graphene films grown on SiC(0001) surfaces (the Si-face). Graphene forms a terraced morphology on the C-face, whereas it forms with a flatter morphology on the Si-face. It is argued that this difference occurs because of differing interface structures in the two cases. For certain SiC wafers, nanocrystalline graphite is found to form on top of the graphene.</p

Low-energy electron reflectivity from graphene

<p>Low-energy reflectivity of electrons from single- and multi-layer graphene is examined both theoretically and experimentally. A series of minima in the reflectivity over the energy range of 0 – 8 eV are found, with the number of minima depending on the number of graphene layers. Using first-principles computations, it is demonstrated that a free standing n-layer graphene slab produces n 1 reflectivity minima. This same result is also found experimentally for graphene supported on SiO2. For graphene bonded onto other substrates it is argued that a similar series of reflectivity minima is expected, although in certain cases an additional minimum occurs, at an energy that depends on the graphene-substrate separation and the effective potential in that space.</p

Graphene Nucleation Density on Copper: Fundamental Role of Background Pressure

<p>In this paper we discuss the effect of background pressure and synthesis temperature on the graphene crystal sizes in chemical vapor deposition (CVD) on copper catalyst. For the first time, we quantitatively demonstrate a fundamental role of the background pressure and provide the activation energy for graphene nucleation in atmospheric pressure CVD (9 eV), which is substantially higher than for low pressure CVD (4 eV). We attribute the difference to a greater importance of copper sublimation in low pressure CVD, where severe copper evaporation likely dictates the desorption rate of active carbon from the surface. At atmospheric pressure, where copper evaporation is suppressed, the activation energy is assigned to the desorption energy of carbon clusters instead. The highest possible temperature, close to the melting point of copper, should be used for large single crystal graphene synthesis. Using these conditions, we have synthesized graphene single crystals approaching 1 mm in size. Single crystal nature of synthesized graphene was confirmed by low energy electron diffraction. We also demonstrate that CVD of graphene at temperatures below 1000 oC shows higher nucleation density on (111) than on (100) and (101) copper surfaces but there is no identifiable preference at higher temperatures.</p