Nishtha Srivastava

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

The morphology of graphene formed on the ( 1 000 ) surface (the C-face) and the (0001) surface (the Si-face) of SiC, by annealing in ultra-high vacuum or in an argon environment, is studied by atomic force microscopy (AFM) and low-energy electron microscopy (LEEM). 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 (ML) 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 (carpet-like) 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 3-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-layer-like 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 surfaces are unintentionally oxidized, which accounts for the inhomogeneous growth.

Low-energy Electron Reflectivity from Graphene

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 1  n 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.

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.

Low-Energy Electron Reflectivity of Graphene on Copper and other Substrates

The reflectivity of low energy electrons from graphene on copper substrates is studied both experimentally and theoretically. Well-known oscillations in the reflectivity of electrons with energies 0 - 8 eV above the vacuum level are observed in the experiment. These oscillations are reproduced in theory, based on a first-principles density functional description of interlayer states forming for various thicknesses of multilayer graphene. It is demonstrated that n layers of graphene produce a regular series of n-1 minima in the reflectance spectra, together with a possible additional minimum associated with an interlayer state forming between the graphene and the substrate. Both (111) and (001) orientations of the copper substrates are studied. Similarities in their reflectivity spectra arise from the interlayer states, whereas differences are found because of the different Cu band structures along those orientations. Results for graphene on other substrates, including Pt(111) and Ir(111), are also discussed.

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, a 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.

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 of 1–3 ML. Effects of a distribution of graphene thicknesses on this method of thickness determination are considered.

Evidence of Electrochemical Graphene Functionalization by Raman Spectroscopy

Electrochemical functionalization of treated epitaxial graphene samples on Si-face 6H-SiC are presented in this work. Three semi-insulating 6H-SiC substrates cut from different boules with varying off cut angle (on axis, 0.5° and 1.0° degrees off axis in the [112‾0] direction) were diced into 10mm x 10mm samples and quality EG grown on top. A home-build electrochemical cell was used with current applied though a 10% H2SO4 solution, with a Pt wire and exposed graphene as the anode and cathode respectively. Functionalization was determined using Raman spectroscopy and measured by an increase in D/G ratio, increase in fluorescence background and introduction of C-H bond peak at ~2930 cm-1. Components of the Raman spectra before and after functionalization of all samples used were analyzed to show a substrate dependent effect on functionalization with values such as D/G ratio and normalized fluorescence slope varying between the substrates.

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

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].

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

The formation of graphene on the SiC(0001¯) surface (the C-face 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-R ± 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-R30° “buffer layer” that forms on the SiC(0001) surface (the Si-face)]. Oxidation of the surface is found to modify (eliminate) the √43 × √43-R ± 7.6° structure, which is interpreted in the same manner as the known “decoupling” that occurs for the Si-face buffer layer.The formation of graphene on the SiC(0001¯) surface (the C-face 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-R ± 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-R30° “buffer layer” that forms on the SiC(0001) surface (the Si-face)]. Oxidation of the surface is found to modify (eliminate) the √43 × √43-R ± 7.6° structure, which is interpreted in the same manner as the known “decoupling” that occurs for the Si-face buffer layer.