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We present epitaxial graphene (EG) growth on nonpolar 6H-SiC-faces by solid-state decomposition of the SiC substrate in the Knudsen flow regime in vacuum. The material characteristics are compared with those known for EG grown on polar SiC-faces under similar growth conditions. X-ray photoelectron spectroscopy (XPS) measurements indicate that nonpolar faces have thicker layers than polar faces. Among nonpolar faces, the m-plane (11̅ 00) has thicker layers than the aplane (112̅ 0). Atomic force microscopy (AFM) shows nanocrystalline graphite features for nonpolar faces, consistent with the small grain size measured by Raman spectroscopy. This is attributed to the lack of a hexagonal template, unlike on the polar Si-and C-faces. These nonpolar face EG films exhibited stress decreasing with increasing growth temperature. These variations are interpreted on the basis of different growth mechanisms on the various faces, as expected from the large differences in surface energy and step dynamics on the various SiC surfaces. Surfaces with smaller grain sizes systematically exhibited thicker layers. Using this observation, we argue that multilayer EG growth, after the nucleation of the first layers, is determined primarily by Si diffusion through grain boundaries and defects, as Si cannot diffuse through a perfect graphene lattice. A greater density of grain boundaries allows more Si to escape during growth, allowing thicker layers of carbon to be grown.

We show ~10x polariton-enhanced infrared reflectivity of epitaxial graphene on 4H-SiC, in SiC's restrahlen band (8-10µm). By fitting measurements to theory, we extract the thickness, N, in monolayers (ML), momentum scattering time (), Fermi level position (E F ) of graphene and estimate carrier mobility. By showing that 1/n s , the carrier concentration/ML, we argue that scattering is dominated by short-range interactions at the SiC/graphene interface. Polariton formation finds application in near-field optical devices such as superlenses.Graphene, a two-dimensional (2D) form of carbon in a honeycomb crystal structure, is the basic building block of other sp 2 carbon nanomaterials, such as carbon nanotubes. It exhibits unusual electronic and optical properties [1][2][3][4][5][6]. Graphene has a dispersionless linear electronic band structure as opposed to the quadratic form observed for most semiconductors. This leads to "massless" Dirac-fermion behavior, and consequently, high electron mobility, as opposed to the usual Schrodinger behavior exhibited by most semiconductors [6][7]. Furthermore, the recent development of epitaxial graphene (EG) formed by the solid-state decomposition of a SiC surface has enabled the systematic production of large area graphene films on a commercial substrate platform. This has prompted the investigation of many high performance electronic devices, such as field effect transistors and p-n junction diodes, photonic devices such as terahertz oscillators, as well as low noise sensors [8][9][10][11]. For all of these applications, knowledge of the optical properties of graphene is important, as it gives insight into the interaction of graphene with external electromagnetic fields.

Electrochemical functionalization and possible hydrogenation of treated epitaxial graphene samples on 6H-SiC are presented. To attract H+ ions to react with the exposed working cathode, a 10% sulfuric acid electrolyte was used with a Pt counter anode. Functionalization was determined using Raman spectroscopy and measured by a marked increase in I(D)/I(G) ratio and introduction of C-H bond peak at ∼2930 cm−1. There was also a marked increase in fluorescence background, which clearly differentiates functionalization from lattice damage in the graphene. Quantifying the fluorescence, we estimate that H-incorporation as high as 50% was achieved based on results on hydrocarbons, although other functional groups cannot be excluded. We further distinguished these functionalization signatures from lattice damage through measurements on nanocrystalline graphene on a and m plane SiC, which displayed very different surface morphologies and no measureable fluorescence. Finally, we show that the extent of functionalization is strongly substrate dependent by using samples cut from three semi-insulating 6H-SiC substrates with similar resistivity but orientations varying from on-axis (∼0.02°), 0.5° to 1.0° off-axis. This functionalization was found to be thermally reversible at ∼1000 °C. Scanning tunneling spectroscopy indicates the presence of sp3-like localized states not present in the starting graphene, further supporting the assertion that functionalization has occurred.

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The doping dependence of dry thermal oxidation rates in n-type 4H-SiC was investigated. The oxidation was performed in the temperature range 1000 0 C to 1200 0 C for samples with nitrogen doping in the range of 6.5×10 15 /cm 3 to 9.3×10 18 /cm 3 , showing a clear doping dependence. Samples with higher doping concentrations displayed higher oxidation rates.The results were interpreted using a modified Deal-Grove model. Linear and parabolic rate constants and activation energies were extracted. Increasing nitrogen led to an increase in linear rate constant pre-exponential factor from 10 -6 m/s to 10 -2 m/s and the parabolic rate constant preexponential factor from 10 -9 m 2 /s to 10 -6 m 2 /s. The increase in linear rate constant was attributed to defects from doping-induced lattice mismatch, which tend to be more reactive than bulk crystal regions. The increase in the diffusion-limited parabolic rate constant was attributed to degradation in oxide quality originating from the doping-induced lattice mismatch. This degradation was confirmed by the observation of a decrease in optical density of the grown oxide films from 1.4 to 1.24. The linear activation energy varied from 1.6eV to 2.8eV , while the parabolic activation energy varied from 2.7eV to 3.3eV, increasing with doping concentration. These increased activation energies were attributed to higher nitrogen content, leading to an increase in effective bond energy stemming from the difference in C-Si (2.82eV) and Si-N (4.26eV) binding energies. This work provides crucial information in the engineering of SiO 2 dielectrics for SiC MOS structures, which typically involve regions of very different doping concentrations, and suggests that thermal oxidation at high doping concentrations in SiC may be defect mediated.

Recently, much attention has been devoted to trilayer graphene because it displays stacking and electric field dependent electronic properties well-suited for electronic and photonic applications [1][2][3][4][5][6][7][8]. Several theoretical studies have predicted the electronic dispersion of Bernal (ABA) and rhombohedral (ABC) stacked trilayers. However, a direct experimental visualization of a well-resolved band structure has not yet been reported. In this work, we obtain large area highly homogenous quasi-free trilayer graphene (TLG) on 6H-SiC(0001) and measure its electronic bands via angle resolved photoemission spectroscopy (ARPES). We demonstrate by low energy electron microscopy measurements that that trilayer domains on SiC extend over areas of tens of square micrometers. By fitting tight-binding bands to the experimental data we extract the interatomic hopping parameters for Bernal and rhombohedral stacked trilayers. For ABC stacks and in the presence of an electrostatic asymmetry, we detect the existence of a bandgap of about 120 meV. Notably our results suggest that on SiC substrates the occurrence of ABC-stacked TLG is significantly higher than in natural bulk graphite. Hence, growing TLG on SiC might be the answer to the challenge of controllably synthesizing ABC-stacked trilayer -an ideal material for the fabrication of a new class of gap-tunable devices.

We present a quantitative study on the growth of multilayer epitaxial graphene (EG) by solid-state decomposition of SiC on polar (c-plane Si and C-face) and non-polar (a and m planes) 6H-SiC faces, with distinctly different defect profiles. The growth rates are slower than expected from a mechanism that involves Si loss from an open and free surface, and much faster than expected for the nucleation of a defect-free EG layer, implying that defects in the EG play a critical role in determining the growth kinetics. We show that a Deal-Grove growth model, which assumes vertical diffusion of Si through these defects as the limiting factor for EG growth, is unsuitable for describing multilayer growth. Instead, we introduce a lateral “adatom” diffusion mechanism for Si out-diffusion, based on a modified Burton, Cabrera, and Frank model. In this model, defects in epitaxial graphene serve as sinks for Si desorption loss, taking the place of reactive sites, such as step edges for nucleation and growth of crystals produced with external precursors. This analysis shows that the surface diffusion of Si atoms to the grain boundaries of EG limits the growth on c-plane C-face and non-polar faces, rather than the purely vertical diffusion of Si through the grain boundaries described in the Deal-Grove model. However, for Si-face c-plane growth, diffusion of Si to the defects, as well as desorption of Si at the grain boundaries are both relevant, leading to a different temperature trend compared with the other faces. This distinct qualitative difference is ascribed to point-defects in Si-face growth, as contrasted with line defects/grain boundaries on the other faces. The size of the EG grains correlates with the surface diffusion length extracted from this model. The longer a Si adatom diffuses, the higher the quality of the grown EG film, an insight that provides valuable information on Si adatom kinetics for optimizing EG growth. We discuss the applicability of this model to growth of multilayer EG in an argon ambient at atmospheric pressure.