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The growth of epitaxial graphene on the SiC(0001) surface is theoretically studied by assuming silicon (Si) sublimation from the surface. Our results indicate that a new graphene sheet grows from the interface between the old graphene sheets and SiC substrate, as found in our previous study on graphene growth by carbon (C) deposition. Graphene growth requires overcoming rather lower energy barriers until 0-monolayer graphene (buffer layer) is formed. Further growth toward formation of 1-monolayer graphene requires overcoming energy barriers higher by about 0.7 eV, which indicates that the growth preferably stops once when 0-monolayer graphene is formed. Compared with the C deposition case, the growth requires overcoming the energy barrier higher by about 0.7 eV, which indicates that the graphene growth is more difficult. In addition, the nonuniform growth of surface C aggregates is thought to degrade the quality of the grown graphene. The C-rich condition is therefore important for obtaining high-quality graphene. The experimental graphene growth is considered to proceed similarly to the C deposition case rather than the Si sublimation case.

The inverting operation of a logic inverter, using two back-gate epitaxial graphene-on-silicon field-effect transistors (GOSFETs), is demonstrated at room temperature. Thanks to the asymmetric ambipolarity of the fabricated GOSFETs, owing to the undesirable back-gate leakage, the on/off state current has been dramatically reduced. This results in a well matched input/output voltage levels for a wide V DD range from more than 1 V down to 0.5 V. The inverting operation of the device is obtained at as low V DD bias as 0.5 V. Voltage gains higher than the unity are also obtained. The experimental results are in good agreement with the simulation results made for the case of asymmetric ambipolar transistors.

The epitaxial graphene growth at the 4H-SiC(0001) surface with intentionally inserted step-free basal plane regions was performed by high temperature annealing in the range of 1600–1900 °C under ultrahigh vacuum. For fabricating inverted-mesa structures with the step-free regions at SiC surfaces, a combined process consisting of a direct laser digging and a Si-vapor etching at 1900 °C was utilized. The graphitized surfaces were characterized by atomic force microscopy, low acceleration voltage (0.1–1.0 kV) scanning electron microscopy and Raman spectroscopy. It was found that the graphene thickness at the SiC step-free surface tends to be suppressed compared with the thickness at background SiC step-terrace surfaces where the steps are intrinsically introduced from intentional/unintentional substrate miscut angles. From the characterization by Raman mapping, 1 ML graphene was obtained at the SiC step-free surface at 1600 °C graphitization in contrast to the case that multilayer graphene was grown at SiC step-terrace surfaces.

[Ren Wen-cai; Gao Li-bo; Ma Lai-peng; Cheng Hui-ming] Chinese Acad Sci, Shenyang Natl Lab Mat Sci, Inst Met Res, Shenyang 110016, Peoples R China.;Ren, WC (reprint author), Chinese Acad Sci, Shenyang Natl Lab Mat Sci, Inst Met Res, Shenyang 110016, Peoples R China;wcren@imr.ac.cn lbgao@imr.ac.cn cheng@imr.ac.cn

In the solid–vapor phase equilibria between SiC and O2 system, there exists a region where the reaction (2+x)SiC+O2→(2+x)Si↑+ 2CO↑+ xC↓ takes place [Y. W. Song and F. W. Smith: J. Am. Ceram. Soc. 88 (2005) 1864]. By tuning the temperature and the oxygen pressure used in the graphitization annealing into this region, we have succeeded in the growth of epitaxial graphene on SiC crystals at 1000 °C, which is lower, by 250 °C or more, than the conventional epitaxial graphene method. The method is especially useful to formation of epitaxial graphene on silicon (GOS), which requires a lower graphitization temperature because of the Si substrate as well as of its mission to attain compatibility with Si technology.

National Natural Science Foundation of China [10702056, 10774124]; Xiamen University; Academic Award for Doctoral Candidates of China

Raman microspectroscopy was used to measure compressive strain within epitaxial graphene (EG) grown on the carbon-terminated SiC(0001¯) face as a function of annealing time for a growth temperature of 1400 °C. A maximum strain of −0.5% was seen at the longest time of 55 min. This differs from the −0.9% expected for strain caused by cooling from the growth temperature due to the differential thermal contraction between the SiC and EG layer, despite good agreement between this model and data on EG on SiC(0001). We suggest that this is due to the different EG bonding mechanisms on the two SiC faces.

The atomic structures of graphene islands on SiC(0001) surfaces are studied theoretically together with their growth mechanism. Two types of embedded graphene island structure are proposed. It is shown that these structures actually act as the graphene island electronically, and that those with zigzag edges have the magnetoelectric effect.

Transmission electron microscopy (TEM) investigations of epitaxial graphene, grown on on-axis and 8° off-axis C-terminated 4H–SiC (0001¯) surfaces are presented. The TEM results provide evidence that the first carbon layer is separated by 3.2 A from the C-terminated SiC surface. It was also found that thick graphene layers grown on on-axis SiC (0001¯) are loosely bound to the SiC substrate. Moreover, the structural observations reveal a certain degree of disorder between the graphene planes, which manifests itself in a rotation of the layers and in an increase in the interplanar spacing between certain carbon layers from 3.35 A, which is characteristic for graphite, up to 3.7 A. Graphene grown on 8° off-axis SiC (0001¯) substrates covers the steps of SiC and as a result disorder seems to be not as pronounced as it is on the on-axis SiC (0001¯) substrate.

Single postdielectric resonators operating on their quasi TE011 modes were used for the measurement of the surface resistance and conductivity of graphene films grown on semi-insulating SiC substrates. With this technique the surface resistance was measured with an uncertainty of ±5% and the conductivity was evaluated with an uncertainty equal to the uncertainty in determining the film thickness. The room temperature conductivity of the graphene films proved to be in the range 5×106 to 6.4×106 S/m.

We report on the observation of inertial-ballistic transport in nanoscale cross junctions fabricated from epitaxial graphene grown on SiC(0001). Ballistic transport is indicated by a negative bend resistance of R12,43 ~ 170 ohm which is measured in a non-local, four-terminal configuration at 4.2 K and which vanishes as the temperature is increased above 80 K., Comment: 10 pages, 3 figures

The quantum Hall effect, with a Berry's phase of $\pi$ is demonstrated here on a single graphene layer grown on the C-face of 4H silicon carbide. The mobility is $\sim$ 20,000 cm$^2$/V$\cdot$s at 4 K and ~15,000 cm$^2$/V$\cdot$s at 300 K despite contamination and substrate steps. This is comparable to the best exfoliated graphene flakes on SiO$_2$ and an order of magnitude larger than Si-face epitaxial graphene monolayers. These and other properties indicate that C-face epitaxial graphene is a viable platform for graphene-based electronics., Comment: Some modifications in the text and figures, 7 pages, 2 figures

Epitaxial graphene films were formed on the Si-face of semi-insulating 4H-SiC substrates by a high temperature sublimation process. A high-k gate stack on epitaxial graphene is realized by inserting a fully oxidized nanometer thin aluminum film as a seeding layer followed by an atomic-layer deposition process. The electrical properties of epitaxial graphene films are sustained after gate stack formation without significant degradation. At low temperatures, the quantum-Hall effect in Hall resistance is observed along with pronounced Shubnikov-de Hass oscillations in diagonal magneto-resistance of gated epitaxial graphene on SiC (0001)., 2 new references added

The epitaxial graphene growth mechanism on SiC(0001) surfaces is studied based on the energetics via the first-principles calculation. It is found that a Si terminated surface is important for the epitaxial growth of thin flat graphene sheets. This surface encourages the growth of graphene sheets because the surface Si atoms act as catalyst. Si desorbed sites trap excess C atoms, and form graphene islands. It is also found that the new graphene sheet prefers to grow just on the Si terminated surface of the SiC substrate even if the surface is covered with other graphene sheets.

The recent success of graphene transistor operation in the giga-hertz range has solidified the potential of this material for high speed electronic applications. Realization of a graphene technology on the production scale; however, requires the ability to synthesize large area graphene, and rapidly characterize the material's structural and electronic quality. We report a direct link between carrier mobility and Raman topography of epitaxial graphene grown on silicon carbide. We have examined epitaxial graphene with mobility values of 25 - 1100 cm2/V-s, and show that the Hall mobility of epitaxial graphene on the Si-face of SiC (SiC(0001)) is not only highly dependent on thickness uniformity, but also on mono-layer strain uniformity. It is not until the thickness and strain uniformity is approaches 50% of the device width that one is able to achieve mobility values higher than 1000 cm2/V-s.

The evolution of silicon carbide (0001) surface reconstruction upon annealing has been studied by Si K-edge near-edge x-ray-absorption fine structure (NEXAFS). With the increase in annealing temperature, the fluorescence yield of Si K-edge NEXAFS clearly indicates an increase in disorder of Si atoms in the much deeper interior beneath the surface due to out diffusion of Si atoms to the surface forming increased Si vacancies. The concentration of Si vacancies beneath the epitaxial graphene formed by high-temperature annealing of SiC is estimated to be as high as 15% to a depth of several micrometers. As acceptors in SiC, the high concentration of Si vacancies could have a significant impact on the electronic properties of epitaxial graphene by charge-transfer doping from the substrate and the introduction of interface states.

Nature Materials 6, 770–775 (2007). The authors wish to make some corrections to the references in the above Article

Scanning tunneling spectroscopy (STS) was used to measure local differential conductance (dI/dV) spectra on nanometer-size graphene islands on an Ir(111) surface. Energy resolved dI/dV maps clearly show a spatial modulation, which we ascribe to a modulated local density of states due to quantum confinement. STS near graphene edges indicates a position dependence of the dI/dV signals, which suggests a reduced density of states near the edges of graphene islands on Ir(111).

7th European Conference on Silicon Carbide and Related Materials, Barcelona, SPAIN, SEP 07-11, 2008; International audience; We report a comparative investigation of Few layers graphene grown on 6H, 4H, and 3C-SiC substrates. We show that the size of the graphitic domains depends more on the < 0001 > SiC Surface orientation than the polytypism.

The adsorption of atomic H above the carbon buffer layer in the graphene/SiC(0001) interface system is investigated within density functional theory through a set of realistic interface models that do not impose large artificial strains on the graphene. We find that hydrogen binding energies above the buffer layer are two to four times higher than on free-standing graphene and display important spatial variations across the unit cell. Adsorption on Si-bonded, fourfold-coordinated C atoms is strongly unfavorable (unstable, or metastable with very low barriers), while all threefold-coordinated C sites lead to stable configurations often showing H binding energies larger than in H-2. We identify the origin of these large binding energies in the local strengthening of the graphene/SiC interface bonding around the adsorption site due to the H-induced deformation of the graphene buffer layer. The most stable adsorption sites are those having two or three C nearest neighbors (almost) atop the underlying surface Si atoms, because the presence of the adsorbed H atom favors the formation of a local sp(3) arrangement of the graphene. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Using density functional calculations, we address the energetics of the interface between the SiC(0001) substrate and the first covalently bonded epitaxial graphene layer. We consider a 6 root 3 x 6 root 3R30 degrees geometry showing the experimental periodicity, a simplified root 3 x root 3R30 degrees geometry presenting a strained graphene layer, and an almost commensurate 4 x 4 geometry where SiC and graphene have the same orientation. The total energies of the structurally relaxed interface systems indicate that the 6 root 3 x 6 root 3R30 degrees geometry is the most stable, in agreement with its experimental occurrence. The binding energy is found to correlate with the vertical spread of the C atoms in the graphene layer, with a larger extension corresponding to a higher binding energy. For the 6 root 3 x 6 root 3R30 degrees geometry, the height variation of the graphene layer displays the experimentally observed modulation with an apparent 6 x 6 periodicity. The charge transfer also correlates with the height of the graphene atoms, being more significant in graphene regions which are strongly attached to the SiC substrate. (C) 2011 Elsevier B.V. All rights reserved.

The study of the electronic band structure of solids is a central task in materials science. Indeed, knowing a material's band structure is key to an understanding of its macroscopic electronic properties. In recent years, a new class of materials has emerged whose unifying feature is an electronic structure that breaks with conventional band theory. Rather than obeying the usual Schrödinger Hamiltonian, the low-energy quasiparticle excitations in the band structure of these materials are better described as chiral, massless Dirac-like carriers, similar in nature to the relativistic quantum particles known from high-energy physics. Prominent members of this class are graphene and topological insulators. Despite being vastly different materials, they share a number of intriguing electronic properties that render them highly attractive for both fundamental and application-oriented studies. The thrust of this thesis work is to investigate the photoexcited Dirac carriers in graphene and in the series of topological insulators (Sb2)m-Sb2Te3 by means of angle-resolved photoemission spectroscopy (ARPES) and XUV laser-based time-resolved ARPES. The aim is to push the research frontier further towards a more complete understanding of the fate of a Dirac carrier when it is brought out of thermodynamic equilibrium by an optical excitation; and what role the substrate material, doping level, laser excitation and the introduction of a band gap play, as well as how one can possibly engineer such a Dirac-like electronic structure. To this end, we take advantage of ARPES' momentum and energy resolving capabilities that can be extended into the time domain by adding femtosecond time-resolution in a pump-probe scheme. By taking the Dirac carriers out of their equilibrium state by a laser pump pulse and watching directly with a second probe pulse how they interact and relax back to equilibrium, a movie of the in situ carrier dynamics is obtained. This dynamical view on the ultrafast scattering processes of photoexcited Dirac carriers in graphene allows us to show a quasi-instant sub 40 femtosecond thermalization to a hot carrier distribution. The carriers cool back to equilibrium on femto- and picosecond timescales via excitations of optical phonons and acoustic phonons assisted by disorder-scattering in supercollisions processes. A method is introduced to accurately extract the hot carriers¿ temperature that is used to quantify the carrier multiplication. Cooling times and carrier multiplication are found to display a remarkable dependence on the doping level of graphene to the extent that a substantial carrier multiplication is possible. The hot carrier dynamics is found to be severely perturbed by the presence of a metal, and also pump-induced electric fields are shown to affect a correct interpretation of TR-ARPES data by masking the intrinsic carrier dynamics. Furthermore, we demonstrate that, in the case of bilayer graphene, the existence of a single-particle band gap in the electronic structure leads to a markedly enhanced carrier lifetime over monolayer graphene, despite the existence of in-gap states. Finally, a complete picture of the evolution of the band structure and Dirac nature of the carriers across a series of compounds, from Sb2Te3 to elemental Sb, is drawn, suggesting a promising strategy for engineering the electronic states.

We use scanning tunneling spectroscopy (STS) at low temperatures to investigate the local electronic structure of mono- and bilayer graphene grown epitaxially on SiC(0001). Already for monolayer graphene, a gap opening is observed in the pi-bands at the Dirac point. The gap size is spatially modulated with the (6 root 3 x 6 root 3)R30 degrees periodicity of the interface structure. We ascribe this effect to a spatially dependent interface potential, which is imprinted into the graphene layer. For bilayer graphene the Dirac gap has a constant size, but a spatially localized mid-gap state is observed within. For both, gap state and pi-bands the intensities are strongly modulated with the atomic periodicity of graphene. (C) 2008 Elsevier B.V. All rights reserved.