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Paradoxically, a very broad diffraction background, named the bell-shaped-component (BSC), has been established as a feature of graphene growth. Recent diffraction studies as a function of electron energy have shown that the BSC is not related to scattering interference. Here, additional experiments are carried out as a function of temperature over the range in which single-layer graphene (SLG) grows. Quantitative fitting of the profiles shows that the BSC follows the increase of the Gr(10) spot, proving directly that the BSC indicates high-quality graphene. Additional metal deposition experiments provide more information about the BSC. The BSC is insensitive to metal deposition, and it increases with metal intercalation, because a more uniform interface forms between graphene and SiC. These experiments support the conclusion that the BSC originates from electron confinement within SLG, and surprisingly, it is an excellent measure of graphene uniformity.

Graphene intercalation is a novel way to control graphene's band structure and generate two-dimensional quantum materials with unusual spintronic and electronic properties. Despite its importance, information about the intercalation mechanism is lacking, especially the role of low density domain boundaries between regions of graphene of different thickness. With high resolution surface diffraction we have systematically studied Pb intercalation on epi-graphene grown on SiC, with domain boundaries between buffer and single layer graphene. By examining the evolution of different diffraction spots as a function of tempertature, the location of Pb and stability of the intercalated phases underneath were determined.

Epitaxial graphene on SiC provides both an excellent source of high-quality graphene as well as an architecture to support its application. Although single-layer graphene on Si-face SiC has garnered extensive interest, many-layer graphene produced on C-face SiC could be significantly more robust for enabling applications. Little is known, however, about the structural properties related to the growth evolution at the buried interface for thick many-layer graphene. Using complementary X-ray scattering and neutron reflectivity as well as electron microscopy, we demonstrate that thick many-layer epitaxial graphene exhibits two vastly different length-scales of the buried interface roughness as a consequence of the Si sublimation that produces the graphene. Over long lateral length-scales the roughness is extremely large (hundreds of A) and it varies proportionally to the number of graphene layers. In contrast, over much shorter lateral length-scales we observe an atomically abrupt interface with SiC terraces. Graphene near the buried interface exhibits a slightly expanded interlayer spacing (∼1%) and fluctuations of this spacing, indicating a tendency for disorder near the growth front. Nevertheless, Dirac cones are observed from the graphene while its domain size routinely reaches micron length-scales, indicating the persistence of high-quality graphene beginning just a short distance away from the buried interface. Discovering and reconciling the different length-scales of roughness by reflectivity was complicated by strong diffuse scattering and we provide a detailed discussion of how these difficulties were resolved. The insight from this analysis will be useful for other highly rough interfaces among broad classes of thin-film materials.

We report scanning tunneling microscopy and spectroscopy (STM and STS) studies of graphene formed from a nitrogen-seeded SiC( 000 1 ¯ ) surface. STM indicates that much of the graphene consists of wide flat plateaus with hexagonal features bounded by pleats and regions with disordered character. Nitrogen impurities are not observed in the epitaxial graphene layers. STS measurements on this surface show peaks corresponding to Landau levels associated with pseudo-magnetic fields as high as 1000 T. The energy distribution of Landau levels is consistent with an electronic model employing a finite bandgap.

In this chapter electronic transport properties such as resistance, mobility, charge density and conductivity of graphene are studied.

Using highly controlled coverages of graphene on SiC(0001), we have studied the structure of the first graphene layer that grows on the SiC interface. This layer, known as the buffer layer, is semiconducting. Using x-ray reflectivity and x-ray standing waves analysis we have performed a comparative study of the buffer layer structure with and without an additional monolayer graphene layer above it. We show that no more than 26\% of the buffer carbon is covalently bonded to Si in the SiC interface. We also show that the top SiC bilayer is Si depleted and is the likely the cause of the incommensuration previously observed in this system. When a monolayer graphene layer forms above the buffer, the buffer layer becomes less corrugated with signs of a change in the bonding geometry with the SiC interface. At the same time, the entire SiC interface becomes more disordered, presumably due to entropy associated with the higher growth temperature., Comment: 12 pages, 9 figures

International audience; Graphene nanoribbons are fundamental components to the development of graphenenanoelectronics. At the nanoscale, electronic confinement effects and electronic edge statesbecome essential to the properties of graphene. These effects depend critically on the ribbonwidth and the nature of the ribbon edge, the control of which at the atomic scale is a majorchallenge. Graphene nanoribbons have been largely studied theoretically, experimentally andwith the perspective of electronic applications. We review the basic properties of graphenenanoribbons and recent progress in fabrication processes, focusing on the question of theelectronic gap. We examine top–down and bottom–up approaches to fabricate graphenenanoribbons by lithographic, catalytic cutting, chemical assembly and epitaxial growthmethods and compare their electronic characteristics.

Epitaxial graphene (EG) is a new material that consists of one or more graphene layers that are grown on single crystal silicon carbide. EG research has for the past 6 years focused on material production and processing. High quality material can now be produced and processing methods are promising. Progress is reviewed here with an emphasis on the recently discovered multilayered epitaxial graphene (MEG) which is multilayered yet exhibits the properties of a single layer. The status of this rapidly developing field is briefly reviewed with an emphasis on properties and effects that are relevant for electronics applications.

While numerous methods have been proposed to produce semiconducting graphene, a significant bandgap has never been demonstrated. The reason is that, regardless of the theoretical gap formation mechanism, disorder at the sub-nanometer scale prevents the required chiral symmetry breaking necessary to open a bandgap in graphene. In this work, we show for the first time that a 2D semiconducting graphene film can be made by epitaxial growth. Using improved growth methods, we show by direct band measurements that a bandgap greater than 0.5 eV can be produced in the first graphene layer grown on the SiC(0001) surface. This work demonstrates that order, a property that remains lacking in other graphene systems, is key to producing electronically viable semiconducting graphene.

We have carried out diffuse X-ray scattering measurements of the growth of Pb nanocrystalline islands on Si(1 1 1). Analysis of our data shows that islands growing on an initially rough wetting layer transforms the portion of the wetting layer below them into ordered fcc sites. Therefore, the islands grow directly on top of the Si surface with a disordered wetting layer occupying the region between the islands and, consequently, the island height responsible for the quantum well depth is one layer thicker than reported by LEED and STM. These islands have an extremely good vertical order until the islands coalesce into a closed film. At that point the disorder of the film increases consistent with misfit strain relaxation.

The comparison between measured vs calculated electronic energy levels in quantum well states (QWS) depends on the dimension of the potential well confining the electrons. The dimensions of the well can be determined with structural probes (high resolution LEED and STM) while the electronic structure can be obtained with angle resolved photoemission and STM spectroscopy. For heteroepitaxial growth the dimension of the well depends on the thickness of the wetting layer between the island and the substrate. For the Pb/Si(1 1 1)-7 × 7 system using SPA-LEED and STM, we have measured the thickness of the layer underneath the islands and the substrate to be 1 ML at all temperatures. For Pb films grown on top of the Si(1 1 1)-7 × 7 at low temperatures T

Despite its importance in theoretical studies of surface diffusion there has been limited experimental progress in developing methods to measure the dynamic structure factor S(q,t). In this work we study gases adsorbed on single-crystal surfaces and show that their equilibrium dynamics is measurable in a diffraction experiment through density fluctuations described by the dynamic correlation function . Using a lattice-gas model with nearest-neighbor repulsive interaction, we demonstrate how to separate out the microscopic dynamics caused by diffusion and adsorption-desorption from the form of . Our analytical results show that decays as an exponential with a time constant {tau}(q). Significant deviations from the hydrodynamic dependence of 1/{tau}(q)=D{sub c}q{sup 2} are found with and without desorption. These deviations have important consequences in extracting diffusion constants from experiments. (c) 2000 The American Physical Society.

We present low-energy electron-diffraction evidence showing that the W(210) surface irreversibly facets into two different orientations: a near-(310) facet and a (530) facet. While the near-(310) facet consists of primarily single-atomic-height steps, the (530) facet is made up of a mixture of single- and double-atomic-height steps. Step doubling is predicted on the basis of existing free-energy models, while faceting has important implications on the nature of the step-step interaction.

We present evidence for equilibrium temporal fluctuations in a high resolution low energy electron diffraction (LEED) experiment. These fluctuations are the reciprocal space analog of current fluctuations in field emission microscopy and therefore can be used to extract surface kinetic information. We show that even when the electron beam illuminates an area larger than its correlation length, time correlated data can be extracted from LEED. To demonstrate this, we present time dependent data from a W(430) surface, which reflects thermal step fluctuations. Our results illustrate the potential of LEED as a real time, ultrafast probe. {copyright} {ital 1998} {ital The American Physical Society }

The understanding of metal nucleation on graphene is essential for promising future applications, especially of magnetic metals which can be used in spintronics or computer storage media. A common method to study the grown morphology is to measure the nucleated island density n as a function of growth parameters. Surprisingly, the growth of Fe on graphene is found to be unusual because it does not follow classical nucleation: n is unexpectedtly high, it increases continuously with the deposited amount θ and shows no temperature dependence. These unusual results indicate the presence of long range repulsive interactions. Kinetic Monte Carlo simulations and density functional theory calculations support this conclusion. In addition to answering an outstanding question in epitaxial growth, i.e., to find systems where long range interactions are present, the high density of magnetic islands, tunable with θ, is of interest for nanomagnetism applications.

We present energy filtered electron emission spectromicroscopy with spatial and wave-vector resolution on few-layer epitaxial graphene on SiC$(000\overline{1})$ grown by furnace annealing. Low-energy electron microscopy shows that more than 80% of the sample is covered by 2--3 graphene layers. C1s spectromicroscopy provides an independent measurement of the graphene thickness distribution map. The work function, measured by photoelectron emission microscopy (PEEM), varies across the surface from 4.34 to 4.50 eV according to both the graphene thickness and the graphene-SiC interface chemical state. At least two SiC surface chemical states (i.e., two different SiC surface structures) are present at the graphene/SiC interface. Charge transfer occurs at each graphene/SiC interface. $k$-space PEEM gives 3D maps of the $|{\mathrm{k\ifmmode \bar{}\else \={}\fi{}}}_{\ensuremath{\parallel}}|$ $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ band dispersion in micron-scale regions showing that the Dirac point shifts as a function of graphene thickness. Bragg diffraction of the Dirac cones via the superlattice formed by the commensurately rotated graphene sheets is observed. The experiments underline the importance of lateral and spectroscopic resolution on the scale of future electronic devices in order to precisely characterize the transport properties and band alignments.

High resolution low-energy electron diffraction measurements are reported that demonstrate the existence of semiordered islands on the clean Pd(110) surface. The islands are stable up to 1000 °C and are approximately 90 atoms long in the 001 direction. A simple model is presented that makes use of step-step interactions to generate the periodic island structure. This model predicts that ordered islands form below the roughening temperature if the step creation energy is small compared to the step-step interaction. A justification for this condition is given for the Pd(110) surface. © 1993 The American Physical Society.

The inability to grow large well-ordered ultra high vacuum (UHV) graphene with a specific number of layers on SiC(0001) is well known. The growth involves several competing processes (Si desorption, carbon diffusion, island nucleation, etc.) and because of the high temperatures, it has not been possible to identify the growth mechanism. Using scanning tunneling microscopy and a vicinal 6H-SiC(0001) sample, we determine that the Si desorption from steps is the main controlling process. Adjacent steps retract with different speeds and the released carbon produces large areas of bilayer graphene with characteristic ``fingers'' emanating from steps. If faster heating rates are used, the different Si desorption rates are avoided and single-layer graphene films extending over many microns are produced.

We present a structural analysis of the graphene-4HSiC(0001) interface using surface x-ray reflectivity. We find that the interface is composed of an extended reconstruction of two SiC bilayers. The interface directly below the first graphene sheet is an extended layer that is more than twice the thickness of a bulk SiC bilayer (~1.7A compared to 0.63A). The distance from this interface layer to the first graphene sheet is much smaller than the graphite interlayer spacing but larger than the same distance measured for graphene grown on the (000-1) surface, as predicted previously by ab intio calculations., Comment: 1 tex file plus 6 figures

We show experimentally that multilayer graphene grown on the carbon terminated SiC(0001[over ]) surface contains rotational stacking faults related to the epitaxial condition at the graphene-SiC interface. Via first-principles calculation, we demonstrate that such faults produce an electronic structure indistinguishable from an isolated single graphene sheet in the vicinity of the Dirac point. This explains prior experimental results that showed single-layer electronic properties, even for epitaxial graphene films tens of layers thick.

We present a structural analysis of the multilayer graphene/$4H\mathrm{Si}\mathrm{C}(000\overline{1})$ system using surface x-ray reflectivity. We show that graphene films grown on the C-terminated $(000\overline{1})$ surface have a graphene-substrate bond length that is very short $(1.62\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$. The measured distance rules out a weak van der Waals interaction to the substrate and instead indicates a strong bond between the first graphene layer and the bulk as predicted by ab initio calculations. The measurements also indicate that multilayer graphene grows in a near turbostratic mode on this surface. This result may explain the lack of a broken graphene symmetry inferred from conduction measurements on this system [C. Berger et al., Science 312, 1191 (2006)].

We show experimentally that multilayer graphene grown on the carbon terminated SiC(0001[over ]) surface contains rotational stacking faults related to the epitaxial condition at the graphene-SiC interface. Via first-principles calculation, we demonstrate that such faults produce an electronic structure indistinguishable from an isolated single graphene sheet in the vicinity of the Dirac point. This explains prior experimental results that showed single-layer electronic properties, even for epitaxial graphene films tens of layers thick.

With expanding interest in graphene-based electronics, it is crucial that high quality graphene films be grown. Sublimation of Si from the 4H-SiC(0001) Si-terminated) surface in ultrahigh vacuum is a demonstrated method to produce epitaxial graphene sheets on a semiconductor. In this paper we show that graphene grown from the SiC$(000\bar{1})$ (C-terminated) surface are of higher quality than those previously grown on SiC(0001). Graphene grown on the C-face can have structural domain sizes more than three times larger than those grown on the Si-face while at the same time reducing SiC substrate disorder from sublimation by an order of magnitude., Submitted to Appl. Phys. Lett

In this paper we discuss how finite data sets influence experimental measurements of the autocovariance function. Autocovariance estimators are biased, meaning that the expectation value for any measured autocovariance function is not identical to the actual autocovariance function. In this work we show that the measured autocovariance function for a finite length time series must become negative for some lag times. We derive analytic corrections to these finite time errors for different types of correlated random sequences. Our results explain the apparent anticorrelated noise observed in experimental observations.

We present the first demonstration of Temporal LEED Spectroscopy’s ability to measure surface dynamics as shown by a study of step dynamics on vicinal Si(001). While we measure activation energies consistent with other work, the observed microscopic mechanism driving the fluctuations is inconsistent with previous claims. We exclude evaporation/condensation of atoms from the steps as the rate limiting kinetics in the temperature range of this experiment. Possible reasons for these differences are discussed.

We present LEED evidence from W(430) showing a phase transition from single-height steps to double-height steps occuring at 940 K. The transition is thought to be driven by the formation of kinks on double steps that are energetically lower compared to kinks on single-height steps. The doubling transition can be predicted in terms of a simple free energy model including step-step interactions.

Graphene grown on the SiC surface is unique. Unlike graphene grown on the (0 0 0 1) surface, graphene grown on the surface has higher electron mobilities and an unusual non-Bernal stacking. Its different electronic properties are associated with its stacking and the graphene–SiC interface. In this paper we discuss what is known about the structure of this material. In particular we will discuss the ordering in this material and how it is related to the interface structure. We update new ideas about the interface and stacking and contrast it with works from other groups. New evidence for how Si is removed from the interface is also given that provides some insight into the growth process and shows that graphene nucleation is not confined to screw dislocations. This has important implications for the viability of patterned graphene growth.

We report high resolution low-energy-electron-diffraction measurements on the Pd(110) surface. We demonstrate that the clean surface consists of semiordered islands. The island structure is stable up to 650°C at which point the island edges roughen and step-step correlations decrease. Above 1100°C, surface evaporation becomes important and the surface becomes kinetically rough. A simple model is presented that makes use of step-step interactions to generate the periodic island structure. This model predicts that ordered islands form below the roughening temperature if the step creation energy is small compared to the step-step interaction. The existence of isolated steps is shown to be consistent with embedded atom calculations that predict a small step-formation energy on Pd(110) surfaces. © 1993 The American Physical Society.

We review the progress towards developing epitaxial graphene as a material for carbon electronics. In particular, we discuss improvements in epitaxial graphene growth, interface control and the understanding of multilayer epitaxial graphene's (MEG's) electronic properties. Although graphene grown on both polar faces of SiC will be discussed, our discussions will focus on graphene grown on the C-face of SiC. The unique properties of C-face MEG have become apparent. These films behave electronically like a stack of nearly independent graphene sheets rather than a thin Bernal stacked graphite sample. The origins of multilayer graphene's electronic behaviour are its unique highly ordered stacking of non-Bernal rotated graphene planes. While these rotations do not significantly affect the inter-layer interactions, they do break the stacking symmetry of graphite. It is this broken symmetry that leads to each sheet behaving like isolated graphene planes.

We have studied the thermal behavior of the Ag(110) surface by synchrotron x-ray diffraction. In-plane diffraction data (Qz ≈ 0) agrees with earlier work and can be fit to a power law form. Out-of-plane data (Qz,max=0.7), however, indicates the existence of two coexisting phases below the roughening transition: flat (110) oriented regions separated by inclined rough regions. The relative coverage of these two phases is found to depend on the temperature. Thus, the roughening process can be viewed as a continuous replacement of the flat faceted regions by the rough phase. Using the Wulff construction, we are able to describe the temperature dependence of the relative phase concentrations and extrapolate to an estimate of the Ag(110) roughening temperature.

The electronic properties of epitaxial graphene grown on SiC have shown its potential as a viable candidate for post-CMOS electronics. However, progress in this field requires a detailed understanding of both the structure and growth of epitaxial graphene. To that end, this review will focus on the current state of epitaxial graphene research as it relates to the structure of graphene grown on SiC. We pay particular attention to the similarity and differences between graphene growth on the two polar faces, (0001) and , of hexagonal SiC. Growth techniques, subsequent morphology and the structure of the graphene/SiC interface and graphene stacking order are reviewed and discussed. Where possible the relationship between film morphology and electronic properties will also be reviewed.

In this report we present a detailed study of the elastic and inelastic scattering of helium from Ni(113). Ni(113) is known to undergo a roughening transition with a roughening temperature of TR = 750 ± 50 K, as determined from line shape analysis. However, we observe a decrease in the one-phonon scattering at a temperature ∼ 600 K for all scattering geometries. The reduced one-phonon scattering is correlated with an increase in the width of single-phonon line shapes from time-of-flight experiments. These effects suggest that the mean lifetime of a surface phonon is decaying 100–150 K below TR. In addition, we also observe the true elastic diffuse scattering to increase beginning near 600 K. These results are discussed in terms of phonon-defect scattering from isolated kinks forming below TR. The formation of these kinks and the temperature dependence of the diffuse elastic scattering is found to be consistent with the temperature singularity expected in a Kosterlitz-Thouless roughening transition.

The roughening of stepped metal surfaces has been demonstrated on Ni(11m) and Cu(11m) surfaces [1, 2, 3]. Roughening of these surfaces proceeds reversibly by the spontaneous formation of kinks along the (001) terraces when T > TR, the roughening temperature (see Fig. 1a). Two Models, the Body-Centered Solid-on-Solid (BCSOS) [4] and the Terrace-Step-Kink (TSK) [5, 6, 7], have been proposed to describe the transition. Both Models predict that the transition to the rough phase is of the Kosterlitz-Thouless (KT) universality class [8]. Indeed, experimental results for Ni(115), Ni(113), and Cu(115) agree quantitatively with several KT predictions. While both models lead to a KT transition, the predicted symmetry of the rough phase is quite different in the two cases. In this review we present evidence from helium diffraction experiments indicating that the rough phase is isotropic, meaning that fluctuations of the terrace edges and the terrace widths are comparable. These experiments support the BCSOS model and suggest that more realistic interaction between kinks reflect the isotropic nature of the BCSOS potential. Time-of-Flight (TOF) experiments show that the transition to the rough phase is consistent with a KT singularity to within the finite resolution of the instrument.

A generic phase diagram is proposed for stepped ($11m$) surfaces of fcc metals, which describes the roughening transition and subsequent changes of the nature of the rough phase in terms of a sequence of universal values for the roughness parameter ${x}_{1}(T, m)$. From a detailed analysis of atom-beam diffraction profiles of Ni(115), for $100 \mathrm{K}lTl900 \mathrm{K}$, it is concluded that Ni(115) is rough in this temperature interval. The roughening temperatures of Ni(113) and Ni(001) are predicted to be 200 \ifmmode\pm\else\textpm\fi{} 50 K and well above 420 K, respectively. Near 420 K, islands appear in the rough (115) surfaces in addition to the meandering steps.