Layer Number Determination and Thickness-Dependent Properties of Graphene Grown on SiC (original) (raw)
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Surface Topography: Metrology and Properties, 2014
A study of the growth of graphene on the silicon-face (0001) and the carbon-face (000 − 1) of SiC is presented. The morphology and layer thickness is investigated using atomic force microscopy and scanning Kelvin probe microscopy and demonstrates the more wrinkled and less uniform thickness of the graphene growth on the C-face compared to the Si-face which shows uniform monolayer growth with some bilayer areas. Raman spectroscopy confirms the predominantly monolayer nature of the Siface graphene and the inhomogeneous nature of the C-face graphene growth. Raman studies on the C-face show overlapping peaks as observed for spectra of Bernal-stacked graphene but we argue that the graphene is turbostratic with nanoscale differences in substrate effects leading to shifting of the Raman modes. Further samples show uniform scanning Kelvin probe maps of the carbon face along with very low bilayer coverage on the Si-face. Epitaxially grown graphene on the Si-face of SiC is reported to have a high carrier concentration and low mobility, however, van Der Pauw measurements demonstrate the low sheet resistance and relatively low carrier concentration of the graphene on the Si-face, in agreement with microwave measurements (Hao et al 2013 Appl. Phys. Lett. 103 123103) and scanning Kelvin probe maps which demonstrate the uniformity of the graphene.
Carbon, 2017
In this work, we report a muti-scale investigation using several nano-, micro and macro-scale techniques of few layer graphene (FLG) sample consisting of large monolayer (ML) and bilayer (BL) areas grown on C-face 4H-SiC (000-1) by high-temperature sublimation. Single 1×1 diffraction patterns are observed by micro-low-energy electron diffraction for ML, BL and trilayer graphene with no indication of out-of-plane rotational disorder. A SiO x layer is identified between graphene and SiC by X-ray photoelectron emission spectroscopy and reflectance measurements. The chemical composition of the interface layer changes towards SiO 2 and its thickness increases with aging in normal ambient conditions. The formation mechanism of the inter
Layer-number determination in graphene on SiC by reflectance mapping
Carbon, 2014
We report a simple, handy and affordable optical approach for precise number-oflayers determination of graphene on SiC based on monitoring the power of the laser beam reflected from the sample (reflectance mapping) in a slightly modified micro-Raman setup. Reflectance mapping is compatible with simultaneous Raman mapping. We find experimentally that the reflectance of graphene on SiC normalized to the reflectivity of bare substrate (the contrast) increases linearly with ~1.7% per layer for up to 12 layers, in agreement with theory. The wavelength dependence of the contrast in the visible is investigated using the concept of ideal fermions and compared with existing experimental data for the optical constants of graphene. We argue also that the observed contrast is insensitive to the doping condition of the sample, as well as to the type of sample (graphene on C-or Si-face of 4H or 6H SiC, hydrogen-intercalated graphene). The possibility to extend the precise layer counting to ~50 layers makes reflectivity mapping superior to low-energy electron microscopy (limited to ~10 layers) in quantitative evaluation of graphene on the Cface of SiC. The method is applicable for graphene on other insulating or semiconducting substrates.
Electronic Structure of Epitaxial Graphene Layers on SiC: Effect of the Substrate
Physical Review Letters, 2007
Recent transport measurements on thin graphite films grown on SiC show large coherence lengths and anomalous integer quantum Hall effects expected for isolated graphene sheets. This is the case eventhough the layer-substrate epitaxy of these films implies a strong interface bond that should induce perturbations in the graphene electronic structure. Our DFT calculations confirm this strong substrate-graphite bond in the first adsorbed carbon layer that prevents any graphitic electronic properties for this layer. However, the graphitic nature of the film is recovered by the second and third absorbed layers. This effect is seen in both the (0001)and (0001) 4H SiC surfaces. We also present evidence of a charge transfer that depends on the interface geometry. It causes the graphene to be doped and gives rise to a gap opening at the Dirac point after 3 carbon layers are deposited in agreement with recent ARPES experiments (T.Ohta et al, Science 313 (2006) 951).
Scientific Reports, 2016
The transport properties of few-layer graphene are the directly result of a peculiar band structure near the Dirac point. Here, for epitaxial graphene grown on SiC, we determine the effect of charge transfer from the SiC substrate on the local density of states (LDOS) of trilayer graphene using scaning tunneling microscopy/spectroscopy and angle resolved photoemission spectroscopy (ARPES). Different spectra are observed and are attributed to the existence of two stable polytypes of trilayer: Bernal (ABA) and rhomboedreal (ABC) staking. Their electronic properties strongly depend on the charge transfer from the substrate. We show that the LDOS of ABC stacking shows an additional peak located above the Dirac point in comparison with the LDOS of ABA stacking. The observed LDOS features, reflecting the underlying symmetry of the two polytypes, were reproduced by explicit calculations within density functional theory (DFT) including the charge transfer from the substrate. These findings demonstrate the pronounced effect of stacking order and charge transfer on the electronic structure of trilayer or few layer graphene. Our approach represents a significant step toward understand the electronic properties of graphene layer under electrical field.
Probing the electrical anisotropy of multilayer graphene on the Si face of 6H-SiC
Physical Review B, 2010
We studied the in-plane magnetoresistance R͑B , T͒ anisotropy in epitaxial multilayer graphene films grown on the Si face of a 6H-SiC substrate that originates from steplike morphology of the SiC substrate. To enhance the anisotropy, a combination of argon atmosphere with graphite capping was used during the film growth. The obtained micro-Raman spectra demonstrated a complex multilayer graphene structure with the smaller film thickness on terraces as compared to the step edges. Several Hall bars with different current/steps mutual orientations have been measured. A clear anisotropy in the magnetoresistance has been observed, and attributed to variations in electron mobility governed by the steplike structure. Our data also revealed that ͑i͒ the graphene-layer stacking is mostly Bernal type, ͑ii͒ the carriers are massive, and ͑iii͒ the carriers are confined to the first 2-4 graphene layers following the buffer layer.
Stacking of adjacent graphene layers grown on C-face SiC
Physical Review B, 2011
Graphene was grown on the C-face of nominally on-axis SiC substrates using high-temperature sublimation with Ar as the buffer inert gas. The results of studies of the morphology, thickness, and electronic structure of these samples using low-energy electron microscopy (LEEM), x-ray photoelectron emission microscopy, photoelectron spectroscopy, angle-resolved photoelectron spectroscopy (ARPES), and low-energy electron diffraction (LEED) are presented. The graphene thickness is determined to vary from 1 or 2 to 6 or 7 monolayers (MLs), depending on the specific growth conditions utilized. The formation of fairly large grains (i.e., crystallographic domains) of graphene exhibiting sharp 1×1 spots in micro-LEED is revealed. Adjacent grains are found to show different azimuthal orientations. Macro-LEED patterns recorded mimic previously published, strongly modulated, diffraction ring LEED patterns, indicating contribution from several grains of different azimuthal orientations. We collected selected area constant initial energy photoelectron angular distribution patterns that show the same results. When utilizing a small aperture size, one Dirac cone centered on each of the six K-points in the Brillouin zone is clearly resolved. When using a larger aperture, several Dirac cones from differently oriented grains are detected. Our findings thus clearly show the existence of distinct graphene grains with different azimuthal orientations; they do not show adjacent graphene layers are rotationally disordered, as previously reported for C-face graphene. The graphene grain size is shown to be different on the different samples. In some cases, a probing area of 400 nm is needed to detect the grains. On one sample, a probing area of 5 μm can be used to collect a 1×1 LEED pattern from a multilayer graphene grain. ARPES is used to determine the position of the Dirac point relative to the Fermi level on two samples that LEEM shows have dominant coverage of 2 and 3 MLs of graphene, respectively. The Dirac point is found to be located within 75 meV of the Fermi level on both samples, which indicates that the electron carrier concentration induced in the second and third graphene layers on the C-face is less than ∼4×10 11 cm −2. Formation of patches of silicate is revealed on some samples, but the graphene formed on such nonhomogenous surfaces can contain fairly large ordered multilayer graphene grains.
Optical and electronic structure of quasi-freestanding multilayer graphene on the carbon face of SiC
We report the optical and electronic properties of multilayer graphene films grown epitaxially on the carbon face (C-face) of 4H-SiC probed using spectroscopic ellipsometry (SE) and angle-resolved photoemission spectroscopy (ARPES). The optical conductivity (σ1) in the energy range from 1.0 to 5.3 eV extracted from SE reveals two important features: the presence of universal conductivity (πe 2 /2h) at the near-infrared region and asymmetric resonant excitons at 4.5 eV. Furthermore, ARPES shows the presence of independent linear electronic dispersion. These features resemble quasi-freestanding properties of multilayer graphene grown on the C-face of SiC.
Morphological and electronic properties of epitaxial graphene on SiC
Physica B: Condensed Matter, 2014
ABSTRACT We report on the structural and electronic properties of graphene grown on SiC by high-temperature sublimation. We have studied thickness uniformity of graphene grown on 4H–SiC (0 0 0 1), 6H–SiC (0 0 0 1), and 3C–SiC (1 1 1) substrates and investigated in detail graphene surface morphology and electronic properties. Differences in the thickness uniformity of the graphene layers on different SiC polytypes is related mainly to the minimization of the terrace surface energy during the step bunching process. It is also shown that a lower substrate surface roughness results in more uniform step bunching and consequently better quality of the grown graphene. We have compared the three SiC polytypes with a clear conclusion in favor of 3C–SiC. Localized lateral variations in the Fermi energy of graphene are mapped by scanning Kelvin probe microscopy. It is found that the overall single-layer graphene coverage depends strongly on the surface terrace width, where a more homogeneous coverage is favored by wider terraces. It is observed that the step distance is a dominating, factor in determining the unintentional doping of graphene from the SiC substrate. Microfocal spectroscopic ellipsometry mapping of the electronic properties and thickness of epitaxial graphene on 3C–SiC (1 1 1) is also reported. Growth of one monolayer graphene is demonstrated on both Si- and C-polarity of the 3C–SiC substrates and it is shown that large area homogeneous single monolayer graphene can be achieved on the Si-face substrates. Correlations between the number of graphene monolayers on one hand and the main transition associated with an exciton enhanced van Hove singularity at ~4.5 eV and the free-charge carrier scattering time, on the other are established. It is shown that the interface structure on the Si- and C-polarity of the 3C–SiC (1 1 1) differs and has a determining role for the thickness and electronic properties homogeneity of the epitaxial graphene.
2012
Using high-temperature annealing conditions with a graphite cap covering the C-face of, both, on axis and 8 ° offaxis 4H-SiC samples, large and homogeneous single epitaxial graphene layers have been grown. Raman spectroscopy shows evidence of the almost free-standing character of these monolayer graphene sheets, which was confirmed by magneto-transport measurements. On the best samples, we find a moderate p-type doping, a high-carrier mobility and resolve the half-integer quantum Hall effect typical of high-quality graphene samples. A rough estimation of the density of states is given from temperature measurements.