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 1 to 3 monolayers. Effects of a distribution of graphene thicknesses on this method of thickness determination are considered.
A simple, non-invasive method using Raman spectroscopy for the estimation of the thickness of graphene layers grown epitaxially on silicon carbide (SiC) is presented, enabling simultaneous determination of thickness, grain size and disorder using the spectra. The attenuation of the substrate Raman signal due to the graphene overlayer is found to be dependent on the graphene film thickness deduced from X-ray photoelectron spectroscopy and transmission electron microscopy of the surfaces. We explain this dependence using an absorbing overlayer model. This method can be used for mapping graphene …
The Moiré structure of epitaxial graphene mono layer grown on 6H-SiC(0001) has been investigated recently by grazing incidence fast atom diffraction, and the results were compared with the calculation of the structure \textit{ab initio} and exact diffraction codes. We review these results and present a complementary approach using the Hard Wall Model to extract information without any \textit{a priori}. Reversely, taking advantage of previous exact calculations we evaluate quantitatively the performance of this simplified approach by comparing predictions using the same potential energy surfac…
Low-energy electron microscopy (LEEM) was used to measure the reflectivity of low-energy electrons from graphitized SiC(0001). The reflectivity shows distinct quantized oscillations as a function of the electron energy and graphite thickness. Conduction bands in thin graphite films form discrete energy levels whose wave vectors are normal to the surface. Resonance of the incident electrons with these quantized conduction band states enhances electrons to transmit through the film into the SiC substrate, resulting in dips in the reflectivity. The dip positions are well explained using tight-bin…
Determination of the Spacing Between Hydrogen-Intercalated Quasi-Free-Standing Monolayer Graphene and 6H-SiC(0001) Using Total-Reflection High-Energy Positron Diffraction
The electronic properties of few-layer graphene grown on the carbon-face of silicon carbide (SiC) are found to be strongly dependent on the number of layers. The carrier mobility is larger in thicker graphene because substrate-related scattering is reduced in the higher layers. The carrier density dependence of the mobility is qualitatively different in thin and thick graphene, with the transition occurring at about 2 layers. The mobility increases with carrier density in thick graphene, similar to multi-layer graphene exfoliated from natural graphite, suggesting that the individual layers are…
We present a structural analysis of the multi-layer graphene-4HSiC(000-1}) system using Surface X-Ray Reflectivity. We show for the first time that graphene films grown on the C-terminated (000-1}) surface have a graphene-substrate bond length that is very short (0.162nm). 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 multi-layer graphene grows in a near turbostratic mode on this surface. This result ma…
We have investigated the atomic structure of superconducting Ca-intercalated bilayer graphene on a SiC(0001) substrate using total-reflection high-energy positron diffraction. By comparing the experimental rocking-curves with ones calculated for various structural models using a full-dynamical theory, we have found that Ca atoms are intercalated in the graphene-buffer interlayer, rather than between the two graphene layers. From transport measurements, the superconducting transition was observed to be at Tc_onset = 4K for this structure. This study is the first to clearly identify the relation…
The crystallographic symmetries and spatial distribution of stacking domains in graphene films on SiC have been studied by low energy electron diffraction (LEED) and dark field imaging in a low energy electron microscope (LEEM). We find that the graphene diffraction spots from 2 and 3 atomic layers of graphene have 3-fold symmetry consistent with AB (Bernal) stacking of the layers. On the contrary, graphene diffraction spots from the buffer layer and monolayer graphene have apparent 6-fold symmetry, although the 3-fold nature of the satellite spots indicates a more complex periodicity in the g…
RF plasma assisted MBE growth of Scandium Nitride (ScN) thin films on GaN (0001)/SiC, AlN (0001)/Al2O3 and on 6H-SiC (0001) hexagonal substrates is found to lead to a face centered cubic (rock-salt) crystal structure with (111) out-of-plane orientation instead of hexagonal orientation. For the first time, cubic (111) twinned patterns in ScN are observed by in-situ electron diffraction during epitaxy, and the twin domains in ScN are detected by electron backscattered diffraction, and further corroborated with X-ray diffraction. The epitaxial ScN films display very smooth, sub nanometer surface …
The morphology of graphene formed on the (000-1) 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 and low-energy electron microscopy. 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 exami…
We present energy filtered electron emission spectromicroscopy with spatial and wave-vector resolution on few layer epitaxial graphene on SiC$(000-1) 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 4.50eV according to both the graphene thickness and the graphene-SiC interface chemical state. At least…
The fracture toughness of Al/SiC nanolaminates with different layer thicknesses (in the range 10 to 100 nm) was measured by means of micropillar splitting and bending of a notched beam. The crack plane was perpendicular to the layers in the former while notched beams with the notch parallel and perpendicular to the layers were milled in the latter. It was found that crack propagation parallel to the layers took place along the metal-ceramic interfaces and the toughness increased with the layer thickness due to the contribution of the plastic deformation of the Al layers. Crack propagation perp…
The present article proposes a data analysis method for experimentally-derived measurements, which consists of an auto-optimization procedure and a sensitivity analysis. The method was applied to the results of a total-reflection high-energy positron diffraction (TRHEPD) experiment, a novel technique of determining surface structures or the position of the atoms near the material surface. This method solves numerically the partial differential equation in the fully-dynamical quantum diffraction theory with many trial surface structures. In the sensitivity analysis, we focused on the experiment…
We measured the local composition and thickness of SiO2-based glass material from diffraction. By using four dimensional scanning transmission electron microscopy (4D-STEM), we obtained diffraction at each scanning point. Comparing the obtained diffraction with simulated diffraction patterns, we try to measure the local composition and thickness. Although this method requires some constraints, this method measured local composition and thickness with 1/10 or less electron dose of EELS.
We propose a table-top method to obtain bilayer quasi-free-standing epitaxial-graphene (QFSEG) on SiC(0001). By applying a microwave annealing in air to a monolayer epitaxial graphene (EG) grown on SiC(0001), the buffer layer is decoupled from the SiC substrate and becomes the second EG layer as confirmed by the low energy electron diffraction, high-resolution transmission electron microscopy, Raman scattering spectroscopy, X-ray photoelectron spectroscopy, and angle-resolved photoelectron spectroscopy. The most likely mechanism of the decoupling is given by the oxidation of the SiC surface, w…
We examine the stacking order of multilayer graphene grown on the SiC$(000\bar{1})$ surface using low-energy electron diffraction and surface X-ray diffraction. We show that the films contain a high density of rotational stacking faults caused by three types of rotated graphene: sheets rotated $30^\circ$ and $\pm 2.20^\circ$ relative to the SiC substrate. These angles are unique because they correspond to commensurate phases of layered graphene, both with itself and with the SiC substrate. {\it Ab intio} calculations show that these rotational phases electronically decouple adjacent graphene l…
We present a systematical study via scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) on the effect of the exposure of Lithium (Li) on graphene on silicon carbide (SiC). We have investigated Li deposition both on epitaxial monolayer graphene and on buffer layer surfaces on the Si-face of SiC. At room temperature, Li immediately intercalates at the interface between the SiC substrate and the buffer layer and transforms the buffer layer into a quasi-free-standing graphene. This conclusion is substantiated by LEED and STM evidence. We show that intercalation occurs th…
An in vacuo thermal desorption process has been accomplished to form epitaxial graphene (EG) on 4H- and 6H-SiC substrates using a commercial chemical vapor deposition reactor. Correlation of growth conditions and the morphology and electrical properties of EG are described. Raman spectra of EG on Si-face samples were dominated by monolayer thickness. This approach was used to grow EG on 50 mm SiC wafers that were subsequently fabricated into field effect transistors with fmax of 14 GHz.
Graphene films prepared by heating the SiC(000-1) surface (the C-face of the {0001} surfaces) in a Si-rich environment are studied using low-energy electron diffraction (LEED) and low-energy electron microscopy (LEEM). Upon graphitization, an interface with rt(43) x rt(43)-R7.6 degree symmetry is observed by in situ LEED. After oxidation, the interface displays rt(3) x rt(3)-R30 degree symmetry. Electron reflectivity measurements indicate that these interface structures arise from a graphene-like "buffer layer" that forms between the graphene and the SiC, similar to that observed on Si-face Si…
The electromagnetic enhancement for surface enhanced Raman spectroscopy (SERS) of graphene is studied by inserting a layer of Al2O3 between epitaxial graphene and Au nanoparticles. Different excitation lasers are utilized to study the relationship between laser wavelength and SERS. The theoretical calculation shows that the extinction spectrum of Au nanoparticles is modulated by the presence of graphene. The experimental results of the relationship between the excitation laser wavelength and the enhancement factor fit well with the calculated results. An exponential relationship is observed be…
This work presents a comparison of the structural, chemical and electronic properties of multi-layer graphene grown on SiC(000-1) by using two different growth approaches: thermal decomposition and chemical vapor deposition (CVD). The topography of the samples was investigated by using atomic force microscopy (AFM), and scanning electron microscopy (SEM) was performed to examine the sample on a large scale. Raman spectroscopy was used to assess the crystallinity and electronic behavior of the multi-layer graphene and to estimate its thickness in a non-invasive way. While the crystallinity of t…
One-Step Epitaxial Access to Rhombohedral Graphene Flat-Band States on Step-Bunched SiC
Institute for Advanced Studies, Wuhan University（武汉大学高等研究院）；Department of Physics, Southern University of Science and Technology（南方科技大学物理系）；School of Physics and Technology, Wuhan University（武汉大学物理科学与技术学院）
The formation of graphene on the SiC(000-1) 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 rt(43) x rt(43)-R\pm7.6° 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 rt(43) x rt(43)-R\pm7.6° …
The effect of oxygen adsorption on the local structure and electronic properties of monolayer graphene grown on SiC(0001) has been studied by means of Low Energy Electron Microscopy (LEEM), microprobe Low Energy Electron Diffraction (\muLEED) and microprobe Angle Resolved Photoemission (\muARPES). We show that the buffer layer of epitaxial graphene on SiC(0001) is partially decoupled after oxidation. The monitoring of the oxidation process demonstrates that the oxygen saturates the Si dangling bonds, breaks some Si-C bonds at the interface and intercalates the graphene layer. Accurate control …
Resistive switching in amorphous silicon carbide (a-SiC) films deposited by a single composite target magnetron sputtering process is reported. Switching performance as a function of thickness of the films (50, 100 and 300 nm) as well as different top metal electrodes (Cu, Pt and Ag) with the bottom electrode fixed as Au, is investigated. The switching parameters (Forming Voltage, Set and Reset voltages and corresponding currents) are found to be dependent on thickness of SiC films and it is observed that 100 nm is the optimal thickness for best endurance. The interface between metal electrode…
Interfacial Coupling Controls Molecular Epitaxy of HMTP on Graphene/SiC
Masaryk University（马萨里克大学）；Brno University of Technology（布尔诺理工大学）；Charles University（查理大学）；Institute of Physical Engineering, Brno University of Technology（布尔诺理工大学物理工程学院）
Intercalation is an established technique for tailoring the electronic structure of epitaxial graphene. Moreover, it enables the synthesis of otherwise unstable two-dimensional (2D) layers of elements with unique physical properties compared to their bulk versions due to interfacial quantum confinement. In this work, we present uniformly Pb-intercalated quasi-freestanding monolayer graphene on SiC, which turns out to be essentially charge neutral with an unprecedented $p$-type carrier density of only $(5.5\pm2.5)\times10^9$ cm$^{-2}$. Probing the low-energy electronic structure throughout the …
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 c…
In this study, we first show that the argon flow during epitaxial graphene growth is an important parameter to control the quality of the buffer and the graphene layer. Atomic force microscopy (AFM) and low-energy electron diffraction (LEED) measurements reveal that the decomposition of the SiC substrate strongly depends on the Ar mass flow rate while pressure and temperature are kept constant. Our data are interpreted by a model based on the competition of the SiC decomposition rate, controlled by the Ar flow, with a uniform graphene buffer layer formation under the equilibrium process at the…
For the example of the SiC(111)-(3x3) reconstruction we show that a holographic interpretation of discrete Low Energy Electron Diffraction (LEED) spot intensities arising from ordered, large unit cell superstructures can give direct access to the local geometry of a cluster around an elevated atom, provided there is only one such prominent atom per surface unit cell. By comparing the holographic images obtained from experimental and calculated data we illuminate validity, current limits and possible shortcomings of the method. In particular, we show that periodic vacancies such as cornerholes …
An investigation of the early stage formation of graphene on the C-face of 6H-SiC is presented. We show that the sublimation of few atomic layers of Si out of the SiC substrate is not homogeneous. In good agreement with the results of theoretical calculations it starts from defective sites, mainly dislocations that define nearly circular flakes, which have a pyramidal, volcano-like, shape with a center chimney where the original defect was located. At higher temperatures, complete conversion occurs but, again, it is not homogeneous. Within the sample surface the intensity of the Raman G and 2D…
The evolution in the surface morphology of epitaxial graphene films and 6H-SiC(0001) substrates is studied by electron channeling contrast imaging. Whereas film thickness is determined by growth temperature only, increasing growth times at constant temperature affect both internal stress and film morphology. Annealing times in excess of 8-10 minutes lead to an increase in the mean square roughness of SiC step edges to which graphene films are pinned, resulting in compressively stressed films at room temperature. Shorter annealing times produce minimal changes in the morphology of the terrace e…
Depth profiling of graphene with high-resolution ion beam analysis is a practical method for analysis of monolayer thicknesses of graphene. Not only is the energy resolution sufficient to resolve graphene from underlying SiC, but by use of isotope labeling it is possible to tag graphene generated from reacted ethylene. Furthermore, we are able to analyze graphene supported by oxidized Si(100) substrates, allowing the study of graphene films grown by chemical vapor deposition on metal and transfered to silicon. This introduces a powerful method to explore the fundamentals of graphene formation.
Epitaxial films of graphene on SiC(0001) are interesting from a basic physics as well as applications-oriented point of view. Here we study the emerging morphology of in-vacuo prepared graphene films using low energy electron microscopy (LEEM) and angle-resolved photoemission (ARPES). We obtain an identification of single and bilayer of graphene film by comparing the characteristic features in electron reflectivity spectra in LEEM to the PI-band structure as revealed by ARPES. We demonstrate that LEEM serves as a tool to accurately determine the local extent of graphene layers as well as the l…
A thermally induced templated synthesis for SiC nanotubes and nanofibers using ammonia or nitrogen as a carrier gas, single wall carbon nanotubes (SWCNT) as templates as well as gaseous Si is presented. The bundles of SWCNT act as both the carbon source and as a nanoframe from which SiC structuctures form. Depending on the duration of the thermally induced templated reaction, for a fixed temperature, carrier gas, and gas pressure, various SiC nanostructures are obtained. These structures include SiC nanorods coated in C, SiC nanorods, SiC nanotubes, and SiC nanocrytals. From our analysis using…
We report a direct correlation between carrier mobility and Raman topography of epitaxial graphene (EG) grown on silicon carbide (SiC). We show the Hall mobility of material on the Si-face of SiC [SiC(0001)] is not only highly dependent on thickness uniformity but also on monolayer strain uniformity. Only when both thickness and strain are uniform over a significant fraction (> 40%) of the device active area does the mobility exceed 1000 cm2/V-s. Additionally, we achieve high mobility epitaxial graphene (18,100 cm2/V-s at room temperature) on the C-face of SiC [SiC(000-1)] and show that carrie…
Contact angle goniometry is conducted for epitaxial graphene on SiC. Although only a single layer of epitaxial graphene exists on SiC, the contact angle drastically changes from 69° on SiC substrates to 92° with graphene. It is found that there is no thickness dependence of the contact angle from the measurements of single, bi, and multi layer graphene and highly ordered pyrolytic graphite (HOPG). After graphene is treated with oxygen plasma, the level of damage is investigated by Raman spectroscopy and correlation between the level of disorder and wettability is reported. By using low power o…
We demonstrate hydrogen assisted growth of high quality epitaxial graphene on the C-face of 4H-SiC. Compared with the conventional thermal decomposition technique, the size of the growth domain by this method is substantially increased and the thickness variation is reduced. Based on the morphology of epitaxial graphene, the role of hydrogen is revealed. It is found that hydrogen acts as a carbon etchant. It suppresses the defect formation and nucleation of graphene. It also improves the kinetics of carbon atoms via hydrocarbon species. These effects lead to increase of the domain size and the…
We investigate electronic band-structure images in reciprocal space of few layer graphene epitaxially grown on SiC(000-1). In addition to the observation of commensurate rotation angles of the graphene layers, the k-space images recorded near the Fermi edge highlight structures originating from diffraction of the Dirac cones due to the relative rotation of adjacent layers. The 21.9° and 27° rotation angles between two sheets of graphene are responsible for a periodic pattern that can be described with a superlattice unit cells. The superlattice generates replicas of Dirac cones with smaller wa…
Large scale, homogeneous quasi-free standing monolayer graphene is obtained on cubic silicon carbide, i.e. the 3C-SiC(111) surface, which represents an appealing and cost effective platform for graphene growth. The quasi-free monolayer is produced by intercalation of hydrogen under the interfacial, (6root3x6root3)R30-reconstructed carbon layer. After intercalation, angle resolved photoemission spectroscopy (ARPES) reveals sharp linear pi-bands. The decoupling of graphene from the substrate is identified by X-ray photoemission spectroscopy (XPS) and low energy electron diffraction (LEED). Atomi…
We report the use of a surfactant molecule during the epitaxy of graphene on SiC(0001) that leads to the growth in an unconventional orientation, namely $R0^\circ$ rotation with respect to the SiC lattice. It yields a very high-quality single-layer graphene with a uniform orientation with respect to the substrate, on the wafer scale. We find an increased quality and homogeneity compared to the approach based on the use of a pre-oriented template to induce the unconventional orientation. Using spot profile analysis low energy electron diffraction, angle-resolved photoelectron spectroscopy, and …
Quasi-free standing graphene (QFG) obtained by the intercalation of a hydrogen layer between a SiC surface and the graphene is recognized as an excellent candidate for the development of graphene based technology. In addition, the recent proposal of a direct equivalence between the $p$-type doping typically found for these systems and the spontaneous polarization (SP) associated to the particular SiC polytype, opens the possibility of tuning the number of carriers in the Dirac cones without the need of external gate voltages. However, first principles calculations which could confirm at the at…
We present a computationally efficient method to incorporate density-functional theory into the calculation of reflectivity in low-energy electron microscopy. The reflectivity is determined by matching plane waves representing the electron beams to the Kohn-Sham wave functions calculated for a finite slab in a supercell. We show that the observed quantum interference effects in the reflectivity spectra of a few layers of graphene on a substrate can be reproduced well by the calculations using a moderate slab thickness.
We present epitaxial graphene (EG) growth on non-polar a-plane and m-plane 6H-SiC faces where material characterization is compared with that known for EG grown on polar faces. Atomic force microscopy (AFM) surface morphology exhibits nanocrystalline graphite like features for non-polar faces, while the polar silicon face shows step like features. This differing behavior is attributed to the lack of a hexagonal template on the non-polar faces. Non-polar faces also exhibit greater disorder and red shift of all Raman peaks (D, G and 2D) with increasing temperature. This is attributed to decreasi…
Grazing incidence X-ray diffraction (GID) was employed to probe the structure of atomically thin carbon layers on SiC(0001): a so-called buffer layer (BL) with a $6(\sqrt{3}\times\sqrt{3})$R30$^\circ$ periodicity, a monolayer graphene (MLG) on top of the BL, and a bilayer graphene (BLG). The GID analysis was complemented by Raman spectroscopy. The lattice parameter of each layer was measured with high precision by GID. The BL possesses a different lattice parameter and corrugation when it is uncovered or beneath MLG. Our results demonstrate that the interfacial BL is the main responsible for t…
Nanoscale materials usually can trap light and strongly interact with it leading to many photonic device applications. The light-matter interactions are commonly probed by optical spectroscopy, which, however, have some limitations such as diffraction-limited spatial resolution, tiny momentum transfer and non-continuous excitation/detection. In this work, using scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) with ultra-wide energy and momentum match and sub-nanometer spatial resolution, we study the optical microcavity resonant spectroscopy in a single S…
Patterning of graphene is key for device fabrication. We report a way to increase or reduce the number of layers in epitaxial graphene grown on the C-face (000-1) of silicon carbide by the deposition of a 120 nm to 150nm-thick silicon nitride (SiN) mask prior to graphitization. In this process we find that areas covered by a Si-rich SiN mask have one to four more layers than non-masked areas. Conversely N-rich SiN decreases the thickness by three layers. In both cases the mask decomposes before graphitization is completed. Graphene grown in masked areas show good quality as observed by Raman s…
Low-loss deposited dielectrics will benefit superconducting devices such as integrated superconducting spectrometers, superconducting qubits and kinetic inductance parametric amplifiers. Compared with planar structures, multi-layer structures such as microstrips are more compact and eliminate radiation loss at high frequencies. Multi-layer structures are most easily fabricated with deposited dielectrics, which typically exhibit higher dielectric loss than crystalline dielectrics. We measured the sub-kelvin and low-power microwave and mm-submm wave dielectric loss of hydrogenated amorphous sili…
Cubic GaN layers were grown by plasma-assisted molecular beam epitaxy on 3C-SiC (001)substrates. In situ reflection high energy electron diffraction was used to quantitatively determine the Ga coverage of the GaN surface during growth. Using the intensity of the electron beam as a probe,optimum growth conditions of c-GaN were found when a 1 ML Ga coverage is formed at the surface. 1 micrometer thick c-GaN layers had a minimum surface roughness of 2.5 nm when a Ga coverage of 1 ML was established during growth. These samples revealed also a minimum full width at half maximum of the (002)rocking…
Electric field, uniform within the slab, emerging due to Fermi level pinning at its both sides is analyzed using DFT simulations of the SiC surface slabs of different thickness. It is shown that for thicker slab the field is nonuniform and this fact is related to the surface state charge. Using the electron density and potential profiles it is proved that for high precision simulations it is necessary to take into account enough number of the Si-C layers. We show that using 12 diatomic layers leads to satisfactory results. It is also demonstrated that the change of the opposite side slab termi…
