We investigate the growth of the graphene buffer layer and the involved step bunching behavior of the silicon carbide substrate surface using atomic force microscopy. The formation of local buffer layer domains are identified to be the origin of undesirably high step edges in excellent agreement with the predictions of a general model of step dynamics. The applied polymer-assisted sublimation growth method demonstrates that the key principle to suppress this behavior is the uniform nucleation of the buffer layer. In this way, the silicon carbide surface is stabilized such that ultra-flat surfaces can be conserved during graphene growth on a large variety of silicon carbide substrate surfaces. The analysis of the experimental results describes different growth modes which extend the current understanding of epitaxial graphene growth by emphasizing the importance of buffer layer nucleation and critical mass transport processes.
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 incommens…
We report a Raman study of the so-called buffer layer with $(6\sqrt3\times6\sqrt3)R30^{\circ}$ periodicity which forms the intrinsic interface structure between epitaxial graphene and SiC(0001). We show that this interface structure leads to a nonvanishing signal in the Raman spectrum at frequencies in the range of the D- and G-band of graphene and discuss its shape and intensity. Ab-initio phonon calculations reveal that these features can be attributed to the vibrational density of states of the buffer-layer.
The atomistic structure of the graphene buffer layer on Si-terminated SiC is studied using a modified environment-dependent interatomic potential (EDIP). The investigation of equilibrium state by conjuguate gradients suffers from a complex multi-minima energy surface. A dedicated procedure is therefore presented to provide a suitable initial configuration on the way to the minimum. The result forms an hexagonal pattern with unsticked rods to release the misfit with the surface. The structure presents an agreement with the global pattern obtained by experiments and even with the details of an a…
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…
We report on an investigation of quasi-free-standing graphene on 6H-SiC(0001) which was prepared by intercalation of hydrogen under the buffer layer. Using infrared absorption spectroscopy we prove that the SiC(0001) surface is saturated with hydrogen. Raman spectra demonstrate the conversion of the buffer layer into graphene which exhibits a slight tensile strain and short range defects. The layers are hole doped (p = 5.0-6.5 x 10^12 cm^(-2)) with a carrier mobility of 3,100 cm^2/Vs at room temperature. Compared to graphene on the buffer layer a strongly reduced temperature dependence of the …
When epitaxial graphene layers are formed on SiC(0001), the first carbon layer (known as the "buffer layer"), while relatively easy to synthesize, does not have the desirable electrical properties of graphene. The conductivity is poor due to a disruption of the graphene pi-bands by covalent bonding to the SiC substrate. Here we show that it is possible to restore the graphene pi-bands by inserting a thin oxide layer between the buffer layer and SiC substrate using a low temperature, CMOS-compatible process that does not damage the graphene layer.
Monolayer epitaxial graphene is an appropriate candidate for a wide variety of electronic and optical applications. One advantage of growing graphene on the Si face of SiC is that it develops as a single crystal, as does the layer underneath, commonly referred to as the interfacial buffer layer. The properties of this supporting layer include a band gap, making it of interest to groups seeking to build devices with on-off capabilities. In this work, using density functional theory, we have calculated the bonding characteristics of the buffer layer to the SiC substrate beneath. These calculatio…
Quasi-free standing epitaxial graphene is obtained on SiC(0001) by hydrogen intercalation. The hydrogen moves between the 6root3 reconstructed initial carbon layer and the SiC substrate. The topmost Si atoms which for epitaxial graphene are covalently bound to this buffer layer, are now saturated by hydrogen bonds. The buffer layer is turned into a quasi-free standing graphene monolayer with its typical linear pi-bands. Similarly, epitaxial monolayer graphene turns into a decoupled bilayer. The intercalation is stable in air and can be reversed by annealing to around 900 degrees Celsius.
The intercalation of epitaxial graphene on SiC(0001) with Ca has been studied extensively, yet precisely where the Ca resides remains elusive. Furthermore, the intercalation of Mg underneath epitaxial graphene on SiC(0001) has not been reported. Here, we use low energy electron diffraction, x-ray photoelectron spectroscopy, secondary electron cut-off photoemission and scanning tunneling microscopy to elucidate the physical and electronic structure of both Ca- and Mg-intercalated epitaxial graphene on 6H-SiC(0001). We find that Ca intercalates underneath the buffer layer and bonds to the Si-ter…
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…
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…
Cubic silicon carbide (3C-SiC) is an emerging material for high power and new generation devices, but the development of high quality 3C-SiC layer still represents a scientific and technological challenge especially when grown on a Si substrate. In the present lecture, we discuss the use of a buffer layer between the epitaxial layer and the substrate in order to reduce the defectiveness and improve the overall quality of the SiC epi-film. In particular, we find that the morphology and the quality of the epi-film depends on the carbonization temperature and the concentration of Ge in close prox…
We report on a comparative structural characterization of two types of high quality epitaxial graphene layers grown by CVD on 4H-SiC(0001). The layers under study are a single layer graphene on top of a buffer layer and a quasi-free-standing graphene obtained by intercalation of hydrogen underneath the buffer layer. We determine the morphology and structure of both layers by different complementary in-situ and ex-situ surface techniques. We found the existence of large islands in both samples but with different size distribution. Photoemission electron microscopy (PEEM) measurements were perfo…
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…
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 atomic and electronic structures of a graphene layer on top of the $(2\times2)$ reconstruction of the SiC (000$\bar{1}$) surface are studied from ab initio calculations. At variance with the (0001) face, no C bufferlayer is found here. Si adatoms passivate the substrate surface so that the very first C layer presents a linear dispersion characteristic of graphene. A small graphene-substrate interaction remains in agreement with scanning tunneling experiments (F.Hiebel et al. {\it Phys. Rev. B} {\bf 78} 153412 (2008)). The stacking geometry has little influence on the interaction which expl…
A recipe on how to engineer a band gap in the energy spectrum for the carriers in graphene is conveyed. It is supported by a series of numerical simulations inspired by an analytical result based on the opening of a band gap in periodically corrugated graphene, e.g. the buffer layer grown on SiC at high temperatures.
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° …
We have investigated electron band structure of epitaxially grown graphene on an SiC(0001) substrate using angle-resolved photoemission spectroscopy. In single-layer graphene, abnormal high spectral intensity is observed at the Dirac energy whose origin has been questioned between in-gap states induced by the buffer layer and plasmaron bands induced by electron-plasmon interactions. With the formation of double-layer graphene, the Dirac energy does not show the high spectral intensity any longer different from the single-layer case. The inconsistency between the two systems suggests that the m…
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…
Interest for layered Ruddlesden-Popper strongly correlated manganites of Pr$_{0.5}$Ca$_{1.5}$MnO$_4$ as well as to their thin film polymorphs is motivated by the high temperature of charge orbital ordering above room temperature. We report on the tailoring of the c-axis orientation in epitaxial RP-PCMO films grown on SrTiO$_3$ (STO) substrates with different orientations as well as the use of CaMnO$_3$ (CMO) buffer layers. Films on STO(110) reveal in-plane alignment of the c-axis lying along to the [100] direction. On STO(100), two possible directions of the in-plane c-axis lead to a mosaic li…
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…
We present a study of quasi-free-standing monolayer graphene obtained by intercalation of Au atoms at the interface between the carbon buffer layer (Bu-L) and the silicon-terminated face (0001) of 4H-silicon carbide. Au intercalation is achieved by deposition of atomically thin Au on the Bu-L followed by annealing at 850 °C in an Argon atmosphere. We explore the intercalation of Au and decoupling of the Bu-L into quasi-free-standing monolayer graphene by surface science characterizations and electron transport in top-gated electronic devices. By gate-dependent magnetotransport we find that the…
Tailoring Ultrathin Magnetic Multilayers at Terraced Topologically Insulating Interfaces for Perpendicularly Magnetized Domains
University of Leeds（利兹大学）；ISIS Neutron and Muon Source, STFC Rutherford Appleton Laboratory（STFC卢瑟福阿普尔顿实验室中子与μ子源）；Diamond Light Source（钻石光源）；Imperial College London（伦敦帝国理工学院）
Graphene halfly doped with H or F possesses local magnetization at the undoped C sites. Thus the Seebeck coefficient is different for each spin channel and its sign also changes depending on the spin polarization. Deposition of doped graphene on the C-face 4H-SiC(0001) with two buffer layers substantially varies the electronic and thermoelectric properties. These properties are efficiently calculated from the semiclassical Boltzmann equations, using the maximally-localized Wannier-functions interpolation of the band structures obtained with the density-functional theory. Our results indicate l…
We present a new fabrication method for epitaxial graphene on SiC which enables the growth of ultra-smooth defect- and bilayer-free graphene sheets with an unprecedented reproducibility, a necessary prerequisite for wafer-scale fabrication of high quality graphene-based electronic devices. The inherent but unfavorable formation of high SiC surface terrace steps during high temperature sublimation growth is suppressed by rapid formation of the graphene buffer layer which stabilizes the SiC surface. The enhanced nucleation is enforced by decomposition of polymer adsorbates which act as a carbon …
We investigate the structural and electronic properties of Li-intercalated monolayer graphene on SiC(0001) using combined angle-resolved photoemission spectroscopy and first-principles density functional theory. Li intercalates at room temperature both at the interface between the buffer layer and SiC and between the two carbon layers. The graphene is strongly $n$-doped due to charge transfer from the Li atoms and two $π$-bands are visible at the $\bar{K}$-point. After heating the sample to 300$^\circ$C, these $π$-bands become sharp and have a distinctly different dispersion to that of Bernal-…
Due to the predominantly surface character of graphene, it is highly suitable for functionalization with external atoms and/or molecules leading to a plethora of new and interesting phenomena. Here we show ferromagnetic properties of hydrogen-functionalized epitaxial graphene on SiC. Ferromagnetism in such a material is not directly evident as it is inherently composed of only non-magnetic constituents. Our results nevertheless show strong ferromagnetism, which cannot be explained by simple magnetic impurities. The ferromagnetism is unique to hydrogenated epitaxial graphene on SiC, where inter…
Realization of post-CMOS graphene electronics requires production of semiconducting graphene, which has been a labor-intensive process. We present tailoring of silicon carbide crystals via conventional photolithography and microelectronics processing to enable templated graphene growth on 4H-SiC{1-10n} (n = 8) crystal facets rather than the customary {0001} planes. This allows self-organized growth of graphene nanoribbons with dimensions defined by those of the facet. Preferential growth is confirmed by Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) measurement…
We report on the charge spill-out and work function of epitaxial few-layer graphene on 6H-SiC(0001). Experiments from high-resolution, energy-filtered X-ray photoelectron emission microscopy (XPEEM) are combined with ab initio Density Functional Theory calculations using a relaxed interface model. Work function values obtained from theory and experiments are in qualitative agreement, reproducing the previously observed trend of increasing work function with each additional graphene plane. Electrons transfer at the SiC/graphene interface through a buffer layer causes an interface dipole moment …
Principal structural defects in graphene layers, synthesized on a carbon-terminated face, i.e. the SiC(000) face of a 4H-SiC substrate, are investigated using microscopic methods. Results of high-resolution transmission electron microscopy (HRTEM) reveal their atomic arrangement. Mechanism of such defects creation, directly related to the underlying crystallographic structure of the SiC substrate, is elucidated. The connection between the 4H-SiC(000) surface morphology, including the presence of the single atomic steps, the sequences of atomic steps, and also the macrosteps, and the correspond…
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…
Heterostructures made of stacked 2D materials with different electronic properties are studied for their potential in creating multifunctional devices. Graphene (G) and hexagonal boron nitride (h-BN) van der Waals (vdW) systems have been extensively researched, including recent studies on synthesizing h-BN on graphene/SiC(0001) templates. These studies suggest that h-BN encapsulation could occur in addition to vdW epitaxy. This work presents theoretical research on G/h-BN heterostructures on SiC(0001) with a carbon buffer layer. The results show an energetic preference for h-BN encapsulation b…
We address the stability of the surface phases that occur on the C-side of 3C-SiC($\bar{1} \bar{1} \bar{1}$) at the onset of graphene formation. In this growth range, experimental reports reveal a coexistence of several surface phases. This coexistence can be explained by a Si-rich model for the unknown (3$\times$3) reconstruction, the known (2$\times$2)$_{C}$ adatom phase, and the graphene covered (2$\times$2)$_{C}$ phase. By constructing an $ab$ $initio$ surface phase diagram using a van der Waals corrected density functional, we show that the formation of a well defined interface structure …
The unique electronic properties of graphene offer the possibility that it could replace silicon when microelectronics evolves to nanoelectronics. Graphene grown epitaxially on silicon carbide is particularly attractive in this regard because SiC is itself a useful semiconductor and, by suitable manipulation of the growth conditions, epitaxial films can be produced that exhibit all the transport properties of ideal, two-dimensional graphene desired for device applications. Nevertheless, there is little or no understanding of the actual kinetics of growth, which is likely to be required for fut…
Graphene films prepared by heating the SiC(000-1) surface (the C-face of the {0001} surfaces) in vacuum or in a Si-rich environment are compared. It is found that different interface structures occur for the two situations. The former yields a well known 3x3 reconstructed interface, whereas the latter produces an interface with rt(43)xrt(43)-R\pm7.6 degrees symmetry. This structure is shown to contain a graphene-like layer with properties similar to the 6rt(3)x6rt(3)-R30 degrees "buffer layer" that forms on the Si(0001) surface (the Si-face).
Independent Tuning of Surface Acoustic-Waves and Spin-Waves via Buffer-Layer Engineering in Co2FeGe Heusler Thin Films
Adam Mickiewicz University（亚当·密茨凯维奇大学）；Universidade do Porto（波尔图大学）；Universidad de Zaragoza-CSIC（萨拉戈萨大学-西班牙国家研究委员会）；NanoBiomedical Centre, Adam Mickiewicz University（纳米生物医学中心，亚当·密茨凯维奇大学）
Graphene, consisting of an inert, thermally stable material with an atomically flat, dangling bond-free surface is by essence an ideal template layer for van der Waals heteroepitaxy of two-dimensional materials such as silicene. However, depending on the synthesis method and growth parameters, graphene (Gr) substrates could exhibit, on a single sample, various surface structures, thicknesses, defects, and step heights. These structures noticeably affect the growth mode of epitaxial layers, e.g. turning the layer-by-layer growth into the Volmer-Weber growth promoted by defect-assisted nucleatio…
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 …
Epitaxial graphene, grown on SiC(0001) surface, has been widely studied both experimentally and theoretically. It was found that first epitaxial graphene layer in such structures is a buffer layer i.e. there are no characteristic Dirac cones in the band structure associated with it. However, C. Riedl et al. (Phys. Rev. Lett. 103, 246804 (2009)) in their experimental work observed recently that hydrogen intercalation of SiC-graphene samples can recover electronic properties typical to selfstanding graphene. The possible scenarios of hydrogen intercalation inducing graphene layer decoupling, inc…
Thermal decomposition of vicinal 6H-SiC(0001) surfaces with off-angles toward the $[1\bar{1}00]$ direction results in the appearance of pairs of (0001) macroterraces and $(1\bar{1}0n)$ macrofacets covered with graphene, as follows. A carpet-like carbon layer grows on the surface, covering both the macroterraces and macrofacets; it forms $(6\sqrt{3} \times 6\sqrt{3})$ buffer layer on the former ones, whereas its partial periodic bonding with the SiC steps on the latter ones generates a pseudo-graphene nanoribbon (pseudo-GNR) array. The nanoribbons have a width of 2 nm and are aligned in the $[1…
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 …
Despite the large literature focused on the growth of graphene (Gr) on 6H-SiC(0001) by chemical vapour deposition (CVD), some important issues have not been solved and full wafer scale epitaxy of Gr remains challenging, hampering applications in microelectronics. With this study we shed light on the generic mechanism which produces the coexistence of two different types of Gr domains, whose proportion can be carefully controlled by tuning the H2 flow rate. For the first time, we show that the growth of Gr using CVD under H2/Ar flow rate proceeds in two stages. Firstly, the nucleation of free-s…
Engineering Ferroelectric Phases in BaTiO$_3$ Films via Buffer-Layer Induced Misfit Strain Reversal
Southern Federal University（南联邦大学）；Federal Research Centre The Southern Scientific Centre of the Russian Academy of Sciences（俄罗斯科学院南方科学中心联邦研究中心）；Jožef Stefan Institute（约瑟夫斯蒂芬研究所）
The potential of graphene to impact the development of the next generation of electronics has renewed interest in its growth and structure. The graphitization of hexagonal SiC surfaces provides a viable alternative for the synthesis of graphene, with wafer-size epitaxial graphene on SiC(0001) now possible. Despite this recent progress, the exact nature of the graphene-SiC interface and whether the graphene even has a semiconducting gap remain controversial. Using scanning tunneling microscopy with functionalized tips and density functional theory calculations, here we show that the interface i…
Graphene on silicon carbide (SiC) bears great potential for future graphene electronic applications because it is available on the wafer-scale and its properties can be custom-tailored by inserting various atoms into the graphene/SiC interface. It remains unclear, however, how atoms can cross the impermeable graphene layer during this widely used intercalation process. Here we demonstrate that, in contrast to the current consensus, graphene layers on SiC are not homogeneous, but instead composed of domains of different crystallographic stacking. We show that these domains are intrinsically for…
Generally, it is supposed that the Fermi level in epitaxial graphene is controlled by two effects: p-type polarization doping induced by the bulk of the hexagonal SiC(0001) substrate and overcompensation by donor-like states related to the buffer layer. In this work, we evidence that this effect is also related to the specific underlying SiC terrace. We fabricated a periodic sequence of non-identical SiC terraces, which are unambiguously attributed to specific SiC surface terminations. A clear correlation between the SiC termination and the electronic graphene properties is experimentally obse…
Nickel intercalation in epitaxial graphene on SiC(0001): a novel platform for engineering two-dimensional heterostructures
Istituto Nanoscienze–CNR（意大利国家研究委员会纳米科学研究所）；NEST-Scuola Normale Superiore（NEST-比萨高等师范学院）；Department of Physical Science, Earth, and Environment, University of Siena（锡耶纳大学地球与环境物理科学学院）；Graphene Laboratories, Istituto Italiano di Tecnologia（意大利理工大学石墨烯实验室）
