Epitaxial Graphene Layers Research Articles

Atomic-scale manipulation of buried graphene–silicon carbide interface by local electric field

Precision of scanning tunneling microscopy (STM) enables control of matter at scales of single atoms. However, transition from atomic-scale manipulation strategies to practical devices encounters fundamental problems in protection of the designer structures formed atop the surface. In this context, STM manipulation of subsurface structures on technologically relevant materials is encouraging. Here, we propose a material platform and protocols for precise manipulation of a buried graphene interface. We show that an electric field from the STM tip reversibly controls breaking and restoring of covalent bonds between the graphene buffer layer and the SiC substrate. The process involves charge redistribution at the atomically sharp interface plane under the epitaxial graphene layer(s). This buried manipulation platform is laterally defined by unit cells from the corresponding (6×6)SiC moiré lattice of the epitaxial graphene. Local and reversible electric-field-induced patterning of graphene heterostructures from the bottom interface creates an alternative architecture concept for their applications.

Growth and atomic structure of twisted bilayer graphene formed on Ni(111) by segregation

Angle-resolved photoelectron spectroscopy and intensity analysis of low-energy electron diffraction were used to explore the growth and atomic structure of twisted bilayer graphene (tBLG) formed on Ni(111) by segregation of dissolved carbon. tBLG was composed of an epitaxial graphene layer in contact with the substrate and an overlaying rotated one with rotation angles of ∼15°. The top rotated graphene layer has been shown to have little effect on the underlying epitaxial graphene/Ni(111) system, indicating that the adjacent graphene layers are effectively decoupled. The process in which the tBLG gradually became dominant on the surface with repeated heating is discussed.

Effect of Polar Faces of SiC on the Epitaxial Growth of Graphene: Growth Mechanism and Its Implications for Structural and Electrical Properties

In this study, epitaxial graphene layers of cm2 sizes were grown on silicon carbide (SiC) substrates by high-temperature sublimation. The behavior of the two crystallographic SiC-polar faces and its effect on the growth mechanism of graphene layers and their properties were investigated. Crystallographic structural differences observed in AFM studies were shown to cause disparities in the electrical conductivity of the grown layers. On the silicon-polar (Si-polar) face of SiC, the graphene formation occurred in spike-like structures that originated orthogonally from atomic steps of the substrate and grew outwards in the form of 2D nucleation with a fairly good surface coverage over time. On the carbon-polar (C-polar) face, a hexagonal structure already formed at the beginning of the growth process. On both polar faces, the known process of step-bunching promoted the formation of nm-scale structural obstacles. Such a step-bunching effect was found to be more pronounced on the C-polar face. These 2D-obstacles account for a low probability of a complete nano-sheet formation, but favor 2D-structures, comparable to graphene nanoribbons. The resulting direction-dependent anisotropic behavior in electrical conductivity measured by four-point probe method mainly depends on the height and spacing between these structural-obstacles. The anisotropy becomes less prudent as and when more graphene layers are synthesized.

Effects of an epitaxial graphene layer for the growth of nickel silicides on a Ni(111) substrate

In this paper, we report on an in-depth study on the growth of nickel silicides, either on a clean Ni(111) substrate or in the presence of a previously-grown epitaxial single graphene (Gr) layer, by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). We demonstrate that two different nickel silicides, namely Ni3Si and Ni2Si, progressively form as the annealing temperature is increased from 450 °C to 600 °C. The presence of the Gr layer does not change the nature of the two silicide phases but rather affects the morphology of the silicide overlayer. Indeed, in the presence of Gr, the deposited silicon atoms intercalate by passing through the Gr defects or domain boundaries and accumulate on specific sample areas, resulting in the formation of multilayer silicide islands. In the absence of Gr, the deposited silicon atoms react uniformly with the nickel substrate, resulting in the formation of homogeneous large scale silicide layers.

Fabrication and Characterization of Epitaxial Graphene Field Effect Transistor Devices Based on a Monolithic Bottom Gate

Current electronics technology increasingly demands higher integration, flexibility, higher efficiency, and performance aspects such as compatibility with higher temperature operation of the semiconductor devices, which may find limitations when silicon is used. The superior intrinsic properties of SiC, eventually combined with the ability of growing monolithically epitaxial, high quality graphene on a SiC wafer (1), makes it a reliable alternative for some electronic applications, such as field effect transistors (FET), radio frequency (RF) power amplifiers, integrated circuits (IC), or sensors. In this work, we describe the fabrication and preliminary electrical characterizations of epitaxial graphene (EG) on a SiC substrate FET devices based on an alternative back gate architecture. We propose a new approach in which the FET device is built on a 4◦ off-axis cut, N+ doped 4H-SiC substrate (the back gate) with, on top of, it a 1μm semi-insulating homoepitaxial layer of SiC compensated with vanadium (the dielectric layer). EG will be used as FET conduction channel. Using this V-compensated dielectric layer is aimed to minimize effects on the FET characteristics such as from defects in the SiC crystal, especially below the FET active areas, which would have occurred when using ion implantation to create a buried gate. The EG film was grown by the high temperature Si sublimation method under an Ar ambient. Raman spectroscopy, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were applied in the structural characterization of epigraphene. The lack of D-peak in Raman spectra, together with SEM and AFM images, indicate that high quality monolayer to few layer epitaxial graphene fully covering the SiC surface is deposited. The electrical characteristics of the EG channel-devices and the functionality of the bottom gate were examined with 2-probe and 4-probe method. The electrical properties of the FET devices were also investigated with 3 terminal configuration.

Environmental effects on layer-dependent dynamics of Dirac fermions in quasicrystalline bilayer graphene

The carrier dynamics in various types of epitaxial graphene layers on SiC substrates was investigated by means of time- and angle-resolved photoemission spectroscopy (TARPES). Layer-dependent electron doping was observed in the Dirac bands of quasicrystalline bilayer graphene after optical pumping, leading to generation of transient voltage between the upper and lower layers. The amount of photoinduced carrier transport depends on the distance from the substrate. A comparison of the TARPES results of single-layer graphene between flat and stepped SiC substrates experimentally indicates that a source of the doping carriers likely originates from interface step states. The dynamic model is described based on the electronic structure, calculated using density functional theory.

Oxygen reduction reaction properties of vacuum-deposited Pt on thermally grown epitaxial graphene layers

We prepared platinum nanostructures on epitaxial graphene layers (EGLs) generated via the thermal annealing of SiC(0001) substrates [Pt/EGLs/SiC(0001)], and the oxygen reduction reaction (ORR) properties were investigated. 4H_ and 6H_SiC(0001) substrates were annealed at 1600 °C in a vacuum to generate EGLs through the sublimation of Si atoms from the Si-terminated SiC(0001) substrate surfaces. Subsequently, a 1.2 nm thick Pt layer was vacuum-deposited on the EGLs/SiC(0001) surfaces at 10−8 Pa to synthesize Pt/EGLs/SiC(0001) nanostructures. Furthermore, the Pt nanostructures were fabricated separately on a 500 eV hydrogen-ion-beam (H2+-beam) irradiated EGL/6H_SiC(0001) [Pt/H2+-EGLs/SiC(0001)]. The ORR properties of the three vacuum-synthesized Pt/EGLs/SiC(0001) samples were evaluated, and the Pt/H2+-EGLs/SiC(0001) surface revealed improved electrochemical structural stability, that is, enhanced ORR durability relative to the non-H2+-irradiated Pt/EGLs/SiC(0001) surfaces.

Elucidating the electronic and magnetic properties of epitaxial graphene grown on SiC with a defective buffer layer

The epitaxial graphene layer (EG) grown on silicon carbide (SiC) is severely affected by the presence of the underlying graphene buffer layer (BL). However, little information is available on the alteration of the magnetic and electronic properties of the top layer when the BL presents structural defects. Herein, by means of first-principle density functional calculations, we investigate the electronic and magnetic properties of the SiC–BL–EG system, with a single vacancy, divacancy or a Stone–Wales defective buffer layer. Our results indicate that new highly stable magnetic states are observed in the SiC–BL–EG non-defective and defective systems, as compared to the SiC–BL models. In addition, the energy differences among the degenerate magnetic states originally found in the SiC-defective systems are further reduced upon inclusion of the EG layer. Interestingly, for the single-vacancy system, a p-type doping is noticed in the spin down channel of the M = 4 µB configuration. This is in sharp contrast with the n-type doping generally measured for EG. Moreover, charge neutrality was observed in two cases namely, the half-semimetal ferromagnetic configurations of the non-defective and the single-vacancy systems. This result may open new avenues to control the electronic doping of the epitaxial graphene layer via defect engineering.

Graphene Blocks Oxidative Segregation of Iron Dissolved in Platinum: A Model Study

AbstractIron–platinum (Fe–Pt) compounds are well known for their interesting magnetic and electrocatalytic properties. However, iron segregation and iron oxides formation under oxidative conditions may influence the characteristics of Fe–Pt systems. Several approaches are used to protect the Fe–Pt compounds from oxidation, the most promising of which involves covering the material with a protective graphitic layer. By performing model‐type ultrahigh vacuum (UHV) studies, it is shown that a layer of epitaxial graphene (Gr) grown on a [111]‐oriented single‐crystal platinum substrate with thermally dissolved iron (Fe–Pt(111) surface alloy) effectively blocks iron segregation and iron oxides formation under oxidative conditions, while still allowing for the adsorption of oxygen atoms underneath the carbon layer. The oxidation is monitored in real time and at the micrometer scale using low energy electron microscopy (LEEM) and local diffraction (μLEED). Notably, a similar result is obtained for a poorly ordered Gr‐like carbon layer grown directly on a Fe–Pt(111) substrate. The findings are rationalized in terms of a locally lowered partial oxygen pressure and inhibited iron oxide growth in a confined space between the carbon layer and the metal support.

Toward Large-Scale Ga2O3 Membranes via Quasi-Van Der Waals Epitaxy on Epitaxial Graphene Layers.

Epitaxial growth using graphene (GR), weakly bonded by van der Waals force, is a subject of interest for fabricating technologically important semiconductor membranes. Such membranes can potentially offer effective cooling and dimensional scale-down for high voltage power devices and deep ultraviolet optoelectronics at a fraction of the bulk-device cost. Here, we report on a large-area β-Ga2O3 nanomembrane spontaneous-exfoliation (1 cm × 1 cm) from layers of compressive-strained epitaxial graphene (EG) grown on SiC, and demonstrated high-responsivity flexible solar-blind photodetectors. The EG was favorably influenced by lattice arrangement of SiC, and thus enabled β-Ga2O3 direct-epitaxy on the EG. The β-Ga2O3 layer was spontaneously exfoliated at the interface of GR owing to its low interfacial toughness by controlling the energy release rate through electroplated Ni layers. The use of GR templates contributes to the seamless exfoliation of the nanomembranes, and the technique is relevant to eventual nanomembrane-based integrated device technology.

Sub-10nm spatial resolution for electrical properties measurements using bimodal excitation in electric force microscopy.

We demonstrate that under ambient and humidity-controlled conditions, operation of bimodal excitation single-scan electric force microscopy with no electrical feedback loop increases the spatial resolution of surface electrical property measurements down to the 5nm limit. This technical improvement is featured on epitaxial graphene layers on SiC, which is used as a model sample. The experimental conditions developed to achieve such resolution are discussed and linked to the stable imaging achieved using the proposed method. The application of the herein reported method is achieved without the need to apply DC bias voltages, which benefits specimens that are highly sensitive to polarization. Besides, it allows the simultaneous parallel acquisition of surface electrical properties (such as contact potential difference) at the same scanning rate as in amplitude modulation atomic force microscopy (AFM) topography measurements. This makes it attractive for applications in high scanning speed AFM experiments in various fields for material screening and metrology of semiconductor systems.

レーザー光励起によるSiC(0001)表面上グラフェン層の剥離：光による単層グラフェン創製

レーザー光励起によるSiC(0001)表面上グラフェン層の剥離：光による単層グラフェン創製

Raman Spectroscopy Imaging of Exceptional Electronic Properties in Epitaxial Graphene Grown on SiC.

Graphene distinctive electronic and optical properties have sparked intense interest throughout the scientific community bringing innovation and progress to many sectors of academia and industry. Graphene manufacturing has rapidly evolved since its discovery in 2004. The diverse growth methods of graphene have many comparative advantages in terms of size, shape, quality and cost. Specifically, epitaxial graphene is thermally grown on a silicon carbide (SiC) substrate. This type of graphene is unique due to its coexistence with the SiC underneath which makes the process of transferring graphene layers for devices manufacturing simple and robust. Raman analysis is a sensitive technique extensively used to explore nanocarbon material properties. Indeed, this method has been widely used in graphene studies in fundamental research and application fields. We review the principal Raman scattering processes in SiC substrate and demonstrate epitaxial graphene growth. We have identified the Raman bands signature of graphene for different layers number. The method could be readily adopted to characterize structural and exceptional electrical properties for various epitaxial graphene systems. Particularly, the variation of the charge carrier concentration in epitaxial graphene of different shapes and layers number have been precisely imaged. By comparing the intensity ratio of 2D line and G line—“I2D/IG”—the density of charge across the graphene layers could be monitored. The obtained results were compared to previous electrical measurements. The substrate longitudinal optical phonon coupling “LOOPC” modes have also been examined for several epitaxial graphene layers. The LOOPC of the SiC substrate shows a precise map of the density of charge in epitaxial graphene systems for different graphene layers number. Correlations between the density of charge and particular graphene layer shape such as bubbles have been determined. All experimental probes show a high degree of consistency and efficiency. Our combined studies have revealed novel capacitor effect in diverse epitaxial graphene system. The SiC substrate self-compensates the graphene layer charge without any external doping. We have observed a new density of charge at the graphene—substrate interface. The located capacitor effects at epitaxial graphene-substrate interfaces give rise to an unexpected mini gap in graphene band structure.

Silicon Carbide Stacking‐Order‐Induced Doping Variation in Epitaxial Graphene

AbstractGenerally, 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 silicon carbide (SiC)(0001) substrate and overcompensation by donor‐like states related to the buffer layer. The presented work is evidence that this effect is also related to the specific underlying SiC terrace. Here a periodic sequence of non‐identical SiC terraces is fabricated, which are unambiguously attributed to specific SiC surface terminations. A clear correlation between the SiC termination and the electronic graphene properties is experimentally observed and confirmed by various complementary surface‐sensitive methods. This correlation is attributed to a proximity effect of the SiC termination‐dependent polarization doping on the overlying graphene layer. These findings open a new approach for a nano‐scale doping‐engineering by the self‐patterning of epitaxial graphene and other 2D layers on dielectric polar substrates.

Investigation of epitaxial graphene via Raman spectroscopy: Origins of phonon mode asymmetries and line width deviations

In this work a comprehensive study is presented for the analysis of epitaxial graphene layers using Raman spectroscopy. A wide range of graphene types is covered, from defective/polycrystalline single layer graphene to multilayer graphene with low defect density. On this basis the influence of strain type, Fermi level and number of layers on the Raman spectrum of graphene is investigated. A detailed view on the 2D/G dispersion and the respective slopes of uniaxially and biaxially strained graphene is given and its implications on the asymmetry of the G peak analyzed. A linear dependency of the phonon mode asymmetry on uniaxial strain is presented in addition to the known Fermi level dependence. Additional impacts on the asymmetry are found to be arising from the defect density and transfer doping of adsorbates. The discovered transfer doping mechanism is contrary to pure phonon excitation through excitons and exhibits increasing asymmetry with increasing Fermi level. A new characteristic correlation between the 2D mode line width and the inverse I(D)/I(G) ratio is introduced that allows the determination of the strain type and layer number and explains the difference between Raman line widths of monolayer graphene on different substrates.

Graphene Quality Assessment Using an Entropy Approach of SEM Images

In this work a new approach of analyzing epitaxial graphene layers on semi-insulating SiC through the gray-scale entropy of SEM images as a measure for the graphene inhomogeneity is demonstrated. Raman spectroscopy as a versatile and the standard tool for graphene characterization allows additionally the determination of the layer properties such as layer count, Fermi level, defect concentration and strain. It is shown that the gray-scale entropy correlates with the defect density derived from Raman measurements and thus can be used as an additional characterization technique with much higher resolution than the conventional Raman spectroscopy allows. As a consequence, the results are used to reflect the two-stepped growth itself and to conclude for advantageous growth conditions.

Layer dependence of graphene-diamene phase transition in epitaxial and exfoliated few-layer graphene using machine learning

The study of the nanomechanics of graphene—and other 2D materials—has led to the discovery of exciting new properties in 2D crystals, such as their remarkable in-plane stiffness and out of plane flexibility, as well as their unique frictional and wear properties at the nanoscale. Recently, nanomechanics of graphene has generated renovated interest for new findings on the pressure-induced chemical transformation of a few-layer thick epitaxial graphene into a new ultra-hard carbon phase, named diamene. In this work, by means of a machine learning technique, we provide a fast and efficient tool for identification of graphene domains (areas with a defined number of layers) in epitaxial and exfoliated films, by combining data from atomic force microscopy (AFM) topography and friction force microscopy (FFM). Through the analysis of the number of graphene layers and detailed Å-indentation experiments, we demonstrate that the formation of ultra-stiff diamene is exclusively found in 1-layer plus buffer layer epitaxial graphene on silicon carbide (SiC) and that an ultra-stiff phase is not observed in neither thicker epitaxial graphene (2-layer or more) nor exfoliated graphene films of any thickness on silicon oxide (SiO2).

Effects of Annealing Parameters on Epitaxial Graphene on SiC Substrates

The effects of annealing on epitaxial graphene on SiC substrates with various conditions are investigated. Results show that high pressure hydrogen atmosphere is more effective to decouple the epitaxial graphene from SiC substrate than that of a relative lower pressure process. Besides, the characteristic 2D-peak of graphene in Raman spectra disappeared with an annealing temperature 1000 °C, which means that the epitaxial graphene layer was decomposed in this condition. The study also shows that the decomposition of graphene can be effectively suppressed by increasing carbon vapor partial pressure through introducing ethylene during high pressure hydrogen annealing at 1000 °C. And the epitaxial graphene is successfully transferred to quasi free standing graphene by the annealing with an appropriate flow of ethylene.

FeSx-graphene heterostructures: Nanofabrication-compatible catalysts for ultra-sensitive electrochemical detection of hydrogen peroxide

Hydrogen peroxide (H2O2) is one of the most important redox-active metabolites in biological processes, from cell signaling to oxidative stress. Incorporation of catalytic nano-phase heterostructures can significantly improve the detection limit of H2O2 electrochemical sensors. Here, we report an iron sulfide (FeSx)/graphene heterostructure for detection of H2O2 down to ∼500 picoMolar pM, with a dynamic range over six orders of magnitude. By relying on the Fenton reaction, the developed catalyst enables selective detection of H2O2 vs. common redox-active metabolites in physiological condition. Electrochemical analysis reveals a strong substrate-dependent performance, where the type and doping of the graphene layer dramatically impact sensitivity. Specifically, for the first time, the electrochemical performance of chemical vapor deposited versus epitaxial graphene (EG) layers is examined and compared in a systematic way. The results demonstrate that a heterostructure of FeSx with chemical vapor deposited graphene (CVD-G) exhibits the highest reduction in charge transfer resistance (more than 16×), leading to a significantly higher sensitivity compared to n-doped and p-doped EG. Furthermore, the excellent sensitivity and material stability of the developed heterostructures enable in situ detection of redox changes due to heat shock-induced oxidative stress on E. coli cells. Since the developed heterostructures are directly-grown, they can be patterned with nanometer-scale resolution. Hence, this work can open up new possibilities for developing ultracompact devices for monitoring redox changes in situ, with high spatial resolution.

Confocal laser scanning microscopy for rapid optical characterization of graphene

Two-dimensional (2D) materials such as graphene have become the focus of extensive research efforts in condensed matter physics. They provide opportunities for both fundamental research and applications across a wide range of industries. Ideally, characterization of graphene requires non-invasive techniques with single-atomic-layer thickness resolution and nanometer lateral resolution. Moreover, commercial application of graphene requires fast and large-area scanning capability. We demonstrate the optimized balance of image resolution and acquisition time of non-invasive confocal laser scanning microscopy (CLSM), rendering it an indispensable tool for rapid analysis of mass-produced graphene. It is powerful for analysis of 1–5 layers of exfoliated graphene on Si/SiO2, and allows us to distinguish the interfacial layer and 1–3 layers of epitaxial graphene on SiC substrates. Furthermore, CLSM shows excellent correlation with conventional optical microscopy, atomic force microscopy, Kelvin probe force microscopy, conductive atomic force microscopy, scanning electron microscopy and Raman mapping.