The paucity of research on hydrogen evolution reaction (HER) under neutral conditions, which is a more sustainable way to produce H2 compared to acidic and alkaline HER, encourages the development of efficient catalytic materials and devices and deeper investigation of the mechanisms behind neutral HER. We present an electrode concept for facilitating HER under neutral conditions. The concept entails the use of annealing-reshaped silver (Ag) nanoparticle array on monolayer epitaxial graphene (MEG) on 4H-SiC. Measurements of HER performance show more positive onset potential of the cathodic HER for Ag-decorated MEG compared to that for pristine MEG, indicating improved water dissociation at Ag/MEG electrodes. Complementary morphological characterization, absorption measurements, and Raman mapping analysis enable us to ascribe the enhanced catalytic performance of electrodes decorated with 2 nm thick annealed Ag on the synergetic effect originating from simultaneous water reduction on circular Ag nanoparticles of 31 nm in diameter and on compressively strained Ag-free graphene regions. The overall results pave the way toward development of stable van der Waals heterostructure electrodes with a tunable metal–carbon interaction for fast HER under neutral conditions.

Thin Ag films, with nominal thickness in the range 2 to 30 nm, are deposited using direct current magnetron sputtering and film morphology is studied by means of plan-view scanning electron microscopy. We find that for 2 mn nominal thickness the film surface consists of isolated circular nanoscale islands, which become interconnected as further material is deposited, leading to a continuous film at a nominal thickness of 30 nm. Our experimental findings are discussed in the context of the density functional theory results, which show that van der Waals forces dominate the interaction between Ag and epitaxial graphene. We also performed micro-Raman analysis and we find that the G and 2D modes of epitaxial graphene exhibit a red-shift upon Ag-layer deposition; which is interpreted as a result of charge transfer at the Ag/graphene interface. Moreover, we observed a pronounced enhancement of the G peak amplitude and area irrespective of the film nominal thickness and morphology, which we attribute to a combination of the charge transfer and plasmonic resonance effects. Our observations provide a critical information on the interaction between Ag and epitaxial graphene, which can be useful to design electronic and sensing devices based on Ag-epitaxial graphene hybrids.

We present a Raman spectroscopy study on epitaxial graphene decorated with thin Ag films (2-15 nm), which are deposited using magnetron sputtering. We find that the presence of Ag on the graphene surface induces doping, the uniformity and efficiency of which is determined by Ag nominal thickness. Deposition of Ag films with thicknesses up to 5 nm favors the effective electron transfer from Ag to epitaxial graphene. A significant redshift and broadening of the 2D peak are observed with increasing the Ag-layer thickness above 5 nm, which is indicative of large strain and doping fluctuations. We also observe a non-trivial linear growth of 2D/G peak intensity ratio with increasing D/G ratio for all Ag-decorated samples, which is explained by increase of peak amplitude due to surface enhanced Raman scattering and charged impurity-induced screening caused by the presence of Ag on the graphene surface.

The integration of epitaxial graphene on 4H-SiC with different metals may allow tunability of electronic and optical properties of graphene, enabling novel high-performance devices. Here we present a Raman spectroscopy study on epitaxial graphene decorated with electrodeposited Pb and Li adatoms and with magnetron sputtered 5 nm-thick Ag nano-island films. We find that the presence of metals on the epitaxial graphene surface generates defects and induces n-type doping, which is evidenced by the observation of the defect related Raman modes (namely D, D' and D + G) and systematic red-shift of the main characteristic modes of graphene. In-depth statistical analysis of the Raman data before and after metal deposition complemented by density functional theory (DFT) calculations allowed to link the interaction strength between the three selected metals and graphene with the metal-induced changes in the vibrational/electronic properties of graphene. Large-area uniform electron doping of epitaxial graphene and surface-enhanced Raman scattering (SERS) effect are reached by room temperature deposition of Ag nano-island films. Very promising results have been obtained from graphene subjected to electrochemical intercalation by Li, which can serve as prerequisites of the construction of Li batteries. The strong interaction between Li or Pb with graphene implies the possibility to exploit the epitaxial graphene as an efficient material for energy storage or for heavy metal sensing, while predominant van der Waals interaction between Ag and graphene favors the formation of extremely thin silver coatings towards two-dimensional metal systems. The present results give better understanding of the nature of epitaxial graphene response to metal deposition and can be useful to design high-performance energy storage devices, optical sensors and heavy metal detection systems. (C) 2019 Elsevier Ltd. All rights reserved.

In this paper, micro-Raman mapping and conductive atomic force microscopy (C-AFM) were jointly applied to investigate the structural and electrical homogeneity of quasi-free-standing monolayer graphene (QFMLG), obtained by high temperature decomposition of 4H-SiC(0001) followed by hydrogen intercalation at 900 °C. Strain and doping maps, obtained by Raman data, showed the presence of sub-micron patches with reduced hole density correlated to regions with higher compressive strain, probably associated with a locally reduced hydrogen intercalation. Nanoscale resolution electrical maps by C-AFM also revealed the presence of patches with enhanced current injection through the QFMLG/SiC interface, indicating a locally reduced Schottky barrier height (ΦB). The ΦB values evaluated from local I-V curves by the thermionic emission model were in good agreement with the values calculated for the QFMLG/SiC interface using the Schottky-Mott rule and the graphene holes density from Raman maps. The demonstrated approach revealed a useful and non-invasive method to probe the structural and electrical homogeneity of QFMLG for future nano-electronics applications.

In this work, we report a multi-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 x 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 SiOx 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 SiO2 and its thickness increases with aging in normal ambient conditions. The formation mechanism of the interface layer is discussed. It is shown by torsion resonance conductive atomic force microscopy that the interface layer causes the formation of non-ideal Schottky contact between ML graphene and SiC. This is attributed to the presence of a large density of interface states. Mid-infrared optical Hall effect measurements revealed Landau-level transitions in FLG that have a square-root dependence on magnetic field, which evidences a stack of decoupled graphene sheets. Contrary to previous works on decoupled C-face graphene, our BL and FLG are composed of ordered decoupled graphene layers without out-of-plane rotation. (C) 2017 Elsevier Ltd. All rights reserved.

We show experimentally that few layer graphene (FLG) grown on the carbon terminated surface (C-face) of 3C-SiC(111) is composed of decoupled graphene sheets. Landau level spectroscopy on FLG graphene is performed using the infrared optical Hall effect. We find that Landau level transitions in the FLG exhibit polarization preserving selection rules and the transition energies obey a square-root dependence on the magnetic field strength. These results show that FLG on C-face 3C-SiC(111) behave effectively as a single layer graphene with linearly dispersing bands (Dirac cones) at the graphene K point. We estimate from the Landau level spectroscopy an upper limit of the Fermi energy of about 60 meV in the FLG, which corresponds to a carrier density below 2.5 × 1011 cm−2. Low-energy electron diffraction μ-LEED) reveals the presence of azimuthally rotated graphene domains with a typical size of ≤200 nm. μ-LEED mapping suggests that the azimuth rotation occurs between adjacent domains within the same sheet rather than vertically in the stack.

Graphene grown by sublimation on C-terminated surface of SiC (0001) has attracted significant attention due its high free carrier mobility and the possibility of wafer scale production. However, C-face graphene typically grows in three-dimensional mode resulting in multi-layer graphene (MLG), with small domains and rough surface. Another issue with C-face graphene is that the individual layers of the MLG show a gradual doping profile with different mobility parameters which limit the use of C-face graphene in device application. Therefore, it is critical to achieve growth of monolayer and bilayer graphene with good surface morphology and controlled electronic properties. In this work, we demonstrated the growth of large-area monoand decoupled bilayer graphene on C-face 4H-SiC (0001). Graphene was grown by high-temperature sublimation at 1950°C in Ar atmosphere. The number of graphene layers was determined using reflectance mapping and low-energy electron microscopy (LEEM). The transport properties were investigated using conductive atomic force microscopy (C-AFM), micro-Raman spectroscopy and mid-infrared optical Hall effect (OHE). The microRaman spectroscopy maps were measured simultaneously with reflectance maps, which allowed precise determination of the graphene layers properties such as doping. The LEEM and reflectance mapping showed large-area monoand decoupled bilayer graphene with size of 600 μm2 and 200 μm2 over the total probed area of 900 μm2, respectively and small domains of thick graphene layers [Fig.1 (a)]. The correlation between the C-AFM, reflectance and micro-Raman spectroscopy showed that the monoand the decoupled bilayer graphene are n-type doped with different electron concentrations: the doping concentration of monolayer graphene was of the order of 1013 cm-2 and the decoupled bilayers were quasi-neutral. The OHE measurements showed Landau transition energies with a root square dependence on the magnetic field [Fig.1 (b)], which can be attributed to monolayer graphene or stacks of decoupled graphene layers. This result was confirmed by microRaman spectroscopy that showed a symmetric 2D Raman peak with a full width at half maximum varying between 26 cm-1 and 40 cm-1. Further a splitting of the Landau levels was observed [Fig.1 (b)], indicating different Fermi velocities. By comparing the doping concentration obtained from C-AFM and micro-Raman spectroscopy on one hand, and the inter-Landau-level transition energies obtained from magneto optical hall effect on the other hand, we attributed the observed Landau levels to the decoupled multilayer graphene layers. The splitting of the Landau levels was attributed to the different strain environment of the layers such as uniaxial strain.

Thin ion transmission detectors can be used as a part of a telescope detector for mass and energy identification but also as a pre-cell detector in a microbeam system for studies of biological effects from single ion hits on individual living cells. We investigated a structure of graphene on silicon carbide (SiC) with the purpose to explore a thin transmission detector with a very low noise level and having mechanical strength to act as a vacuum window. In order to reach very deep cavities in the SiC wafers for the preparation of the membrane in the detector, we have studied the Inductive Coupled Plasma technique to etch deep circular cavities in 325μm prototype samples. By a special high temperature process the outermost layers of the etched SiC wafers were converted into a highly conductive graphitic layer. The produced cavities were characterized by electron microscopy, optical microscopy and proton energy loss measurements. The average membrane thickness was found to be less than 40μm, however, with a slightly curved profile. Small spots representing much thinner membrane were also observed and might have an origin in crystal defects or impurities. Proton energy loss measurement (also called Scanning Transmission Ion Microscopy, STIM) is a well suited technique for this thickness range. This work presents the first steps of fabricating a membrane structure of SiC and graphene which may be an attractive approach as a detector due to the combined properties of SiC and graphene in a monolithic materials structure.

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 (0001), 6H–SiC (0001), and 3C–SiC (111) 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 (111) 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 ~4.5eV 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 (111) differs and has a determining role for the thickness and electronic properties homogeneity of the epitaxial graphene.

The graphene and silicon carbide can create a viable platform, for example by a monolithic transistor that uses the entire material system epitaxial graphene on silicon carbide [2]. This was shown to have an on/off ratio exceeding 10 4 and no damping at megahertz frequencies. The fabrication process requires, in the most simple realization, only one lithography step to build transistors, diodes, resistors and eventually integrated circuits without the need of metallic interconnects.

Epitaxial graphene (EG) grown by sublimation epitaxy on SiC holds great promise for large-scale production of next generation fast electronic devices. Despite significant progress and intense research efforts in the field, state-of-the-art EG shows mobility parameters that are still orders of magnitude lower than those of exfoliated graphene. Understanding the physical origin of the substantially different transport properties of epitaxial and free-standing graphene remains one of the major issues, and prevents further technological advances. The key point is to identify and control how the substrate affects graphene uniformity, thickness and carrier mobility. EG on the hexagonal polytypes of SiC has been extensively studied. On the other hand, cubic 3C-SiC substrates have not been explored, mostly due to the fact that they are not commercially available. However, 3C-SiC can offer advantages over the other poltytypes in terms of isotropic growth surfaces and device performance. Mapping nonuniformities in EG on a large-scale and possible interrelation to its electronic and transport properties will be beneficial for both studying substrate effects and device production. However, the simultaneous mapping of these properties presents a significant challenge due to the domain structure of EG and the fact that characterization techniques with different footprints need to be employed. In this work, we report large-area micro-ellipsometry mapping of thickness and electronic properties of EG grown on thick bulk-like 3C-SiC(111) layers. We explore both Si and C polarities of the substrate and discuss in detail the determining factors of EG thickness uniformity in relation to its electronic and transport properties. The EG layers were grown by high temperature sublimation in Ar2 atmosphere [1] under optimized conditions on the Siand C-face of home-grown 3C-SiC(111). The thick (few hundreds of micrometers) 3C-SiC layers were grown by sublimation epitaxy on 6H-SiC (0001) [2]. Spectroscopic ellipsometry mapping (SE) from 1.25 eV up to 5.45 eV was performed with an M2000 rotating compensator ellipsometer from J. A. Woollam Co. on a circular area of the samples with a diameter of 0.5 cm and with a microspot of 30x30μm. Details about the experimental and modeling procedures can be found in Refs. 3-4. SE thickness maps [Figs 1(a) and (c)] demonstrate that 1 monolayer (ML) graphene can be achieved on Siand C-polarities of the 3C-SiC(111) substrates in good agreement with low-energy electron microscopy and Raman scattering spectroscopy. Large domains with homogeneous ML coverage with size of ~2x2 mm are grown at the Si-face 3C-SiC [Fig. 1(a)]. In this case, few thick graphite-like islands are formed and their nucleation sites can be correlated with an increased interface roughness of the substrate. On the C-polar 3C-SiC the formation of the thick graphite islands is suppressed [Fig. 1(c)]. However, the areas of EG with homogeneous thickness have considerably smaller size and are randomly distributed indicating a different formation mechanism than for EG on the Si-face of 3C-SiC. Our results indicate that the polarity of the 3C-SiC substrates critically affects the formation mechanism and growth kinetics of EG, which will be discussed in relation to substrate defects and surface status. Furthermore, the maps of the free-charge carrier scattering time show that higher mobility can be achieved in the homogeneous areas of 1 ML EG, while the thicker graphite islands show that the carrier mobility drastically decreases [Figs. 1(b) and (d)]. Finally, correlation between the number of MLs and the energy of the critical point (CP) associated with an exciton enhanced Van Hove singularity at ~4.5 eV may be established for both substrate polarities (Fig.2). The CP energy positions and dielectric function shape will be further discussed in view of strain and the interaction of EG with the substrate. The reported results can be used in future works on the application of optical micro-spectroscopy techniques to study electronic properties and monitor graphene thickness homogeneity.

Growth of epitaxial graphene was performed on the Si face of SiC substrates in an inductively heated furnace at a temperature of 2000°C and at an ambient argon pressure of 1 atm. We have studied thickness uniformity and wrinkling on graphene grown on 4H-SiC(0001), 6H-SiC(0001), and 3C-SiC(111) substrates. Graphene surface morphology, thickness, band structure, structure and electronic properties have been assessed by using AFM, LEEM, ARPS, STM, and STS respectively. Graphene formation has been analyzed in respect to step bunching and surface decomposition energy differences created by the SiC basal plane stacking sequence on different SiC polytypes. 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. The uniformity of silicon sublimation is an important factor for obtaining large area homogenous graphene. 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 demonstrated a monolayer (ML) graphene growth on all SiC polytypes, but larger area, over 50 50 m 2 , on cubic SiC (Fig. 1a). To study the surface restructuring during SiC sublimation we examined around 300 steps for each sample using AFM. The corresponding histogram of the step height for 4H-SiC (Fig. 1b) indicates that two bilayer-height steps are the most probable and four bilayer-height steps show a significant probability. For the 6H-SiC sample (Fig. 1c) two and three bilayer-height steps dominate. On the 3C-SiC graphene sample one Si-C bilayer height has the highest percentage (48%) of appearance although some larger steps are present (Fig. 1d). The sublimation rate of 3C-SiC is the same over the whole defect-free substrate surface due to the similar decomposition energy on all step terraces, this providing a uniform source of C on the surface which results in a superior uniformity of the grown graphene layer. It is worth noting that C contained in one unit cell (three Si-C bilayers) of 3C-SiC is sufficient to feed the formation of 1ML graphene. The 6H-SiC polytype shows close quality of graphene to that on the 3C-SiC polytype, because half of the unit cell contains three Si-C bilayers. The results for the 4H-SiC substrate coverage by graphene show that graphene formation process has narrower window of growth parameters. We have found that single Si-C bilayer steps with the same decomposition energy in the beginning of the graphene formation are the controlling factors for the uniformity of Si subtraction. Having a rather low step height distribution is one advantage of our results, since it has been reported that the resistance of epitaxial graphene on SiC increases linearly with step height on the substrate [1]. Wrinkling is a very general phenomenon in nature with dimensions spanning across length scales from meters down to nanometers. Graphene wrinkles (Fig. 2a) easily and often. They are larger in dimension and form by compressive strain induced during cooling from the growth temperature due to the difference in thermal expansion coefficients of graphene and SiC. Wrinkles are linear defects which can cause carrier scattering and decrease mobility. [2] Deep understanding and sufficient control of the wrinkle appearance are central to our current research interest. By modifying substrate conditions we have been able to change the wrinkle orientation from a random network to a full alignment (Fig. 2b) in a particular direction or radial, by partially reducing strain. By these results we have found out how the step size and point defect can rule wrinkle morphology. The behaviour of wrinkles during thermal cycling at the same temperature and different temperatures and also cooling down to 4 K has been studied. We also examine to what extent the electronic and structural integrity of graphene is preserved upon wrinkle formation by STM and STS. We will also present, from typical AFM images, an approximate evaluation of strain for the top graphene layer forming wrinkle network on different SiC samples with diverse wrinkle morphology and size. We observed that the wrinkles appear very rare in SiC wafers in comparison with small size of SiC substrates. It seems that the larger size of graphene sheets can sustain more compressive strain and avoid wrinkling. Wrinkling was studied in a series of computer simulation. The simulations were performed with the molecular dynamic method using AIREBO forcefield which is quite suitable for simulation of hydrocarbons and the Tersoff potential for Si-C simulation. The simulated samples have about 150,000 atoms including both graphene and substrates. The results confirmed the experimental findings of wrinkle formation.

One of the main challenges in the fabrication of device quality graphene is the achievement of large area monolayer graphene that is processing compatible. Here, the impact of the substrate properties on the thickness uniformity and electronic characteristics for epitaxial graphene on SiC produced by high temperature sublimation has been evidenced and discussed. Several powerful techniques have been used to collect data, among them large scale ellipsometry mapping has been demonstrated for the first time. The study is covering all three SiC polytype, e.g. 4H-, 6H- and 3C-SiC in order to reveal eventual peculiarities that have to be controlled during graphene growth. The advantage of the cubic polytype is unambiguously demonstrated.

Microfocal spectroscopic ellipsometry mapping of the electronic properties and thickness of epitaxial graphene grown by high-temperature sublimation on 3C-SiC (111) substrates is 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(111) differs and has a determining role for the thickness and electronic properties homogeneity of the epitaxial graphene.

Graphene samples were grown on the C-face of SiC, at high temperature in a furnace and an Ar ambient, and were investigated using LEEM, XPEEM, LEED, XPS and ARPES. Formation of fairly large grains (crystallographic domains) of graphene exhibiting sharp (1x1) patterns in mu.LEED was revealed and that different grains showed different azimuthal orientations. Selective area constant initial energy photoelectron angular distribution patterns recorded showed the same results, ordered grains and no rotational disorder between adjacent layers. A grain size of up to a few mu.m was obtained on some samples

Silicon carbide (SiC) is a wide band gap semiconductor satisfying requirements to replace silicon in devices operating at high power and high frequency at high temperature, and in harsh environments. Hexagonal polytypes of SiC, such as 6H-SiC and 4H-SiC are available on the power device markets. However, the cubic SiC (3C-SiC) polytype is still not industrially used, essentially due to the lack of 3CSiC substrates. This is mainly because of a high density of defects appearing in the crystals. Thus, it is critical to understand material growth and defect formation, and learn to control their appearance. Ensuring, that growth methods capable of large scale industrial production can be applied. The aim of this work was to develop operation conditions for fabrication of 3C-SiC crystals via understanding fundamentals of the growth process and to explore structural and electrical properties of the grown material, including its suitability for substrate applications. The physical vapor transport or sublimation process has already shown a capability to produce substantial quantities of large area and high quality hexagonal SiC substrates. In the present study a similar growth principle, but in a different geometry, namely sublimation epitaxy, was applied. Using this method very high growth rates (up to 1 mm/h) can be achieved for hexagonal polytypes while maintaining high material quality. Additionally, the growth process does not require expensive or hazardous materials, thus making the method very attractive for industrial use. When growing 3C-SiC directly on 6H-SiC, the substrate roughness does not have significant influence on the yield and quality of 3C-SiC. This is mostly due to the growth of homoepitaxial 6H-SiC which appears before the 3C-SiC. Structural characterization showed that 3C-SiC grown directly on 6HSiC exhibited the highest quality as compared with other substrate preparation, such as annealing or deposition of a 3C-SiC buffer layer. Thus, further investigation was devoted to the growth of 3C-SiC on 6H-SiC substrates. The parameter window for the growth of 3C-SiC is quite narrow. The temperature interval is from ~1675oC, where the material starts to nucleate, to ~1850oC, where an uncontrolled growth process begins. Si-rich conditions (high Si/C ratio) and high supersaturation are needed in the growth chamber for preferable 3C-SiC nucleation. Deviation from these parameters leads to the growth of homoepitaxial 6HSiC in spiral or 2D island mode along with cubic SiC with high defect density. Nucleation is the most important step in the growth process. The growth on 6H-SiC substrates commences from homoepitaxial 6H-SiC growth in spiral mode, which makes the surface perfect for 3CSiC nucleation. At temperature of ~1675oC the supersaturation is high enough and the 3C-SiC nucleation initiates in two-dimensional islands on the 6H-SiC spiral terraces. Control of the homoepitaxial 6H-SiC growth is a key element in the growth of 3C-SiC. SiC is a polar material having surfaces terminated by either silicon or carbon atoms, called Si- and C-face, respectively. The growth is different on both faces due to the different free surface energies. The lower surface free energy on the C-face causes more uniform nucleation of 3C-SiC and thereafter more uniform twinned domain distribution. Additionally, calculations showed that increase of growth temperature from 1675oC to 1775oC does not change the supersaturation ratio on the C-face due to a much higher surface diffusion length. This results in appearance of pits in the 3C-SiC layer with a 6H-SiC spiral. The pits were not observed on Si-face material as the supersaturation ratio was much higher. Pits formed in the early stages of growth were overgrown more effectively during the later stages. Characterization by transmission electron microscopy showed that transformation from 6H-SiC to 3C-SiC is not abrupt and can appear in two different modes. The first one is forming a few micrometers of polytypic transition zone consisting predominantly of 15R-, 6H- and 3C-SiC. The second one appears due to a competition between 3C-SiC and 6H-SiC resulting in a step-like intermixing zone between these polytypes. Four-fold twins were observed, which resulted in depressions at the surface of 3C-SiC. These defects expand proportionally to the layer thickness, thus drastically reducing usable area of thick layers. Electrical measurements revealed carrier mobility ~200 cm2/Vs at room temperature and the dominant charge carrier scattering is by neutral centers and phonons. The neutral centers originate from extended defects, such as 6H-SiC inclusions, stacking faults and twin boundaries. By growing 3C-SiC on atomically flat and vicinal substrates a preferential orientation of twin boundaries (TBs) was achieved. The mobility was higher in the material with twin boundaries parallel to the current flow, and lower when twin boundaries were perpendicular to the current flow. This was less pronounced at higher temperature as relatively fewer carriers have to overcome barriers created by TBs. Finally, the substrate capability of the 3C-SiC (111) was demonstrated by growth of a monolayer graphene, which was compared with graphene grown on hexagonal SiC poytypes. The quality of the graphene in terms of thickness uniformity and pit appearance was the best when grown on 3C-SiC. The lower quality on hexagonal substrates was attributed to a more difficult process control which is due to the more complex crystal structure.

Epitaxially grown single layer and multi layer graphene on SiC devices were fabricated and compared for response towards NO2. Due to electron donation from SiC, single layer graphene is n-type with a very low carrier concentration. The choice of substrate is demonstrated to enable tailoring of the electronic properties of graphene, with a SiC substrate realising simple resistive devices tuned for extremely sensitive NO2 detection. The gas exposed uppermost layer of the multi layer device is screened from the SiC by the intermediate layers leading to a p-type nature with a higher concentration of charge carriers and therefore, a lower gas response. The single layer graphene device is thought to undergo an n–p transition upon exposure to increasing concentrations of NO2 indicated by a change in response direction. This transition is likely to be due to the transfer of electrons to NO2 making holes the majority carriers.

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 electronmicroscopy (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 x 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 mu m can be used to collect a 1 x 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 similar to 4x10(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.

Received 23 September 2011DOI:https://doi.org/10.1103/PhysRevB.84.129902©2011 American Physical Society