Role of extended defected SiC interface layer on the growth of epitaxial graphene on SiC

An extended layer of defected SiC has been observed in SiC subjected to heat treatments at 850 and 1050 °C prior to growth of graphene by thermal decomposition. This layer is found to strongly affect the graphene thickness, surface morphology, and Raman spectrum of graphene grown on it. By comparing the strength of the XPS signal associated with this layer it was found that the samples with stronger defected layer signal had the least number of surface pits but also showed the increase in Raman D to G band ratio. The shifts in 2D and G peaks are associated with varying amounts of strain and unintentional doping induced by the SiC defected interface layer, respectively.     

The effect of a SiC cap on the growth of epitaxial graphene on SiC in ultra high vacuum

Thin and homogeneous graphenes with excellent thickness uniformity were produced on the carbon-rich surface of a SiC crystal using an ultra high vacuum technique. The sample surface was capped by another SiC substrate with a silicon-rich face to form a shallow cavity between them. During the graphene growth by high temperature annealing, silicon atoms sublimated from the capped sample were trapped inside the cavity between the two substrates. The confined vapor phase silicon maintains a relatively high partial pressure at the sample surface which significantly reduces the extremely high growth rate of epitaxial graphene to an easily controllable range. The structure and morphology of the graphene samples grown with this capping method are characterized by low energy electron diffraction and Raman spectroscopy and the results are compared with those of layers grown on an uncapped sample surface. The results show that capping yields much thinner graphene with excellent uniformity.     

Mechanism of diamond epitaxial growth on silicon

According to this study and to our previously reported nucleation model, we can explain the mechanism of diamond heteroepitaxial growth as follows: microscopic nucleation sites, i.e. etching scars, bunching atomic steps, and grooves formed by growing SiC, are formed on the substrate surface at the beginning of the bias application and are related to the crystal orientation of the substrate. Carbon atoms that reach the substrate surface are trapped at the nucleation sites and form clusters. Since these clusters are in the embryonic form during ion irradiation, deformation, rotation and migration can easily take place to form clusters with shapes that correspond to the shapes of these sites. At the same time, the conversion from sp² to sp³ progresses, and diamond nuclei of critical sizes are thought to be formed. This phenomenon strongly suggests that the heteroepitaxial growth of diamond involves a graphoepitaxial process in the formation of a critical nucleus size. In this study, we examined in detail the effect of bias on substrates to clarify the mechanism of heteroepitaxial growth of diamond.     

Wafer-scale homogeneity of transport properties in epitaxial graphene on SiC

Magnetotransport measurements on Hall bar devices fabricated on purely monolayer epitaxial graphene on silicon carbide (SiC/G) show a very tight spread in carrier concentration and mobility across wafer-size dimensions. In contrast, SiC/G devices containing bilayer graphene domains display variations in their electronic properties linked to the amount of bilayer content. The spread in properties among devices patterned on the same SiC/G wafer can thus be understood by considering the inhomogeneous number of layers often grown on the surface of epitaxial graphene on SiC.     

SiC surface orientation and Si loss rate effects on epitaxial graphene

We have explored the properties of SiC-based epitaxial graphene grown in a cold wall UHV chamber. The effects of the SiC surface orientation and silicon loss rate were investigated by comparing the characteristics of each formed graphene. Graphene was grown by thermal decomposition on both the silicon (0001) and carbon (000-1) faces of on-axis semi-insulating 6H-SiC with a "face-down" and "face-up" orientations. The thermal gradient, in relation to the silicon flux from the surface, was towards the surface and away from the surface, respectively, in the two configurations. Raman results indicate the disorder characteristics represented by I_D/I_G down to < 0.02 in Si-face samples and < 0.05 in C-faces over the 1 cm² wafer surface grown at 1,450°C. AFM examination shows a better morphology in face-down surfaces. This study suggests that the optimum configuration slows the thermal decomposition and allows the graphene to form near the equilibrium. The Si-face-down orientation (in opposition to the temperature gradient) results in a better combination of low disorder ratio, I_D/I_G, and smooth surface morphology. Mobility of Si-face-down orientation has been measured as high as approximately 1,500 cm²/Vs at room temperature. Additionally, the field effect transistors have been fabricated on both Si-face-down and C-face-down showing an ambipolar behavior with more favorable electron conduction.     

Field-induced recovery of massless Dirac fermions in epitaxial graphene on SiC

We report first-principles calculations of atomic and electronic structures of epitaxial single-layer graphene on Si-terminated 4H-SiC(0 0 0 1) surface under homogeneous transverse electric fields. We find that atomic positions are insensitive to applied electric fields, but the electronic band structures of the graphene layer are shifted in energy, depending strongly on the applied electric fields, while those of the buffer layer are almost unchanged. This effect finally results in field-induced closing of the energy gap at the Dirac energy point and recovery of the conic feature of the low-energy band structures of free-standing graphene, which are verified and analyzed further with a tight-binding model consisting of the single-layer and the buffer-layer graphene only. The recovery of conical dispersion of the single-layer graphene and ambipolar field-effect behavior, despite the band-gap closure under electric field, makes epitaxial single-layer graphene one of the promising alternatives to current state-of-the-art transistors for radiofrequency applications.     

Origin of growth defects in CVD diamond epitaxial films

Three types of growth defects commonly found epitaxial diamond films grown by chemical vapour deposition (CVD), namely unepitaxial crystals (UCs), hillocks with flat top (FHs) and pyramidal hillocks (PHs), were etched using hydrogen/oxygen plasma to discuss their origin. UCs formed at random locations on the grown layer without any apparent relation with the substrate. Their nucleation might be due to contaminants and their development controlled by the growth conditions in the plasma. In contrast, dislocations formed from impurities segregated at the interface between the substrate and the CVD layer, were found to be the origin of the FHs and the PHs. A simple crystal model that involves micro-faceting or twinning at an intrinsic stacking fault originating from the dislocation core is proposed to explain the formation and the evolution of the growth defects.     

Multidimensional characterization,Landau levels and Density of States in epitaxial graphene grown on SiC substrates

Using high-temperature annealing conditions with a graphite cap covering the C-face of, both, on axis and 8° off-axis 4H-SiC samples, large and homogeneous single epitaxial graphene layers have been grown. Raman spectroscopy shows evidence of the almost free-standing character of these monolayer graphene sheets, which was confirmed by magneto-transport measurements. On the best samples, we find a moderate p-type doping, a high-carrier mobility and resolve the half-integer quantum Hall effect typical of high-quality graphene samples. A rough estimation of the density of states is given from temperature measurements.     

Long-range interactions of bismuth growth on monolayer epitaxial graphene at room temperature

Long-range electronic interaction between Bismuth (Bi) adatoms on graphene formed on a 4H-SiC (0 0 0 1) substrate are clearly observed at room temperature (T = 300 K). Using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we have demonstrated that such oscillatory interaction results mainly from the mediation of graphene Dirac-like electrons and the effect of the corrugated surface of SiC substrate. These two factors cause the observed oscillatory interaction with characteristic distribution distances and linear arrangements of Bi adatoms. The present study sheds light on understanding and controlling the nucleation of adatoms and subsequent growth of nanostructures on graphene surface.     

Microstructure and growth mechanism of multi-layer graphene standing on polycrystalline SiC microspheres

Multi-layer graphene standing on polycrystalline SiC microspheres was prepared by pyrolyzing liquid polysilacarbosilane. The lateral dimension of the multi-layer graphene is ∼100 nm and the average diameter of the microspheres is ∼0.9 μm. The growth of the multi-layer graphene is proposed to be initiated by phase separation of the microspheres, and facilitated with both crystallization inside and chemical vapor deposition outside. This method offers an alternative way to prepare multi-layer graphene on SiC without the need for 4H– or 6H–SiC crystals.     

Heavy phosphorus doping by epitaxial growth on the (111) diamond surface

Semiconducting n-type diamond can be fabricated using phosphorus as a substitutional donor dopant. The dopant activation energy level at 0.58 eV is deep. At high dopant concentrations of 10²⁰ cm^− 3 the activation energy reduces to less than 0.05 eV. Phosphorus doping at concentrations of 10²⁰ cm^− 3 or higher has been achieved with epitaxial growth on the (111) diamond crystallographic surface. In this work epitaxial growth of diamond with high phosphorus concentrations exceeding 10²⁰ cm^− 3 is performed using a microwave plasma-assisted chemical vapor deposition process with process conditions that include a pressure of 160 Torr. This pressure is higher than previous phosphorus doping reports of (111) surface diamond growth. The other growth conditions include a feedgas mixture of 0.25% methane and 500 ppm phosphine in hydrogen, and a substrate temperature of 950–1000 °C. The measured growth rate was 1.25 μm/h. The room temperature resistivity of the heavily phosphorus doped diamond was 120–150 Ω-cm and the activation energy was 0.027 eV.     

Dislocations and impurities introduced from etch-pits at the epitaxial growth resumption of diamond

The fabrication of diamond-based electronic devices requires that several active layers with different doping concentrations are grown in different reactors. In this paper, we have investigated the effect of interrupting and resuming the epitaxial growth of a homoepitaxial diamond film using high-power plasma CVD. It was found that long lifetime blue phosphorescence which is localised on regions with a high dislocation density is generated. Such phosphorescence is related to a higher uptake of impurities at the interface between two subsequent films, due to an increased surface roughness from etching at the epitaxial growth resumption. Etching was found to occur preferentially on threading dislocations leaving typical etch-pits. Cathodoluminescence helps identify boron as the main background impurity. Besides, the formation of new dislocations was observed on the facets of these etch-pits. The continuation of epitaxy on a roughened surface thus comes with a substantial decrease in crystal quality.     

Recent IBIC measurements on epitaxial CVD diamond

Frontal IBIC (Ion Beam Induced Current) mesurements have been carried out on new single crystal epitaxial CVD diamond. The sample consists of about 100 μm synthetic diamond grown by microwave CVD on a 300 μm thick, low cost, HPHT diamond substrate (see Balducci et al. – this conference). Both proton and alpha microbeams of energies 3 and 4.5 MeV have been used, with a beam diameter spot of about 1.5–3 μm. Scanned areas varied from 450 μm × 450 μm down to 150 μm × 150 μm and the homogeneity of charge collection efficiency (cce) was suitably monitored. At voltage bias of 80–100 V, the average cce was in the range 42–50%. Depending on the scanned surface area and on the beam type, energy resolutions FWHM from 1.3% to 4.1% FWHM have been obtained, even at counting rates as high as 700 cps.     

Frictional characteristics of exfoliated and epitaxial graphene

To determine the friction coefficient of graphene, micro-scale scratch tests are conducted on exfoliated and epitaxial graphene at ambient conditions. The experimental results show that the monolayer, bilayer, and trilayer graphene all yield friction coefficients of approximately 0.03. The friction coefficient of pristine graphene is less than that of disordered graphene, which is treated by oxygen plasma. Ramping force scratch tests are performed on graphene with various numbers of layers to determine the normal load required for the probe to penetrate graphene. A very low friction coefficient and also its high pressure resistance make graphene a promising material for antiwear coatings.     

Research on maximizing the diamond content of diamond/SiC composite

Diamond content is a key factor affecting diamond/SiC composite performance, especially thermal and mechanical properties, but the composite with high diamond content manufacturing is still challenging issues. Hot mold pressing combined with liquid silicon infiltration to make diamond/SiC composites with high diamond content and relative density has been proposed in this paper. In addition, the effect of diamond particle size on the maximization of diamond content as well as properties of the composites were evaluated. The experiment shows that the content of diamond in the composites increases with the increase of the diamond particle size. When the particle size of diamond is 400 µm, the volume fraction of diamond reaches 59.08%. The highest thermal conductivity (d_dia= 300 µm) and highest bending strength (d_dia= 50 µm) are 616.77 W/m K (It is the maximum TC of diamond/SiC prepared by pressureless infiltration at present) and 380 MPa, respectively. This work provides a novel and efficient preparation method for further improving the thermal conductivity of diamond/SiC composites.     

The influence of mixing on the epitaxial synthesis of diamond

The effect of intermixing on the synthesis of two concurrent phases, i.e. diamond and graphite, is discussed. An experimental unit is described, and experimental results are presented.     

Naturally formed epitaxial diamond crystals in rubies

Materials inspired by nature comprise a running theme of modern science. Among the crystals that can be formed, diamond is perhaps most emblematic. In the conventional thinking, natural diamonds form only under high-pressure and high-temperature conditions. Here we show a new, natural form of diamond crystals of high quality that are epitaxial with their ruby substrate. Diamonds in rubies are rare; heteroepitaxial diamonds are twice as unexpected. Epitaxy suggests that the natural diamonds in the rubies were formed after ruby crystallization in a thermodynamically diamond stable region. This striking natural control over diamond epitaxy suggests a general strategy by which to form naturally-inspired, gem-quality crystals.     

Silicon substrate preparation for epitaxial diamond crystals

Using the microwave plasma-assisted chemical-vapour deposition technique (MPCVD), we study the role of the (100) silicon substrate preparation prior to the ultra short bias enhanced nucleation (USBEN) step via various pre-treatments. We study the effect of the silicon HF cleaning coupled with the hydrogen plasma exposure in order to determine the efficiency of these pre-treatments to eliminate the native oxide layer on the silicon surface prior to the bias step. We show that the residual oxygen content in the gas phase during the hydrogen plasma exposure can strongly affect the nucleation density because of the formation of an oxide layer which is detrimental for the synthesis of highly oriented diamond (HOD) films. Moreover, we study the effect of the carburation step, often used for the synthesis of HOD films and show that it raises the percentage of epitaxial crystals and the crystal density. Thus, we show that the achievement of high epitaxial crystal density is not only due to the BEN parameters but also to the silicon substrate preparation. As a conclusion, it appears that the silicon substrate preparation prior to BEN is fundamental for controlling the quality of epitaxial diamond films.     

SiC涂层对金刚石刀头性能的影响

探索了在金刚石表面镀覆SiC涂层的工艺方法,并以机械合金化铁合金粉末为基体,采用热压烧结工艺制备了长条形金刚石刀头,测试分析了刀头的硬度、抗弯强度和微观组织.结果表明:用金刚石+Si+I2混合粉末(工艺A)、或金刚石+聚碳硅烷(PCS)溶液(工艺B)于1000℃～1200℃真空反应,均能在金刚石表面制备出SiC涂层;在基体中添加Zn、Sn等低熔点元素,会降低刀头的硬度和强度;而添加少量B4C,可以起弥散强化的作用;对金刚石先镀Ti、再镀SiC,可使刀头的硬度和强度进一步提高,最高硬度为HRB118,抗弯强度为543MPa.     

Tunable bands in biased multilayer epitaxial graphene

We have studied the electronic characteristics of multilayer epitaxial graphene under a perpendicularly applied electric bias. Ultraviolet photoemission spectroscopy measurements reveal that there is notable variation of the electronic density-of-states in valence bands near the Fermi level. Evolution of the electronic structure of graphite and rotational-stacked multilayer epitaxial graphene as a function of the applied electric bias is investigated using first-principles density-functional theory including interlayer van der Waals interactions. The experimental and theoretical results demonstrate that the tailoring of electronic band structure correlates with the interlayer coupling tuned by the applied bias. The implications of controllable electronic structure of rotationally fault-stacked epitaxial graphene grown on the C-face of SiC for future device applications are discussed.