Bilayer graphene grown on 4H-SiC (0001) step-free mesas

We demonstrate the first successful growth of large-area (200 × 200 μm(2)) bilayer, Bernal stacked, epitaxial graphene (EG) on atomically flat, 4H-SiC (0001) step-free mesas (SFMs) . The use of SFMs for the growth of graphene resulted in the complete elimination of surface step-bunching typically found after EG growth on conventional nominally on-axis SiC (0001) substrates. As a result heights of EG surface features are reduced by at least a factor of 50 from the heights found on conventional substrates. Evaluation of the EG across the SFM using the Raman 2D mode indicates Bernal stacking with low and uniform compressive lattice strain of only 0.05%. The uniformity of this strain is significantly improved, which is about 13-fold decrease of strain found for EG grown on conventional nominally on-axis substrates. The magnitude of the strain approaches values for stress-free exfoliated graphene flakes. Hall transport measurements on large area bilayer samples taken as a function of temperature from 4.3 to 300 K revealed an n-type carrier mobility that increased from 1170 to 1730 cm(2) V(-1) s(-1), and a corresponding sheet carrier density that decreased from 5.0 × 10(12) cm(-2) to 3.26 × 10(12) cm(-2). The transport is believed to occur predominantly through the top EG layer with the bottom layer screening the top layer from the substrate. These results demonstrate that EG synthesized on large area, perfectly flat on-axis mesa surfaces can be used to produce Bernal-stacked bilayer EG having excellent uniformity and reduced strain and provides the perfect opportunity for significant advancement of epitaxial graphene electronics technology.     

Tunable Reverse‐Biased Graphene/Silicon Heterojunction Schottky Diode Sensor

A new chemical sensor based on reverse‐biased graphene/Si heterojunction diode has been developed that exhibits extremely high bias‐dependent molecular detection sensitivity and low operating power. The device takes advantage of graphene's atomically thin nature, which enables molecular adsorption on its surface to directly alter graphene/Si interface barrier height, thus affecting the junction current exponentially when operated in reverse bias and resulting in ultrahigh sensitivity. By operating the device in reverse bias, the work function of graphene, and hence the barrier height at the graphene/Si heterointerface, can be controlled by the bias magnitude, leading to a wide tunability of the molecular detection sensitivity. Such sensitivity control is also possible by carefully selecting the graphene/Si heterojunction Schottky barrier height. Compared to a conventional graphene amperometric sensor fabricated on the same chip, the proposed sensor demonstrated 13 times higher sensitivity for NO₂ and 3 times higher for NH₃ in ambient conditions, while consuming ～500 times less power for same magnitude of applied voltage bias. The sensing mechanism based on heterojunction Schottky barrier height change has been confirmed using capacitance‐voltage measurements.     

Long spin relaxation times in wafer scale epitaxial graphene on SiC(0001)

We developed an easy, upscalable process to prepare lateral spin-valve devices on epitaxially grown monolayer graphene on SiC(0001) and perform nonlocal spin transport measurements. We observe the longest spin relaxation times τ(S) in monolayer graphene, while the spin diffusion coefficient D(S) is strongly reduced compared to typical results on exfoliated graphene. The increase of τ(S) is probably related to the changed substrate, while the cause for the small value of D(S) remains an open question.     

SiC陶瓷与Zn界面结合特性的第一性原理研究

低温焊接SiC陶瓷是金属/陶瓷连接领域非常重要的研究方向,而与之相关的理论研究相对匮乏,同时,通过实验手段难以描述金属/陶瓷界面原子之间的相互作用。为研究低温Zn基钎料与SiC陶瓷的界面结合方式,采用第一性原理方法,计算了Zn(0001)和SiC(0001)的表面能,6种不同堆垛方式的Zn(0001)/SiC(0001)界面模型的分离功,并分析了其中最稳定两种模型的电荷密度图、电荷密度差分图和Mulliken布局。结果表明:Zn/SiC界面只形成了Zn-Si离子键,Si终端孔穴型界面的Zn-Si键结合强度高于C终端孔穴型。     

Ti/n型6H-SiC(0001)接触界面的研究

用同步辐射光电子能谱(SRPES)和X射线光电子能谱(XPS)的方法研究了Ti/n型6H-SiC(0001)的接触界面。Ti/n型6H-SiC(0001)样品采用磁控溅射的方法获得,然后将表面的Ti用氩离子刻蚀的方法慢慢刻蚀掉,Ti2p3/2用XPS测得,结合能从刻蚀时间为245 min的457.86 eV逐渐移动到刻蚀时间为255 min时的457.57 eV,移动约为0.3 eV。Si2p用同步辐射光测得,结合能从刻蚀245 min时的101.12 eV移动到干净的100.67 eV,峰形状未发生变化,表明Ti与衬底之间没有发生化学反应,SiC的价带发生弯曲,形成的势垒高度为0.89 eV。向SiC上蒸Si 2.5 min,退火30 min,观察LEED花样,发现当发射电流为30mA,能量37 eV时,SiC表面有√3*√3重构,发射电流为40 mA时,有6√3*6√3的重构。     

Graphene epitaxy by chemical vapor deposition on SiC

We demonstrate the growth of high quality graphene layers by chemical vapor deposition (CVD) on insulating and conductive SiC substrates. This method provides key advantages over the well-developed epitaxial graphene growth by Si sublimation that has been known for decades. (1) CVD growth is much less sensitive to SiC surface defects resulting in high electron mobilities of ～1800 cm(2)/(V s) and enables the controlled synthesis of a determined number of graphene layers with a defined doping level. The high quality of graphene is evidenced by a unique combination of angle-resolved photoemission spectroscopy, Raman spectroscopy, transport measurements, scanning tunneling microscopy and ellipsometry. Our measurements indicate that CVD grown graphene is under less compressive strain than its epitaxial counterpart and confirms the existence of an electronic energy band gap. These features are essential for future applications of graphene electronics based on wafer scale graphene growth.     

SiC颗粒增强镁基复合材料界面稳定性的第一性原理计算

针对SiC颗粒增强镁基复合材料界面的理论研究较少,大多研究仅停留在表征层面等问题。本文采用第一性原理方法,计算了四种不同SiC(0001)/Mg(0001)界面模型的电荷密度、布局分析和界面分离功。结果表明:对于同种终端的SiC(0001)/Mg(0001)界面模型中,顶位型结构比心位型结构的稳定性好;不同终端的SiC(0001)/Mg(0001)的界面模型中,Si终端结构比C终端结构更加稳定,其中Si终端顶位型结构稳定性最好,其分离功为W_sep=3.297 J/m²,界面间距为d₀=2.651nm。界面的键合方式主要为C-Mg共价键和Mg-Si离子键。     

Atomic-scale transport in epitaxial graphene

The high carrier mobility of graphene is key to its applications, and understanding the factors that limit mobility is essential for future devices. Yet, despite significant progress, mobilities in excess of the 2×10(5) cm(2) V(-1) s(-1) demonstrated in free-standing graphene films have not been duplicated in conventional graphene devices fabricated on substrates. Understanding the origins of this degradation is perhaps the main challenge facing graphene device research. Experiments that probe carrier scattering in devices are often indirect, relying on the predictions of a specific model for scattering, such as random charged impurities in the substrate. Here, we describe model-independent, atomic-scale transport measurements that show that scattering at two key defects--surface steps and changes in layer thickness--seriously degrades transport in epitaxial graphene films on SiC. These measurements demonstrate the strong impact of atomic-scale substrate features on graphene performance.     

Evidence of Plasmonic Coupling in Gallium Nanoparticles/Graphene/SiC

Graphene is emerging as a promising material for plasmonics applications due to its strong light-matter interactions, most of which are theoretically predicted but not yet experimentally realized. Therefore, the integration of plasmonic nanoparticles to create metal nanoparticle/graphene composites enables numerous phenomena important for a range of applications from photonics to catalysis. For these applications it is important to articulate the coupling of photon-based excitations such as the interaction between plasmons in each of the material components, as well as their charge-based interactions dependent upon the energy alignment at the metal/graphene interface. These coupled phenomena underpin an active application area in graphene-based composites due to nanoparticle-dependent surface-enhanced Raman scattering (SERS) of graphene phonon modes. This study reveals the coupling of a graphene/SiC support with Ga-nanoparticle-localized surface plasmon resonance, which is of particular interest due to its ability to be tuned across the UV into the near-IR region. This work is the first demonstration of the evolving plasmon resonance on graphene during the synthesis of surface-supported metal nanoparticles, thus providing evidence for the theoretically predicted screening revealed by a damped resonance with little energy shift. Therefore, the role of the graphene/substrate heterojunction in tailoring the plasmon resonance for nanoplasmonic applications is shown. Additionally, the coupled phenomena between the graphene-Ga plasmon properties, charge transfer, and SERS of graphene vibrational modes are explored.     

Measurement of interfacial residual stress in SiC fiber reinforced Ni-Cr-Al alloy composites by Raman spectroscopy

Raman spectroscopy was used to measure Raman spectra of the inner SiC fibers and surface C-rich layers of SiC fibers, composite precursors and SiCf/Ni-Cr-Al composites. The residual stresses of the inner SiC fibers and surface C-rich layers were calculated, and the effect of the(Al + Al_2O_3) diffusion barrier layer on the interfacial residual stress in the composites was analyzed in combination with the interface microstructure and energy disperse spectroscopy(EDS) elements lining maps. The results show that the existence of(Al + Al_2O_3) diffusion barrier improves the compatibility of the SiCf/Ni-Cr-Al interface,inhibits the adverse interfacial reaction, and relieves the residual stress inside SiC fibers and at the interface of composite material. Heat treatment can reduce the residual stress at the interface. As the heat treatment time increases, the residual stress at the interface decreases.     

Electronic structures of an epitaxial graphene monolayer on SiC(0001) after gold intercalation: a first-principles study

The atomic and electronic structures of an Au-intercalated graphene monolayer on the SiC(0001) surface were investigated using first-principles calculations. The unique Dirac cone of graphene near the K?point reappeared as the monolayer was intercalated by Au atoms. Coherent interfaces were used to study the mismatch and the strain at the boundaries. Our calculations showed that the strain at the graphene/Au and Au/SiC(0001) interfaces also played a key role in the electronic structures. Furthermore, we found that at an Au coverage of 3/8?ML, Au intercalation leads to a strong n-type doping of graphene. At 9/8?ML, it exhibited a weak p-type doping, indicative that graphene was not fully decoupled from the substrate. The shift in the Dirac point resulting from the electronic doping was not only due to the different electronegativities but also due to the strain at the interfaces. Our calculated positions of the Dirac points are consistent with those observed in the ARPES experiment (Gierz et al 2010 Phys. Rev. B 81 235408).     

Epitaxial graphene transistors: enhancing performance via hydrogen intercalation

We directly demonstrate the importance of buffer elimination at the graphene/SiC(0001) interface for high frequency applications. Upon successful buffer elimination, carrier mobility increases from an average of 800 cm(2)/(V s) to >2000 cm(2)/(V s). Additionally, graphene transistor current saturation increases from 750 to >1300 mA/mm, and transconductance improves from 175 mS/mm to >400 mS. Finally, we report a 10× improvement in the extrinsic current gain response of graphene transistors with optimal extrinsic current-gain cutoff frequencies of 24 GHz.     

Surface morphology control of the SiC (0001) substrate during the graphene growth

Abstract

The dependence of surface morphology of the SiC(0001) substrate on the rate with which it is heated up to the temperature of graphene growth was studied by three techniques: atomic force microscopy, Raman spectroscopy and Kelvin probe force microscopy. The study was carried out for the rates of substrates heating ranging from 100?°C/min to 320?°C/min. As a result, it was found out that both the width of the terraces forming on the surface of SiC substrate and the uniformity of the graphene layers covering these terraces significantly depend on the applied rate of the heating. It was also shown that the most homogeneous monolayer graphene with the minimum of double-layers inclusions is formed if the rate of SiC heating is about 250?°C/min.     

Effects of Pb Intercalation on the Structural and Electronic Properties of Epitaxial Graphene on SiC

The effects of Pb intercalation on the structural and electronic properties of epitaxial single‐layer graphene grown on SiC(0001) substrate are investigated using scanning tunneling microscopy (STM), noncontact atomic force microscopy, Kelvin probe force microscopy (KPFM), X‐ray photoelectron spectroscopy, and angle‐resolved photoemission spectroscopy (ARPES) methods. The STM results show the formation of an ordered moiré superstructure pattern induced by Pb atom intercalation underneath the graphene layer. ARPES measurements reveal the presence of two additional linearly dispersing π‐bands, providing evidence for the decoupling of the buffer layer from the underlying SiC substrate. Upon Pb intercalation, the Si 2p core level spectra show a signature for the existence of Pb? Si chemical bonds at the interface region, as manifested in a shift of 1.2 eV of the bulk SiC component toward lower binding energies. The Pb intercalation gives rise to hole‐doping of graphene and results in a shift of the Dirac point energy by about 0.1 eV above the Fermi level, as revealed by the ARPES measurements. The KPFM experiments have shown that decoupling of the graphene layer by Pb intercalation is accompanied by a work function increase. The observed increase in the work function is attributed to the suppression of the electron transfer from the SiC substrate to the graphene layer. The Pb intercalated structure is found to be stable in ambient conditions and at high temperatures up to 1250 °C. These results demonstrate that the construction of a graphene‐capped Pb/SiC system offers a possibility of tuning the graphene electronic properties and exploring intriguing physical properties such as superconductivity and spintronics.     

Simultaneous N-intercalation and N-doping of epitaxial graphene on 6H-SiC(0001) through thermal reactions with ammonia

Surface functionalization of epitaxial graphene overlayers on 6H-SiC(0001) has been attempted through thermal reactions in NH₃. X-ray photoelectron spectroscopy and micro-region low energy electron diffraction results show that a significant amount of N is present at the NH₃-treated graphene surface, which results in strong band bending at the SiC surface as well as decoupling of the graphene overlayers from the substrate. The majority of the surface N species can be removed by annealing in vacuum up to 850 °C, weakening the surface band bending and resuming the strong coupling of graphene with the SiC surface. The desorbed N atoms can be attributed to the intercalated species between graphene and SiC. Low temperature scanning tunneling spectroscopy and density functional theory simulations confirm the presence of N dopants in the graphene lattice, which are in the form of graphitic substitution and can be stable above 850 °C. This is the first report of simultaneous N intercalation and N doping of epitaxial graphene overlayers on SiC, and it may be employed to alter the surface physical and chemical properties of epitaxial graphene overlayers.      

Scalable templated growth of graphene nanoribbons on SiC

In spite of its excellent electronic properties, the use of graphene in field-effect transistors is not practical at room temperature without modification of its intrinsically semimetallic nature to introduce a bandgap. Quantum confinement effects can create a bandgap in graphene nanoribbons, but existing nanoribbon fabrication methods are slow and often produce disordered edges that compromise electronic properties. Here, we demonstrate the self-organized growth of graphene nanoribbons on a templated silicon carbide substrate prepared using scalable photolithography and microelectronics processing. Direct nanoribbon growth avoids the need for damaging post-processing. Raman spectroscopy, high-resolution transmission electron microscopy and electrostatic force microscopy confirm that nanoribbons as narrow as 40 nm can be grown at specified positions on the substrate. Our prototype graphene devices exhibit quantum confinement at low temperatures (4 K), and an on-off ratio of 10 and carrier mobilities up to 2,700 cm(2) V(-1) s(-1) at room temperature. We demonstrate the scalability of this approach by fabricating 10,000 top-gated graphene transistors on a 0.24-cm(2) SiC chip, which is the largest density of graphene devices reported to date.     

Photoemission studies of the vicinal SiC(100) 4° surface and the Cs/SiC(100) 4° interface

Photoemission studies of the electronic structure of the vicinal SiC(100) 4° surface, which was grown using a new substrate atom substitution method, and the Cs/SiC(100) 4° interface have been performed for the first time. The modification of spectra of the valence band and C 1s and Si 2p core levels in the process of formation of the Cs/SiC(100) 4° interface was analyzed. The suppression of the surface SiC state with a binding energy of 2.8 eV and the formation of a cesium-induced state with a binding energy of 10.5 eV were observed. The modification of the complex component structure in the spectrum of C 1s core level has been detected and examined for the first time. It was found that Cs adsorption on the vicinal SiC(100) 4° surface results in intercalation of graphene islands on SiC(100) 4° with Cs atoms.     

Improvement of carrier mobility of top-gated SiC epitaxial graphene transistors using a PVA dielectric buffer layer

The effects of treatment with polyvinyl alcohol (PVA) and a dielectric film of HfO(2) on the properties of SiC based epitaxial graphene have been explored and analyzed. We have characterized the carrier mobility of graphene on Si-face and C-face SiC with a layer of HfO(2), with or without an initial PVA treatment on the device active layer. Epitaxial graphene grown on the C-face displays a higher mobility than a film grown on the silicon face. Also, the mobility in the presence of the PVA treatment with HfO(2) dielectric layer has been improved, compared with the mobility after deposition of only gate dielectric: ～20% in C-face graphene and ～90% in Si-face graphene. This is a major improvement over the degradation normally observed with dielectric/graphene systems.     

Formation of microcrystalline SiC films by chemical transport with a high-pressure glow plasma of pure hydrogen

The formation of microcrystalline 3C-SiC films on Si substrates by the plasma-enhanced chemical transport method was investigated using a pure hydrogen glow plasma at 0.027 MPa. In this method, no source gas was necessary. Instead, the erosion products of a sintered 3C-SiC plate in a hydrogen plasma were used as the deposition source. By Fourier transform infrared (FT-IR) absorption gas analysis, the species generated by the hydrogen etching of sintered SiC were found to be SiH₄ and CH₄, which can serve as precursors for SiC film formation. The etch rate of sintered SiC by hydrogen plasma decreased with increasing source temperature. The maximum etch rate of the sintered SiC was 450 nm/min at an input power of 47 W/cm². Films prepared by this method at substrate temperatures (T_sub) of 600 and 1073 K were analyzed by FT-IR absorption spectroscopy. An absorption peak at 800 cm^- 1 related to Si-C bonds was clearly observed, but no significant hydrogen-related absorption peaks, such as C-H and Si-H, were observed in the prepared films. The deposition rate of SiC was about 8 nm/min, independent of T_sub. The SiC films had a columnar structure, and their surface morphologies revealed faceted growth. With decreasing T_sub, the lateral grain size became large. The current-voltage characteristics of a prepared SiC/Si heterojunction np diode showed rectifying behavior, demonstrating that the doping of an SiC film can be achieved without a doping gas source. The dopant distribution near the SiC/Si interface deduced from capacitance-voltage measurements suggests that the precise control of the initial growth stage is important to obtain a good SiC/Si interface.     

Josephson Coupling in Junctions Made of Monolayer Graphene Grown on SiC

Chemical vapor deposition has proved to be successful in producing graphene samples on silicon carbide (SiC) homogeneous at the centimeter scale in terms of Hall conductance quantization. Here, we report on the realization of co-planar diffusive Al/ monolayer graphene/ Al junctions on the same graphene sheet, with separations between the electrodes down to 200 nm. Robust Josephson coupling has been measured for separations not larger than 300 nm. Transport properties are reproducible on different junctions and indicate that graphene on SiC substrates is a concrete candidate to provide scalability of hybrid Josephson graphene/superconductor devices.