C-terminal Face Research Articles

Ultrafine Si evaporation control technology to inhibit the island nucleation of epitaxial graphene on SiC (000-1)

The theoretically high mobility of graphene on C-terminated face of silicon carbide (SiC (000-1)) makes it a suitable candidate for electronic devices. However, the quality of graphene grown on SiC (000-1) is always poor. In this paper, we clarify that the partial pressure of silicon is the core parameter to control the nucleation and growth of graphene on SiC (000-1). Therefore, we propose a new system with SiC cap and optimize the height between the SiC cap and the SiC sample to control the partial pressure of silicon. Finally, the high-quality graphene with almost no D peak in Raman spectrum has been obtained. The nanosized domain underlying the top graphene layer has been demonstrated as the source of the defects, which can be inhibited by high partial pressure of silicon. Herein, a special electronic detective mode of electrically biased tapping mode (eb-TM) has been successfully implemented to characterize of the nano-domains underside. Furthermore, in our system, the suitable temperature is from 1350 °C to 1500 °C and the pressure is from 750 Pa to 40 kPa. This broad growth window makes the SiC cap system has greater advantages in the large-scale and stable batch production of epitaxial graphene in the future.

The CELL NUMBER REGULATOR FW2.2 protein regulates cell-to-cell communication in tomato by modulating callose deposition at plasmodesmata.

FW2.2 (standing for FRUIT WEIGHT 2.2), the founding member of the CELL NUMBER REGULATOR (CNR) gene family, was the first cloned gene underlying a quantitative trait locus (QTL) governing fruit size and weight in tomato (Solanum lycopersicum). However, despite this discovery over 20 yr ago, the molecular mechanisms by which FW2.2 negatively regulates cell division during fruit growth remain undeciphered. In the present study, we confirmed that FW2.2 is a membrane-anchored protein whose N- and C-terminal ends face the apoplast. We unexpectedly found that FW2.2 is located at plasmodesmata (PD). FW2.2 participates in the spatiotemporal regulation of callose deposition at PD and belongs to a protein complex which encompasses callose synthases. These results suggest that FW2.2 has a regulatory role in cell-to-cell communication by modulating PD transport capacity and trafficking of signaling molecules during fruit development.

Oxidation differences on Si- versus C-terminated surfaces of SiC during planarization in the fabrication of high-power, high-frequency semiconductor device

Silicon carbide (SiC) wafers have attracted attention as a material for advanced power semiconductor device applications due to their high bandgap and stability at high temperatures and voltages. However, the inherent chemical and mechanical stability of SiC poses significant challenges in the chemical mechanical planarization (CMP) process, an essential step in reducing defects and improving surface flatness. SiC exhibits different mechanical and chemical properties depending on SiC terminal faces, affecting SiC oxidation behavior during the CMP process. Here, we investigate the process of oxide layer formation during the CMP process and how it relates to the SiC terminal faces. The results show that under the same conditions, the C-terminated face (C-face) exhibits higher oxidation reaction kinetics than the Si-terminated face (Si-face), forming an oxide layer of finer particles. Due to the different oxidation kinetic tendencies, the oxide layer formed on the C-face has a higher friction coefficient and more defects than the oxide layer formed on the Si-face. This results in a higher removal rate during CMP for the C-face than the Si-face. Furthermore, by controlling the physicochemical properties of the oxide film, high removal rates can be achieved by friction with the pad alone, without the need for nanoparticle abrasives.

Structures of the holo CRISPR RNA-guided transposon integration complex

CRISPR-associated transposons (CAST) are programmable mobile genetic elements that insert large DNA cargos using an RNA-guided mechanism1–3. CAST elements contain multiple conserved proteins: a CRISPR effector (Cas12k or Cascade), a AAA+ regulator (TnsC), a transposase (TnsA–TnsB) and a target-site-associated factor (TniQ). These components are thought to cooperatively integrate DNA via formation of a multisubunit transposition integration complex (transpososome). Here we reconstituted the approximately 1 MDa type V-K CAST transpososome from Scytonema hofmannii (ShCAST) and determined its structure using single-particle cryo-electon microscopy. The architecture of this transpososome reveals modular association between the components. Cas12k forms a complex with ribosomal subunit S15 and TniQ, stabilizing formation of a full R-loop. TnsC has dedicated interaction interfaces with TniQ and TnsB. Of note, we observe TnsC–TnsB interactions at the C-terminal face of TnsC, which contribute to the stimulation of ATPase activity. Although the TnsC oligomeric assembly deviates slightly from the helical configuration found in isolation, the TnsC-bound target DNA conformation differs markedly in the transpososome. As a consequence, TnsC makes new protein–DNA interactions throughout the transpososome that are important for transposition activity. Finally, we identify two distinct transpososome populations that differ in their DNA contacts near TniQ. This suggests that associations with the CRISPR effector can be flexible. This ShCAST transpososome structure enhances our understanding of CAST transposition systems and suggests ways to improve CAST transposition for precision genome-editing applications.

Sublimation Anisotropic Etching of Silicon Carbide in Aluminum Nitride Vapors

Specific features of sublimation etching of crystalline silicon carbide (SiC) in the presence of aluminum nitride (AlN) vapors have been reported. The etching is shown to be essentially anisotropic. Abnormally high speed of sublimation in a lateral direction, which weakly depends on surface orientation and polarity, is revealed through increasing artificial holes and flattening out surface elevations prepared in the shape of rectangular mesa structures. A high-rate etching is observed at outcrops of micropipes and closed-core screw dislocations, which leads to the formation of pits on () C-terminated faces. During annealing, the pits evolve into holes that might provoke the formation of pores inside free-standing AlN layers grown in one process with the sublimation of SiC substrates. The sublimation kinetics of a Si-terminated face is evidently different from that of a C-terminated face. It is found that a liquid phase is formed on the etched surfaces and it plays a key role in interpreting the etching anisotropy on SiC crystal faces of different orientations. It is shown that the sublimation of mesa-shaped SiC substrates can be used with the aim of optimizing the fabrication of thick freestanding AlN layers.

The role of charge distribution on the friction coefficients of epitaxial graphene grown on the Si-terminated and C-terminated faces of SiC

The friction coefficients of single-layer epitaxial graphene grown on the Si-terminated and C-terminated faces of Silicon Carbide (SiC) substrate were measured under ambient conditions using Friction Force Microscope (FFM). The lateral friction force measurements acquired in the applied normal force range between 4.0 and 16.0 nN showed that the friction coefficient of graphene on the C-terminated face of SiC is about two times smaller than the one grown on its Si-terminated face. The lateral friction was found to be decreased as the average of root mean square roughness increases suggesting the observed difference in the friction coefficients cannot be related to the roughness of the graphene layers. DFT calculations demonstrated that the altered periodicity of charge distribution on graphene due to the specific interactions with two distinct polar faces of SiC substrate might explain the observed difference in the friction coefficients.

ReaxFF molecular dynamics simulation of oxidation behavior of 3C-SiC in O2 and CO2

This paper studies the oxidation behavior of nanoscale 3C-SiC in O2 and CO2 with a reactive force field molecular dynamics (ReaxFF MD) simulation. The first step of the oxidation of 3C-SiC involved five polar faces (C(100), Si(100), C(111), Si(111), m(11¯0)) in O2 which was simulated to explore the effect of polar faces on the oxidation process. It was observed that C-terminated face possesses a lower oxidative stability. The effect of oxygen concentration on the oxidation behavior of C(111) face was also checked. It seems that there exists little influence of variation of oxygen concentration on the activation energy of 3C-SiC. Then the oxidation process of C(111) face in CO2 was studied. The results reflected a much higher rate of activation energy of 3C-SiC in CO2 compared with that in O2. It can be inferred that the chemical reaction between 3C-SiC with CO2 was much slower than that in O2. Finally, the oxidation behavior of C(111) face in a mixed gas including O2, and CO2 was investigated while the effect of the ratio of concentration between O2 and CO2 on the oxidation behavior of 3C-SiC was further explored. The results indicate that there is no obvious coupling effect between CO2 and O2 during the process of chemical reaction.

The N- or C-terminal cytoplasmic regions of P4-ATPases determine their cellular localization.

Mammalian P4-ATPases specifically localize to the plasma membrane and the membranes of intracellular compartments. P4-ATPases contain 10 transmembrane domains, and their N- and C-terminal (NT and CT) regions face the cytoplasm. Among the ATP10 and ATP11 proteins of P4-ATPases, ATP10A, ATP10D, ATP11A, and ATP11C localize to the plasma membrane, while ATP10B and ATP11B localize to late endosomes and early/recycling endosomes, respectively. We previously showed that the NT region of ATP9B is critical for its localization to the Golgi apparatus, while the CT regions of ATP11C isoforms are critical for Ca2+-dependent endocytosis or polarized localization at the plasma membrane. Here, we conducted a comprehensive analysis of chimeric proteins and found that the NT region of ATP10 proteins and the CT region of ATP11 proteins are responsible for their specific subcellular localization. Importantly, the ATP10B NT and the ATP11B CT regions were found to harbor a trafficking and/or targeting signal that allows these P4-ATPases to localize to late endosomes and early/recycling endosomes, respectively. Moreover, dileucine residues in the NT region of ATP10B were required for its trafficking to endosomal compartments. These results suggest that the NT and CT sequences of P4-ATPases play a key role in their intracellular trafficking.

Исследование методом наноиндентирования твердости и модуля Юнга в тонких приповерхностных слоях карбида кремния со стороны Si- и C-граней

This paper presents the results of a nanoindentation study of the hardness and Young’s modulus of hexagonal silicon carbide SiC-4H, obtained by the modified Lely method, in thin surface layers near the C-terminated and Si-terminated faces at small penetration depths of the indenter. It is shown that differences in the elastic properties and hardness of SiC propagate from the surface into the crystal to a depth of about 60 nm. The Young's modulus at the C-terminated face practically coincides with the Young's modulus of the bulk SiC-4H sample (~ 400 GPa), which is approximately 2.3 times higher than the Young's modulus at the Si-terminated face at a depth of 0 to 35 nm (~ 170 GPa). The value of the SiC hardness is approximately 1.5 times higher at the surface of the C-terminated face than at the Si-terminated face, on average, at a depth of 0 to 60 nm. It is concluded from the obtained data that the surface energy of the C-terminated face is also approximately 1.5 times higher than the surface energy of the Si-terminated face since a new surface is formed upon deformation or cracking of the crystal

Comparison of Casimir forces and electrostatics from conductive SiC-Si/C and Ru surfaces

Comprehensive knowledge of Casimir forces and associated electrostatics from conductive SiC and Ru surfaces can be essential in diverse areas ranging from micro/nanodevice operation in harsh environments to multilayer coatings in advanced lithography technologies. Hence, the Casimir force was measured between an Au-coated microsphere and N-doped SiC samples with Si- and C-terminated faces, and the results were compared with the measurements using the same microsphere and a metallic Ruthenium surface. Electrostatic calibration showed that the Si- and C-faces behave differently with a nearly ~0.6-0.7 V difference in the contact potentials V0Si/C. We attribute this to a higher incorporation of N on the C-terminated face in the near surface region resulting in the formation of NOx and an increased work function compared to the Si-terminated surface which is in agreement with x-ray photoelectron spectroscopy data. Notably, the contact potential of the SiC-C face was closer to the metallic Ru-Au system. However, the measured optical properties of the SiC-Si/C terminated surfaces with ellipsometry did not show any substantial differences indicating that the effective depth of the Si/C terminating surface layers are significantly smaller than the photon penetration depth not leading to any differences in the calculated forces via Lifshitz theory. Nonetheless, the measured Casimir forces, after compensation of the electrostatics contributions, showed differences between the Si/C faces, whereas the comparison with the Lifshitz theory prediction shows better agreement for the SiC-Si face. Finally, comparison of the Casimir forces below 40 nm separations between the SiC-Si/C and Ru surfaces indicated that the short-range roughness effects on the Casimir force increase in magnitude with increasing metallic behavior of the plate surface.

Structures of NHBA elucidate a broadly conserved epitope identified by a vaccine induced antibody.

Neisserial heparin binding antigen (NHBA) is one of three main recombinant protein antigens in 4CMenB, a vaccine for the prevention of invasive meningococcal disease caused by Neisseria meningitidis serogroup B. NHBA is a surface-exposed lipoprotein composed of a predicted disordered N-terminal region, an arginine-rich region that binds heparin, and a C-terminal domain that folds as an anti-parallel β-barrel and that upon release after cleavage by human proteases alters endothelial permeability. NHBA induces bactericidal antibodies in humans, and NHBA-specific antibodies elicited by the 4CMenB vaccine contribute to serum bactericidal activity, the correlate of protection. To better understand the structural bases of the human antibody response to 4CMenB vaccination and to inform antigen design, we used X-ray crystallography to elucidate the structures of two C-terminal fragments of NHBA, either alone or in complex with the Fab derived from the vaccine-elicited human monoclonal antibody 5H2, and the structure of the unbound Fab 5H2. The structures reveal details on the interaction between an N-terminal β-hairpin fragment and the β-barrel, and explain how NHBA is capable of generating cross-reactive antibodies through an extensive conserved conformational epitope that covers the entire C-terminal face of the β-barrel. By providing new structural information on a vaccine antigen and on the human immune response to vaccination, these results deepen our molecular understanding of 4CMenB, and might also aid future vaccine design projects.

Spin transport in epitaxial graphene on the C-terminated (0001¯)-face of silicon carbide

We performed a temperature dependent study of the charge and spin transport properties of epitaxial graphene on the C-terminated (0001¯) face of silicon carbide (SiC), a system without a carbon buffer layer between the graphene and the SiC. Using spin Hanle precession in the nonlocal geometry, we measured a spin relaxation length of λS = 0.7 μm at room temperature, lower than in exfoliated graphene. We show that the charge and spin diffusion coefficient, DC and DS, respectively, increasingly deviate from each other during electrical measurements up to a difference of a factor 4. Thus, we show that a model of localized states that was previously used to explain DC ≠ DS, can also be applied to epitaxial graphene systems without a carbon buffer layer. We attribute the effect to charge trap states in the interface between the graphene and the SiC.

Graphene on C-terminated face of 4H-SiC observed by noncontact scanning nonlinear dielectric potentiometry

We studied graphene synthesized on the C-terminated face (C-face) of a 4H-SiC substrate by noncontact scanning nonlinear dielectric potentiometry. As already reported by other researchers, multilayer graphene sheets with moiré patterns were observed in our sample, which indicates the existence of rotational disorder between adjacent layers. We found that the potentials of graphene on the C-face are almost neutral and significantly smaller than those observed on the Si-terminated face (Si-face). In addition, the neutrality of potentials is not affected by various topographic features underlying the multilayer graphene sheets. These results indicate that graphene on the C-face of SiC is decoupled or screened from the underlying structures and substrate, unlike graphene on the Si-face.

ReaxFF molecular dynamics study on oxidation behavior of 3C-SiC: Polar face effects**Project supported by the 111 Project (Grant No. B07050) and the National Natural Science Foundation of China (Grant No. 11402206).

The oxidation of nanoscale 3C-SiC involving four polar faces (, Si(100), , and Si(111)) is studied by means of a reactive force field molecular dynamics (ReaxFF MD) simulation. It is shown that the consistency of 3C-SiC structure is broken over 2000 K and the low-density carbon chains are formed within SiC slab. By analyzing the oxygen concentration and fitting to rate theory, activation barriers for , Si(100), , and Si(111) are found to be 30.1, 35.6, 29.9, and 33.4 kJ·mol−1. These results reflect lower oxidative stability of C-terminated face, especially along [111] direction. Compared with hexagonal polytypes of SiC, cubic phase may be more energy-favorable to be oxidized under high temperature, indicating polytype effect on SiC oxidation behavior.

Kinetics of nitrogen incorporation at the SiO2/4H-SiC interface during an NO passivation

Nitridation of the SiO2-4H-SiC interface using post-oxidation annealing in nitric oxide (NO) is the most established process for obtaining high quality 4H-SiC MOS interfaces. In this paper, a detailed study of the interfacial nitrogen uptake kinetics is reported for the (0001) Si-terminated and (000-1) C-terminated crystal faces of 4H-SiC. The results indicate a significantly faster kinetics for the C-face compared to the Si-face. For the first time, the correlation between N-uptake and the interface trap density reduction was observed on the C-face. First-order kinetics models are used to fit the nitrogen up-take. Empirical equations predicting N coverage at the SiO2-4H-SiC interface via NO annealing have been established. The result also shows that the N-uptake rate is associated with the oxidation rate.

Molecular dynamics simulation of graphene growth by surface decomposition of 6H-SiC(0001) and

Much attention has been paid to graphene growth by the surface decomposition of SiC. A SiC substrate surface contains a Si-terminated (0001) face and a C-terminated face. It was reported that graphene layers on these two faces have different structures and electronic properties. We studied the effects of the SiC substrate surface, i.e., of the Si-face and C-face, and annealing temperature on graphene growth using classical molecular dynamics (MD) simulation. It was found that quality of graphene on the Si-face was better than that on the C-face. In addition, graphene coverage was high at a high annealing temperature.

Epitaxial graphene on SiC{0001}: advances and perspectives

We review here recent progress on epitaxial graphene grown on a SiC substrate. Epitaxial graphene can be easily grown by heating the SiC single crystal in a high vacuum or in an inert gas atmosphere. The SiC surfaces used for graphene growth contain Si- and C-terminated faces. On the Si-face, homogeneous and clean graphene can be grown with a controlled number of layers, and the carrier mobility reaches as high as several m(2) V s(-1), although this is reduced by the presence of the substrate steps. On the C-face, although the number of layers is not homogeneous, twisted bilayer graphene can be grown, which is expected to be the technique of choice to modify the electronic structure of graphene. From the application point of view, graphene on SiC will be the platform used to fabricate high-speed electronic devices and dense graphene nanoribbon arrays, which will be used to introduce a bandgap.

Exploring graphene formation on the C-terminated face of SiC by structural, chemical and electrical methods

The properties of epitaxial graphene on the C-face of SiC are investigated using comprehensive structural, chemical and electrical analyses. By matching similar nanoscale features on the surface potential and Raman spectroscopy maps, individual domains have been assigned to graphene patches of 1–5 monolayers thick, as well as bare SiC substrate. Furthermore, these studies revealed that the growth proceeds in an island-like fashion, consistent with the Volmer-Weber growth mode, illustrating also the presence of nucleation sites for graphene domain growth. Raman spectroscopy data shows evidence of large area crystallites (up to 620nm) and high quality graphene on the C-face of SiC. A comprehensive chemical analysis of the sample has been provided by X-ray photoelectron spectroscopy investigations, further supporting surface potential mapping observations on the thickness of graphene layers. It is shown that for the growth conditions used in this study, 5 monolayer thick graphene does not form a continuous layer, so such thickness is not sufficient to completely cover the substrate.

Optimising the Growth of Few-Layer Graphene on Silicon Carbide by Nickel Silicidation

Few-layers graphene films (FLG) were grown by local solid phase epitaxy on a semi-insulating 6H-SiC substrate by annealing Ni films deposited on the Si and C-terminated faces of the SiC. The impact of the annealing process on the final quality of the FLG films is studied using Raman spectroscopy. X-ray photoelectron spectroscopy was used to verify the presence of graphene on the sample surface. We also demonstrate that further device fabrication steps such as dielectric deposition can be carried out without compromising the FLG films integrity.

Electronic properties of epitaxial graphene

It has been known for almost 25 years that high temperature treatment of polar faces of SiC crystals lead to the graphitisation of the surface as a consequence of Si preferential sublimation. The group of Walt de Heer in Atlanta has proposed to use this procedure to obtain macroscopic multilayer graphene samples. This has lead to a renewal of interest for the study of the graphitic layers grown on the SiC surfaces. Due to the polar nature of the material, the 6H(4H)-SiC substrate present two non-equivalent surfaces: the (0001) is known as the Si face, and the (000-1) one is the C face. It turns out that the structure and the properties of the multilayer graphene samples obtained on those two surfaces are rather different. In particular the multilayer graphene samples grown on the C-terminated face presents electronic properties that are rather similar to those of monolayer graphene with very high mobility. This is shown by transport and optical experiments under high magnetic field. This surprising result is explained by the rotational stacking sequence, observed experimentally, which leads to an electronic decoupling as demonstrated by ab initio band structure calculations.