SiC Bilayer Research Articles

Surface Structuring of Patterned 4H-SiC Surfaces Using a SiC/Si/SiC Sandwich Approach

Mesa- and trench-patterned surfaces of 4H-SiC(0001) 4°off wafers were structured in macrosteps using Si melting in a SiC-Si-SiC sandwich configuration. Si spreading difficulties were observed in the case of trench-patterned samples while the attempts on mesa-patterned ones were more successful. In the latter case, parallel macrosteps were formed on both the dry-etched and unetched areas though these macrosteps rarely cross the patterns edges. The proposed mechanism involved preferential etching at Si-C bilayer step edges and fast lateral propagation along the [1120] direction.

Extraction of interfacial thermal resistance across organic/semiconductor interface using optical-interference contactless thermometry

Abstract We have developed an experimental method to extract interfacial thermal resistance (ITR) at organic/semiconductor interface based on optical-interference contactless thermometry. The proposed technique was applied to the SU-8/SiC bilayer sample, and clear oscillations in reflectivity induced by optical interference during pulse heating and cooling were observed. Measured ITR increased with the temperature of SU-8 around interface from 170 mm2KW-1 at 304 K to 515 mm2KW-1 at 369 K. The increasing ITR also leads to an increase in the temperature drop at the interface from 7 K to 73 K.

Computational insight of lithium adsorption and intercalation in bilayer TiC3

Lithium-ion batteries (LIBs) have gained significant attention owing to their long lifespan. However, these batteries offer unmatched energy storage capacity and suffer from restricted lithium-ion mobility within the electrodes. Here, we employ first-principles calculation to investigate the two-dimensional TiC3 bilayer material. The results exhibit a remarkably high specific capacity of 1277 mAh/g and a low diffusion energy barrier of 0.12 eV. The TiC3 bilayer is anticipated to show high electrical conductivity, maintaining its metallicity due to strong bonding with four Li atoms. Additionally, its high thermal and dynamic stabilities are expected to significantly enhance the battery performance. Notably, the AB stacking bilayer TiC3 experiences a mere 6.01 % increase in volume, considerably smaller compared to the 28 % increase observed in the SiC bilayer. This suggests that TiC3 bilayers remain intact even at the highest concentration of lithium adsorptions. We also explored the solid-electrolyte interface (SEI) formation at the outset of battery operation using reactive force field molecular dynamics simulation. The reactive products of SEI are nicely matched with previous experimental and theoretical findings. All these intriguing properties position the TiC3 bilayer as an exceptionally promising material for use in LIBs.

Construction of a ceramic coating with low residual stress on C/CA composites for thermal protection at ultra-high temperatures

Carbon fiber reinforced carbon aerogel (C/CA) composites are promising candidates for thermal protection at ultra-high temperatures. However, the poor anti-oxidative performance and difficulties in preparing a dense and crack-free coating without damaging the substrate have limited their use in oxidizing environments. Here, we have prepared a dense SiC coating on C/CA by chemical vapor deposition under the assistance of a SiC gradient layer derived from in-situ ceramization of the CA matrix. By courtesy of the gradual compaction and composition, the SiC bilayer coating with 440 μm thickness is crack-free and has low residual stress. Resultantly, the coating has a high interfacial bonding strength of 6.79 MPa due to the difficult crack initiation and propagation as well as the limited strain localization. The coated C/CA shows good ablation resistance with a linear ablation rate of 0.62 μm/s at 1900 °C for 600 s and remarkable thermal insulation with a large temperature drop of 930 °C in 10 mm thickness. The strategy of a combination of dense CVD and gradient ceramization layers can be extended to other components with excellent high temperature properties such as ZrC, HfC, which will facilitate the improvement of the ablation performance of C/CA in ultra-high temperatures.

Energy barriers for Dy and H penetrating graphene on 6H-SiC(0001) and freestanding bilayer graphene from first-principles calculations

Currently, intercalation of foreign guest atoms into two-dimensional (2D) layered van der Waals materials is an active research area motivated in part by the development of next-generation energy-storage technologies and optoelectronic devices. One such extensively studied 2D material is the graphene-on-SiC system. To realize and control the desired intercalated structures, it is fundamentally important to understand the kinetic process of intercalation. For the intercalation of a guest atom into graphene layers on SiC substrate, a critical kinetic parameter is the energy barrier of a guest atom penetrating the perfect graphene top layer into the gallery under it. However, accurate theoretical calculations for such penetration barriers are unavailable in literature. From our first-principles density functional theory calculations, we obtain the global energy barriers of 3.47 and 1.80 eV for single Dy and H atoms penetrating the graphene top layer on a graphene buffer layer supported by a Si-terminated 6H-SiC(0001) substrate, respectively. For comparison as well as for examining the lateral strain effects, we also obtain the global barriers of 5.05 and 1.50 eV for single Dy and H atoms penetrating freestanding bilayer graphene with a tensile strain of about 8.8% to match our model for supported graphene, as well as the global barriers of 7.21 and 4.18 eV for penetrating unstrained freestanding bilayer graphene, respectively. From corresponding minimum energy paths with multiple energy minima and saddle points, we can also obtain various local energy barriers and the global backward barrier from the graphene gallery back to the top surface.

Gas-Sensing Properties of the SiC Monolayer and Bilayer: A Density Functional Theory Study.

Using density functional theory calculations, the adsorption of gaseous molecules (NO, NO2, NH3, SO2, CO, HCN, O2, H2, N2, CO2, and H2O) on the graphitic SiC monolayer and bilayer has been investigated to explore the possibilities in gas sensors for NO, NO2, and NH3 detection. The strong adsorption of NO2 and SO2 on the SiC monolayer precludes its applications in nitride gas sensors. The nitride gases (NO, NO2, and NH3) are chemisorbed on the SiC bilayer with moderate adsorption energies and apparent charge transfer, while the other molecules are all physisorbed. Further, the bilayer can effectively weaken the adsorption strength of NO2 and SO2 molecules, that is, NO2 molecules are only weakly chemisorbed on the SiC bilayer with an Eads of −0.62 eV, while SO2 are physisorbed on the bilayer. These results indicate that the SiC bilayer can serve as a gas sensor to detect NO, NO2, and NH3 gases with excellent performance (high sensitivity, high selectivity, and rapid recovery time). Moreover, compared with other molecular adsorptions, the adsorption of NH3 molecules significantly changes the work function of the SiC monolayer and bilayer, indicating that they can be used as optical gas sensors for NH3 detection.

Surface Morphology of 4H-SiC after Thermal Oxidation

Step-controlled growth of 4H-SiC epitaxial layers leads to the formation of a step-bunched morphology along the surface with larger macrosteps, composed of smaller microsteps of several Si-C bilayer heights. As thermal oxidation is an orientation-dependent process, a multi-faceted surface is expected to exhibit a different oxidation behavior compared to a perfectly planar surface. In this work, step-bunched surfaces after oxidation are investigated by high-resolution atomic force microscopy (HR-AFM) and transmission electron microscopy (TEM) indicating a morphological change in the early stages of thermal oxidation. An orientation-dependent oxidation model is used to correctly describe variations of the oxide thicknesses at isolated macrosteps.

Stability of multiple Shockley type basal plane stacking faults in heavily nitrogen-doped 4H-SiC crystals

The stability of various types of multiple Shockley type basal plane stacking fault in heavily nitrogen-doped 4H-SiC crystals was theoretically investigated on the basis of the quantum well action (QWA) mechanism. The energy levels of confined electrons within multiple Shockley stacking faults (SSFs) were calculated using the simple quantum well model and compared to the experimental values estimated from photoluminescence measurements. The formation energies of the stacking faults were also calculated on the basis of the axial next nearest neighbor Ising (ANNNI) model for SiC polytypes. Based on these physical and electronic parameters, the stability of multiple SSFs in heavily nitrogen-doped 4H-SiC crystals was discussed, and it was revealed that the stacking sequence of multiple SSFs largely affects their stability; SSFs having a cubic stacking sequence of medium thickness (five to six Si-C bilayer thickness) are stable in heavily nitrogen-doped 4H-SiC crystals at the typical growth and device processing temperatures of SiC, i.e., 1500–2600 K.

Structure and evolution of semiconducting buffer graphene grown on SiC(0001)

Using highly controlled coverages of graphene on SiC(0001), we have studied the structure of the first graphene layer that grows on the SiC interface. This layer, known as the buffer layer, is semiconducting. Using x-ray reflectivity and x-ray standing waves analysis we have performed a comparative study of the buffer layer structure with and without an additional monolayer graphene layer above it. We show that no more than 26\% of the buffer carbon is covalently bonded to Si in the SiC interface. We also show that the top SiC bilayer is Si depleted and is the likely the cause of the incommensuration previously observed in this system. When a monolayer graphene layer forms above the buffer, the buffer layer becomes less corrugated with signs of a change in the bonding geometry with the SiC interface. At the same time, the entire SiC interface becomes more disordered, presumably due to entropy associated with the higher growth temperature.

Theoretical and experimental investigation of the atomic and electronic structures at the 4H-SiC(0001)/SiO2 interface

Density functional theory calculations are carried out to investigate the atomic and electronic structures of the $4H\text{-SiC}(0001)/{\mathrm{SiO}}_{2}$ interface. We find two characteristic interface atomic structures in scanning transmission electron microscopy images: One is an interface in which the density of atoms at the first interfacial SiC bilayer is greater than that in the SiC substrate, while the other is an interface where the density of atoms at the first interfacial SiC bilayer is lower. Density functional theory calculations reveal that the difference in the scanning transmission electron microscopy images is a reflection of the atomic structures of these two interfaces. In addition, it has been reported that the floating states, which appear at the conduction band edge of a $4H\text{-SiC}(0001)/{\mathrm{SiO}}_{2}$ interface, affect the electronic structure of the interface and cause marked scattering of the electrons flowing along the interface [S. Iwase, C. J. Kirkham, and T. Ono, Phys. Rev. B 95, 041302(R) (2017)]. Interestingly, we find that the floating states do not appear at the conduction band edge of one of the two interfaces. These results provide physical insights into understanding and controlling the electronic structure and carrier mobility of electronic devices using wide-band-gap semiconductors.

Design and synthesis of diffusion-modified HfC/HfC SiC bilayer system onto WC-Co substrate for adherent diamond deposition

Abstract A diffusion-modified HfC/HfC SiC bilayer system was innovatively designed and synthesized onto cemented carbide (WC-Co) substrate by reactive double glow plasma surface alloying (DGPSA) technique for adherent diamond deposition. The microstructure, phase composition and adhesion of the bilayer system were investigated, as well as its effect on the diamond coating system. The results showed that the bilayer system was made up of a 1.0-μm-thick nanostructured HfC-dominant inner layer and a 0.7-μm-thick HfC SiC mixed outer layer. As the gradient element distributions existed at the bilayer/substrate interface, the bilayer system displayed a superior adherence to the substrate. And the surface microhardness improved from 1802 HV0.1 to 2857 HV0.1 after the reactive DGPSA treatment. Subsequently, a uniform and dense nanocrystalline diamond coating with thickness of 4.0 μm was successfully deposited onto the HfC/HfC SiC interlayered substrate. Benefit from the effective suppression of Co diffusion by the interlayer and the reasonable design in its hardness and thermal expansion coefficient (TEC), the obtained diamond coating possessed an excellent adhesion. Therefore, the diffusion-modified HfC/HfC SiC bilayer systems synthesized via reactive DGPSA technique can be considered as a novel interlayer option for depositing adherent diamond coatings on WC–Co substrates.

Atomically resolved microscopy of ion implantation induced dislocation loops in 4H-SiC

During high temperature electrical activation of ion-implanted dopant species in SiC, extrinsic dislocation loops are formed on the basal planes of the SiC lattice. Investigations have suggested Si-based loops are caused in accordance with the well-known +1 model. Herein we apply aberration corrected STEM to resolve the atomic structure of these loops. It is shown that the dislocation loops formed during annealing of Al-implanted SiC consist of an extra inserted Si-C bilayer of the (0001) polar sense, which upon insertion into the lattice causes a local extrinsic stacking fault. The +1 model thus needs to be expanded for binary systems.

4H-SiC Surface Structures and Oxidation Mechanism Revealed by Using First-Principles and Classical Molecular Dynamics Simulations

The 4H-SiC(000-1)C and (0001)Si surface reconstructed structures and the oxidation processes of these surfaces are investigated using a first-principles calculation method. The most stable reconstructed 4H-SiC(000-1)C and (0001)Si surfaces have p-bonded chains. In the topmost SiC bilayer, half of Si and C atoms exchange their positions and C-C or Si-Si bonds formed densely below the surfaces. When we place a SiO2 layer on the p-bonded chain (000-1)C surface, C-C bonds are formed more densely below the interface. We simulate a sequence of O2 molecules arrivals at an interface of tridymite-phase-SiO2 and 4H-SiC(000-1)C. Dissociated O atoms at the interface tended to make bonds with Si atoms. The C-C bonds in the SiC substrate break easily and a local C surface occasionally appears. We have examined how the surface structure changes through an O2 molecule exposure by using a classical molecular dynamics simulation program and confirmed the formation of C-C bonds below the surface and the interface.

Charge neutrality of quasi-free-standing monolayer graphene induced by the intercalated Sn layer

It has been confirmed by angle-resolved photoemission spectroscopy that decoupled quasi-free-standing monolayer graphene (QFMLG), obtained by Sn intercalation between the buffer layer and the 6H-SiC(0 0 0 1) substrate, is charge-neutral, i.e. the Dirac point matches with the Fermi level. By combined studies of scanning tunneling microscopy/spectroscopy and core-level/valence-band photoemission spectroscopy on this system, it has been found that the intercalated Sn atoms, bonding with the Si atoms of the top Si-C bilayer on the substrate comprise a hexagonal layer, which turns out to be metallic. Such a metallic character, which has never been found in intercalation using different elements, is a major cause of charge neutrality of QFMLG, since conduction electrons of the Sn layer compensate completely spontaneous polarization charges of 6H-SiC(0 0 0 1). This charge-neutral QFMLG is stable at a high temperature of 850 °C.

Hydrogenation of the buffer-layer graphene on 6H-SiC (0001): A possible route for the engineering of graphene-based devices

The hydrogenation at various temperatures of the (6√3 × 6√3)R30° reconstruction of SiC(0001), the so-called buffer layer graphene (BLG), is investigated. For the BLG, a significant concentration of remaining dangling bonds related to unsaturated Si atoms of the outermost SiC bilayer is evidenced in the inverse photoemission spectra. These dangling bonds give rise to a peak around 1 eV above the Fermi level, associated with the upper single-electron states of a Mott-Hubbard insulator, which vanishes upon hydrogenation. Hydrogen atoms adsorbed at ambient temperature remain covalently bound to BLG (H-BLG) up to temperatures of ∼500 °C. They induce additional C-Si bonds at the BLG/SiC interface that saturate the remaining Si dangling bonds, as evidenced in both IPES and Auger electron spectra. The H-BLG further shows a large energy gap and an excess n-type doping in comparison to the pristine BLG. Upon hydrogen exposure at higher temperature (> 700 °C), hydrogen atoms intercalate at the BLG/SiC interface, inducing the formation of a single layer of quasi-free-standing graphene (QFSG) lying on top of a hydrogenated (√3 × √3)R30° reconstruction as supported by IPES. We suggest that the high-stability and the distinct electronic structure of both BLG-derived structures, H-BLG and QFSG, may open a route for the engineering of graphene-based devices.

Formation mechanism of threading-dislocation array in AlN layers grown on 6H-SiC (0001) substrates with 3-bilayer-high surface steps

We grew AlN layers on 6H-SiC (0001) substrates with three Si-C bilayer high (0.75 nm) steps. In the AlN layers, most of the threading dislocations (TDs) were arranged in rows. The TD row consisted of arrays of a half-loop dislocation, which was formed by an AlN/SiC interfacial dislocation along the step edges of the SiC substrate surfaces and a TD pair at both ends. The configuration of the interfacial dislocation was highly relevant with two-dimensional AlN nuclei at the initial stage of growth. We concluded that the half-loop dislocation arrays were generated in the AlN nucleus coalescence over the SiC step edges.

Atomic configuration of irradiation-induced planar defects in 3C-SiC

The atomic configuration of irradiation-induced planar defects in single crystal 3C-SiC at high irradiation temperatures was shown in this research. A spherical aberration corrected scanning transmission electron microscope provided images of individual silicon and carbon atoms by the annular bright-field (ABF) method. Two types of irradiation-induced planar defects were observed in the ABF images including the extrinsic stacking fault loop with two offset Si-C bilayers and the intrinsic stacking fault loop with one offset Si-C bilayer. The results are in good agreement with images simulated under identical conditions.

Large-scale uniform bilayer graphene prepared by vacuum graphitization of 6H-SiC(0001) substrates

We report on the preparation of large-scale uniform bilayer graphenes on nominally flat Si-polar 6H-SiC(0001) substrates by flash annealing in ultrahigh vacuum. The resulting graphenes have a single thickness of one bilayer and consist of regular terraces separated by the triple SiC bilayer steps on the 6H-SiC(0001) substrates. In situ scanning tunneling microscopy reveals that suppression of pit formation on terraces and uniformity of SiC decomposition at step edges are the key factors to the uniform thickness. By studying the surface morphologies prepared under different annealing rates, it is found that the annealing rate is directly related to SiC decomposition, diffusion of the released Si/C atoms and strain relaxation, which together determine the final step structure and density of defects.

Nucleation of 3C-SiC associated with threading edge dislocations during chemical vapor deposition

Transmission electron microscopy and atomic force microscopy were used to study the origin of a preferred nucleation site on the atomically flat 3C-SiC mesas leading to the formation of tetrahedral hillocks. The hillocks exhibit a “wedding cake” structure consisting of concentric triangular terraces with a step height of 0.25nm corresponding to the thickness of a single Si-C bilayer. KOH etching revealed the presence of a threading dislocation at the center of the hillock. Its Burgers vector was shown to lie in the basal plane with no component along the surface normal. It is argued that the strain fields around a threading edge dislocation make it a preferred nucleation site.

Epitaxial graphene on SiC(0001¯): Stacking order and interfacial structure

The fundamental structural properties of multilayer epitaxial graphene (MEG) on C-face SiC(0001¯) were revealed in a straightforward manner using cross-sectional transmission electron microscopy (TEM) and scanning TEM (STEM). The AB-stacking and the azimuthal rotational disorder of the graphene layers were directly identified by selected area electron diffraction and high-resolution TEM. The directly interpretable STEM revealed that the interlayer spacing between the first graphene layer and the top SiC bilayer is substantially larger than that of the bulk graphite. Such a large interlayer spacing combined with the regional partially decomposed top bilayers of the SiC substrate provides a plausible explanation to the weak bonding between the MEG film and the SiC(0001¯) substrate.