C-terminated Surfaces Research Articles

An investigation on the surface properties of B4C for advancing its nuclear applications

B4C is an important material in diverse nuclear applications. However, a systematic examination of its surface properties is still missing. In this work, we employ first-principles simulations to investigate the energetic stability of 16 distinct slab models representing (001), (100), (101), (110), and (111) surfaces, which are constructed by minimizing dangling bonds. Our results show that C-terminated (001) surface exhibits significantly greater stability than other surfaces under both the carbon and boron-rich conditions. Besides, we also study the defect formation energies on the C-terminated (001) surface and compare them with the cases in bulk. The high formation energies of the defects suggest a low likelihood of their occurrence on this surface, despite their formation energies being lower compared to bulk cases. Furthermore, mid-gap surface states are revealed for the top atomic layers of the C-terminated (001) surface, which are deduced at the deeper layers, and the band structures of the middle layers of this slab recover to the bulk band gap. These surface mid-gap states allow electron excitation from the valence band to these states, resulting in a reduced optical band gap compared to the bulk band gap of B4C. This provides a plausible explanation for the significantly smaller band gap observed in experiments compared to the larger gap predicted by theoretical models. Our study not only sheds light on the surface properties of B4C but also lays the groundwork for advancing this material for more advanced nuclear applications.

First-principles study of Ti adsorption on Al4C3 (0001) surface

Al4C3 phase is hypothesized to undergo a solid–solid phase transition into TiC, thereby facilitating the heterogeneous nucleation of α-Al. However, capturing this transition process experimentally is challenging, which undermines the credibility of this transition theory. In this study, first-principles calculations are employed to investigate the feasibility of Ti atom adsorption on the exposed (0001) surface of the Al4C3 crystal, providing theoretical support for the phase transition hypothesis. The findings indicate that Ti adatoms can effectively adsorb on the C-terminated (0001) surface, whereas adsorption on the Al-terminated surface is thermodynamically unfavorable. Upon structural optimization, Ti adatoms at bridge site B, top site T, and hollow site H1, irrespective of coverage, migrate towards the most stable hollow H2 site. Electronic structure analysis reveals that the bonding between the Ti adatom at the H2 site and the surface layer C atoms is primarily covalent, while the bonding with subsurface layer Al atoms is metallic. Additionally, Ti atom adsorption induces polarization of the bonding electron cloud between Al and C atoms. This study provides theoretical support for the Ti-induced transformation of the Al4C3 to TiC phase via surface adsorption, paving the way for the development of high-performance master alloys based on phase transition theory.

Research on the electronic properties of TiC/γ-Fe and TiB2/γ-Fe interfaces based on first-principles calculations

Addressing the unclear understanding of the bonding mechanisms at the TiC/γ-Fe and TiB2/γ-Fe interfaces, the atomic structure, adhesive work, and electronic properties of the TiC (111)/Fe (111) and TiB2 (001)/Fe (111) interfaces were investigated using first-principles calculations. The results reveal that the C-terminated surface of TiC (111) and the B-terminated surface of TiB2 (001) exhibit high surface activity, facilitating their binding with the Fe (111) surface. The adhesive energy at the TiC (111)/Fe (111) interface is 13.04 J/m2, with a wetting angle of 41.32°, while at the TiB2 (001)/Fe (111) interface, the adhesive energy and wetting angle are 6.08 J/m2 and 74.69°, respectively. TiC demonstrates better wettability and stronger binding strength with γ-Fe surface. At the interface, the C-p orbitals and B-p orbitals hybridize with Fe-d orbitals to form bonds. The population of Fe-C bonds and Fe-B bonds is 0.46 and 0.02, respectively, indicating that Fe-C bonds exhibit covalent characteristics, while Fe-B bonds display ionic characteristics, with Fe-C bonds being stronger than Fe-B bonds. Statistical analysis of charge distribution and charge difference reveals electron accumulation near C and B atoms at the interface. Moreover, the electron transfer at the TiC (111)/Fe (111) interface is significantly greater than that at the TiB2 (001)/Fe (111) interface, which is the fundamental reason for the higher binding strength at the TiC (111)/Fe (111) interface.

Preferential orientation of diamond formation on TaC: Diamond(111)//TaC(111)

Experimental results showed that for the diamond film prepared by hot filament chemical vapor deposition (HFCVD) using Ta filament, TaC existed between diamond and the silicon substrate, and diamond grew directly on TaC, while the inherent mechanism was not clear. Here, a special coherent interface Diamond(111)//TaC(111) is observed using high resolution transmission electron microscopy, and then we explore the effects of the TaC with different lattice planes on the diamond formation by first-principle calculations. The results show that C tends to adsorb on the TaC(111) C-terminated surface. The strong covalent bond between C from diamond and Ta from TaC is formed in the Diamond(111)//TaC(111) interface, while only C–C covalent bonds are formed at the Graphite(002)/TaC(111). This makes diamond thermodynamically more stable than graphite on the TaC surfaces. Our investigations provide critical information to understand the complex diamond formation mechanism, especially with the presence of TaC.

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.

Determination of Work Function for p- and n-Type 4H-SiC Single Crystals via Scanning Kelvin Probe Force Microscopy

Silicon carbide (SiC) is a promising platform for fabricating high-voltage, high-frequency and high-temperature electronic devices such as metal oxide semiconductor field effect transistors in which many junctions or interfaces are involved. The work function (WF) plays an essential role in these devices. However, studies of the effect of conductive type and polar surfaces on the WF of SiC are limited. Here, we report the measurement of WFs of Si- and C-terminated polar surfaces for both p-type and n-type conductive 4H-SiC single crystals by scanning Kelvin probe microscopy (SKPFM). The results show that p-type SiC exhibits a higher WF than n-type SiC. The WF of a C-terminated polar surface is higher than that of a Si-terminated polar surface, which is further confirmed by first-principles calculations. By revealing this long-standing knowledge gap, our work facilitates the fabrication and development of SiC-based electronic devices, which have tremendous potential applications in electric vehicles, photovoltaics, and so on. This work also shows that SKPFM is a good method for identifying polar surfaces of SiC and other polar materials nondestructively, quickly and conveniently.

Adhesion and electronic properties of 4H-SiC/α-Al2O3 interfaces with different terminations calculated via first-principles methods

To comprehensively reveal the microscopic properties of the 4H-SiC/α-Al2O3 interface, not only the properties of 4H-SiC and α-Al2O3 surfaces, but also the interface separation work, interface energies, band offsets, charge transfer, interfacial bonding, and partial density of states for different types of 4H-SiC/α-Al2O3 interfaces were studied using the first-principles method. Considering the two distinct crystal faces of 4H-SiC (Si-terminated and C-terminated surfaces) and the three distinct terminal configurations on the α-Al2O3 side (single Al (Al1-terminated), double Al (Al2-terminated), and O-terminated surface), we built six different types SiC/Al2O3 interfaces (Si-Al1, Si-Al2, Si-O, C-Al1, C-Al2, and C-O interfaces). The results of the interface separation work for the O-terminated interfaces (Si-O and C-O interfaces) revealed that the O-terminated interfaces were significantly greater compared to Al1- and Al2-terminated interfaces, indicating the pronounced dominance of O-terminated interfaces in terms of bonding strength across the six interfacial structures. Moreover, the charge transfer, interfacial bonding, and electronic properties for the interfaces showed that the O-terminated interfaces had larger charge transfer and more interfacial bonds. Additionally, the larger charge transfer at the O-terminated (especially Si-O interface) interfaces caused the larger conduction band offsets (CBOs), which could effectively prevent leakage current and improve the reliability of SiC/Al2O3 devices.

A systematic investigation on the surface properties of Ti2AlC via first-principles calculations

In this study, the surface properties of Ti2AlC surface with different configurations were systematically investigated based on the analysis of surface energy and electronic structure via first-principles calculations. 18 Ti2AlC surface models of basal, prismatic or pyramidal planes were constructed and calculated. The results show that either the Ti(C)- or Al-terminated (0001) surface exhibits the lowest surface energy and highest thermodynamic stability depending on the element chemical potential. While the C-terminated (0001) surface exhibits the lowest thermodynamic stability independent of the element chemical potential. Since the atomic structures of (101¯0) and (112¯0) prismatic planes are similar to those of (101¯1)∼(101¯6) and (112¯1)∼(112¯6) pyramidal planes, respectively, they have similar surface energy and stability. The electronic structure calculations revealed that the highest thermodynamic stability of Ti(C)- or Al-terminated (0001) surface is due to their less structure relaxations. The present first-principles calculation results are well consistent with previous experimental results, and could provide theoretical guidance for the further engineering application of Ti2AlC phase.

Ultrathin epitaxial MgB2 on SiC: Substrate surface-polarity-dependent properties

High-quality, ultrathin superconducting films are required for advanced devices such as hot-electron bolometers, superconducting nanowire single-photon detectors, and quantum applications. Using hybrid physical-chemical vapor deposition, we show that ${\mathrm{MgB}}_{2}$ films as thin as 4 nm can be fabricated on the carbon-terminated $6H$-SiC (0001) surface with a superconducting transition temperature above 33 K and a rms roughness of 0.7 nm. Remarkably, the film quality is a function of the SiC surface termination, with the C-terminated surface preferred to the Si-terminated surface. To understand the ${\mathrm{MgB}}_{2}$ thin film/SiC substrate interactions giving rise to this difference, we characterized the interfacial structures using Rutherford backscattering spectroscopy/channeling, electron-energy-loss spectroscopy, and x-ray photoemission spectroscopy. The ${\mathrm{MgB}}_{2}$/SiC interface structure is complex and different for the two terminations. Both terminations incorporate substantial unintentional oxide layers influencing ${\mathrm{MgB}}_{2}$ growth and morphology, but with a different extent, intermixing, and interface chemistry. In this paper, we report measurements of transport, resistivity, and the critical superconducting temperature of ${\mathrm{MgB}}_{2}$/SiC that are different for the two terminations, and they link interfacial structure variations to observed differences. The result shows that the C face of SiC is a preferred substrate for the deposition of ultrathin superconducting ${\mathrm{MgB}}_{2}$ films.

Molecular Dynamics Study of Interdiffusion for Cubic and Hexagonal SiC/Al Interfaces

The mechanical properties of the SiC/Al interface are crucial in estimating the overall strength of this ceramic-metal composite. The present work investigates the interdiffusion at the SiC/Al interface using molecular dynamics simulations. One cubic and one hexagonal SiC with a higher probability of orientations in contact with Al are examined as two samples of metal-matrix nanocomposites with whisker and particulate reinforcements. These reinforcements with the Si- and C-terminated surfaces of the SiC/Al interfaces are also studied. The average main and cross-interdiffusion coefficients are evaluated using a single diffusion couple for each system. The effect of temperature and annealing time are analysed on the self- and interdiffusion coefficients. It is found that the diffusion of Al in SiC is similar in cubic and hexagonal SiC and as expected, the interdiffusion coefficient increases as the temperature and annealing time increase. The model after diffusion can be used to evaluate the overall mechanical properties of the interface region in future studies.

Mechanism of Co atom point defects on different termination surfaces of WC–Co/diamond interface by first-principles

Based on the density functional theory, the first-principles method had been used to calculate the binding energy, the charge density of the interface, Mulliken population, density of states, band structure and phonon dispersion analysis of Co atom point defects on the W-terminated surface and C-terminated surface of the WC–Co/Diamond film-base interface. The results showed that the binding energy of W-GD, W-VD, W-SD sites on the W termination surface with WC–Co/Diamond interface was gradually weakened. And the binding energy of C-GD, C-SD, C-VD sites on the C termination surface was the same as the former. From the charge density, Co atom on the W-terminated surface of WC(100) had a stronger influence on the film-base interface than the C-terminated surface. From the analysis of the Mulliken population, the ability of Co to transfer charge on W-terminated surface was stronger than C-terminated surface. According to the density of states analysis, the Fermi level state density of the W terminal surface was higher than that of the C terminal surface and the activity of the W terminal surface was higher than that of the C terminal surface. Through the analysis of the electronic energy band structure, the system indicated significant metallicity. The phonon spectrum showed that the presence of Co atoms on the W-terminated surface and C-terminated surface of the WC–Co/Diamond film-base interface is stable.

Zinc-Blende GeC Stabilized on GaN (001): An Ab Initio Study

First-principle calculations have been performed to explore the initial stages of the zinc blende-like germanium carbide epitaxial growth on the gallium nitride (001)-(2 × 2) surface. First, we studied the Ge/C monolayer adsorption and incorporation at high symmetry sites. Results show that the adsorptions at the top and hcp1 sites are the most stable structures of C and Ge, respectively. Different terminated surfaces were used on the GeC epitaxial growth. According to the surface formation energies, only the first two bilayers are stable; therefore, the GeC epitaxial growth is favorable only under N-rich conditions on a Ge-terminated surface and with Ge bilayers terminated. In addition, it is demonstrated that GeC bilayers on the C-terminated surfaces are unstable and preclude the epitaxial growth. Electronic properties have been investigated by calculating the density of states (DOS) and the projected density of states (PDOS) of the most favorable structures.

The wettability of molten aluminum droplets on the 3C–SiC surface: Molecular dynamics study

AlSiC composites is an irreplaceable material for high temperature semiconductor devices due to its unique performance of high thermal conductivity, low thermal expansion coefficient and bending strength. Understanding of the atomic level interactions between SiC and Al is paramount important. This leads to appropriate control on the transport process at the solid-liquid interface in the preparation of SiC reinforced aluminum matrix composites. To characterize the solid-liquid interaction, the aim is to perform molecular dynamics study on the wetting behavior of molten aluminum droplets on the 3C–SiC surface. The wettability of four crystal surface structure including Si-terminated, C-terminated, rectangular groove, wedge groove was characterized. The corresponding contact angles θ were 113.52°, 80.11°, 108.93° and 132.83° respectively. It was found that the atomic surface terminals and crystal surface morphology play an important role in the wetting behavior of 3C–SiC. High hydrophilic was observed on the C-terminated surface, while the wedge-shaped surface showed high hydrophobic. These theoretical findings demonstrate the effective modeling strategies when using wetting behavior as an interface modeling framework.

First-principles insights into ammonia decomposition on WC (0001) surface terminated by W and C

In order to investigate the adsorption and dehydrogenation process of ammonia on WC (0001) surfaces terminated by W and C atoms, the density functional theory (DFT) was used along with periodic slab models. All possible adsorption configurations and energies of relevant intermediates on WC (0001) surfaces were identified. The results of the adsorption energy and transition states showed that the C-terminated WC surface was more favorable for hydrogen production by the decomposition of ammonia. Conversely, the WC surface terminated by W atoms was susceptible to be poisoned by the adsorbed N atoms, leading to the combinative desorption of N atoms as the rate-limiting step in the entire reaction. The unique differences between WC (0001) surfaces terminated by W and C atoms suggested that a structure-sensitive reaction was ammonia decomposition over the WC catalyst.

A model to partition the constituent element’s atomic energy in TiC crystal and to characterize the interface properties of α-Ti(0001)/TiC(111)

A periodic slab model was proposed to characterize the TiC(111)/α-Ti(0001) interface properties. The model provides an effective method to partition the total energy of TiC crystalline into its constituent element’s atomic energy by constructing a periodic TiC(111)/α-Ti(0001) interface structure with a specific atomic stacking sequence on the interface. The calculation result shows that Ti and C atom possess −116.65009 Ry and −12.06893 Ry in TiC crystal respectively, which is in good agreement with the TiC total energy of −128.71901 Ry, indicating a satisfactory accuracy better than ~10−4 Ry. The information of the constituent element’s atomic energy provides a direct solution to quantitatively characterize TiC(111) surface energy, or rather, the surface energy on an arbitrary crystallographic plane of TiC crystal. The calculated Ti- and C-terminated surface energies on TiC(111) surface are −0.154 J/m2 and 8.014 J/m2 respectively. Under the atomic configuration in the present periodic model, the Ti-Ti and Ti-C interface energies were calculated to be −1.705 J/m2 and −1.891 J/m2, respectively. The investigation on the ideal work of adhesion indicated that the Ti-Ti interface is the most vulnerable region in the TiC(111)/α-Ti(0001) interface structure on account of the weaker metallic bonding between the Ti-Ti atomic layers.

First-Principles Study of Graphene-6H SiC Surface Interactions

Interactions of graphene with 6H-SiC {0001} surfaces are numerically investigated from first principles. In order to describe the bulk structure and its 6 bilayer thick surfaces correctly, bare and dipole-corrected atomic relaxations are considered. The obtained lattice parameters and bulk modulus are in good agreement with experimental values. The calculated indirect band gap width of 2.10 eV is smaller than the experimental value due to the nature of the computational method. Geometrical optimization of the surfaces, where dipole correction is applied, reveals that the first two bilayers displace significantly, where the relaxations of the very top bilayer is more pronounced. Band structures of the {0001} surfaces possess two flat bands around the Fermi level due to unsaturated bonds on opposite faces. When one layer of C atoms are introduced on the Si-terminated surface, it behaves as a tightly-bound buffer layer. This is also the case for the C-terminated surface when van der Waals interactions are taken into account. In contrast, disregarding these interactions yields free-standing graphene like behavior for the first C overlayer. On both surfaces, the second C overlayer is free-standing where the corresponding band structures incorporate Dirac-cone like features.

Effects of irradiation defects on the adsorption of oxygen on 3C-SiC low index surfaces

Silicon carbide (SiC) is widely regarded as a potential accident tolerant fuel cladding material for the next generation nuclear power system. Oxidation is the leading cause of failure of SiC in a high temperature environment. Adsorption of oxygen on SiC surface is the first stage of SiC oxidation. Effects of irradiation defects, such as vacancy and antisite atom, on the adsorption of oxygen on 3C-SiC low index surfaces are investigated by first principles calculation method. The results show that the oxygen molecule tends to dissociate spontaneously into two O atoms when adsorbed on the low index surfaces. The adsorption stability of oxygen molecule on the Si-terminated surface is higher than that on the C-terminated surface. As a whole, irradiation defects decrease the stability of oxygen adsorption on C-(100) and Si-(100) surfaces but increase the stability of oxygen adsorption on (110), (111), and (1¯1¯1¯) surfaces. Except for (110) surface, new covalent bonds as the precursors of oxides are formed on the SiC surfaces due to the irradiation defects, which changed the oxide species of other surfaces during the initial oxidation. Bader charge analysis and the local density of state results show that charge transfer plays a crucial role in the adsorption process.

Effect of SiC nano-size fillers on the aging resistance of XLPE insulation: A first-principles study

Effective strategies to prevent the electrical tree growth and extend the lifetime of cross-linked polyethylene (XLPE) insulation are of crucial importance for the development of power cable industry. The first-principles study with DFT method was performed to understand the mechanism of aging resistance for SiC/XLPE nanocomposite. The Bader charge redistribution calculations between different types of silicon carbide and XLPE suggest that SiC fillers have the ability of capturing hot electrons to suppress the accumulation of space charges because of their unsaturated electronic structures and magnetism on the Si- or C-terminated surfaces. The interfacial behavior between SiC and XLPE was investigated based on the physical interaction and chemical reactivity. The 3C, 4H and 6H-type SiC with flat Si-terminated surfaces are proposed to be favorable additives by showing relatively strong physical interaction with XLPE to limit its movement, and high activation energy for the hydrogen migration reaction to protect the XLPE.

Magnetic field induced enhanced absorption using a gated graphene/1D photonic crystal hybrid structure: Quantum regime

The terahertz (THz) absorption properties of a gated graphene monolayer placed on top of a one-dimensional photonic crystal is investigated in the presence of a perpendicular magnetostatic bias. The response of electrons to the magnetic field is inspected in the quantum regime, due to the low doping level of graphene grown on the C-terminated surface of silicon-carbide. It has been shown that there is the possibility of achieving enhanced absorption at low magnetic fields for certain states of circular polarization of light. Furthermore, adjusting the gate voltage of the graphene provides another method of tuning absorption in the proposed structure. Therefore, one can obtain enhanced absorption with the appropriate choices of magnetic and electric biases. We believe that these properties make our structure suitable for designing tunable graphene-based THz absorbers.

Influence of Fe on Al4C3 as a heterogeneous nucleation substrate: a first-principles study

To elucidate the influence of Fe on the crystal structure of Al4C3 and its carbonaceous refining effect on primary Mg grains, the adsorption energy and electronic structure of adsorption were investigated via first-principles calculations. The results show that H1 and are the most stable adsorption sites for the Al-terminated and C-terminated Fe/Al4C3(0 0 0 1) adsorption systems. Based on their formation energies, the adsorption system is more stable than the H1 adsorption system, indicating that Fe atoms should adsorb on the C-terminated surface rather than on the Al-terminated surface. Analyzing the atomic structure of the adsorption system revealed that the surface structure of Al4C3(0 0 0 1) is reconstructed when the adsorption coverage exceeds 0.25 ML of Fe atoms. Thus, Fe adsorption may weaken the heterogeneous nucleation potency of Al4C3 particles. This explains the observed experimental phenomenon that excessively high Fe contents negatively influence the refinement of Mg alloys with added carbon.