SiC Clusters Research Articles

Mechanical behavior of zirconium diboride–silicon carbide–boron carbide ceramics up to 2200°C

The mechanical properties of hot pressed zirconium diboride–silicon carbide–boron carbide (ZrB 2 –SiC–B 4 C) ceramics were characterized from room temperature up to 2200 °C in an argon atmosphere. The average ZrB 2 grain size was 3.0 μm. The SiC particles segregated into clusters, and the largest clusters were >30 μm in diameter. The room temperature flexural strength was 700 MPa, decreasing to 540 MPa at 1800 °C and to 260 MPa at 2200 °C. The strength was controlled by the SiC cluster size up to 1800 °C. At higher temperatures, strength was controlled by formation of liquid phases, and precipitation of large BN and B–O–C–N inclusions. The mechanical behavior of these materials changes at ∼1800 °C, meaning that extrapolation of properties from lower temperatures is not accurate. Mechanical behavior in the ultra-high temperature regime was dominated by impurities and changes in microstructure. Therefore, the use of higher purity materials could lead to significant improvements in ultra-high temperature strength.

용융 금속 TSV 충전을 위한 저열팽창계수 SiC 복합 충전 솔더의 개발

Among through silicon via (TSV) technologies, for replacing Cu filling method, the method of molten solder filling has been proposed to reduce filling cost and filling time. However, because Sn alloy which has a high coefficient of thermal expansion (CTE) than Cu, CTE mismatch between Si and molten solder induced higher thermal stress than Cu filling method. This thermal stress can deteriorate reliability of TSV by forming defects like void, crack and so on. Therefore, we fabricated SiC composite filling material which had a low CTE for reducing thermal stress in TSV. To add SiC nano particles to molten solder, ball-typed SiC clusters, which were formed with Sn powders and SiC nano particles by ball mill process, put into molten Sn and then, nano particle-dispersed SiC composite filling material was produced. In the case of 1 wt.% of SiC particle, the CTE showed a lowest value which was a <TEX>$14.8ppm/^{\circ}C$</TEX> and this value was lower than CTE of Cu. Up to 1 wt.% of SiC particle, Young's modulus increased as wt.% of SiC particle increased. And also, we observed cross-sectioned TSV which was filled with 1 wt.% of SiC particle and we confirmed a possibility of SiC composite material as a TSV filling material.

Effect of substrate temperature on SiC interlayers for diamond coatings deposition on WC-Co substrates

SiC films were synthesized on WC-Co substrate as interlayers for improving the adhesion of diamond coatings. The influence of the substrate temperature on the SiC films was investigated. The results showed that when the temperature was 600 °C, the film was formed of loose agglomerates piled up by SiC nanoparticles. As the temperature increased to 850 °C, shell structure composed of SiC particles and graphite replaced the agglomerates. When the temperature exceeded 950 °C, flower-like SiC clusters and graphite spherical particles coexisted in the surface, and Co2Si particles embedded at the SiC/substrate interface. SiC films deposited at 600 °C, 850 °C and 950 °C were typically used as interlayer for diamond deposition. The coatings exhibited the similar nano-diamond structure. However, the diamond coating on the interlayer produced at 850 °C possessed the best adhesion; and the diffusion of Co was effectively inhibited by SiC interlayer which would not interfere with the process of diamond deposition.

Structures and stabilities of small carbon interstitial clusters in cubic silicon carbide

Knowledge of the configurations and stabilities of defect clusters in SiC is important for understanding radiation damage in this material, which is relevant to its nuclear and electronic applications. Such information is, however, often difficult to obtain experimentally. In this study, we perform Monte Carlo basin-hopping simulations with both empirical potential and density functional theory (DFT) calculations to search for the ground state (GS) configurations of small carbon interstitial clusters and carbon–antisite-based defects in cubic SiC (3C–SiC). Educated guesses of possible GS configurations have also been made. Our new approach can successfully identify many hitherto unknown GSs and energetically highly competitive metastable structures. For carbon penta- and hexa-interstitials, the GS structures predicted by DFT and empirical potential differ, and the plausible origin of this discrepancy is discussed from the chemical bonding point of view. Surprisingly, the GS structures of large carbon interstitial clusters in SiC are disjointed and composed of di- and tri-interstitial clusters as fundamental building blocks. Based on the present results, a possible mechanism for the carbon tri-interstitial defect to grow into the extended {111} planar interstitial defects observed in neutron-irradiated 3C–SiC is proposed. Furthermore, we show that C interstitial clusters can get trapped at a carbon antisite and form very stable complexes in SiC.

Mechanical behavior of zirconium diboride–silicon carbide ceramics at elevated temperature in air

The mechanical properties of zirconium diboride–silicon carbide (ZrB 2 –SiC) ceramics were characterized from room temperature up to 1600 °C in air. ZrB 2 containing nominally 30 vol% SiC was hot pressed to full density at 1950 °C using B 4 C as a sintering aid. After hot pressing, the composition was determined to be 68.5 vol% ZrB 2 , 29.5 vol% SiC, and 2.0 vol% B 4 C using image analysis. The average ZrB 2 grain size was 1.9 μm. The average SiC particles size was 1.2 μm, but the SiC particles formed larger clusters. The room temperature flexural strength was 680 MPa and strength increased to 750 MPa at 800 °C. Strength decreased to ∼360 MPa at 1500 °C and 1600 °C. The elastic modulus at room temperature was 510 GPa. Modulus decreased nearly linearly with temperature to 210 GPa at 1500 °C, with a more rapid decrease to 110 GPa at 1600 °C. The fracture toughness was 3.6 MPa·m ½ at room temperature, increased to 4.8 MPa·m ½ at 800 °C, and then decreased linearly to 3.3 MPa·m ½ at 1600 °C. The strength was controlled by the SiC cluster size up to 1000 °C, and oxidation damage above 1200 °C.

Kinetic Monte Carlo Simulation of Impurity Effects on Nucleation and Growth of SiC Clusters on Si(100)

The influence of attractive and repulsive impurities on the nucleation process of the SiC clusters on Si(100) surface was investigated. Kinetic Monte Carlo simulations of the SiC clusters growth show that that increase of the impurity concentration (both attractive and repulsive) leads to decrease of the mean cluster size and rise of the nucleation density of the clusters.

Using an Au interlayer to enhance electron field emission properties of ultrananocrystalline diamond films

We observe that an Au interlayer markedly enhances the electrical field emission (EFE) properties of ultrananocrystalline diamond (UNCD) films on Si substrates. The EFE properties of UNCD/Au/Si films can be turned on at a lower field and attain a higher current density than in UNCD films grown on Si substrates without an Au interlayer. Transmission electron microscopy reveals that the Au interlayer induces the formation of SiC clusters, preventing the formation of a resistive amorphous carbon layer that nucleates the diamond clusters. This improves the diamond-to-substrate interfacial conductivity. Moreover, there is an abundant nano-graphite phase, which is presumably induced by the coalescence of nano-sized diamond clusters. The percolation of the nano-graphite clusters helps transport electrons, improving the conductivity of the UNCD films. We believe that the simultaneous increase in the conductivity of the UNCD-to-Si interface and the bulk of the UNCD films is the main factor enhancing electrical conductivity and EFE properties of the films.

Carbon tri-interstitial defect: A model for the DIIcenter

Using a combination of random configuration sampling, molecular dynamics simulated annealing with empirical potential, and ensuing structural refinement by first-principles density functional calculations, we perform an extensive ground-state search for the most stable configurations of small carbon interstitial clusters in SiC. Our search reveals a ``magic'' cluster number of three atoms, where the formation energy per interstitial shows a distinct minimum. A carbon tri-interstitial cluster with trigonal ${C}_{3v}$ symmetry is discovered, in which all carbon atoms are fourfold coordinated. In addition to its special thermodynamic stability, its localized vibrational modes are also in a very good agreement with the experimental photoluminescence spectra of the D${}_{\mathrm{II}}$ center in both 3C- and 4H-SiC. The D${}_{\mathrm{II}}$ center is one of the most persistent defects in SiC, and we propose that the discovered carbon tri-interstitial is responsible for this center.

Theoretical predictions of a bucky-diamond SiC cluster

A study of structural relaxations of SinCm clusters corresponding to different compositions, different relative arrangements of Si/C atoms, and different types of initial structure, reveals that the SinCm bucky-diamond structure can be obtained for an initial network structure constructed from a truncated bulk 3C-SiC for a magic composition corresponding to n = 68 and m = 79. This study was performed using a semi-empirical Hamiltonian (SCED–LCAO) since it allowed an extensive search of different types of initial structures. However, the bucky-diamond structure predicted by this method was also confirmed by a more accurate density functional theory (DFT) based method. The bucky-diamond structure exhibited by a SiC-based system represents an interesting paradigm where a Si atom can form three-coordinated as well as four-coordinated networks with carbon atoms and vice versa and with both types of network co-existing in the same structure. Specifically, the bucky-diamond structure of the Si68C79 cluster consists of a 35-atom diamond-like inner core (four-atom coordinations) suspended inside a 112-atom fullerene-like shell (three-atom coordinations).

Investigation on mechanical properties of silicon nitride composite reinforced by SiC nanoparticles

The design of silicon nitride reinforced by silicon nitride (SiC) nanoparticles is attractive due to its improved high-temperature strength and resistance to oxidation. The high-temperature mechanical behaviors of these composites are difficult to be investigated because it is hard to measure these structural changes in the mechanical test. In this paper, we applied the molecular dynamics simulations to study the effects of size and quantity of SiC clusters on mechanical properties of silicon nitride-based composites. The results show the model with a SiC diameter of 2.4 nm has the largest stress and Young’s modulus, and the stress and Young’s modulus tend to be smaller when the quantity of SiC clusters increases. With an increasing temperature, the Young’s modulus of the structure with the largest clusters decreases before 1300 K and then jumps to 97 GPa at 1500 K, both the loss and jump in Young modulus is related to the interfacial effects between matrix and clusters.

The reduction of the substitutional C content in annealed Si/SiGeC superlattices studied by dark-field electron holography

Si/Si(1 − x − y)GexCy superlattices are used in the construction of new microelectronic architectures such as multichannel transistors. The introduction of carbon in SiGe allows for compensation of the strain and to avoid plastic relaxation. However, the formation of incoherent β-SiC clusters during annealing limits the processability of SiGeC. This precipitation leads to a modification of the strain in the alloy due to the reduction of the substitutional carbon content. Here, we investigated the strain in annealed Si/Si0.744Ge0.244C0.012 superlattices grown by reduced pressure chemical vapour deposition using dark-field electron holography. The variation of the substitutional C content was calculated by correlating the results with finite-element simulations. The obtained values were then compared with Fourier-transformed infrared spectrometry measurements. It was shown that after annealing for 2 min at 1050 °C carbon no longer has any influence on strain in the superlattice, which behaves like pure SiGe. However, a significant proportion of substitutional C atoms remain in a third-nearest neighbour (3nn) configuration. It was deduced that the influence of 3nn C on strain is negligible and that only isolated atoms have a significant contribution. It was also proposed that the 3nn configuration is an intermediary step during the formation of SiC clusters.

Microstructure evolution and elemental diffusion of SiCp/Al–Cu–Mg composites prepared from elemental powder during hot pressing

15vol%SiCp/Al–Cu–Mg composites were fabricated by hot pressing method using pure elemental powders. Microstructure evolution and elemental diffusion of Cu and Mg were studied. The microstructure of as-hot pressed composites and the elemental distribution of the composites before and after solution treatment were also investigated. The results showed that there were two types of eutectic liquid phases with different compositions after the compact was heated to 580 °C. After the compact was held at 580 °C for 60 min, the eutectic liquid was absorbed into the Al matrix and some equilibrium liquid phases formed in the boundaries of the initial Al particles. Meanwhile, Cu was homogeneously distributed in the Al particles while Mg tended to be distributed near the boundaries of the initial Al particles and in the SiC clusters. The presence of Al2Cu, Mg2Si, and some oxides of Mg was identified in the as-hot pressed composite. After solution treatment, Al2Cu dissolved into the Al matrix, however, some Mg-rich compounds (silicide and oxide of Mg) did not dissolve into the matrix completely.

Quantum Monte Carlo vs. Density Functional Methods for the Prediction of Relative Energies of Small Si-C Clusters

In the present paper, we assess the accuracy of popular and widely used approaches based on density functional theory by relating them to the most accurate at present quantum Monte Carlo calculations. As the test case, we consider the relative stability of small SinCm isomers. We find out that none of the studied DFT approaches employing local, semilocal, or even hybrid functionals are able to predict correctly the relative stability of the isomers.

Investigation of Defects in Solar Cells and Wafers by Means of Magnetic Measurements

Elaborated characterization tools play an important role for the further improvement of solar material and the development of solar cells. Besides the huge variety of highly advanced methods mainly based on optical and electrical measurements, the direct measurement of surface currents by the detection of their induced magnetic fields has gained less attention. The novel method current-analysis-by-inductive-coils (CAIC) based on an inductive coil detector and is reviewed and compared with already established methods, which are light-beam-induced-current (LBIC) and dark-lock-in-thermography (LIT). The detector reveals complementary information at high resolution. The LIT measurements depicted shunting defects in forward and reverse current. Because of the high spatial resolution of the CAIC measurement technique it was determinable that in some cases the positions of current sinks in the CAIC maps are not corresponding with the microstructure. The analyses of the superpositions reveals macroscopic precipitates like SiC and Si3N4 filaments and clusters as an origin of some of the shunts.

Processing, microstructure and mechanical properties of magnesium matrix nanocomposites fabricated by semisolid stirring assisted ultrasonic vibration

Particulate reinforced magnesium matrix nanocomposites were fabricated by semisolid stirring assisted ultrasonic vibration. Compared with the as-cast AZ91 alloy, the grain size of matrix alloy in the SiCp/AZ91 nanocomposite stirring for 5 min was significantly decreased due to the addition of SiC nanoparticles. SiC nanoparticles within the grains exhibited homogeneous distribution although some SiC clusters still existed along the grain boundaries in the SiCp/AZ91 nanocomposite stirring for 5 min. With increasing the stirring time, agglomerates of SiC nanoparticles located along the grain boundaries increased. The ultimate tensile strength, yield strength and elongation to fracture of the SiCp/AZ91 nanocomposite stirring for 5 min were simultaneously improved compared with the as-cast AZ91 alloy. However, the ultimate tensile strength and elongation to fracture of the SiCp/AZ91 nanocomposite decreased with increasing the stirring time.

Growth of SiC nanowhiskers from wooden precursors, separation, and characterization

We have achieved a catalyst free, low cost, easy synthesis of straight and uniform, high-aspect ratio β-SiC nanowhiskers (of 25–100 nm diameters), and a suitable separation technique to produce large quantities of individual whiskers. We conclude that these nanowhiskers nucleate on initially formed SiC clusters by a sequential gas–gas-surface reaction. The SiC clusters are formed initially as the primary product of the contact reaction between carbon obtained from wood powder (saw dust) and SiO 2 from tetraethyl orthosilicate (TEOS).

Designing novel Sn-Bi, Si-C and Ge-C nanostructures, using simple theoretical chemical similarities.

A framework of simple, transparent and powerful concepts is presented which is based on isoelectronic (or isovalent) principles, analogies, regularities and similarities. These analogies could be considered as conceptual extensions of the periodical table of the elements, assuming that two atoms or molecules having the same number of valence electrons would be expected to have similar or homologous properties. In addition, such similar moieties should be able, in principle, to replace each other in more complex structures and nanocomposites. This is only partly true and only occurs under certain conditions which are investigated and reviewed here. When successful, these concepts are very powerful and transparent, leading to a large variety of nanomaterials based on Si and other group 14 elements, similar to well known and well studied analogous materials based on boron and carbon. Such nanomaterias designed in silico include, among many others, Si-C, Sn-Bi, Si-C and Ge-C clusters, rings, nanowheels, nanorodes, nanocages and multidecker sandwiches, as well as silicon planar rings and fullerenes similar to the analogous sp2 bonding carbon structures. It is shown that this pedagogically simple and transparent framework can lead to an endless variety of novel and functional nanomaterials with important potential applications in nanotechnology, nanomedicine and nanobiology. Some of the so called predicted structures have been already synthesized, not necessarily with the same rational and motivation. Finally, it is anticipated that such powerful and transparent rules and analogies, in addition to their predictive power, could also lead to far-reaching interpretations and a deeper understanding of already known results and information.

Simulation of doping and primary radiation damage to the SiC(111) surface under bombardment by Si N atomic and cluster ions (N = 1, 5, and 60) using classical molecular dynamics

The features of the cascade of atomic collisions, the spatial distribution of dopes, and primary radiation damage in a near-surface region of cubic silicon carbide under bombardment by SiN ions and clusters (N = 1, 5, and 60) in the case of the same energy per one atom of the particle-projectile (200 and 1000 eV/atom) are studied in this paper. The study is carried out using classical molecular dynamics. As a result, several features of the low-energy implantation of polyatomic clusters in SiC(111) are revealed, namely, a relatively weak effect of the size of the implanted cluster on the distribution of ranges of incorporated atoms, a low degree of nonlinear effects at the cascade and postcascade stages, and formation of amorphous regions in the target during cluster implantation.

Ag diffusion in cubic silicon carbide

The diffusion of Ag impurities in bulk 3C–SiC is studied using ab initio methods based on density functional theory. This work is motivated by the desire to reduce transport of radioactive Ag isotopes through the SiC boundary layer in the Tristructural-Isotropic (TRISO) fuel pellet, which is a significant concern for the Very High Temperature Reactor (VHTR) nuclear reactor concept. The structure and stability of charged Ag and Ag-vacancy clusters in SiC are calculated. Relevant intrinsic SiC defect energies are also determined. The most stable state for the Ag impurity in SiC is found to be a Ag atom substituting on the Si sub-lattice and bound to a C vacancy. Bulk diffusion coefficients are estimated for different impurity states and values are all found to have very high activation energy. The impurity state with the lowest activation energy for diffusion is found to be the Ag interstitial, with an activation energy of approximately 7.9 eV. The high activation energies for Ag diffusion in bulk 3C–SiC cause Ag transport to be very slow in the bulk and suggests that observed Ag transport in this material is due to an alternative mechanism (e.g., grain boundary diffusion).

A low temperature surface preparation method for STM nano-lithography on Si(1 0 0)

Registration markers are crucial in connecting scanning tunneling microscope (STM) lithographed nano- and atomic-scale devices to the outside world. In this paper we revisit an ultra high vacuum annealing method with a low thermal budget that is fully compatible with etched registration markers and results in clean 2 × 1 reconstructed Si(1 0 0) surfaces required for STM lithography. Surface contamination is prevented by chemically stripping and reforming a protective silicon oxide layer before transferring the sample to the vacuum system. This allows for annealing temperatures of only 900 °C, where normally carbon contaminants result in the formation of SiC clusters on the surface at annealing temperatures below 950 °C. Reactive ion etched marker structures with an etch depth of 60 nm and a typical lateral dimension of only 150 nm survive a 900 °C flash anneal.