A quantum-mechanical model of dilatation dipoles in topochemical synthesis of silicon carbide from silicon

Physics of the Solid State - The interaction between a silicon vacancy and a carbon atom formed in silicon during the topochemical synthesis of silicon carbide from silicon has been calculated...     

Structural heteroepitaxy during topochemical transformation of silicon to silicon carbide

Silicon carbide samples synthesized from silicon by topochemical substitution of atoms are studied by the ion channeling method. The results of the analysis unambiguously demonstrate the occurrence of structural heteroepitaxy. The lattice of synthesized silicon carbide of hexagonal polytype 6H is epitaxially matched in the 〈0001〉 direction with the lattice grating grid array network of an initial substrate silicon in the 〈111〉 direction. The main features of structural self-coupling matching in this epitaxial heterocomposite are revealed. Despite the very large silicon carbide and silicon lattice parameter mismatch, the misfit dislocation density at the interface is low, which is a feature of the topochemical substitution method leading to comparable structures.     

Epitaxial growth of cadmium telluride films on silicon with a buffer silicon carbide layer

An epitaxial 1–3-μm-thick cadmium telluride film has been grown on silicon with a buffer silicon carbide layer using the method of open thermal evaporation and condensation in vacuum for the first time. The optimum substrate temperature was 500°C at an evaporator temperature of 580°C, and the growth time was 4 s. In order to provide more qualitative growth of cadmium telluride, a high-quality ~100-nm-thick buffer silicon carbide layer was previously synthesized on the silicon surface using the method of topochemical substitution of atoms. The ellipsometric, Raman, X-ray diffraction, and electron-diffraction analyses showed a high structural perfection of the CdTe layer in the absence of a polycrystalline phase.

     

First-order phase transition through an intermediate state

The theory of first-order phase transitions in systems where the direct formation of nuclei of a new phase is inhibited for any reason, for example, because of the extremely high elastic energy, has been constructed using the example of the silicon-silicon carbide phase transition due to the chemical reaction with carbon monoxide. It has been shown that, in this case, the phase transition occurs through an intermediate state, which significantly promotes the formation of new-phase nuclei. For the silicon-silicon carbide phase transition, such an intermediate state is the “pre-carbide” state of silicon saturated with dilatation dipoles, i.e., pairs formed by a carbon atom and a silicon vacancy that are strongly attracted to each other. The model dependence of the potential energy of systems with an intermediate phase on the reaction coordinates has been investigated. The kinetics of transformation of the intermediate state into a new phase has been described.     

IR spectra of carbon-vacancy clusters in the topochemical transformation of silicon into silicon carbide

Using infrared (IR) spectroscopy and spectral ellipsometry, we experimentally confirmed the previously predicted mechanochemical effect of the stoichiometric composition disorder leading to the formation of carbon-vacancy structures in silicon carbide (SiC) films grown on silicon substrates by the atom substitution method. It was found that a band at 960 cm^–1 in the IR spectra of SiC films on silicon, corresponding to “carbon-vacancy clusters” is always present in SiC films grown under pure carbon monoxide (CO) or in a mixture of CO with silane (SiH₄) on Si substrates of different orientation and doping level and type. There is no absorption band in the region of 960 cm–1 in the IR spectra of SiC films synthesized at the optimum ratio of the CO and trichlorosilane (SiHCl₃) gas pressures. The previously predicted mechanism of the chemical reaction of substitution of Si atoms for carbon by the interaction of gases CO and SiHCl₃ on the surface of the silicon substrate, which leads to the formation of epitaxial layers of single-crystal SiC, is experimentally confirmed.     

Evolution of the symmetry of intermediate phases and their phonon spectra during the topochemical conversion of silicon into silicon carbide

A symmetry analysis of the crystal structure and the phonon spectrum during continuous topochemical conversion of silicon into silicon carbide has been carried out. The transformation of the symmetry of phonons at high-symmetry points of the Brillouin zone upon the transition from the initial cubic structure of silicon (diamond) through an intermediate cubic structure of silicon carbide to the trigonal structure of SiC has been determined. The selection rules for the infrared and Raman spectra of all the three phases under investigation have been established.     

X‐ray absorption near‐edge structure anomalous behaviour in structures with buried layers containing silicon nanocrystals

Substructure and phase composition of silicon suboxide films containing silicon nanocrystals and implanted with carbon have been investigated by means of the X‐ray absorption near‐edge structure technique with the use of synchrotron radiation. It is shown that formation of silicon nanocrystals in the films' depth (more than 60 nm) and their following transformation into silicon carbide nanocrystals leads to abnormal behaviour of the X‐ray absorption spectra in the elementary silicon absorption‐edge energy region (100–104 eV) or in the silicon oxide absorption‐edge energy region (104–110 eV). This abnormal behaviour is connected to X‐ray elastic backscattering on silicon or silicon carbide nanocrystals located in the silicon oxide films depth.     

New shaped ceramics based on silicon carbide

New techniques have been developed for producing inexpensive shaped SiC ceramics with certain structure and porosity for a wide variety of applications. These techniques are based on the interaction of silicon melt with carbon from a previously pressed blank of definite composition (carbon, silicon carbide, organic bond) and porosity.     

Determination of size and concentration of critical clusters of silicon carbide in the vapour phase in homogeneous nucleation process

The concentrations of clusters of various size in the atmosphere during silicon carbide crystal growth have been calculated on the basis of fundamental ideas of homogeneous nucleation theory, taking into account the specific parameters of silicon carbide. It has been shown that the cluster concentration are sufficiently high to conclude that this is the dominant influence during the initial stages of crystal growth. In this way the assumption of the polymer theory of polytypism, namely that the polytype properties of silicon carbide can be determined from the composition of the gas phase, containing sufficiently large clusters with various polytype structures, has been confirmed.     

Formation of silicon carbide nanocrystals by high-temperature carbonization of porous silicon

Based on X-ray diffraction analysis, Auger spectroscopy, and Raman scattering, it is shown that carbonization of porous silicon at temperatures of 1200–1300°C results in formation of silicon carbide nanocrystals 5–7 nm in size. The growth of 3C-SiC nanocrystals of fixed size d proceeds as follows. Silicon nanocrystals with d = 3–7 nm pass into the liquid phase, thereby effectively participating in the growth of silicon carbide. After the size of a crystallite has achieved a critical value determined by the equality of its melting point and environmental temperature, the crystallite solidifies and virtually ceases to grow. As a result, a nanocrystalline Si-SiC-amorphous SiC heterostructure is obtained.     

连续碳纤维增强碳化硅的辐照效应

连续碳纤维增强碳化硅材料除了具有碳化硅材料固有的低中子活化性能,低衰变热性能和低氚渗透性能等优点以外,还具有密度低、线性膨胀系数小、高比强度、高比模量、耐高温、抗氧化、抗蠕变、抗热震、耐化学腐蚀、耐盐雾、优良的电磁波吸收特性等一系列优异性能,是各类核工程重要的潜在候选材料。在核聚变工程应用领域,连续碳纤维增强碳化硅材料作为第一壁材料不可避免地会受到各种辐射粒子的影响。研究清楚这些辐射粒子对它的辐照效应对其在核工程领域的安全使用至关重要。采用蒙特卡罗方法与分子动力学方法进行模拟计算,研究了氕、氘、氚和氦四种粒子对连续碳纤维增强碳化硅的辐照效应。SRIM和LAMMPS计算结果表明:当入射原子能量为100 eV,连续碳纤维增强碳化硅中碳的浓度在80%~85%时,氕、氘、氚和氦原子的溅射率存在最小值;入射粒子的种类对溅射率的影响显著,氦原子的溅射率大于氘原子和氚原子,而氘原子和氚原子的溅射率相差不大但均显著大于氕原子;溅射率随入射能量的增加先迅速增加后逐渐减小,氕、氘、氚和氦原子入射能量分别在200,400,600和800 eV时存在溅射率最大值;当氦原子入射能量为100 eV时,溅射率随入射角度的增加而逐渐减少。这些结果对连续碳纤维增强碳化硅材料在核工程上的应用具有一定的参考意义。Continuous carbon fiber reinforced silicon carbide material has the low neutron activation, low decay heat performance and tritium permeability, which are inherent performance of silicon carbide materials. It also has other advantages such as low density, small linear expansion coefficient, specific strength and specific modulus, high temperature resistance, oxidation resistance, creep resistance, thermal shock, resistance to chemical corrosion, salt fog resistance, excellent electromagnetic wave absorption properties, etc. It is an important potential candidate material in various field of nuclear engineering. In the field of nuclear fusion engineering applications, continuous carbon fiber reinforced silicon carbide as the first wall material will inevitably be bombarded by a variety of radiation particles. The radiation effect is critical to its safe use in nuclear engineering. The Monte Carlo method and the molecular dynamics method were used to study the radiation effect of protium, deuterium, tritium and helium on continuous carbon fiber reinforced silicon carbide. The SRIM and LAMMPS simulation results show that when the incident energy is 100 eV and the concentration of carbon in the continuous carbon fiber reinforced silicon carbide is about 80% ~ 85%, the sputtering yield of protium, deuterium, tritium and helium atoms have the minimum values. The kind of incident particle has a significant effect on the sputtering yield. The sputtering yield of helium atoms is larger than that of tritium atoms and deuterium atoms. There is not much difference between the sputtering yield of deuterium atoms and tritium atoms, and both the sputtering yield of deuterium atoms and tritium atoms are larger than that of protium atoms. The sputtering yield initially increases rapidly with the increase of the incident energy and then decreases gradually. The incident energy of the protium, deuterium, tritium and helium atoms has the maximum value of the sputtering yield at 200, 400, 600 and 800 eV, respectively. When the incident energy of helium atoms is 100 eV, the sputtering yield decreases while the increase of the incident angle. These results can provide a certain reference for the application of continuous carbon fiber reinforced silicon carbide materials in nuclear engineering.     

New method for growing silicon carbide on silicon by solid-phase epitaxy: Model and experiment

A new method of solid-state epitaxy of silicon carbide (SiC) on silicon (Si) is proposed theoretically and realized experimentally. Films of various polytypes of SiC on Si(111) grow through a chemical reaction (at T = 1100–1400°C) between single-crystal silicon and gaseous carbon oxide CO (at p = 10–300 Pa). Some silicon atoms transform into gaseous silicon oxide SiO and escape from the system, which brings about the formation of vacancies and pores in the silicon near the interface between the silicon and the silicon carbide. These pores provide significant relaxation of the elastic stresses caused by the lattice misfit between Si and SiC. X-ray diffraction, electron diffraction, and electron microscopy studies and luminescence analysis showed that the silicon carbide layers are epitaxial, homogeneous over the thickness, and can contain various polytypes and a mixture of them, depending on the growth conditions. The typical pore size is 1 to 5 μm at film thicknesses of ～20 to 100 nm. Thermodynamic nucleation theory is generalized to the case where a chemical reaction occurs. Kinetic and thermodynamic theories of this growth mechanism are constructed, and the time dependences of the number of new-phase nuclei, the concentrations of chemical components, and the film thickness are calculated. A model is proposed for relaxation of elastic stresses in a film favored by vacancies and pores in the substrate.     

Influence of silicon cluster on epitaxial growth of silicon carbide

Precursor concentration dependences of growth rate, doping concentration and surface morphology have been investigated in the epitaxial growth of 4H-SiC(0001) epilayers with horizontal hot-wall CVD system using various precursor concentrations under constant C/Si ratio. Form the experimental data it is found that silicon cluster which is formed through gas phase nucleation plays an important role in controlling the doping concentration and epitaxial growth rate of the silicon carbide. It was observed that t...     

表面改性碳化硅基底反射镜加工技术现状

针对表面改性SiC基底反射镜在空间光学系统中的应用,总结了该类反射镜在国内外的研究现状。概括了碳化硅基底反射镜的发展趋势。介绍了常用的碳化硅材料,分析了它们的性质。给出了几种常用的碳化硅镜坯制备工艺,包括成型、改性和不同的抛光技术。通过对国内现有加工工艺和改性技术的分析,总结出了适应我国的表面改性碳化硅反射镜加工的发展方向。     

Synthesis of epitaxial silicon carbide films through the substitution of atoms in the silicon crystal lattice: A review

A review of recent advances in the field of epitaxial growth of SiC films on Si by means of a new method of epitaxial substitution of film atoms for substrate atoms has been presented. The basic statements of the theory of the new method used for synthesizing SiC on Si have been considered and extensive experimental data have been reported. The elastic energy relaxation mechanism implemented during the growth of epitaxial SiC films on Si by means of the new method of substitution of atoms has been described. This method consists in substituting a part of carbon atoms for silicon matrix atoms with the formation of silicon carbide molecules. It has been found experimentally that the substitution for matrix atoms occurs gradually without destroying the crystalline structure of the matrix. The orientation of the film is determined by the “old” crystalline structure of the initial silicon matrix rather than by the silicon substrate surface only, as is the case where conventional methods are used for growing the films. The new growth method has been compared with the classical mechanisms of thin film growth. The structure and composition of the grown SiC layers have been described in detail. A new mechanism of first-order phase transformations in solids with a chemical reaction through an intermediate state promoting the formation of a new-phase nuclei has been discussed. The mechanism providing the occurrence of a wide class of heterogeneous chemical reactions between the gas phase and a solid has been elucidated using the example of the chemical interaction of the CO gas with the single-crystal Si matrix. It has been shown that this mechanism makes it possible to grow a new type of templates, i.e., substrates with buffer transition layers for growing wide-band-gap semiconductor films on silicon. A number of heteroepitaxial films of wide-band-gap semiconductors, such as SiC, AlN, GaN, and AlGaN on silicon, whose quality is sufficient for the fabrication of a wide class of micro- and optoelectronic devices, have been grown on the SiC/Si substrate grown by solid-phase epitaxy.     

Ferromagnetism in Cu-doped silicon carbide

Cu doped silicon carbide is shown to be ferromagnetic based on experiment results and first-principles calculations. The magnetization value of the Cu doped silicon carbide decreased as the Cu concentration increased. When the films were annealed at 800 °C, the ferromagnetic signal was increased. Reduction of the C vacancy concentration will introduce a large total moment in the system. Theoretically, compared with the case of one Cu atom replacing one Si atom, increasing the Cu doping, changing the Cu atom location or including carbon vacancies in the calculations for the system all make the ferromagnetic moment decrease. One Cu atom replacing one Si atom with the addition of one C vacancy makes the energy band gap of the system disappear. Our investigations suggest that the ferromagnetism arises from the hybridization between Cu 3d orbital and C 2p orbital. Ferromagnetic moment is influenced by a symmetry-lowering distortion of the surrounding lattice by the Cu dopant.     

Doping of silicon with carbon during laser ablation process

The effect of laser ablation on properties of remaining material in silicon was investigated. It was found that laser cutting of wafers in the air induced the doping of silicon with carbon. The effect was more distinct when using higher laser power or UV radiation. Carbon ions created bonds with silicon atoms in the depth of the material. Formation of the silicon carbide type bonds was confirmed by SIMS, XPS and AES measurements. Modeling of the carbon diffusion to clarify its depth profile in silicon was performed. Photochemical reactions of such type changed the structure of material and could be the reason of the reduced machining quality. The controlled atmosphere was applied to prevent carbonization of silicon during laser cutting. PACS 52.38.Mf; 82.50.Hp; 82.80.Ms; 82.80.Pv     

Epitaxial growth of cadmium sulfide films on silicon

A 300-nm-thick cadmium sulfide epitaxial layer on silicon was grown for the first time. The grown was performed by the method of evaporation and condensation in a quasi-closed volume at a substrate temperature of 650°C and a growth time of 4 s. In order to avoid a chemical reaction between silicon and cadmium sulfide (at this temperature, the rate constant of the reaction is ~10³) and to prevent etching of silicon by sulfur, a high-quality silicon carbide buffer layer ~100 nm thick was preliminarily synthesized by the substitution of atoms on the silicon surface. The ellipsometric, Raman, electron diffraction, and trace element analyses showed a high structural perfection of the CdS layer and the absence of a polycrystalline phase.     

光声谱研究多孔碳化硅的能带特性

报道了用UV光照射和不用UV光照射条件下形成的p型α PSC以及原始SiC的光声光谱（PAS）.从光声Rosenwaig Gersho理论出发，计算出多孔SiC的吸收系数与能量的关系，得到多孔SiC的能隙低于原始SiC的能隙，并深入分析了能隙的变化原因，同时，对吸收边附近的吸收情况进行了讨论. 关键词： 光声光谱 多孔碳化硅 能隙     

Development of a model of silicon carbide thermodestruction for preparation of graphite layers

A three-stage scheme of the silicon carbide thermodestruction resulting in surface graphitization, which was proposed earlier (based on structural studies), is discussed. A theoretical analysis shows, however, that this process occurs in two stages, namely, thermodesorption of silicon atoms from the two outer Si-C bilayers followed by condensation of carbon atoms on the Si(0001) face of silicon carbide, thus giving rise to the formation of a two-dimensional graphite structure (graphene).