The energy levels in quantum wells formed at the contacts between cubic and hexagonal polytypes of silicon carbide

The energies of the ground (ε₀) and the first excited state (ε₁) in a quantum well (QW) are evaluated within the framework of the triangular QW model. It is shown that the ε₀ level in QWs at the contacts between cubic (3C) and hexagonal (NH, N = 2, 4, 6, 8) polytypes of silicon carbide (SiC) can be effectively controlled only by means of doping a wide-bandgap n-NH-SiC polytype with shallow donors.     

Gaps in the spectrum of epitaxial graphene formed on silicon carbide polytypes

Silicon carbide NH-SiC polytypes with N = 2, 4, 6, and 8 are considered as substrates for the epitaxial formation of graphene. The density of states for the substrates is described using the Haldane-Anderson model. It is shown that this model always leads to the appearance of two gaps in the graphene spectrum, which are adjacent to the valence and conduction bands of the substrate. The gap widths are determined by the ratio of the energy of interatomic interaction in the free-standing graphene sheet and the energy of graphene-substrate interaction. If this ratio is very small, the gap widths may increase so as to jointly cover almost the entire bandgap of the substrate; on the contrary, if this ratio is extremely large, both gaps exhibit narrowing and become negligibly small.     

Correlation between layer thickness and periodicity of long polytypes in silicon carbide

The layer widths and repeat spacing of long-period polytypes (LPPs) have been determined using synchrotron radiation source (SRS) X-ray diffraction topography (XRDT). This method has proved to be a powerful tool in investigating the spatial extent of one-dimensional disorder (1DD), long-period polytypes (LPPs) and the boundaries of polytype layers in silicon carbide (SiC). The resulting neighbourhood coalescence models have confirmed the validity of the sandwich rule even in the limit of two arbitrarily long LPPs, as well as the unique nature of the 6H polytype. A significant empirical trend is reported here that relates the thickness of LPP layers to the periodicity of the repeat stacking sequence measured on the topographs. A good correlation between the data suggests that this behaviour is governed by a simple mathematical expression t = kNⁿ. Values for k and n have been determined that relate the polytype thickness (t in microns) to the number of hexagonal layers (N) in the polytype stacking repeat. These values can be used to prompt questions about the limits of polytypism and disorder in SiC.     

Determination of the ratios of polytypes in a silicon carbide grit using neutron diffraction

The distribution of polytypes in a sample of silicon carbide has been measured by the technique of neutron powder diffraction profile analysis. Three polytypes are shown to be present and their relative proportions have been calculated. The neutron technique was used because it was desired to examine the bulk properties of the sample and to use large (～ 1cm) samples.     

Effect of contact area on synthesis of silicon carbide through carbothermal reduction of silicon dioxide

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Fabrication and characterization of silicon quantum dots in Si-rich silicon carbide films

Amorphous Si-rich silicon carbide films were prepared by magnetron co-sputtering and subsequently annealed at 900-1100 degrees C. After annealing at 1100 degrees C, this configuration of silicon quantum dots embedded in amorphous silicon carbide formed. X-ray photoelectron spectroscopy was used to study the chemical modulation of the films. The formation and orientation of silicon quantum dots were characterized by glancing angle X-ray diffraction, which shows that the ratio of silicon and carbon significantly influences the species of quantum dots. High-resolution transmission electron microscopy investigations directly demonstrated that the formation of silicon quantum dots is heavily dependent on the annealing temperatures and the ratio of silicon and carbide. Only the temperature of about 1100 degrees C is enough for the formation of high-density and small-size silicon quantum dots due to phase separation and thermal crystallization. Deconvolution of the first order Raman spectra shows the existence of a lower frequency peak in the range 500-505 cm(-1) corresponding to silicon quantum dots with different atom ratio of silicon and carbon.     

Development of a spinodal decomposition model for the example of a heterostructure based on silicon carbide polytypes

The transition region of a 3C-SiC/4H-SiC heterostructure constituted by layers of the 3C and 4H polytypes has been studied. A previously proposed spinodal decomposition model was used to estimate the thickness ratio of 4H and 3C layers in comparison with the image furnished by transmission electron microscopy.     

Study on the growth of heteroepitaxial cubic silicon carbide layers in atmospheric-pressure H2-based plasma

The heteroepitaxial growths of cubic silicon carbide (3C-SiC) layers on Si(001) substrates are studied at a temperature of 800 degrees C in atmospheric-pressure (AP) plasma excited by a 150 MHz, very high-frequency (VHF) power using a porous carbon electrode. The effect of a very large C/Si ratio (-400) of the source molecules on the improvement of crystallinity of the resultant SiC layer is mainly investigated. For this purpose, we utilize the chemical transport of Si induced by AP H2/CH4 plasma instead of using SiH4 as the Si source. The layer crystallinity is characterized using reflection high-energy electron diffraction, transmission electron microscopy and infrared absorption spectroscopy. The results show that the SiC layer exhibits the (001) 3C-SiC growth aligned to the Si matrix epitaxially. Although the SiC layer contains a high density of defects originating presumably from anti-phase boundaries and twin boundaries, the layer crystallinity has been considerably improved in comparison with that of the layer grown with C/Si = 10. It is also demonstrated that the moderate dilution of H2 with He leads to a further improvement of the layer crystallinity.     

立方碳化硅CMP过程中机械作用分子动力学仿真

为了更好地理解立方碳化硅在化学机械抛光(CMP)过程中原子层面的材料去除机理,利用分子动力学(MD)方法建立了金刚石磨粒刻划碳化硅的原子模型,仿真研究了金刚石磨粒半径、刻划深度和刻划速度对碳化硅表面形貌、晶体结构、摩擦力和原子去除率的影响规律,并与无定型二氧化硅氧化膜的机械刻划作用的仿真结果进行了对比分析.结果发现:碳化硅在机械刻划过程中局部会出现非晶态变化;刻划深度增大会导致切削力和切削温度增大,原子去除率也随之增加;刻划速度的改变会影响温度和原子去除率,而对切削力几乎无影响;磨粒半径的增加会导致切削力和温度的增加,在压入深度相同的情况下对原子去除率影响不大;碳化硅表面生成的二氧化硅膜能大幅度降低切削力,但由于其结构的影响,机械刻划作用仅使氧化膜产生明显的致密化,而不产生磨屑.     

Structural changes in tungsten wire and their effect on the properties of hydrogenated nanocrystalline cubic silicon carbide thin films

We investigated the structural changes in tungsten wire heated to 1800 °C in SiH₄/CH₄/H₂/N₂ atmosphere and the effect of the aging tungsten wire on the properties of N-doped hydrogenated nanocrystalline cubic silicon carbide (nc-3C-SiC:H) thin films. The aged tungsten wire had two parts: hot parts of the middle of the wire and relatively cold parts connected to clamps. Tungsten carbide (W₂C) layer formed in the wire of the hot parts, while crystalline silicon and cubic silicon carbide (c-Si/3C-SiC) layer deposited on the wire of the cold parts. N-doped nc-3C-SiC:H thin films were deposited for 5 min (thickness: ~ 30 nm) after the tungsten wire was heated under the same condition as during the film deposition for given times (exposure time). No changes in the structural, electrical and optical properties of the nc-3C-SiC:H thin films were observed for the exposure time up to 450 min.     

Effect of infiltrants on the electrical resistivity of reaction-sintered silicon carbide

The effect of infiltrants on the electrical resistivity of reaction-sintered silicon carbide, at temperatures ranging from RT to 1000°C, has been studied. Electrical resistivity decreases with increase in temperature up to 1000°C in VC and MoSi₂, whereas minimum electrical resistivity is observed at ∼600°C in B₄C infiltrant.     

Effect of oxide additives on the morphology of fine-particle silicon carbide

Silicon carbide powders were prepared from pelletized mixtures of fine-particle SiO₂ and petroleum coke with CaO, Fe₂O₃, and Al₂O₃ additions at temperatures from 1770 to 1870 K and atmospheric pressure. The Si loss in the form of SiO and the SiC yield were measured. The materials with CaO additions consisted of needlelike crystallites and a small amount of submicron-sized particles. The materials containing Fe₂O₃ consisted of roughly equal amounts of needlelike crystallites and submicron-sized particles. The introduction of CaO-Fe₂O₃ (10-66% Fe₂O₃) combined additions resulted in materials consisting mainly of submicron-sized particles. In the presence of A1₂O₃, needlelike crystallites did not grow.     

SiC量子点荧光材料光谱特性影响因素的研究

通过腐蚀法制备SiC量子点荧光标记材料,以硝酸和氢氟酸为腐蚀剂,通过超声空化破碎分散及超重力场层析剪裁,得到SiC量子点水相溶液,研究了制备工艺参数对光学特性的影响,结果表明,降酸清洗次数、超声时长对量子点光致发光峰值(荧光强度)的影响最为显著,而腐蚀剂成分及配比、超重力系数在一定程度上影响了其光致发光强度及发射光波长,同时腐蚀剂成分中氢氟酸含量的增加使得光致发光峰值位置发生红移,而超声时长及超重力系数的增加使得SiC量子点光致发光峰值位置发生蓝移。并对SiC颗粒腐蚀过程相关机制也进行了探讨。     

温度对化学气相沉积碳化硅涂层的影响

以三氯甲基硅烷和氢气为气源,研究了化学气相沉积碳化硅过程中,温度(850-1350℃)对沉积速率、反应物消耗效应、涂层形貌和相结构的影响.用磁悬浮天平在线实时称量基体质量变化进行动力学研究;采用扫描电镜和X射线衍射对样品做了表征.结果表明,沉积过程存在四个控制机理:a区(<1000℃)为表面反应动力学控制;b区(1000-1050℃)主要是HCl对沉积的抑制作用;c区(1050-1300℃)是表面化学反应和传质共同控制;d(>1300℃)为传质为限速步骤.由于不同的控制机制,导致所得涂层的形貌存在差异.含碳含硅中间物质浓度的减小、HCl增多和MTS的分解共同导致反应物消耗效应.涂层由热解碳和碳化硅两相组成,温度的升高使热解碳相减少,碳硅比接近1.     

Effect of hydrogen exposures on the strength of sintered alpha silicon carbide

The room-temperature strength distributions of sintered alpha silicon carbide (SASC) were determined for polished bars. The bars were broken in the as-polished condition after exposure to hydrogen only and after exposure to hydrogen under two different static stresses. The hydrogen exposures were at 1370° C for times up to 50 h. The strength distributions displayed a dependence on both the exposure time and the magnitude of the applied stress. In stressed exposures, the strength distributions were apparently controlled by competing mechanisms. Under the lower applied stress, the material was weaker than the as-polished control group, as was the material exposed to hydrogen with no stress applied. Under the higher applied stress, the material was stronger than the control group.     

Effect of soft substrate on the indentation damage in silicon carbide deposited on graphite

Indentation-induced damage is investigated in silicon carbide (SiC) deposited on graphite substrate. The SiC films have been grown by LPCVD (Low Pressure Chemical Vapor Deposition) method using MTS (CH₃SiCl₃) as a source gas and H₂ as a diluent gas to provide highly dense deposited layer and strong interfacial bonding. The elastic-plastic mismatch is very high to induce distinctive damages in the coating and the substrate layer. The specimens with various coating thicknesses are prepared by changing CVD condition or mechanical polishing. Indentation damages with different sizes are introduced by controlling indentation load in Nanoindentation, Vickers indentation and Hertzian indentation test. Basic mechanical properties such as hardness, toughness, elastic modulus are evaluated against coating thickness. Mechanical properties are sensitive to the indentation load and coating thickness. The results indicate that coating thickness has a vital importance on the design of hard coating/soft substrate system because the soft substrate affects on the mechanical properties.     

Effect of frequency on high-temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers, Si₃N₄SiC_W, has been undertaken at elevated temperature during static and dynamic loading at increasing K and ΔK respectively. It is shown that cyclic sub-critical crack growth rates are lower than static crack growth rates. The increased crack growth rate during static far field loading is attributed to the stress relaxation of the inter-granular glass phase which allows time-dependent processes to occur ahead of the crack tip which lead to enhanced sub-critical crack growth rates. During cyclic fatigue the glass phase has insufficient time to relax and glassy ligaments are able to bridge the crack wake thereby shielding the crack tip from the full force of the applied load. Also, at particular temperatures, bridging between the surfaces of the crack wake by the inter-granular glass phase results in increased strength and fatigue retardation. The extent of ‘crack wake healing’ is shown to be time and temperature dependent. The viscosity of the glass phase is directly related to the temperature and the bonding force associated with glass phase bridging is observed to reduce with increasing temperature. The results from a previous study at room temperature are compared to those found during this investigation.