Atomic-scale study of the role of carbon on boron clustering

Boron (BF₂, 20 keV, 3.14/cm²) and carbon (13 keV, 10¹⁵/cm²) implanted silicon annealed at 800 °C during 30 min or at 1000 °C during 10 s has been investigated using a laser-assisted wide-angle tomographic atom probe (LaWaTAP) instrument. Boron-silicon clusters containing ~ 1.3 at.% of boron atoms have been observed in boron implanted silicon with a concentration exceeding the solubility limit. Often identified as BICs, they are interpreted as a metastable phase. Furthermore, addition of carbon clearly reduced the clustering of boron. This was interpreted as a diminution of boron diffusion or as an increase of the solubility limit of boron. Carbon-silicon clusters containing ~ 1.5 at.% of carbon atoms were observed, maybe the precursors of the SiC phase.     

Oxidation behaviour and protection of carbon/carbon composites

The oxidation behaviour of carbon/carbon composite materials and graphite (in cube form), in flowing air, has been studied in the temperature range 500 to 1100 °C. Gasification for unprotected samples occurred at temperatures around 500 °C. SiC coatings offered only limited protection below their intrinsic protection range due to the diffusion of oxygen along microcracks. Diffusional control was more significant for thicker coatings. However, the use of boron oxide applied on an underlayer of SiC, gave good protection for extended periods at temperatures up to 1000 °C, due to microcrack sealing. The use of borate coatings, both with and without an SiC underlayer, was limited by the volatility of the borate.     

Substrate effects on the structure and optical properties of GaN epitaxial films

The phase composition and luminescent properties of GaN films grown by molecular beam epitaxy on (0001) sapphire and 6H-SiC substrates were studied. The films grown on SiC were found to consist only of the hexagonal phase and contain a lower concentration of impurities. Grains of cubic GaN, as well as donor and acceptor impurities, were found in the GaN film grown on sapphire. The formation of impurity centers is caused by the diffusion of oxygen and aluminum from the sapphire substrate during crystal growth.     

Effects of the SiC/Al interface reaction on fracture behavior of a composite conductor using SiC fiber reinforced aluminum for next generation power equipment

Electrical power demands are increasing every year, meaning that lightweight electric cable is needed which has high transmission capacity, high thermal resistance and low sag. Tokyo Electric Power Co., Chubu Electric Power Co. and Hitachi Cable Ltd. have been breaking new ground in the field of electric cable through the development of a SiC fiber reinforced aluminum conductor. In this work, the SiC/Al interface reaction during the manufacturing process and the electricity transmission temperature were studied by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX) and field emission-Auger electron spectroscopy (FE-AES) for long-term reliability assessment. No reaction products were detected at the SiC/Al interface of elemental wire consisting of 7 SiC/Al preformed wires, indicating that the wire manufacturing process was reliable. An Al₄C₃ product was detected locally at the SiC/Al interface of the wire which had been thermally treated in molten Al under unfavorable conditions. The activation energy, Q, of Al₄C₃ growth at the SiC/Al interface was about 190 kJ/mol. In the temperature range of electricity transmission, Al atoms diffused into SiC fiber during heat treatment, and the amount of the diffused Al increased with increasing treatment temperature and holding time. The activation energy of Al diffusion through the SiC/Al interface to SiC fiber was about 78 kJ/mol. Strength deterioration was not induced by Al diffusion into SiC fiber, but strength strongly depended on the formation of Al₂SiO₅ compound at the SiC/Al interface above 400°C transmission temperatures. Kinetics calculations indicated that the rate of strength deterioration of the composite cable, held at 300°C for 36 years, was about 5%, so that practical use of SiC/Al composite cable should not be far in the future.     

Boron doping of silicon rich carbides: Electrical properties

Boron doped multilayers based on silicon carbide/silicon rich carbide, aimed at the formation of silicon nanodots for photovoltaic applications, are studied. X-ray diffraction confirms the formation of crystallized Si and 3C-SiC nanodomains. Fourier Transform Infrared spectroscopy indicates the occurrence of remarkable interdiffusion between adjacent layers. However, the investigated material retains memory of the initial dopant distribution. Electrical measurements suggest the presence of an unintentional dopant impurity in the intrinsic SiC matrix. The overall volume concentration of nanodots is determined by optical simulation and is shown not to contribute to lateral conduction. Remarkable higher room temperature dark conductivity is obtained in the multilayer that includes a boron doped well, rather than boron doped barrier, indicating efficient doping in the former case. Room temperature lateral dark conductivity up to 10^?3 S/cm is measured on the multilayer with boron doped barrier and well. The result compares favorably with silicon dioxide and makes SiC encouraging for application in photovoltaic devices.     

Nano-precipitation in hot-pressed silicon carbide

Heat treatments at 1300°C, 1400°C, 1500°C, and 1600°C in Ar were found to produce nanoscale precipitates in hot-pressed silicon carbide containing aluminum, boron, and carbon sintering additives (ABC-SiC). The precipitates were studied by transmission electron microscopy (TEM) and nano-probe energy-dispersive X-ray spectroscopy (nEDS). The precipitates were plate-like in shape, with a thickness, length and separation of only a few nanometers, and their size coarsened with increasing annealing temperature, accompanied by reduced number density. The distribution of the precipitates was uniform inside the SiC grains, but depleted zones were observed in the vicinity of the SiC grain boundaries. A coherent orientation relationship between the precipitates and the SiC matrix was found. Combined high-resolution electron microscopy, computer simulation, and nEDS identified an Al₄C₃-based structure and composition for the nano-precipitates. Most Al ions in SiC lattice exsolved as precipitates during the annealing at 1400 to 1500°C. Formation mechanism and possible influences of the nanoscale precipitates on mechanical properties are discussed.     

Interface characterization by TEM,AES and SIMS in tough SiC (ex-PCS) fibre-SiC (CVI) matrix composites with a BN interphase

The fibre-matrix interfacial zone formed during the isothermal/isobaric chemical vapour infiltration processing of SiC fibres (ex-polycarbosilane)/boron nitride/SiC matrix composites has been analysed by TEM/electron energy loss spectroscopy, Auger electron spectroscopy, and secondary ion mass spectroscopy. In the composites, the boron nitride interphase (deposited from BF₃-NH₃) is made of turbostratic boron nitride, almost stoichiometric but containing some oxygen (less than 5 at %). The boron nitride layer stacks are randomly orientated except in the very vicinity of the fibre surface where they lie almost parallel to the substrate. The long chemical vapour infiltration treatment at 1000 °C used to infiltrate the SiC matrix acts as an annealing treatment for the metastable ex-polycarbosilane fibres which gives rise to the growth of an SiO₂/carbon amorphous double layer at the boron nitride/fibre interface. Deflection of microcracks arising from the failure of the brittle SiC-matrix occurs at the boron nitride/SiO₂ interface considered to be the weaker link in the matrix/boron nitride interphase/SiO₂/carbon/fibre sequence. It is suggested that the combination of the boron nitride layered interphase and SiO₂/carbon fibre decomposition products might play an important role in determining the propagation path of microcracks in the fibre/matrix interfacial zones and could be responsible, at least to some extent, for the non-brittle behaviour of such composites.     

Thermal expansion of high-modulus fibers

A resistance-heating technique was used to measure the axial thermal expansion of high-modulus boron and silicon carbide (SiC) fibers from room temperature to 700 and 1500°C, respectively. Both types of fibers investigated in this study were manufactured by a chemical vapor deposition (CVD) process. The boron fibers examined here are composed of boron and a tungsten boride core arising from reaction of deposited boron with a tungsten wire substrate. The composition of the SiC fibers consists of a SiC sheath with a carbon-rich outer coating surrounding an unreacted pyrolytic graphite coated carbon core. The thermal expansion of boron fibers was found to increase parabolically with temperature up to 700°C. Above this temperature the fiber contracted due to void migration and subsequent residual stress relaxation. For SiC fibers, a relatively small initial expansion from room temperature to 450°C was observed. Above 450°C the expansion was found to increase linearly with temperature up to 1300°C, where a hysteresis effect was observed involving a 50% reduction in expansion. Possible explanations for this hysteresis effect were considered and different theories presented. Volume percentage of carbon core was varied and found to have negligible effect on expansion. The conclusion was reached that expansion of these SiC fibers is controlled by the SiC sheath.Paper presented at the Tenth International Thermal Expansion Symposium, June 6–7, 1989, Boulder, Colorado, U.S.A.     

FABRICATION OF SiCp-AI COMPOSITE MATERIALS

Liquid phase fabrication methods for aluminum matrix composites reinforced with SiC whiskers, or SiC particles have been investigated and the mechanical properties of fabricated composites have been evaluated. Three kinds of liquid phase fabrication methods; hot extrusion, hot pressing and pressure infiltration, were studied. Commercial SiC whiskers and SiC powders of alpha type and beta type were used as the reinforcements for an aluminum matrix. Among the fabrication methods investigated, the best results were achieved by the pressure infiltration. The mechanical properties and the wear resistance of the fabricated composites were measured. The SiC whisker reinforced aluminum matrix composites have high strength, so that they can be used as high specific strength materials. The SiC particulate reinforced aluminum matrix composites are not strong as the SiC whisker reinforced composites. However, the SiC particulate reinforced aluminum matrix composites have a good potential for use as wear resistant material. The hardening effect of beta type particles on the aluminum matrix was larger than that of alpha type particles.     

SiC matrix composites reinforced with internally synthesized TiB2

A new process of preparing particulate-reinforced ceramic composites by internal synthesis has been developed. SiC powder mixed with TiN and amorphous boron was hot-pressed above 2000° C in an argon atmosphere. The boron molar content in the mixture was designed to be more than twice that of TiN. In the process of hot-pressing, the following reaction took place between 1100 and 1700° C TiN+2B TiB₂+1/2N₂ The synthesis of TiB₂ was followed by the densification of SiC matrix with the aid of the excess boron. The new process provides SiC matrix composites in which fine TiB₂ particulates are dispersed. Compared with hot-pressed monolithic SiC, the composite containing 20 vol % TiB₂ exhibits a 80% increase in fracture toughness and about the same flexural strength of 490 MPa at 20° C in air and 750 MPa at 1400° C in a vacuum.     

Modeling of the transient enhanced diffusion of boron implanted into preamorphized silicon: the case of BF2 implantation

We have simulated transient enhanced diffusion (TED) in the presence of end-of-range (EOR) defects produced by Ge amorphization followed by BF₂⁺ implantation. Ostwald ripening of EOR defects has been taken into account. A comparison of annealed profiles with equivalent B⁺ implantation shows that the existing models are not sufficient to simulate the BF₂⁺ experimental profiles where the boron diffusion depth is very low. We have proposed that the presence of fluorine can act as sinks for interstial boron and, hence, reduces the boron diffusion depth in order to obtain a good approximation of experimental profiles.     

β → α Phase transformation in sintered SiC involving feather formation

The beta-alpha phase transformation has been studied in hot pressed boron doped SiC (HP SiC). The microstructure has been characterized using optical microscopy, scanning and transmission electron microscopy. The influence of the sintering times and temperatures on the crystalline structure of the materials has been checked. The phase transformation which occurs during the sintering involves the formation of feathers. These feathers, constituted by two adjacent alphas grains, can be described as mechanical twins. Three different twinning laws have been observed. To check the influence of the external pressure on the feathers formation, the previous study in pressureless boron doped SiC (PS SiC) has been completed by a new investigation. The results obtained in both PS and HP boron doped SiC show that the external pressure has no influence on the microstructure of the feathers.     

Dopant diffusion and activation induced by sub-melt laser anneal within the co-implanted p+ polycrystalline silicon gate used in CMOS technologies

Due to the continuous CMOS transistor scaling requirements, highly doped shallow junctions with improved activation have been widely investigated in recent CMOS technologies. In this scope, sub-melt millisecond laser annealing has been introduced in the integration flows to enhance dopant activation, without any additional detrimental diffusion. This MSA step impacts not only the transistor junction properties, but also the polysilicon gate depletion. This paper is devoted to the study of the MSA influence on boron and germanium co-implanted polysilicon films. A sensitive boron diffusion occurring during the laser anneal step, with or without an initial spike annealing step, has been observed. The activation energy of the boron diffusivity extracted from SIMS profiles in the laser only sequence has been found equal to 4.05 eV. In addition, it was shown that either a high temperature laser anneal sequence or a spike anneal followed by a laser anneal sequence can reach the same activation levels.     

Formation of Complexes of Phosphorus and Boron Impurity Atoms in Silicon

Inorganic Materials - We have studied the effect of high phosphorus concentration on the diffusion of boron impurity atoms in silicon and, vice versa, the effect of boron on phosphorus diffusion in...     

Depth profiles of aluminum and sodium near surfaces: Nuclear resonance method

Depth profiles of aluminum and sodium implanted into silicon carbide, silicon and silicon dioxide have been measured by means of sharp resonances in the reactions ²⁷Al(p,γ)²⁸Si and ²³Na(p,γ)²⁴Mg. The absolute number of impurity atoms has been determined and compared with that indicated by charge integration during implantation. Adjacent areas of some specimens have been measured by the Cameca ion-beam mass spectrometer and the nuclear resonance method; results are compared. A depth resolution of less than 20 Å has been demonstrated for Al very near the surface of SiC. Information concerning the migration of sodium in SiO₂ under ion bombardment is presented. Depth profiles are extracted from gamma-ray yield curves taking into account the beam energy distribution, the resonance shape, the average proton energy loss in the sample and the energy loss straggling.     

Preparation and dielectric properties of B-doped SiC powders by combustion synthesis

The SiC(B) solid solution powders were synthesized via combustion reaction of Si/C system in Ar atmosphere, using boron powder as the dopant and polytetrafluoroethylene as the chemical activator, which were investigated by X-ray diffraction (XRD), scanning electronic microscope (SEM) and Raman spectra. Results show that the prepared powders are C-enriched SiC with C antisites and sp² carbon defects in which the sp² carbon is transformed to the sp³ carbon due to boron doping. The electric permittivities of the prepared powders were determined in the frequency range of 8.2-12.4 GHz. The dielectric real part ?′ and dielectric loss tan δ of undoped powder have maximum values (?′ = 5.5-5.3, tan δ = 0.23-0.20), and decrease with increasing boron content. The mechanism of dielectric loss by doping has been discussed.     

The oxidation behaviour of two- and three-dimensional C/SiC thermostructural materials protected by chemical-vapour-deposition polylayers coatings

The oxidation behaviour of two- and three-dimensional C/SiC protected by a chemicalvapour-deposition (CVD) ceramic coating was studied. The elements used to achieve the surface protection were silicon, boron and carbon, preferably forming SiC, B or B₄C. The best results were obtained with the trilayer coatings, that is with, SiC as the internal layer, boron or boron carbide, as the intermediate layer and an external SiC layer. To get a good protection in a large temperature range, from 450 to 1500 °C, the total thickness of the trilayers must be higher than 160 m and the intermediate layer thickness must be higher than 5 m. Morphological characterization of oxidized samples has shown that, for intermediate oxidation temperatures, a glass was produced in the cracks. When the oxidation temperature was equal to or higher than 1300 °C, sealing of the cracks was rarely observed, but the oxidation resistance remained satisfactory.     

电子束熔炼多晶硅对杂质铝去除机制研究

采用电子束熔炼方式,利用铝的蒸发系数较大的特点通过蒸发去除硅中的杂质铝。将实验的实测值与理论计算的蒸发量、损失量等加以比较,得到了铝在电子束下的蒸发去除速率由其在硅中扩散过程所决定的结论,并对铝的去除量与硅的损失量之间关系进行分析。     

硼酸铝晶须氮化特性研究

针对硼酸铝晶须增强铝基复合材料的界面反应问题,经热力学预测,首次提出对硼酸铝晶须进行氮化处理这一新工艺。采用Ｘ射线光电子能谱（ＸＰＳ）研究了硼酸铝晶须的氮化特性。实验结果表明：硼酸铝晶须中已渗入氮元素;同时,硼酸铝晶须中硼元素向ＢＮ转变。硼酸铝晶须的氮化反应受扩散控制,为一热激活过程,其反应活化能为３０６．３４ｋＪ／ｍｏｌ。     

Diffusion of aluminum into silicon nitride films

Aluminum diffusion into silicon nitride films at temperatures in the range 450–530°C was studied by Auger electron spectroscopy in conjunction with depth profiling. The activation energy for the diffusion of aluminum and the diffusion coefficient were found to be 2.0±0.3 eV and (7.3±3.5) x 10^-3 cm² s^-1, respectively. The chemical effects in the KLL aluminum Auger spectra together with the compositional depth profiles suggest that the migration of aluminum is dominated by volume diffusion which involves the reaction of aluminum with oxygen.