Carbothermal synthesis of nanoparticulate silicon carbide in a self-contained protective atmosphere

We report a process for the preparation of ultrafine silicon carbide powder in a self-contained protective atmosphere. The protective atmosphere is ensured by the silicon monoxide released during the carbothermal synthesis. A reactor design is proposed in which an excess silicon monoxide pressure is generated in order to prevent atmospheric oxygen from entering the reactor. The reactor makes it possible to synthesize silicon carbide in furnaces operating in air. The use of pure starting materials has allowed us to obtain silicon carbide nanofibers ? 100 nm in diameter, containing no more than 0.01 wt % impurities.     

Vapor-solid-solid Si nano-whiskers growth using pure hydrogen as the source gas

This paper describes the growth mechanism and structure of Si whiskers grown on a Si substrate in a tungsten hot filament chemical vapor deposition reactor with pure hydrogen as source gas using a two step process. In the first step, atomic hydrogen etched the silicon surface, forming silicon hydrides that react with tungsten from the filament. The resulting silicide particles deposited on the silicon surface forming a mesh-like pattern. The particles work as an etching mask against hydrogen radical etching of the silicon surface and inverted-pyramids or V-groove-shaped surface texture were obtained. In the second step the filament current was reduced and whiskers grew onto the substrate due to the interaction of the silicon hydrides with the particles and subsequent precipitation of saturated Si. The whiskers were found to have tungsten silicide particles on their tip, suggesting the whisker growth was through the Vapor-Solid-Solid (VSS) mechanism. A balance between the hydrogen radical etching effect and the supply of silicon hydride from the etching reaction on the silicon surface is crucial for the growth of dense silicon whiskers.     

Autowave synthesis of cast Mo-W-Si silicides

This work examines general trends in the autowave synthesis of cast Mo-W-Si ceramics in a reactor at an elevated gas pressure using highly exothermic mixtures of molybdenum oxide, tungsten oxide, aluminum, and silicon. The results indicate that the gas (N₂, Ar) pressure in the reactor and reactant ratio have a strong effect on the combustion process. We have obtained cast solid solutions with a controlled MoSi₂: WSi₂ ratio.     

Decomposition of silicon tetrachloride by microwave plasma jet at atmospheric pressure

A new method for destroying silicon tetrachloride has been developed, which based on a microwave plasma jet that operates at atmospheric pressure using hydrogen as work gas. Experiments showed that the silicon tetrachloride was dissociated into silicon and hydrogen chloride under the effect of the plasma jet. The silicon was deposited on the molybdenum substrate of the plasma reactor, which was yellow and typical nanosized particles. These solid samples have been analyzed by SEM, EDX, XRD, FT-IR and the active particle in the plasma is detected by atomic emission spectroscopy. The results show that the silicon tetrachloride is mainly transformed into nano-silicon with size of 54 nm. The dissociation efficiency reaches 50%.     

Kinetics and properties of chemically vapour-deposited tungsten films on silicon substrates

The kinetic parameters for the deposition of tungsten films on silicon substrates were evaluated. The reactions (hydrogen reduction or silicon reduction of tungsten hexafluoride) were performed in a resistance-heated horizontal reactor. Film structure and composition were examined by X-ray diffraction, scanning electron microscopy and electron microprobe analysis. The physical, optical, chemical and electrical properties of the films were also measured and are discussed.     

Numerical study of SiC CVD in a vertical cold-wall reactor

The conventional heat and mass transport model is extended to describe silicon cluster formation in the gas phase and is employed for a numerical analysis of SiC chemical vapor deposition in a commercial vertical rotating disc reactor. The model is verified by comparing the computed growth rate with available experimental data. The growth rate is studied as a function of precursor flow rates varied in a wide range of values. It is found that the growth rate is limited by the gas mixture depletion in silicon atoms due to homogeneous nucleation. The secondary phase formation on the growing surface is analyzed. The SiC growth window depending on the precursor flow rates is calculated, and a significant influence of the homogeneous nucleation on the window width is shown. The model results predict that the Si/C ratio on the wafer can considerably differ from that at the reactor inlet.     

Ultra-low pressure chemical vapour deposition of polycrystalline and amorphous silicon

The deposition of undoped polycrystalline and amorphous silicon in an ultra-low pressure chemical vapour deposition system capable of achieving operating pressure of 0.03 Pa is discussed. It is found that at deposition temperatures in the region of 630 °C and reactor pressures of less than 1.3 Pa very large grained polycrystalline silicon films are obtained, and in this regime the growth rate is independent of the reactor pressure. Excellent uniformity is obtained and the process can be easily scaled up for large substrates and high volume batch production.     

Evaluation of the optimal carrier gas flow rate for the carbon nanotubes growth

Arrays of aligned carbon nanotubes (CNTs) were synthesized in a gas-phase flow reactor by thermal decomposition of reaction mixture (2% solution of ferrocene in toluene) on the surface of silicon substrates heated to 800°C. Variation of the height of the CNT array as a function of position of the substrate in the reactor and carrier gas flow rate was registered. The difference in the obtained dependences and temperature distribution in the reactor points to the necessity of taking into account the change in the concentration of the active carbon component in the gas mixture. An expression associating the parameters of synthesis and thickness of the CNT array being formed on the substrate is offered.     

Synthesis of amorphous silicon carbide nanoparticles in a low temperature low pressure plasma reactor

Commercial scale production of silicon carbide (SiC) nanoparticles smaller than 10?nm remains a significant challenge. In this paper, a microwave plasma reactor and appropriate reaction conditions have been developed for the synthesis of amorphous SiC nanoparticles. This continuous gas phase process is amenable to large scale production use and utilizes the decomposition of tetramethylsilane (TMS) for both the silicon and the carbon source. The influence of synthesis parameters on the product characteristics was investigated. The as-prepared SiC particles with sizes between 4 and 6?nm were obtained from the TMS precursor in a plasma operated at low temperature and low precursor partial pressure (0.001-0.02?Torr) using argon as the carrier gas (3?Torr). The carbon:silicon ratio was tuned by the addition of hydrogen and characterized by x-ray photoelectron spectroscopy. The reaction mechanism of SiC nanoparticle formation in the microwave plasma was investigated by mass spectroscopy of the gaseous products.     

Advanced APCVD-processes for high-temperature grown crystalline silicon thin film solar cells

Crystalline silicon thin film (cSiTF) solar cells based on the epitaxial wafer-equivalent (EpiWE) concept combine advantages of wafer-based and thin film silicon solar cells. In this paper two processes beyond the standard process sequence for cSiTF cell fabrication are described. The first provides an alternative to wet chemical saw damage removal by chemical vapor etching (CVE) with hydrogen chloride in-situ prior to epitaxial deposition. This application decreases the number of process and handling steps. Solar cells fabricated with different etching processes achieved efficiencies up to 14.7%. 1300 degrees C etching temperature led to better cell results than 1200 degrees C. The second investigated process aims for an improvement of cell efficiency by implementation of a reflecting interlayer between substrate and active solar cell. Some characteristics of epitaxial lateral overgrowth (ELO) of a patterned silicon dioxide film in a lab-type reactor constructed at Fraunhofer ISE are described and first solar cell results are presented.     

Incoherent radiative processing of titanium silicides

Titanium silicide thin films were formed after short-time processing of thin films of metallic titanium over single-crystal silicon and polycrystalline silicon. Radiation from high intensity lamps provided a directional driving force for the reaction, which was carried out both in the presence of oxygen in the reactor and under vacuum. The effect of oxygen on the reaction was monitored using sheet resistance, X-ray diffraction and Auger electron spectroscopy (AES) measurements. The film quality was found to be strongly influenced by an oxygen partial pressure in the reactor. The effect of the processing time was also assessed and the optimum time and power input interval were determined. High quality, low resistivity films with TiSi₂ as the major phase were obtained after 10 s under a roughing vacuum. AES studies indicated that most of the oxygen and other contaminants remained in a narrow surface layer after processing.     

Fourier transform infrared detection in miniaturized total analysis systems for sucrose analysis

In this work, a flow system containing a micromachined lamella-type porous silicon reactor and a novel mid-IR fiber-optic flow cell were used for the enzymatic determination of sucrose in aqueous solution. The method relies on the enzymatic hydrolysis of sucrose to fructose and glucose catalyzed by β-fructosidase and on the acquisition of FT-IR spectra before and after complete reaction. β-Fructosidase was covalently bound to the porous silicon surface of the channels in the microreactor. The porous silicon was achieved by anodization of the silicon reactor in a HF/ethanol mixture. For the measurement of small amounts of aqueous solution, a miniaturized flow cell was developed which consisted of two AgCl(x)Br(1)(-)(x) fiber tips (diameter, 0.75 mm) coaxially mounted in a PTFE block at a distance of 23 μm. The flowing stream was directed through the gap of the two fiber tips which served to define the optical path length and to bring the focused mid-IR radiation to the place of measurement. Using this construction, a probed volume of ～10 nL was obtained. The calibration curve was linear between 10 and 100 mmol/L sucrose. Furthermore, the potential of this method was demonstrated by the analysis of binary sucrose/glucose mixtures showing no interference from glucose and by the successful determination of sucrose in real samples.     

SiCf/SiC陶瓷复合材料的研究进展

SiCf/SiC陶瓷复合材料具有良好的力学性能、高温抗氧化性和化学稳定性，是航空航天和原子能等领域理想的新一代高温结构材料。本文概述了增强体SiCt的发展状况及存在的问题，对SiCt/SiC材料的制备工艺、界面相的研究状态、材料的损伤破坏机理和目前的应用研究进展做了综述，并分析了SiCf/SiC陶瓷复合材料的研究重点和发展前景。     

Low-temperature preparation of polycrystalline silicon from silicon tetrachloride

Polycrystalline silicon has been prepared by zinc reduction of silicon tetrachloride at a low temperature (≈550° C) in a vertical vapour phase reactor. Characterisation of the remelted polycrystalline ingot by X-ray, SEM and electrical methods shows that the material is p-type with an average grain size of 0.5 μm having a room-temperature resistivity in the range 1.0–1.5 Ω cm suitable for solar cell fabrication.     

Non-intrusive plasma diagnostics for the deposition of large area thin film silicon

Plasma diagnostics for large area, industrial RF parallel-plate reactors can be useful for process optimization and monitoring, provided that their implementation is practical and non-intrusive. For instance, Fourier transform infrared (FTIR) absorption spectroscopy and/or time-resolved optical emission spectroscopy (OES) can easily be retro-fitted into the pumping line of a reactor. Both techniques were used to measure the fractional depletion of silane in silane/hydrogen plasmas. By means of a simple analytical plasma chemistry model, it is shown that the silane depletion is related to the silicon thin film properties such as microcrystallinity. Uses of the diagnostics are demonstrated by two examples: (i) the optimal plasma parameters for high deposition rate of microcrystalline silicon, along with efficient gas utilization, are shown to be high input concentration and strong depletion of silane; and (ii) the optimal reactor design, in terms of fast equilibration of the plasma chemistry, is shown to be a closed, directly-pumped showerhead reactor containing a uniform plasma.     

MODELING OF PURE AND IN SITU DOPED POLYCRYSTALLINE SILICON DEPOSITION UN LPCVD REACTORS

A global mathematical model of Low Pressure Chemical Vapor Deposition (LPCVD) reactors has been developed. The use of the model can be of practical interest for both the choice of operating conditions and reactor design. Calculations of the wafer temperature have been carried out providing better insights in the radiation heat transfer phenomena, occurring inside the reactor. The presence of injectors, often used in the industrial practice, has also been taken into account. This model turned out to be successful for predicting growth rates of pure and in situ boron doped polycrystalline silicon. The case of in situ phosphorus doped poly-Si, involving complex homogeneous chemistry and a consequent radial nonuniformity, needs a more appropriate local approach, which is briefly discussed.     

A study of pyrolysis of polymethylsiloxanes by Fourier transform infrared

This paper is dedicated to a comparative study of pyrolysis of decamethylcyclopentasiloxane and hexamethyldisiloxane, widely used as precursors for CVD of silicon dioxide films. The pyrolysis process was carried out in a hot-wall horizontal tube reactor made from quartz within the temperature range 25-1000 degrees C. FTIR spectroscopy has been used for the analysis of gaseous reaction products in the exhaust line of the reactor. It has been found that transformation of DMPSO was initiated by the open ring in the precursor molecules with its further transformation to linear biradicals followed by the chain's growth due to radical reactions. HMDSO transformation is connected with separation of silanon, silyl and methyl radicals with following multi-type interactions of siloxane radicals and formation of non-rigorously organized three-dimensional molecules.     

Preparation of silicon carbide whiskers from silicon nitride

Silicon carbide whiskers have been prepared by sintering silicon nitride powder in a graphite reactor at 1800°C under a nitrogen atmosphere. The whiskers differ in morphology: tubular needles, hollow faceted fibers with a square cross section, and solid fibers with a triangular cross section. The average diameter of the needles is 0.5?5 μm, and that of the faceted fibers is up to 20 μm. The fibers range in length up to several millimeters. Such silicon carbide whiskers can be used as reinforcing agents for structural ceramics based on nonoxide materials.     

Effect of HCl addition on the crystalline fraction in silicon thin films prepared by hot-wire chemical vapor deposition

In an effort to increase the crystalline fraction of silicon films directly deposited on a glass substrate by hot-wire chemical vapor deposition, the effect of HCl addition was studied. The silicon film was deposited on a glass substrate at 320 °C under a reactor pressure of 1333 Pa at the wire temperature of 1600 °C with 10%SiH₄–90%He at a fixed flow rate 100 standard cubic centimeter per minute (sccm) and HCl varied at 0, 10, 16 and 28 sccm. With increasing HCl, the crystalline fraction of silicon was increased as revealed by Raman spectra but the growth rate was decreased.     

The influence of reactor height adjustment on properties in GaN films grown on 6H-SiC by metal organic chemical vapor deposition

The influence of reactor height adjustment on properties in GaN films grown on 6H-SiC by metal organic chemical vapor deposition (MOCVD) was investigated. The property of GaN epilayer was investigated by atomic force microscopy, X-ray diffraction, low-temperature (10 K) photoluminescence and the Raman scattering. It is found that, as the spacing between showerhead and susceptor decreased, the growth rate increased and the tensile stress decreased. This result may be useful to control the stress in GaN thin films grown on silicon carbide substrate by MOCVD.