Showing papers on "Silicon carbide published in 1990"

Sublimation of silicon carbide to produce large, device quality single crystals of silicon carbide

Abstract: The present invention is a method of forming large device quality single crystals of silicon carbide. The sublimation process is enhanced by maintaining a constant polytype composition in the source materials, selected size distribution in the source materials, by specific preparation of the growth surface and seed crystals, and by controlling the thermal gradient between the source materials and the seed crystal.

Method of preparing silicon carbide surfaces for crystal growth

Abstract: The invention is a method of forming a substantially planar surface on a monocrystalline silicon carbide crystal by exposing the substantially planar surface to an etching plasma until any surface or subsurface damage caused by any mechanical preparation of the surface is substantially removed. The etch is limited, however, to a time period less than that over which the plasma etch will develop new defects in the surface or aggravate existing ones, and while using a plasma gas and electrode system that do not themselves aggravate or cause substantial defects in the surface.

Method of improving mechanically prepared substrate surfaces of alpha silicon carbide for deposition of beta silicon carbide thereon and resulting product

Abstract: The invention is a method of improving a mechanically prepared surface of alpha silicon carbide for increasing the polytype purity of an epitaxial layer of beta silicon carbide grown thereon. The method comprises decreasing the frequency of exposed higher energy lattice positions along a mechanically prepared {0001} surface of a 6H alpha silicon carbide substrate by adding atoms to vacant lattice positions and by increasing the average height and separation between steps on a mechanically prepared {0001} surface of 6H silicon carbide.

Active‐to‐Passive Transition in the Oxidation of Silicon Carbide and Silicon Nitride in Air

Abstract: The active-to-passive transition in the oxidation of SiC and Si3N4 was determined in a flowing air environment as a function of temperature and total pressure. The experimentally observed transition temperatures ranged from a low of 1347 C to a high of 1543 C for partial pressures of oxygen of 2.5 and 123.2 Pa, respectively. The SiC and Si3N4 samples had approximately the same transition point for a given pressure. In general, the higher the flow rate, the higher the transition temperature for a given pressure. The transitions for SiC measured in this study agree with previous data for the transition of SiC measured in pure oxygen at reduced pressures and in oxygen inert gas mixtures.

Volatility Diagrams for Silica, Silicon Nitride, and Silicon Carbide and Their Application to High‐Temperature Decomposition and Oxidation

Abstract: Volatility diagrams—isothermal plots showing the partial pressures of two gaseous species in equilibrium with the several condensed phases possible in a system—are discussed for the Si-O and Si-N systems, and extended to the Si-N-O and Si-C-O systems, in which the important ceramic constituents SiO2, Si3N4, Si2N2O, and SiC appear as stable phases. Their use in understanding the passiveactive oxidation transitions for Si, Si3N4, and SiC are demonstrated.

Hydrophobically coated abrasive grain

Abstract: The strength and water resistance of the binding of abrasive grain on the basis of corundum or silicon carbide into synthetic resin bonded abrasive agents is improved by a surface treatment with microdispersed hydrophobic silicon dioxide.

Infrared spectra and electron spin resonance of vanadium deep level impurities in silicon carbide

Abstract: Trace impurities of vanadium in Lely‐grown silicon carbide single crystals have been detected by their strong, polytype‐specific photoluminescence in the 1.3–1.5 μm near‐infrared spectral range, as well as by infrared absorption. The spectra arise from the intra‐3d‐shell transitions 2E(3d1)→2T2(3d1) of V4+Si(3d1). Electron spin resonance reveals that VSi in SiC acts as a deep acceptor, V4+Si(3d1)/V3+Si(3d2)−A0/A−, and possibly also as a deep donor. The role of vanadium as minority‐carrier lifetime killer in SiC‐based optoelectronic devices is suggested from these data.

Liquid‐Phase Reaction‐Bonding of Silicon Carbide Using Alloyed Silicon‐Molybdenum Melts

Abstract: The authors have investigated reaction-forming of silicon carbide by the infiltration of carbonaceous preforms using alloyed silicon melts, in order to synthesize composite materials free of the residual silicon phase that has previously limited mechanical properties and upper use temperatures. In this approach, rejection of the alloying component(s) from the primary silicon carbide phase into the remaining melt results in the formation of a secondary refractory phase, such as a silicide, in place of residual free silicon. Experiments conducted in the Si-Mo melt system show that relatively dense ({gt}90%) silicon carbide-molybdenum silicide materials free of residual silicon and residual carbon can be obtained. A model for reactive infiltration based on time-dependent permeabilities is proposed. Processing variables important for control of the reaction rate relative to the infiltration rate, and associated processing flaws, are discussed.

Metal/ceramic joining.

Abstract: This review article on metal/ceramic joining is subdivided into the description of research activities in the fields of active metal brazing and diffusion bonding published in the last decade. Informations are given on active metal brazing of oxide, nitride, and carbide ceramics and on diffusion bonding of alumina, zirconia, magnesia, silicon nitride, aluminum nitride and silicon carbide ceramics to metals. Ultra high vacuum diffusion bonding and experiments using the model combination Nb/alumina are also regarded. Emphasis is laid on a concise reproduction of experimental data concerning the bonding conditions and the determintion of bond strength. The review demonstrates that much effort was devoted to studies on the formatin of interfacial reaction layers and on the efficiency of interlayers additonally introduced between the ceramic and the metal part to reduce internal stresses caused by thermal expansion misfit of the materials to be bonded.

Light emitting diode device and method for producing same

Abstract: A light emitting diode device comprises an n type silicon carbide substrate having first and second major surfaces opposite to each other at least inclined at a predetermined angle not less than 3° from a {0001} plane, an n type silicon carbide layer grown on the first major surface, a p type silicon carbide layer grown on the n type silicon carbide layer, a p type ohmic electrode formed on a partial area of the p type silicon carbide layer, and an n type ohmic electrode formed on a partial area of the second major surface. The diode element has a substantially trapezoidal form in a cross section orthogonal to the first major surface. The diode element has the side of the p type silicon carbide layer broader than the side of the second major surface and is supported at the side of the type silicon carbide layer fixed to a supporting stem.

Toughening Behavior of Silicon Carbide with Additions of Yttria and Alumina

Abstract: The fracture-toughness-determining mechanism of silicon carbide with additions of yttria and alumina was studied. Observations of indentation crack profiles revealed that significant crack deflection had occurred. Median deflection angles increased with increased volume fractions of the second phases, which was accompanied by increased fracture toughness.

The nucleation and morphology of diamond crystals and films synthesized by the combustion flame technique

Abstract: The nucleation and morphology of diamond crystals and films synthesized by the use of a combustion flame have been investigated By operating an oxy-acetylene torch in a fuel-rich mode, diamond crystals and films have been deposited on mechanically abraded molybdenum, on in situ created molybdenum carbide, and on thin diamond-like carbon (DLC) layers synthesized on molybdenum Scanning electron microscopy, Auger and Raman spectroscopy have been used to characterize the films and crystals Diamond is found to be uniformly deposited in the region of the substrate that intersects the inner, acetylene-rich region of the flame The nucleation density, the growth rate, and the morphology of the diamond crystals and films are found to be strongly influenced by the surface condition of the substrate On mechanically abraded molybdenum, abraded with 600 mesh silicon carbide, and on molybdenum carbide, well-formed cubo-octahedrons of diamond, up to 45 µm in diameter, are formed for deposition times of 90 min Film formation is seldom observed under these conditions To enhance nucleation, thin layers of DLC were formed on molybdenum substrates by reducing the O2/C2H2 ratio in the gas mix to ~ 075 for short periods of time under 30 s This was followed by increasing the O2/C2H2 ratio to conditions that produce diamond (an O2/C2H2 ratio of ~ 09) Under these conditions the nucleation density of diamond was increased by an order of magnitude and the growth rates by about 60%, as compared to diamond deposited on abraded molybdenum and molybdenum carbide In addition, the morphology of the diamond crystals and films was substantially affected with indications of dendritic growth The DLC layer is effective in promoting diamond nucleation due to the high surface defect density and the high hydrogen concentration of these films The combination of surface defects, in the form of dangling bonds, and the evolution of hydrogen from the DLC layer during the diamond deposition process, which is characterized by higher temperatures, result in a high concentration of active surface sites for diamond nucleation The nucleation density, the distribution on the substrate, and the morphology of diamond crystals and films are not driven by the transport of reactive specie in the flame to the substrate, but rather by nucleation processes, temperature distribution across the surface, and attendant surface phenomena

Electrode for use in the treatment of an object in a plasma

Abstract: An electrode for use in the treatment of an object such as a semiconductor wafer through plasma reaction has at least a surface layer formed of silicon carbide. The electrode comprises a base, and the surface layer of silicon carbide is formed on a surface of the base by CVD coating process.

Effects of Carbon as a Sintering Aid in Silicon Carbide

Abstract: Self-diffusion data are collected from the literature in an attempt to better understand the strong effects of carbon as a sintering aid in SiC. These data indicate that the presence of excess carbon, in addition to reducing the native SiO2 layer of the SiC, probably enhances the rate-controling bulk self-diffusion rate of SiC by a factor of about 100.

Reaction-Layer Interfaces in SiC-Fiber-Reinforced Glass-Ceramics: A High-Resolution Scanning Transmission Electron Microscopy Analysis

Abstract: The reaction layers between silicon carbide continuous fibers (Nicalon) and a calcium aluminosilicate glass-ceramic (anorthite composition) matrix in hot-pressed composites have been characterized both structurally and chemically using high-resolution, field-emission scanning transmission electron microscopy. Chemical compositions at 10-nm spacings, with a resolution of ∼5 nm, were collected across the fiber-matrix interface zone. The reaction sequence in the material is silicon carbide (fiber)—carbon (in this case, graphite)—silica-rich glass—anorthite. The composition of the carbon layer is constant across its width; the interfaces between the four phases are planar. This morphology and the chemical gradients observed are consistent with the simple, ‘carbon-condensed’ oxidation displacemetn reaction, SiC + O2→ SiO2+ C, being responsible for interface phase formation in the composites. The planar interfaces indicate that the rate-limiting process in the interface formation reaction is the diffusion of oxygen through the matrix and silica glass layer; a corollary of this conclusion is that the diffusion of silicon through the carbon layer is a relatively faster process.

Toughening of Silicon Nitride Matrix Composites by the Addition of Both Silicon Carbide Whiskers and Silicon Carbide Particles

Abstract: Si3N4 matrix composites reinforced by SiC whiskers, SiC particles, or both were fabricated using the hot-pressing technique. The mechanical properties of the composites containing various amounts of these SiC reinforcing materials and different sizes of SiC particles were investigated. Fracture toughness of the composites was significantly improved by introducing SiC whiskers and particles together, compared with that obtained by adding SiC whiskers or SiC particles alone. On increasing the size of the added SiC particles, the fracture toughness of the composites reinforced by both whiskers and particles was increased. Their fracture toughness also showed a strong dependence on the amount of SiC particles (average size 40 μm) and was a maximum at the particle content of 10 vol%. The maximum fracture toughness of these composites was 10.5 MPa·m1/2 and the flexural strength was 550 MPa after addition of 20 vol% of SiC whiskers and 10 vol% of SiC particles having an average particle size of 40 μm. These mechanical properties were almost constant from room temperature to temperatures around 1000°C. Fracture surface observations revealed that the reinforcing mechanisms acting in these composites were crack deflection and crack branching by SiC particles and pullout of SiC whiskers.

Joining of ceramics demonstrated by the example of SiC/Ti

Abstract: The reaction behaviour of titanium sputtered on silicon carbide was investigated in a series of tests. It was ascertained that, in the temperature range between 1250 and 1500° C, mainly the ternary phase Ti3SiC2 is formed (>90%). In joining experiments of silicon carbide with 1–3 μm thick titanium layers at 1450° C at compaction pressures between 5 and 30 MPa the formation of the ternary phase leads to a high joining strength. The achieved bending strength (σM ≈ 286 MPa; m ≈ 10) is comparable to that of the starting materials (σM ≈ 303 MPa; m ≈ 15).

Grain size dependence of the thermal conductivity of polycrystalline chemical vapor deposited β-SiC at low temperatures

Abstract: Thermal conductivity data of polycrystalline chemical vapor deposited cubic silicon carbide are calculated from thermal diffusivity and heat capacity data in the temperature region of 80–300 K. Below 200 K, a linear dependence of the thermal conductivity with the product of grain size of the silicon carbide and cubic temperature is observed. This is explained in terms of phonon scattering by the grain boundaries.

Apparatus for eliminating residual nitrogen contamination in epitaxial layers of silicon carbide and resulting product

Abstract: The invention is a method, and associated apparatus and product, of forming extremely pure epitaxial layers of silicon carbide by reducing the carrier concentration of residual nitrogen in silicon carbide formed by chemical vapor deposition processes. The method comprises placing a substrate upon which an epitaxial layer of silicon carbide will form upon a susceptor, and in which the susceptor is formed of a material that will not generate undesired nitrogen-containing out gases at the temperatures at which chemical vapor deposition of silicon carbide will take place from appropriate source gases. The substrate is heated to a temperature at which chemical vapor deposition of silicon carbide will take place from appropriate source gases by inductively heating the susceptor using an induction frequency that heats the susceptor material. Silicon-containing and carbon-containing source gases are then introduced that will form an epitaxial layer of silicon carbide upon the heated substrate.

Diamond, silicon carbide and related wide bandgap semiconductors

Abstract: The exact definition of wide bandgap semiconductors versus conventional semiconductors is rather ambiguous. A variety of different semiconductors have been discussed in the present symposium, but the major emphasis, of course dictated by the participant response, has been placed on diamond and silicon carbide. Simply stated, diamond as an emerging material has the greater potential for many of these applications; however, silicon carbide is in a much more advanced stage of development. With the high level of activity in diamond research and development, it may be expected that many of the technological hurdles facing this material will be overcome in the years ahead. On the other hand, if certain key problems cannot be resolved (i.e., a shallow n-type dopant and a heteroepitaxial substrate), or if economic considerations are not favorable, then silicon carbide may be the wide bandgap semiconductor of choice for many of the aforementioned applications. Furthermore, other materials (some discussed in this symposium) will continue to be developed and compete for niches in this fast paced, rapidly growing market.

Vitrified bonded grinding wheel with mixtures of sol gel aluminous abrasives and silicon carbide

Abstract: Vitrified bonded grinding wheels which contain a mixture of sintered sol gel aluminous abrasive and silicon carbide abrasive are substantially and surprisingly better than those with either abrasive component alone, in their ability to abrade certain metals.

Silicon Carbide (SIC) - Recent Results in Physics and in Technology

Abstract: Silicon carbide provides promising physical properties which urge this wide band gap semiconductor to be reinspected as material for a possible use in highpower, high-speed, high-temperature, and high-radiation resistant devices. This demand for a second look is supported by progress in the understanding of the physical properties, by the recent improvement of the crystalline quality of bulk material and of epitaxially-grown films, and by the development of suitable process technologies necessary for the device fabrication. A brief review of recent results on the topics mentioned above is given in the present paper.

Silicon carbide bodies having high toughness and fracture resistance and method of making same

Abstract: A sintered silicon carbide ceramic body preferably produced from a uniform mixture comprising from about 82 percent to about 99.4 percent by weight silicon carbide, from about 0.5 percent to about 10 percent by weight of a nitrogen containing aluminum compound and from about 0.1 to about 8 percent of a rare earth oxide, both reacted with oxygen, wherein said sintered ceramic body has a density greater than 90% percent of theoretical and a fracture toughness, as measured by a single edge notched beam test, of more than 7 MPam 1/2 and method of making the same.

Chemically vapor deposited silicon carbide (SiC) for optical applications

Abstract: In this paper we present important physical, thermal, mechanical, and optical properties of cubic (β) silicon carbide produced via a bulk chemical vapor deposition (CVD) process developed at CVD Incorporated. This CVD SiC has been identified as the leading mirror material for high energy synchrotron radiation because of its high thermal conductivity, low thermal expansion, high polishability, and high reflectance in the vacuum UV. However, it has been difficult to obtain high quality, monolithic, CVD SiC mirrors in large sizes, i.e., greater than 10–20 cm. Recently, CVD Incorporated has been successful in scaling an SiC CVD process to produce large monolithic pieces of SiC up to 60 cm (24 in.) in diameter and plates up to 76 cm (30 in.) long by 46 cm (18 in.) wide with thicknesses up to 13 mm (0.5 in.). The properties of this material that make it attractive for optical applications, such as synchrotron optics, will be discussed.

Study of Fracture Behavior of Very Fine-Grained Silicon Carbide Ceramics

Abstract: Dense, fine-grained silicon carbide (SiC) ceramics were fabricated by a hot-pressing technique using pyrolyzed polycarbosilane powders. Hot-isostatic pressing treatments were also applied to some of these hot-pressed samples. The grainsize range of the obtained sintered bodies was from 0.2 to 1.4 μm, which was much finer than that of ordinary sintered SiC ceramics. Relationships among sintering conditions, microstructures, and fracture toughness of the obtained ceramics were investigated. A clear grain-size dependence of fracture toughness was observed in this very fine-grain region (0.2 to 1.4 μm). Fracture toughness showed its maximum (5.1 MPa.m1/2) at the average grain size of ∼0.7 μm. Also, the fracture toughness of the samples having similar grain sizes increased with increasing relative density.

Burner Rig Hot Corrosion of Silicon Carbide and Silicon Nitride

Abstract: A number of commercially available SiC and Si3N4 materials were exposed to 1000 C for 40 h in a high-velocity, pressurized burner rig as a simulation of an aircraft turbine environment. Na impurities (2 ppm) added to the burner flame resulted in molten Na2SO4 deposition, attack of the SiC and Si3N4, and formation of substantial Na2O+x(SiO2) corrosion product. Room-temperature strength of the materials decreased as a result of the formation of corrosion pits in SiC and grain-boundary dissolution and pitting in Si3N4.

Interfacial Structure of Metal-Ceramic Joints

Abstract: On joining of metals to ceramics, chemical reaction at the bonding interfaces is unavoidable in most of the combinations of ceramics and metals including brazes except for a few special cases. Thermodynamic possibility of the chemical reaction and the resulted interfacial structure are compared with the experimental results in the cases of silicon nitride and silicon carbide ceramics. The influence of the reaction products on the bond strength of the joints and the effect of impurities on the interfacial reaction are discussed.

Surface micromachined linear thermal microactuator

Abstract: A linear thermal actuator based on controlled resistive heating of a suspended, micromachined, polysilicon (and silicon carbide) serpentine has been demonstrated. Maximum linear displacements of 1.5 mu m were measured for a polysilicon serpentine microbridge consisting of six bars (500 mu m*10 mu m*2 mu m). These displacements occur when sections of the micro-serpentine have been resistively heated to an average temperature change Delta T of 200 K to 500 K. Although this approach to linear actuation produces only small displacements, these thermally generated forces are significantly larger than those which can be obtained by electrostatic actuation methods. This novel thermal microactuator design is compatible with current IC processing techniques and may have applications for precision linear positioning or mechanical switching. >