Showing papers on "Silicon carbide published in 1991"

Thin film deposition and microelectronic and optoelectronic device fabrication and characterization in monocrystalline alpha and beta silicon carbide

Abstract: The deposition of silicon carbide thin films and the associated technologies of impurity incorporation, etching, surface chemistry, and electrical contacts for fabrication of solid-state devices capable of operation at temperatures to 925 K are addressed. The results of several research programs in the United States, Japan and the Soviet Union, and the remaining challenges related to the development of silicon carbide for microelectronics are presented and discussed. It is concluded that the combination of alpha -SiC on alpha -SiC appears especially viable for device fabrication. In addition, considerable progress in the understanding of the surface science, ohmic and Schottky contacts, and dry etching have recently been made. The combination of these advances has allowed continual improvement in Schottky diode p-n junction, MESFET, MOSFET, HBT, and LED devices. >

Silicon oxycarbide glasses: Part II. Structure and properties

Abstract: Silicon oxycarbide glass is formed by the pyrolysis of silicone resins and contains only silicon, oxygen, and carbon. The glass remains amorphous in x-ray diffraction to 1400 °C and shows no features in transmission electron micrographs (TEM) after heating to this temperature. After heating at higher temperature (1500–1650 °C) silicon carbide lines develop in x-ray diffraction, and fine crystalline regions of silicon carbide and graphite are found in TEM and electron diffraction. XPS shows that silicon-oxygen bonds in the glass are similar to those in amorphous and crystalline silicates; some silicons are bonded to both oxygen and carbon. Carbon is bonded to either silicon or carbon; there are no carbon-oxygen bonds in the glass. Infrared spectra are consistent with these conclusions and show silicon-oxygen and silicon-carbon vibrations, but none from carbon-oxygen bonds. 29Si-NMR shows evidence for four different bonding groups around silicon. The silicon oxycarbide structure deduced from these results is a random network of silicon-oxygen tetrahedra, with some silicons bonded to one or two carbons substituted for oxygen; these carbons are in turn tetrahedrally bonded to other silicon atoms. There are very small regions of carbon-carbon bonds only, which are not bonded in the network. This “free” carbon colors the glass black. When the glass is heated above 1400 °C this network composite rearranges in tiny regions to graphite and silicon carbide crystals. The density, coefficient of thermal expansion, hardness, elastic modulus, index of refraction, and viscosity of the silicon oxycarbide glasses are all somewhat higher than these properties in vitreous silica, probably because the silicon-carbide bonds in the network of the oxycarbide lead to a tighter, more closely packed structure. The oxycarbide glass is highly stable to temperatures up to 1600 °C and higher, because oxygen and water diffuse slowly in it.

Vapor-phase fabrication and properties of continuous-filament ceramic composites.

Abstract: The continuous-filament ceramic composite is becoming recognized as necessary for new, high-temperature structural applications. Yet because of the susceptibility of the filaments to damage from traditional methods for the preparation of ceramics, vapor-phase infiltration has become the fabrication method of choice. The chemical vapor infiltration methods for producing these composites are now being studied in earnest, with the complexity of filament weaves and deposition chemistry being merged with standard heat and mass-transport relationships. Two of the most influential effects on the mechanical properties of these materials are the adhesion and frictional force between the fibers and the matrix, which can be controlled by a tailored interface coating. A variety of materials are available for producing these composites including carbide, nitride, boride, and oxide filaments and matrices. Silicon carbide-based materials are by far the most advanced and are already being used in aerospace applications.

Some New Perspectives on Oxidation of Silicon Carbide and Silicon Nitride

Abstract: This study provides new perspectives on why the oxidation rates of silicon carbide and silicon nitride are lower than those of silicon and on the conditions under which gas bubbles can form on them The effects on oxidation of various rate-limiting steps are evaluated by considering the partial pressure gradients of various species, such as O2, CO, and N2 Also calculated are the parabolic rate constants for the situations when the rates are controlled by oxygen and/or carbon monoxide (or nitrogen) diffusion These considerations indicate that the oxidation of silicon carbide and silicon nitride should be mixed controlled, influenced both by an interface reaction and diffusion

A Model of Silicon Carbide Chemical Vapor Deposition

Abstract: This paper presents a model describing the interacting gas phase and surface chemistry present during the steady-state chemical vapor deposition (CVD) of silicon carbide (SiC). In this work, the authors treat the case of steady-state deposition of SiC from silane (SiH{sub 4}) and propane (C{sub 3}H{sub 8}) mixtures in hydrogen carrier gas at one atmosphere pressure. Epitaxial deposition is assumed to occur on a pre-existing epitaxial silicon carbide crystal. Pyrolysis of SiH{sub 4} and C{sub 3}H{sub 8} is modeled by 83 elementary gas-phase reactions. A set of 36 reactions of gas- phase species with the surface is used to simulate the deposition process. Rates for the gas/surface reactions were obtained from experimental measurements of sticking coefficients in the literature and theoretical estimates. The authors' results represent the first simulation of a silicon carbide deposition process that includes detailed description of both the gas phase and surface reactions. The chemical reaction mechanism is also combined with a model of a rotating disk reactor (RDR), which is a convenient way to study the interaction of chemical reactions with fluid mechanics. Transport of species from the gas to the surface is accounted for using multicomponent transport properties. Predictions of deposition rates as amore » function of susceptor temperature, disk rotation rate, and reactant partial pressure are presented. In addition, velocity, temperature, and concentration profiles normal to the heated disk for 41 gas-phase species are determined using reactor conditions typical of epitaxial silicon carbide deposition on silicon substrates.« less

Reaction-formed silicon carbide

Abstract: The reaction bonding of silicon carbide (SiC) typifies liquid-solid reaction processes for the synthesis of refractory ceramic composites. These processes have particular advantages over conventional sintering and hot-pressing techniques in their lower processing temperatures, shorter times and near-net shape fabrication capabilities. Two particular modifications that we have employed in order to improve the mechanical properties and the ultimate use temperature of reaction-bonded SiC are the use of microporous carbon pre-forms derived from polyfurfural alcohol for refinement of microstructure, and the use of alloyed melts in order to replace detrimental residual silicon with a refractory silicide. The control of reaction rate is always a key issue in reaction processing. We have studied the kinetics and mechanisms of the liquid SiC reaction. Experiments on carbon fibers and plates show that the principle mechanism is one of solution-reprecipitation. There is an increased solubility of carbon at very fine SiC particles formed by the spallation of the misfitting carbide from the carbon interface, leading to reprecipitation of SiC at defective seed crystals. Molybdenum and boron at low concentrations (3.2 mol.%) have little effect on reaction kinetics, whereas aluminum is able to impede the reaction through the formation of an interfacial carbide layer.

Mechanical and Electrical Properties of Silicon Nitride–Silicon Carbide Nanocomposite Material

Abstract: Mechanical and electrical properties of nanocomposite materials composed of a Si3N4 matrix and nanometer-sized SiC particles are described. Composites containing less than 10 vol% SiC particles have the same order of resistivity and dielectricity as the non-SiC material as well as highly improved mechanical properties. The composites are promising materials for use under harsh conditions.

Energy dependence of electron damage and displacement threshold energy in 6H silicon carbide

Abstract: The frequency response of silicon carbide (SiC) light-emitting diodes has been used to measure the energy dependence of displacement damage produced in 6H SiC by energetic electrons. The minimum electron energy required to produce displacement damage was determined to be 108+or-7 keV, corresponding to an atomic displacement of silicon atoms. For electrons of energies greater than 0.5 MeV, the damage constant for lifetime degradation in SiC is lower than that for GaAs by more than three orders of magnitude, indicating a greatly superior resistance of SiC to displacement damage in most radiation environments. >

Silicon carbide field-effect transistor with improved breakdown voltage and low leakage current

Abstract: A silicon carbide field-effect transistor is provided which includes a semiconductor substrate, a channel formation layer of silicon carbide formed above the substrate, source and drain regions provided in contact with the channel formation layer, a gate insulator disposed between the source and drain regions, and a gate electrode formed on the gate insulator, wherein a first contact between the channel formation layer and the drain region exhibits different electric characteristics from those of a second contact between the channel formation layer and the source region. Also provided is a method for producing such a silicon carbide field-effect transistor.

High sensitivity ultraviolet radiation detector

Abstract: A high sensitivity radiation detecting photodiode formed in silicon carbide comprises a monocrystalline silicon carbide substrate; a first monocrystalline portion of silicon carbide upon the substrate and having a first conductivity type; a second monocrystalline portion of silicon carbide adjacent the first portion and having the opposite conductivity type from the first portion; and a p-n junction between the adjacent first and second portions. The photodiode provides a dark current density of no more than about 1×10-9 amps/cm2 at a reverse bias of -1.0 volts and at temperatures of 170° C. or less.

Surface modification and dispersion of silicon nitride and silicon carbide powders

Abstract: A surface modification technique using controlled hydrolysis and polymerization of Al-alkoxide is presented. It was found by measuring the electrokinetic behaviour and the adsorption properties that a minimum amount of about 0·5 mg Al/m2 was necessary to give SiC and Si3N4 powders alumina-like surface properties. This permanent surface coating improved the dispersability of Si3N4 in cyclohexane using a commercial dispersant with an acidic head-group.

Junction field-effect transistor formed in silicon carbide

Abstract: A junction field-effect transistor is disclosed that comprises a bulk single crystal silicon carbide substrate having respective first and second surfaces opposite one another, the substrate having a single polytype and having a concentration of suitable dopant atoms so as to make the substrate a first conductivity type. A first epitaxial layer of silicon carbide is formed on the first surface of the substrate, and having a concentration of suitable dopant atoms that give the first epitaxial layer the first conductivity type. A second epitaxial layer of silicon carbide is formed on the first epitaxial layer, the second epitaxial layer having a concentration of suitable dopant atoms to give the second epitaxial layer a second conductivity type opposite from the first conductivity type. A higher conductivity region of silicon carbide is formed on the second epitaxial layer, A trench is formed in the second epitaxial layer and higher conductivity region extending entirely through the higher conductivity region and partially into the second epitaxial layer toward the first surface of the substrate for defining a gate region in the second epitaxial layer between the trench and the first epitaxial layer. The trench divides the second epitaxial layer and higher conductivity region into respective first and second regions with the trench therebetween.

Ceramic composites: TiB2-TiC-SiC

Abstract: In the ternary system TiB2-TiC-SiC, the different two-phase composites, TiC-TiB2, SiC-TiB2 and SiC-TiC exhibit remarkable mechanical properties in regard with the single phase ceramics. The evolution of those properties, i.e. modulus of rupture σf, fracture toughnessK 1c, critical flaw sizea c, hardnessHv, coefficient of thermal expansion α and electrical resistivity ρ, over the complete ternary diagram was investigated. A methodology of research using optimal design was used to minimize the number of composites to be elaborated. In this study, 16 samples were sufficient to empirically determine a provisional mathematical model for each property. A model, then, enables the plot of isoresponse curves in the ternary diagram. The samples were hot pressed and the optimal hot-pressing cycles were determined using densification rates against temperature curves. The concordance between computed and experimental values is excellent, e.g. a sample containing 20 mol % of TiB2, 55 mol % of TiC and 25 mol % of SiC has σfexp = 1080 M Pa, σfcomp=1070 MPa;K 1cexp=6.7 MPa m1/2,K 1ccomp=6 M Pa m1/2;Hv exp=1 6.6 G Pa,Hv comp=17.3 GPa; and ρexp=57.4 μΩ cm, ρcomp=55 μΩm cm. Although titanium diboride does not react with silicon carbide, a strong interface bond is developed between titanium diboride and titanium carbide, and between titanium carbide and silicon carbide. This explains the bend strength evolution in the ternary system, and more particularly the fact that, in the area σf > 1000 MPa andK 1c > 6 MPam1/2, to high SiC contents correspond to low TiB2 contents and conversely. The relevant microstructures will be discussed.

Synthesis of nitrogen ceramic povvders by carbothermal reduction and nitridation Part 1 Silicon nitride

Abstract: Silicon nitride powders of high α content were prepared from mixtures of very fine amorphous silica and carbon powders in flowing nitrogen at 1400–1500°C. The higher the reaction temperature within this range the lower the yield of silicon nitride in spite of a higher rate of SiO generation. The overall reaction rate increased with increasing nitrogen flowrate. An increase of carbon content of the starting mixtures increased the reaction rate by increasing the rate of SiO formation as well as the rate of Si3N4 nucleation. It is thought that the effect of the carbon particle size on the reaction rate is more significant than that of the specific surface area. The amount of silicon carbide formed and the α/β ratio in the reaction products were strongly affected by a change of the source of carbon used in the starting mixtures.MST/1346a

Thermochemical Analysis of the Silicon Carbide‐Alumina Reaction with Reference to Liquid‐Phase Sintering of Silicon Carbide

Abstract: The stabilities of different phases in the Si-Al-C-O system are calculated from thermodynamic considerations with the objective of identifying the liquid phases formed during sintering of SiC in the presence of Al2O3. It is shown that a liquid phase can form at the sintering temperatures by the reaction of SiC with Al2O3. Depending on the carbon activity, the liquid can be either of the following: Al2O3 + Al4C3, SiC + Al4C3, or molten aluminum. The stability of the aluminosilicate melts that can form by the reaction of Al2O3 with the surface silica layer on SiC powders is also evaluated. Several factors that influence liquid-phase sintering, such as the solubility of SiC in the melts and the generation of gases during sintering, are discussed. The results of the thermodynamic analysis are compared with the observed sintering behavior for SiC.

Experimental examination of the push-down technique for measuring the sliding resistance of silicon carbide fibers in a ceramic matrix

Abstract: The use of a push-down technique to measure the sliding resistance of NICALON fibers (SiC) in a lithium aluminosilicate (LAS) matrix has been examined experimentally. Tests have been conducted on more than 300 fibers in 2 different samples of the as-processed SiC/LAS III composite. Results show that the push-down measurements on an individual sample are reproducible, that the sliding resistance can vary significantly between samples, and that Poisson's expansion does not affect push-down measurements of fibers in this system.

Properties and Applications of Silicon Carbide Ceramics

Abstract: Silicon carbide is a promising candidate for high-temperature structural materials and wear-resistant materials. We have developed pressureless-sintered silicon carbide ceramics. The properties, applications and related technologies of silicon carbide ceramics are described.

Converting ceramic materials to electrical conductors and semiconductors

Abstract: In a preferred embodiment room temperature electrically conductive or semiconductive ceramic paths or areas are produced on carbide and nitride ceramic substrates by a process of controlled oxidation using localized thermal heating (e.g., laser heating) by tracing desired paths onto the substrates, where air is the source of oxygen. In another embodiment, nitride and carbide ceramic substrates are converted to electroconductive or semiconductive ceramics where the substrate is characterized as whiskers, fibers, flakes or platelets whose dimensions are in the micron range, by controlled oxidation as prescribed by laser beam processing. The resulting conductive or semiconductive paths or surfaces of the substrate comprise electrically conductive or semiconductive nonstoichiometric aluminum-nitrogen-oxygen ceramic, when the initial ceramic substrate material is aluminum nitride(A1N); and electrically conductive or semiconductive nonstoichiometric silicon-carbon-oxygen ceramic, when the initial ceramic material used is silicon carbide (SiC). The path cut into the surface on a flat substrate can serve e.g. as electrical interconnects akin to printed circuitry on a wiring board and patterns of semiconductors formed can serve e.g. as semiconductive devices akin to rectifier devices. In the instance of the whiskers, fibers, flakes or platelets, the electrically conductive surfaces thereof may be used directly or enhanced for example, by coating other conductor metal or alloys onto the surface for uses e.g. as composite materials in matrices at the microstructural level.

In Situ Synthesis of Silicon Carbide Whiskers from Silicon Nitride Powders

Abstract: Silicon carbide whiskers were synthesized in situ by direct carbothermal reduction of silicon nitride with graphite in an argon atmosphere. Phase evolution study reveals that the formation of β-SiC was initiated at 1400° to 1450°C; above 1650°C silicon was formed when carbon was deficient. Nevertheless, Si3N4 could be completely converted to SiC with molar ratio Si3N4:C = 1:3 at 1650°C. The morphology of the SiC whiskers is needlelike, with lengths and diameters changing with temperature. SiC fibers were produced on the surface of the sample fired at 1550°C with an average diameter of 0.3 μm. No catalyst was used in the syntheses, which minimizes the amount of impurities in the final products. A reaction mechanism involving the decomposition of silicon nitride has been proposed.

Near-interface characterization of diamond films on silica and silicon

Abstract: The near-interface structure of diamond films grown from a methane and hydrogen gas mixture by microwave plasma enhanced chemical vapor deposition has been studied. Freestanding diamond films grown on both silica and silicon at two different methane concentrations were analyzed by scanning and transmission electron microscopies, electron diffraction, Raman spectroscopy, and secondary ion mass spectroscopy. It was found that the substrate chemistry greatly influenced the nature of the carbon initially deposited on the substrate surface. Diamond formed large flat contact areas on silicon, whereas on silica a particulate type of intermediate layer formed first because of the chemical reactions occurring on and/or with the surface. It was found that the phase content of the films was greatly affected by the methane concentration in hydrogen. At the low (1.0% or less) methane concentrations in hydrogen, phase pure diamond formed; while at the high (5.0%) methane concentration in hydrogen, graphite and disordered carbon were codeposited along with diamond during the early growth stages. Silicon carbide was detected at the diamond interfaces which appeared in discrete areas on silica as opposed to a rather continuous layer as is believed to form on silicon.

Thermodynamic Brazing Alloy Design for Joining Silicon Carbide

Abstract: Using solution thermodynamic theory, a Ni-Cr Si alloy, based on the Ni/Cr ratio of AWS BNi-5 (Ni-18Cr-19Si (atom%)), was designed for brazing SiC ceramics. The optimum thermodynamic composition was computed to be Ni-14.3Cr-36Si (atom%). Brazing experiments were conducted to assess the effect of changing the Si content away from this composition on the joint microstructures. For alloys containing less than 36 atom% Si, an excessively vigorous joining reaction occurred, resulting in the formation of a porous reaction zone at the brazing alloy/SiC interface, the amount of porosity decreasing as the brazing alloy composition approached the thermodynamically predicted optimum. It was found that the most likely mechanism for the formation of the porous zone was CO evolution during the SiC decomposition reaction. The best microstructures were attained for 40 atom% Si alloy joints, closely agreeing with the thermodynamic model, whereas higher Si content alloys exhibited localized debonding of the brazing alloy from the SiC.