Browsing by Author "Chiu, Chienchia"

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    Chemical vapor deposition of β-SiC thin films on Si(100) in a hot wall reactor
    Chiu, Chienchia (Virginia Tech, 1994-01-12)
    A systematic method was developed for the deposition of β-SiC thin films on Si(100) substrates in a hot wall reactor, using low pressure chemical vapor deposition (LPCVD). Due to poor adhesion resulting from lattice mismatch and difference in thermal expansion coefficients between the (SiC films and the Si(100) substrates, the feasibility of forming a SiC buffer layer on the Si(100) surface before beginning the chemical vapor deposition (CVD) process was investigated. The SiC buffer layers were formed with either a smooth or porous morphology. A nonporous Si(100) substrate with a 35Å thick SiC buffer layer was formed when the Si surface was heated at 1050°C in an atmosphere of C₂H₂ and H₂. A porous surface was obtained when the Si substrate was heated at 1000°C in C₂H₂ alone. The porous defects were correlated to the out—diffusion of Si in the carburizing process. On smooth Si(100) substrates, polycrystalline and stoichiometric β-SiC thin films with the (111) planes paralleling the Si(100) substrates were grown from a CH₃SiCl₃ (MTS)—H₂ mixture at 1050°C. At high H₂/MTS ratios and/or low deposition pressures, no etching on the Si substrates of the β-SiC films was observed, resulting in a smooth topography. Degradation in film morphology, changes in the preferred orientation, and etching of the Si substrates were observed at higher pressures, temperatures, and H₂/MTS ratios. The etching of the Si substrate was due to the out—diffusion of Si atoms from the substrate and the presence of Cl—containing radicals, which resulted from the decomposition of MTS molecules before arriving at the substrates. A model of the deposition mechanism is proposed which predicts the deposition rates in a hot wall CVD reactor and agrees very well with the experimental data. On the Si(100) substrate with a porous topography, epitaxial β-SiC(100) thin films were grown from MTS—H₂ at 1150°C. The crystallinity of the deposited films was influenced by the deposition time. With increasing deposition time, rotational β-SiC(100) crystals and polycrystalline β-SiC with a highly preferred orientation of (100) and/or (111) were obtained. At a lower temperature of 1100°C, poor morphology and polycrystalline β-SiC thin films were observed. Finally, a new approach to the calculation of the local equilibrium CVD phase diagrams, which represent the most stable phases above the substrates in a hot wall reactor, for SiC deposition from the MTS—H₂ gas mixture by coupling the depletion effects to the equilibrium thermodynamic computer code SOLGASMIX—PV. The calculated CVD phase diagrams were also compared with experimental and the literature data. Although the local equilibrium CVD phase diagrams predicted the deposition of single phase SiC better than established CVD phase diagrams, the experimental regions for depositing single phase SiC are larger than those calculated from local CVD phase diagrams. This may be because of the high linear velocity of the gas flux under low pressure and the polarity of the Si—containing intermediate species.
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    Low temperature synthesis and properties of lead ferroniobate Pb(Fe0.5Nb0.5)O₃
    Chiu, Chienchia (Virginia Tech, 1990-11-05)
    Pure, single phase stoichiometric Pb(Fe0.5Nb0.5)O₃ (PFN) powders were successfully formed by molten salt synthesis using mixture of NaCl and KCI salts. Lower temperatures and shorter times (0.5 hour at 800°C) were needed for single phase PFN formation from molten salts relative to those required for solid-state methods (4 hours at 1000°C). A systematic study indicating the effects of process parameters, such as temperature, time, and amount of flux with respect to starting oxides, on the PFN formation mechanism and its resulting powder characteristics is reported. The particle size increased with increasing synthesis temperature, the rate of increase is greatest above 900°C which is close to the melting point of lead oxide. PFN powders formed by molten salt synthesis were cuboidal, and were free from agglomerates. The sinterability, microstructure, and dielectric properties of these powders have been studied for the pure form and with the presence of lead oxide or lithium carbonate. The dielectric properties were sensitive to as-sintered density, the type of additive and the amount of additive. For pure PFN, the highest valve of dielectric constant is 12,270 at 1MHz, which is sintered at 1100°C for 13 hours. Ceramics sintered with lead oxide additive exhibited inferior dielectric properties although lead oxide served as a sintering aid to increase the as-sintered density. The dielectric properties of PFN with lithium carbonate sintered at 1000°C were attractive: the dielectric constant was increased to 14,000 at 1MHz and the D.C. conductivity was reduced. This reduction in the D.C. conductivity was interpreted in terms of the substitution of lithium for iron.