436-zheng-1990-854 - Oxidation of Single-Crystal Silicon...

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Oxidation of Single-Crystal Silicon Carbide Part I. Experimental Studies Z. Zheng, R. E. Tressler, and K. E. Spear* Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 ABSTRACT The oxidation of single crystal SiC in dry oxygen (10-3-1 atm and 1200~176 followed parabolic kinetics. Two differ- ent apparent activation energies were calculated for oxidation of the (0001) C faces of SiC, approximately 120 k J/tool below 1350~ and 260 kJ/mol above 1350~ Two regimes were not apparent for oxidation of the (0001) Si faces, and appar- ent activation energies lay between 223 and 298 kJ/mol. Double oxidation experiments using 1~O2 and 1802 indicated that the process is dominated by the transport of molecular oxygen at lower temperatures (< 1300~ with a substantial contri- bution from diffusion of ionic oxygen at higher temperatures. Epitaxially grown Si 13C films on alpha-Si 12C substrates via CVD were used to study carbon transport behavior during oxidation of SiC. Depth profiles for carbonaceous species using SIMS showed that carbon can transport quickly through the oxide layer, which eliminates the possibility that transport of carbonaceous species is rate controlling in the oxidation of SiC. Oxidation mechanisms of SiC are discussed on the basis of these results. Silicon carbide is one of the most important high-tem- perature structural materials. The long-term reliability of high-performance structural ceramics under stress de- pends critically upon the environmental corrosion of these materials. The rate and extent to which corrosion occurs depend upon the details of the type(s) and rate(s) of the thermodynamically favorable chemical reactions between the ceramic and its environment. Thermal oxidation studies of SiC in dry oxygen at high temperatures (above 1000~ are numerous with reports of various apparent activation energies ranging from 85 to 500 kJ/mol (1-11). Direct comparison of these oxidation ki- netics data is impossible because different materials with different impurities and manufacturing techniques, and thus different microstructures, were used. In addition, a variety of oxidation conditions was investigated. In gen- eral, the oxidation of silicon carbide in the passive regime has been shown to follow a parabolic law. Therefore, the rate-controlling step in the growth of the SiO2 layer is ex- pected to be a diffusion process, i.e., either oxidants diffus- ing inward or the oxidation by-products diffusing outward through the oxide layer. There are two common ways by which oxygen is trans- ported through the silica layer. One is the permeation of molecular oxygen and another is the diffusion of ionic net- work oxygen. For the permeation of molecular oxygen in vitreous silica, the activation energy values reported (around 100 kJ/mol) are basically consistent with one an- other (12, 13). However, for the diffusion of ionic oxygen in vitreous silica, a large difference exists among the re- ported activation energy values; they vary from about 83 to 300 kJ/mol (14-17). These values were derived from isotope exchange experiments. Based on the similarity of oxida-
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This note was uploaded on 12/10/2009 for the course IF PFIS1200 taught by Professor Antonio during the Spring '09 term at Universidade Federal do Rio de Janeiro.

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436-zheng-1990-854 - Oxidation of Single-Crystal Silicon...

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