Oxidation of silicon carbide and the formation of silica polymorphs
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The oxidation of both single crystal and relatively pure polycrystalline silicon carbide, between 973 and 2053 K, resulted in the formation of cristobalite, quartz, or tridymite, which are the stable crystalline polymorphs of silica (SiO2) at ambient pressure. The oxide scales were found to be pure SiO2 with no contamination resulting from the oxidizing environment. The only variable affecting the occurrence of a specific polymorph was the oxidation temperature. Cristobalite was formed at temperatures 艌1673 K, tridymite between 1073 and 1573 K, and quartz formed at 973 K. The polymorphs were determined using electron diffraction in a transmission electron microscope. These results were further confirmed using infrared and Raman spectroscopies. Cristobalite was observed to grow in a spherulitic fashion from amorphous silica. This was not the case for tridymite and quartz, which appeared to grow as oriented crystalline films. The presence of a thin silicon oxycarbide interlayer was detected at the interface between the SiC substrate and the crystalline silica using x-ray photoelectron spectroscopy.
I. INTRODUCTION
Single-crystal silicon carbide (SiC) is an excellent candidate for the development of electronic devices capable of operating under high power, high frequency, and high temperatures. These attractive properties are due to its wide energy band gap (>3.0 eV), high breakdown voltage (∼30 × 105 V cm−1), high electron drift velocity (2 × 107 cm s−1), and high temperature stability (sublimation occurs >3000 K). Moreover, like Si, it is possible to grow a thermal oxide on SiC, although the oxidation rate is in general considerably slower. The parabolic rate constant for SiC was found to be 2.5 times lower than the value of the rate constant for the oxidation of Si at 1470 K, for example.1 No direct comparison can be made between the oxidation of SiC and Si in the higher temperature range because the oxidation of Si is often conducted below 1570 K (Si melts at 1683 K). Changes in the activation energy for SiC oxidation around 1620 K were reported by several groups, e.g. Ref. 2. It is thought that molecular oxygen is the dominant permeating oxidant species at lower temperatures, and that ionic oxygen diffusion becomes more important at lower oxygen partial pressures and at higher temperatures. Higher activation
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0317 2550 J. Mater. Res., Vol. 21, No. 10, Oct 2006 http://journals.cambridge.org Downloaded: 16 Mar 2015
energies, upturn in Arrhenius plots, thicker oxide scales, and pronounced porosity, all observed to increasing extents at higher temperatures, are thought to be indicative of enhanced oxidation kinetics in impure environments.1 Although this native SiO2 oxide could enable SiC metal oxide semiconductor (MOS) technology to follow the highly successful path of Si MOS technology, there are nevertheless important differences in insulator quality and device processing that are preventing SiC-based devices from rea
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