| Silicon carbides (SiC) are a kind of wide bandgap compound semiconductor material which have greater saturated electron drift velocity,higher critical breakdown electric field and higher thermal conductivity. The ability to grow thermal oxides on SiC gives significant advantages over other compound semiconductors and also makes it compatible with other silicon technologies. Therefore, SiC is a promising material for fabricating high-temperature, high-power, high-frequency and anti-irradiate electronic devices. Power MOS devices need high quality of SiO2 insulating film.The interface state of SiO2/SiC and structure of SiO2 are main factors to determine quality of the gate layer of a MOS device.Recently, most of the researches focused on the traps of SiO2/SiC interface. Some researches successfully improved the quality of the interface by nitric oxide postoxidation. A few studies, however, noticed the breakdown voltage of the SiO2 layer. (SIMS) and (XPS) were the common methods to detect impurity atoms, such as carbon, in the SiO2 layer.In this article, positron annihilation Doppler-spread-spectroscopy and infrared reflection spectroscopy were used to study the void defect in the SiO2 layer, which was thermally fabricated on SiC, before and after annealing. Furthermore, the reason of low SiO2/SiC oxide film quality even low breakdown voltage of SiO2 /SiC MOS structure had also been studied.High frequency C-V techniqe was employed to measure the flat-band characteristics of the SiO2/SiC structure, which were very close to the reported values, thus indicated that the interface traps density of the samples prepared in thiswork were already satisfied. However, the breakdown voltage monitored by I-V measurement showed that the typical value of critical breakdown electric field of SiO2/SiC structure was 2-2.5 XI07 V/cm, which was less than that of SiO2/Si structure(about 4X107 V/cm) frabricated at same condition.Positron annihilation Doppler-spread-spectroscopy is very sensitivity to the void defects. By using this technique it could be confirmed the existence of void defects in SiO2 on SiC. In this work.it had been observed that a number of the void-like defects in the SiO2 layer obviously decreased after annealing. The shift of the peak (around 1085cm-1) that is due to the vibration of Si-0 bond in the spectrum mesaured by infrared reflectance spectroscopy has suggested that the SiO2 layer became tightness after annealing. Even in this case, however, the breakdown voltage of the SiO2 layer had just slightly improved.The above experiments indicated that, the existence of void-like defects in SiO2 was the reason of low breakdown voltage of SiO2 /SiC MOS structure. It was also indicated that post-annealing processing could improve the quality of the SiO2 layer. Because of the multiformity of the defects caused by carbon and the complexity of the SiO2 layer, the role of nitrogen in the post annealling to get higher breakdown voltage of SiO2 /SiC MOS structure was still not clear. Therefore, further studies are needed for a multi-steps annealing process.
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