First-principles calculation of oxidation process of SiC substrate

The atomic structure of 4H-SiC(0001)/SiO$_2$ interface is still under discussion since the conclusion derived by HRTEM is different from that by SIMS and XPS. The oxidation processes as well as the CO desorption of a SiC substrate are investigated by first-principles calculations. We employ SiC surface and SiC/SiO$_2$ interface models to imitate initial and middle stages of oxidation, respectively. O atoms are inserted between Si-C bond sequentially and the energy gain of the CO desorption is calculated by removing a C atom as a form of CO molecule. We found that the CO desorption becomes preferable when the number of inserted O atoms is three because the perfect SiO$_2$ network remains after the CO desorption and the stress is relaxed by removing the CO molecule from the SiC/SiO$_2$ interface. When C atoms are not removed as CO molecules at the interface, the unoxized Si-C bond remains in SiO$_2$. We also investigate the CO desorption from SiO$_2$. The energy gain of the CO desorption indicates that the CO desorption is unfavorable because the rearrangement of Si-O bonds costs more energy than the CO desorption. Thus, C atoms are immediately emitted from the interface as CO molecules and not kicked out from SiO$_2$. Our results support the conclusion obtained by SIMS and XPS.