Defects and Passivation Mechanism of the Suboxide Layers at SiO₂/4H-SiC (0001) Interface: A First-Principles Calculation

The origination of poor quality remains debating at the as-grown SiO2/4H-SiC (0001) interface during the thermal oxidation process. A low electron density layer (SiO$_{x \text {(0.3} \,{<; \,x} \, {\,{<; \,2}}{)}{}}$ ) is observed at the Si-terminated SiO2/4H-SiC (0001) interface in experiment, different from the previous reports on carbon-related defects. In this article, the SiO2/4H-SiC (0001) interface modeled with an interfacial SiO$_{x {(0.3} \, {<; \,\,x} \, {\,{<; 2)}}}$ suboxide layer (~1 nm) is systematically studied by first-principles calculations. According to the calculated electronic structures, the Si-Si antibonding structures in the SiOx layer are the dominating defects that cause obvious gap states, while the oxygen and silicon vacancy are not. The energy positions of the defect states are located at ~0.72 eV above the valence band maximum (VBM), which indicates the Si-Si defect is the potential influence factor for the p-channel Silicon carbide (SiC)-MOSFET. We further study several passivation schemes by introducing nitrogen, phosphorus, boron, and aluminum at the interface to analyze the passivation mechanism. According to the calculated results of the passivation models, nitrogen passivation is an effective method, fully removing the gap states from Si-Si defects by forming Si3N, while boron passivation works for vacancy defect at the suboxide layers. The theoretical results prove the dominated defect structures at the interfacial SiOx layer and suggest that a combination passivation strategy (nitrogen and boron) may be an effective method to further improve the SiO2/4H-SiC interface.