Theory of Carbon-Vacancy Diffusion at the SiO₂/4H-SiC Interface

Experimental data indicate that carbon vacancies incorporated in active regions of SiC devices are important electrical defects, responsible for device limiting effects such as carrier lifetime reduction. For field-effect transistors that include a 4H-SiC/SiO₂ interface, such as at the gate, the oxidation process is understood to introduce native defects to the SiC, including injection of carbon self-interstitials and vacancies, that diffuse into the active layer and interact with other defects and impurities. It is therefore important to understand the migration behaviour of primary native defects such as V_C in the vicinity of 4H-SiC/SiO₂ interfaces. We report here the results of a density-functional theory investigation into the diffusion of the carbon vacancy in such a region. We conclude that the migration of V_C is significantly hindered in the immediate vicinity of the interface, with the energy of diffusion barrier being approximately 15% greater than the corresponding diffusion in bulk 4H-SiC.