A change in the interface state density in 4H-SiC metal–oxide–semiconductor (MOS) structures by incorporation of various elements was systematically investigated. B, N, F, Al, P, and Cl ions were implanted prior to the oxidation and introduced at the SiO2/SiC interface by subsequent thermal oxidation. Interface state density near the conduction band edge for Al-, B-, F-, and Cl-implanted MOS capacitors increased with implantation dose. On the other hand, a strong reduction of the interface state density was observed for N- and P-implanted samples when the implantation dose was larger than 5.0 × 1012 cm−2. It was found that the interface state density can be reduced by P as well as N.