Boron (B) is one of the promising group-IIIA acceptor impurities for SiC and forms a shallow acceptor level (285-390 meV) and deep level (540-720 meV). The B-related deep defect can lead to the degradation of a devices electrical characteristics over the long terms, therefore, a suitable process is needed to suppress the B-related deep defect level, known as the D-center. The C/B sequential implantation technique, a site-competition effect, was applied to suppress the formation of the D-center. The thermal admittance spectroscopy was used to detect the electrically active shallow and deep defect levels introduced by the C/B sequential implantation of 4H-SiC. The C/B sequential implantation was found to be effective in suppressing the formation of the D-center. However, the concentration of the co-implanted C-atoms is very sensitive as the increasing C content leads to the decrease in the active hole concentration by quenching the shallow acceptor level due to the formation of a deep defect level at about 428 meV. A complex microstructure in which C-atom bonds with the B atom at the Si-lattice site was proposed to account for the experimentally observed new B-related deep defect level.