Ionicity has been raised as an important factor in discussing defect formation in Hf-based oxides. It has been elucidated from our first-principles calculations, that the stability of defects is dominated by Coulomb interactions between charged defects and surrounding ions. For instance, the formation energy of positively charged oxygen vacancies (Vo+2) is markedly decreased when they are coupled with substitutional N atoms at O sites, as the nominal charges of N3- are negatively greater than those of the oxygen ions O2-. Our computational results have further revealed that the ionic character of Hf causes generation of a low-lying Si dangling bond level in Si doped HfSiOx. These results suggest that one must take into account the ionicity of the Hf-based compounds, which possess totally different properties from the conventional gate insulator SiO2 in terms of defect formation.