Formation of an oxygen vacancy-dinitrogen complex in nitrogen-doped hafnium oxide

Nitrogen in a thin HfO_2-xN_x layer was observed in the form of dinitrogen molecules by atom probe microscope measurements. To evaluate the chemical state, an ab initio calculation based on monoclinic HfO ₂ was performed with using oxygen vacancy models combined with interstitial or substitutional nitrogen. The formation of a hafnium dinitrogen complex near the oxygen vacancy was found to considerably lower the system energy. This result contrasts the substitutional nitrogen complex passivating the oxygen vacancy level in the cubic structure and suggests that the structural stability enhanced by the oxygen vacancy-dinitrogen complex restrains the formation of grain boundaries in amorphous HfO₂, which are responsible for the large leakage current.