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A stoichiometry-driven phase transition from rocksalt to “nitrogen-rich” structure exists in group-IVB transition metal nitride films. As this phase transition is critical in controlling the film properties it has attracted numerous studies. However, researchers are still divided with regard to the structural identity of this “nitrogen-rich” phase, not to mention detailed exploration of the phase transition mechanisms. In this study, we confirmed that the “nitrogen-rich” phase in hafnium nitride (HfN x ) films had a cubic Th 3 P 4 structure of space group symmetry of I- 43 d (220), namely c -Hf 3 N 4 . The confirmation was obtained by combining the first-principle calculations with a series of experiments: Selected Area Electron Diffraction, High Resolution Transmission Electron Microscopy, Raman, Gracing Incident X-ray Diffraction and X-ray Photoelectron Spectroscopy. The mechanisms of the phase transition were elucidated through calculations on enthalpy of formation ( EOF ). The experimental results agree well with the theoretical calculations. We conclude that with increasing nitrogen, phase transition takes place from rocksalt ( δ -HfN) to c -Hf 3 N 4 through three stages of structural evolution: δ -HfN (containing Hf vacancies) → mixture of ( δ -HfN + c -Hf 3 N 4 ) → c -Hf 3 N 4 . The driving force of the phase transition is energy minimization. The three stages of structural evolution are explained by comparing the EOF of the δ -HfN and c -Hf 3 N 4 phases. As the phase transition takes place, the hafnium nitride film morphs from a conductive and opaque metal into an insulating and transparent semiconductor. [ABSTRACT FROM AUTHOR]

Although point defects are found to exist commonly in non-stoichiometric group-IVB transition metal nitrides, the identification of primary point defects still causes some disagreement. The formation mechanism and influence of primary point defects on electronic structures are not yet well explored. This study finds that the types and formation mechanism of primary point defects in rocksalt hafnium nitride (δ-HfN x ) films can be identified by a combination of first-principles calculations and grazing incidence X-ray diffraction, Raman and high-resolution transmission electron microscopy experiments. It is shown that the primary point defects in sub- and over-stoichiometric δ-HfN x films are N and Hf vacancies, respectively, which arise preferentially because they are thermodynamically more stable than other types of point defects, such as interstitials and antisites, because they have much lower formation energy and equilibrium formation enthalpy. Furthermore, it is found that the formation of N and Hf vacancies have an important role in changing electronic structures. N vacancies act as donor-like defects and can add extra free electrons to the conduction band at a rate of an electron per N vacancy, while Hf vacancies serve as acceptor-like defects and efficiently reduce free electrons at a rate of two electrons per Hf vacancy. Additionally, the formation of N vacancies can induce two new interband absorption bands centered at ∼0.81 and ∼2.27 eV, while the incorporation of Hf vacancies create an additional interband absorption band at ∼3.75 eV. These new insights are demonstrated by good agreement between calculations and experiments. [ABSTRACT FROM AUTHOR]