The Effects Of Oxygen Vacancies On Ferroelectric Phase Stability Of HfO₂-Based Thin Film From First-Principle

Compared with the traditional ferroelectric materials,the newly discovered hafnium oxide-based ferroelectric thin film is fully compatible with the standard Si technical platform and has a large forbidden band gap（⁵.7 eV）,exhibiting excellent ferroelectricity with a thickness even below 10 nm.So the hafnium oxide-based film is expected to be the new generation of the most promising ferroelectric film for memory application.However,many experimental and computational studies have shown that the ferroelectric orthorhombic phase of the hafnium oxide-based film is not the most stable phase structure under usual conditions.Therefore,the mechanism of ferroelectric orthorhombic phase transition of the hafnium oxide-based film has been confusing to the researchers,which also hinders the development and application of hafnium oxide-based ferroelectric film.Studies have shown that the presence of oxygen vacancy affects the ferroelectric orthorhombic phase transition of hafnium oxide-based films.And the wake-up effects,fatigue effects and imprint effects of the film are related to oxygen vacancy.But the specific effects of oxygen vacancy on the structure and properties of hafnium oxide-based film are still unclear.Therefore,by performing the first-principles calculation based on density functional theory,the effects of oxygen vacancies and complex defects formed by the combination of oxygen vacancies and typical impurities on phase stability were systematically studied.Due to the ambiguous effects of TiN/HfO₂ interfaces on the ferroelectricity of hafnium oxide-based film,therefore,this thesis also focuses on the influence of the oxygen content of TiN/HfO₂ interface.The research results obtained in this thesis mainly include the following three aspects:（1）The effect of oxygen vacancies on phase stability and electrical properties.It was found that the energy difference between the perfect ferroelectric orthorhombic phase and monoclinic phase is 2.64 eV（per 8 unit cells）.When there are oxygen vacancies in the bulk of hafnium oxide-based film,the energy difference between the ferroelectric orthorhombic phase and monoclinic phase is reduced to 1.21 eV,which is beneficial to the stability of the ferroelectric orthorhombic phase.The polarization magnitude of hafnium oxide was enhanced with the increasing of oxygen vacancy concentration.And the energy barrier for polarization switching decreases first and then increases.In addition,compared with the electronic structure of the perfect ferroelectric orthorhombic phase,the ferroelectric orthorhombic phase with oxygen vacancies shows the results that Fermi-level shifted from the top of valence band and a new trap-level arises in the band gap.The trap-level generated by the tri-coordinated oxygen vacancies is closer to the bottom of the conduction band than that of the tetra-coordinated oxygen vacancies,which is more likely to cause an increase of leakage current.（2）The effect of oxygen vacancies combined with the impurity of N、Ti、La and Ta on phase stability.When there is impurity in the bulk,the decrease of the energy difference between the doped ferroelectric orthorhombic phase and monoclinic phase is only within 1 eV.So we further studied the effects of the combination of oxygen vacancies and impurity on ferroelectric orthorhombic phase stability.It was found that impurities,such as N and La,have a strong combination with the oxygen vacancies,forming a defect dipole along the polarization direction.These complex defects reduce the energy difference between the ferroelectric orthorhombic phase and monoclinic phase by about 2 eV,which means the energy of ferroelectric orthorhombic phase is substantially equal to that of the monoclinic phase.（3）The effect of oxygen vacancies in ferroelectric/electrode interface on the phase evolution of thin film.The formation energy of oxygen vacancies at the interface is much smaller than that of the oxygen vacancies inside the film（approximately 2 eV）,which means that oxygen vacancies are more easily to appear at the interface.Within the perfect structures,the ferroelectric orthorhombic phase is the most stable one in this case with the energy of the 4 layers of HfO₂ unit cells 2.8eV lower than that of the monoclinic phase.When there is one oxygen vacancy at the interface,the ferroelectric orthorhombic phase and monoclinic phase are comparable in the energy,and the tetragonal phase is easy to transform into the ferroelectric orthorhombic phase,which is in good agreement with the evolution of structural observed in the wake-up process.As the numbers of oxygen vacancies increase furtherly,the monoclinic phase increases in the film,leading to fatigue.