Ab Initio Molecular Dynamics Study On The Origin Of Ferroelectricity Of Hafnium-Based Thin Films

Due to its superior radiation resistance,high speed,wide temperature range,low power consumption and long life,ferroelectric memory has great potential and very large application requirements.HfO₂-based ferroelectric materials are a new type of ferroelectric material that is fully compatible with silicon technology.Its discovery can break through the technical bottleneck of the development of ferroelectric memory.Though possessing significant advantages,the HfO₂-based ferroelectric memory is still in the laboratory stage in the world.This is because the origin mechanism of ferroelectricity of HfO₂ has not been well understood yet,which hinders the improvement of ferroelectric properties of HfO₂-based films.In this thesis,the formation mechanism of ferroelectric phase and the polarization switching proerties of HfO₂ under different stress/strain and external electric field conditions were systematically studied by using Ab Initio Molecular Dynamics method.The research achievements of this thesis include the following aspects:?1?Stress-temperature equilibrium phase diagram of HfO₂ under hydrostatic pressure.It was found that,under standard atmospheric pressure,HfO₂transforms from monoclinic phase to tetragonal phase,and then to cubic phase,when the temperature is increasing to 1900 K and 2700 K,respectively.At room temperature,with the increase of hydrostatic pressure,the monoclinic phase was transformed into the orthogonal phase Pbca at 26 GPa.It is found that the monoclinic phase of HfO₂ is most stable within 5 GPa,and the ferroelectric phase Pca2₁ can be obtained after annealing under 5 GPa.Tetragonal phase is the most stable phase between 10 and 15 GPa,while Pbca is most stable between 20 GPa and 30 GPa.For the Hf_0.5Zr_0.5O₂,the tetragonal phase is the most stable phase within 10 GPa,and the Pbca in the orthorhombic phase is most stable above 15 GPa.The results show that the ferroelectric phase Pca21 in HfO2 can be obtained under by annealing under a small hydrostatic pressure.?2?Stress-temperature equilibrium phase diagram of HfO₂ under in-plane constraint.The results of annealing simulation show that the in-plane polarized ferroelectric phase Pca2₁,with the energy lower than that of the monoclinic phase,is the most stable phase under the in-plane compressive strain.While the monoclinic phase is most stable under the in-plance tensile strain.The addition of Zr is beneficial to the stability of Pbca.The ferroelectric phase Pca2₁ with the polarization perpendicular to the constraint plane does not appear in the plane-strain equilibrium phase diagram.This means that the ferroelectric phase Pca2₁ with polarization along the out-of-plane direction may exist as metastable state under plane strain condition.?3?The structural and polarization responses of HfO₂ under the applied electrical field.Due to the ferroelectric phase Pca2₁ with polarization direction perpendicular to the strain plane cannot be obtained in the plane-strain equilibrium phase diagram,the phase stability and polarization switching properties of HfO₂ under external electric field are further studied.The results show that under the activation of 45 MV/cm electric field,HfO₂ under-2%¹%equiaxial plane strain will be transformed from the in-plane polarized ferroelectric phase into a tetragonal phase.The activated tetragonal phase exhibits antiferroelectric property under periodic electric field.However,in the case of 3%⁴%equiaxial plane strain,HfO₂can be transformed into the metastable out-of-plane polarized ferroelectric phase by a small activation electric field of 15 MV/cm.The activated ferroelectric phase shows good ferroelectric properties under the periodic electric field.These results are in good agreement with the experiments.