Theoretical Study On Ferroelectricity Of Hafnia-based Thin Films

Ferroelectric materials are widely used in non-volatile memory,ferroelectric field effect tubes and ferroelectric tunnel junctions due to their reversible conversion between two polarization states with the electric fields.However,it is difficult to integrate ferroelectric materials under nanoscale linewidth by traditional perovskite materials.As a wide band gap semiconductor,the transition metal oxides HfO₂has high dielectric constant and good thermal stability,which has replaced the traditional SiO₂ materials and promoted the development of ferroelectric memory.Experimentally,the doping of various elements in HfO₂ can make it show good ferroelectric properties,such as Si,Al,La,Y,Gd,Zr,Sr,etc.Doped elements with ionic radius slightly lower than Hfusually exhibit ferroelectric characteristics over a narrower doping range,while ionic radius slightly greater than Hfdoped elements exhibit ferroelectric characteristics over a wider process window.Although the experimental results show that the ferroelectric properties of doped HfO₂ have been significantly improved,which makes it have greater application potential,the theoretical study on the properties of doped HfO₂ ferroelectric materials is still lacking.In this thesis,the ferroelectric properties of doped hafnium-based thin films are studied theoretically through first principles calculation.The purpose of this paper is to theoretically explain the mechanism of different element doping on the ferroelectric properties of HfO₂.The content of this paper is mainly discussed as follows:（1）Set out the basic properties and research status of HfO₂ based ferroelectric materials.It includes the basic properties and classification of ferroelectric materials,the electronic structure and ferroelectric properties of HfO₂,including the effect of doping and annealing temperature on the ferroelectric properties of HfO₂.（2）The theoretical basis and calculation method of first principles calculation are expounded,such as density functional theory,GGA-1/2 new algorithm and VASP software.In addition,the new algorithm is applied to calculate the basic properties of the bulk phase of HfO₂,and the reliability of the algorithm is verified.（3）Stabilization of ferroelectric phase doped with HfO₂.The formation energy and free energy of monoclinic phase,cubic phase and ferroelectric phase of Hf_xAl_1-xO₂ and Hf_xLa_1-xO₂ with different doping concentrations were systematically studied,and the effects of doping elements and doping concentrations on the stability of ferroelectric phase were analyzed.It was found that the ferroelectric properties of HfO₂ films were better when Al doping concentration was lower and La doping concentration was higher.（4）The calculation of spontaneous polarization intensity and ferroelectric property of doped HfO₂ were analyzed.The spontaneous polarization intensity of HfO₂ primitive cell was calculated by means of Born effective charge and Berry phase method,and the results were 68.20 and 19.40μC/cm²,respectively.The result of HfO₂enlargement calculated by Berry phase method is 43.84μC/cm²,which is in good agreement with the theoretical calculation.The spontaneous polarization intensity of HfO₂ doped was calculated by Berry phase method.When the doping concentration of Al was 2.08 f.u.%,the maximum polarization intensity was 48.44μC/cm²,and when the doping concentration of La was 2.08f.u.%,the maximum spontaneous polarization intensity was 49.19μC/cm².