First-principles Study On Phase Stability Of <111> Orientations Hafnium Oxide Ferroelectric Thin Films

Ferroelectric memory has attracted people's attention since a long time ago because of its high read-write rate and low power consumption.However,due to the obvious size effect of ferroelectric materials and the conflict between ferroelectric materials and modern silicon integrated circuit technology,it has not been able to make significant progress.In 2011,hafnium oxide(HfO₂)with ferroelectric properties was discovered.Due to its mature CMOS process,unique ferroelectric properties and obvious miniaturization effect,the project of ferroelectric information storage,which was originally difficult to develop,has been revitalized.However,the metastability of the ferroelectric phase of hafnium oxide and the origin of ferroelectricity remain unsolved.In this paper,by means of first principles calculation,the phase stability and polarization origin of the resently discovered<111>-oriented hafnium oxide based ferroelectric thin films were studied.We obtained the following conclusions:(1)In 2018,ferroelectric properties were observed in the[111]-oriented hafnium oxide thin film,which was attributed to rhombohedral phase with R3m space group.However,the microscopic mechanism of the ferroelectric polarization and phase stability the R3m phase remains unclear.Based on the calculation,it is found that the out-of-plane Hf-O bonds are alternately broken under the compressive strain in(111)-plane,leading to the broken symmetry of spatial inversion and spontaneous polarization of about 25?C/cm².The phase stability of the R3m is significantly imporved in the[111]-oriented hafnium oxide thin film with a small thickness due to the lowest surface energy.More interestingly,when the thickness of the film decreases,the in-plane lattice constants of the[111]-oriented R3m film automatically shrinks,resulting in an considerable spontaneous in-plane compressive strain,which is in accordance with the experiments.Our calculation results reveal the phase stability of the ferroelectric phase of hafnium oxide and the mechanism of ferroelectric origin.(2)In order to clarify the interface effects on the phase stability of<111>-oriented hafnium oxide thin films,the hexagonal InN substrate was chosen to form the InN-HfO₂ heterostructures.It is found that energy difference between the orthorhombic ferroelectric phase(Pca2₁)and most stable monoclinic phase(P2₁/c)of HfO₂ thin films is substantially reduced on the InN substrate.When the in-plane lattice constant is greater than 7.12(?),the Pca2₁ phase of hafnium oxide becomes the structure with the lowest free energy.But,the R3m phase mentioned above does not exist stably on InN substrate.The ferroelectric properties of orthorhombic phase hafnium oxide on InN substrate maintains well under in-plane strain.It is noteworthy that the polarization reversal barrier increases with the increase of in-plane tensile strain.However,even if the in-plane lattice constant is increased to7.19(?)and the polarization switching barrier is 0.19 eV/unit cell,this value is still smaller than the switching energy barrier of hafnium oxide in the bulk.These results indicate that the hexagonal substrate is conducive to improving the ferroelectric phase stability and ferroelectricity of HfO₂ thin films,providing effective solution for the development of high performance HfO₂-based ferroelectric films.