Ferroelectric Domains And Its Switching Behavior Of Nanoscale Hf_0.5Zr_0.5O₂ Grains

Among various ferroelectric materials,Hf O₂-based ferroelectric material is considered to be an ideal new generation of ferroelectric materials due to its excellent shrinkable characteristics and good compatibility with existing silicon process platforms.However,Hf O₂-based ferroelectric thin films usually have a coexisting structure of ferroelectric phase and non-ferroelectric phase.Experiments show that the grain size is the key factor affecting the ferroelectric phase content.In order to explore grain size effect on Hf O₂ ferroelectric domain structure and its switching property,thus finally clarify the inherent physical mechanism of grain size effect.In this paper,the relationship between grain size and the ferroelectricity of the polycrystalline Hf_0.5Zr_0.5O₂?HZO?ferroelectric thin film is studied by piezoresponse force microscopy?PFM?.It is found through calculation that surface energy is a key factor affecting the formation of ferroelectric phases for small-sized grains,and this paper using the phase field theory to further study the effect of surface energy on the ferroelectric phase transition of Hf O₂-based ferroelectric thin film.The specific research content is as follows:1. Transmission electron microscope?TEM?,X-ray diffractometer?XRD?and atomic force microscope?AFM?are used to study the crystal and properties of polycrystalline HZO ferroelectric thin films prepared by RF magnetron sputtering.The longitudinal size of the grains in the film is 14 nm,and the lateral size is 50?148 nm.It is confirmed that the grains are flat and penetrate the entire ferroelectric layer.The pattern of TEM and the diffraction peaks of XRD confirm the existence of ferroelectric phase.The polarization value and relative dielectric constant of the HZO ferroelectric thin film are measured using the ferroelectric analyzer,and the existence of a non-ferroelectric phase is confirmed while characterizing the ferroelectricity.The potential of the surface is about 1.2 V.The finite element software COMSOL is used to calculate the distribution of the electric field of the PFM conductive probe in the film,which the lateral size is about 20 nm.PFM is used to characterize the initial domain structure of the HZO ferroelectric film.Under inversion,it is found that the ferroelectric domains in the polycrystalline HZO are smaller than 10 nm in size and the polarization orientation is randomly distributed,and these nanodomains show excellent switching under an applied electric field.2. The switching spectroscopy PFM of different size grains in polycrystalline HZO ferroelectric thin films are characterized.It is found that the smaller the size of the grains,the stronger the ferroelectricity.We used Litho-PFM to study the switching performance of grains of different sizes.At the same voltage,the switchable part of small-sized grains is more than that of large-sized grains,that is,the ferroelectric domain content of small-sized grains is higher.3. The density functional theory is used to study the energy of non-ferroelectric monoclinic and ferroelectric phases in Hf O₂ materials,the volume energy of monoclinic phase is lower than that of ferroelectric phase,but the surface energy of monoclinic phase is higher than that of ferroelectric phase.Then the energy of grains of different sizes is calculated.When the grain size is smaller,the energy of the ferroelectric phase is lower.When the grain size is larger,the monoclinic phase is more likely to exist stably.Using phase field method,a coexisting structure with ferroelectric phase and non-ferroelectric phase of Hf O₂ thin film is constructed.By introducing surface energy into the model,it is found that the surface energy is the key factor to promote the ferroelectric phase transition of HfO₂.