Investigation On Thermal,Electrical,and Optical Properties Of Ferroelectric Hafnia

Recently,the discovery of the emerging ferroelectric hafnia（referred to as ferroelectric HfO₂ hereafter）has attracted considerable attention.Compared to conventional ferroelectrics,ferroelectric HfO₂ exhibits robust ferroelectricity at nanometer thickness and excellent compatibility with the complementary metal-oxide-semiconductor process,benefiting the development of various nanoscale device applications including energy harvesting and nonvolatile memories.With the development of ferroelectric physics,several questions have arisen in theoretical studies and practical applications of ferroelectric HfO₂.For instance,ferroelectric HfO₂ shows an unusual piezoelectric effect with longitudinal and transverse piezoelectric stress coefficients both being negative,and it will contract or expend in all dimensions in response to the electric field,which is known as the electric auxetic effect,while the mechanisms of the negative piezoelectric effect and the electric auxetic effect in ferroelectric HfO₂ remain elusive.In addition,ferroelectric HfO₂ has been considered as a promising candidate for ferroelectric photovoltaic applications due to its switchable photoelectric response,while its optical properties need to be further explored.More importantly,the thermal management of ferroelectrics is facing great challenges with the gradual miniaturization of electronic devices,while the mechanisms and tailoring strategies of thermal transport in ferroelectric HfO₂ remain elusive.Solving these problems requires investigations of thermal,electrical,and optical properties,as well as an in-depth understanding of the fundamental mechanisms underlying the energy transport in ferroelectric HfO₂,which may benefit the top-level designs of ferroelectrics.In this work,we focus on the spontaneous polarization,a feature specific to ferroelectrics,and investigate the thermal,electrical,and optical properties of ferroelectric HfO₂.We aim to dig out the correlation between these basic physical properties and the spontaneous polarization,establishing the structure-property relation in ferroelectric HfO₂,which may provide useful insights for the designs and tailoring strategies of ferroelectric materials.Using first-principles calculations,we first investigate the thermal transport in ferroelectric HfO₂.We find that the thermal conductivity is positively correlated with the spontaneous polarization due to the ionicity in ferroelectric HfO₂ and related fluorite-structure ferroelectrics,namely,a large ionicity corresponds to a large ferroelectric distortion favoring a large spontaneous polarization and a strong bond strength facilitating the thermal conduction.Applying a hydrostatic pressure can effectively tune the thermal conductivity and the spontaneous polarization in ferroelectric HfO₂,that is,under a hydrostatic pressure,the lattice becomes increasingly harmonic with an enhanced spontaneous polarization,facilitating the thermal transport.We further demonstrate such a correlation in three other representative ferroelectrics（i.e.,tetragonal PbTiO₃,wurtzite AlN,and hexagonal ABC ferroelectric LiBeSb）,indicating that this may be a universal feature in ferroelectrics.Then,we investigate the electrical properties in ferroelectric HfO₂ and related fluoritestructure ferroelectrics in order to reveal the variation of spontaneous polarization in response to the stress or strain,known as the piezoelectric effect.We find that all these fluorite-structure ferroelectrics have the rare negative longitudinal piezoelectric coefficients,and the larger the"ionicity",the stronger the negative longitudinal piezoelectric effect.Fluorite-structure ferroelectrics exhibits a unique response mechanism to the electric field.For example,they will contract in all dimensions in response to an electric field along the polar axis（defined as a positive electric field）,and their longitudinal piezoelectric coefficients will decrease.Such a novel electrical response may provide more opportunities for turning the properties of ferroelectrics by electric fields.Furthermore,based on the understanding of the thermal and electrical properties of ferroelectric HfO₂,we propose to tune the thermal conductivity in ferroelectric HfO₂ by applying a finite electric field.Via first-principles calculations,we investigate the response of the thermal conductivity to an electric field.We find that a positive electric field is beneficial to the thermal transport,while the negative one is detrimental.This is attributed to the fact that a positive electric field facilitates the ferroelectric distortion corresponding to a larger spontaneous polarization,and suppresses the lattice anharmonicity resulting in a larger thermal conductivity.By inspecting the structural deviations of the polar ferroelectric phase from the nonpolar paraelectric phase,we define a geometric parameter to describe such a structureproperty（the spontaneous polarization and the thermal conductivity）relation in ferroelectric HfO₂,which may benefit the designs and thermal management of ferroelectrics.Finally,we investigate the optical properties of ferroelectric HfO₂ and its response to an electric field.We determine the first-principles calculation method for the dielectric function at finite temperature by comparing with spectroscopic ellipsometry measurements of barium titanate,and further calculate the temperature-dependent and electric field-dependent dielectric function of ferroelectric HfO₂.We find that the redshifted dielectric function coincides with the reduction of the spontaneous polarization at elevated temperatures.The origin is attributed to the temperature-driven ferroelectric-paraelectric phase transition.Similar regulation can be achieved by a finite electric field,i.e.,a positive（negative）electric field will facilitate（suppress）the ferroelectric distortion,resulting in the blueshift（redshift）of the imaginary part of the dielectric function.