Synthesis And Physical Properties Of Hybrid Improper Ferroelectricity Ca₃Ti₂O₇ Thin Films

In recent years,significant attention has been paid to the study of the hybrid improper ferroelectricity（HIF）,because the spontaneous electric polarization in those HIF materials is not the primary order parameter in the phase transition,they are called“inproper”feiToelectricis,Such a concept paves an avenue for pursuing room-temperature multiferroicity with strong magnetoelectric coupling via modulating the octahedral tilt and rotation by electric field.According to the first principle calculation,Ca₃Ti₂O₇（CTO）has the characteristics of RP structure and is predicted to have the rotation of oxygen octahedron and the octahedral tilting non-polar distortion.These two non-polar distortions will lead to the generation of a polarization mode in the Brillouin zone,causing a deviation of the A-position Ca²⁺,ions from the equilibrium position,resulting in the appearance of ferroelectric polarization.In this study,we have prepared a series of Ca₃Ti₂O₇ films by pulsed laser deposition method,and characterized and the crystal structure and surface morphology of the Ca₃Ti₂O₇ films by X-ray diffraction（XRD）.atomic force microscopy and piezoelectric microscopy,ferroelectric domain morphology and electrical transport properties,etc.Themain content and conclusions include:Firstly,using Ca₃Ti₂O₇ ceramic as the target in the pulsed laser deposition process,the Ca₃Ti₂O₇ films were deposited onto the Pt（111）substrates at various substrate temperatures and different oxygen pressures.The crystal structure analysis of the CTO films showed that the Ca₃Ti₂O₇ samples were epitaxially grown along the（001）direction on the Pt（111）substrate.And as the substrate temperature increased during the deposition process the diffraction peaks of CTO thin films shifted obviously to the high angle of two theta.This indicates that the substrate temperature could significantly affect the unit cell parameters of the grown Ca₃Ti₂O₇ films.However,no significant shift is observed for the XRD diffraction peaks of the CTO films prepared under different oxygen pressures,indicating that the oxygen pressure during the urowth process of the CTO films has no significant effect on the lattice parameters of CTO thin films.The calculation of strain shows that as the substrate temperature increases,the in-plane compressive strain gradually decreases,while the in-plane tensile strain gradually decreases.The morphology and roughness analysis of CTO thin films deposited with different substrate temperatures show that when the substrate temperature is 650℃,the surface roughness of the growth film is relative smaller than that of other film samples,indicating that the surface of the sample is the smoothest.Morphology and roughness analysis of CTO films with different growth pressures shows that the oxygen pressure of 30 Pa is most suitable condition for obtaining the flat film surface with the smallest Rq and Ra values.Secondly,the piezoelectric response of Ca₃Ti₂O₇ films were characterized by PFM piezoelectric force microscope（PFM）.The piezoelectric response signals with different amplitudes and phases can be obtained in different regions of Ca₃Ti₂O₇ films by changing the magnitude and direction of the voltage applied to the conductive probe tip of PFM.This indicates that there is a ferroelectric domain in the Ca₃Ti₂O₇ epitaxial films,and the applied electric field（E～1000 kV/cm）can induce the switching of the ferroelectric domain in the Ca₃Ti₂O₇ film.The amplitude curves with "butterfly" shape and the phase hysteresis loop were successfully obtained by using PFM to measure the piezoelectric effect in different regions of the Ca₃Ti₂O₇ films,which confirmed the exist of ferroelectricity and ferroelectric domain in the Ca₃Ti₂O₇ thin films.Using the PFM technique,the longitudinal piezoelectric coefficient d33 of the Ca₃Ti₂O₇ film on the Pt substrate wras probed to be around 5.5 pm/V.Thirdly,employing conductive atomic force microscopy（C-AFM）the current（I）-voltage（V）characteristics of the Ca₃Ti₂O₇ films were characterized for first time at room temperature.The electrical current hysteresis loops and electricoresistance effect（external electric field induced change of electric resistance of CTO films）can be clearly observed in the I-V curve for most CTO thin films.It is found that the appearance of the current loops and electroresistance effect show dependence on the amplitude of biased external electric fields,that is,when the biased external electric field is lower than 1000 kV/cm.the current curves almost coincide with each other for the positively and negatively scans bias.When the applied electric field is greater than 1000 kV/cm.the current loops begins to appear and becomes larger with the increase of the applied electric field.This indicates that a higher applied electric field could lead to a larger electrioresistance effect in CTO thin films.The diode-like conductivity,the electro-resistive effect,and the current loop are all originated from the controllable switching of the ferroelectric domain in the Ca₃Ti₂O₇ films under the action of an external electric field.