Polarization Control Of Ferroelectric Thin Films

Ferroelectric thin films are widely used in various fields,including non-volatile ferroelectric random access memory,sensors,field-effect transistors,and pyroelectric infrared detector.The application of these ferroelectric effects depends on the control of the size,direction,and stability of polarization.It is well known that the polarization direction of ferroelectric thin films will be reversed under the action of an external electric field.However,sometimes it is difficult to polarize ferroelectric thin films by electrical stimulation,such as when there is no top or bottom electrode in the ferroelectric device.Therefore,studying how to control the growth process of thin films so that they do not require any polarization processing technology and exhibit macroscopic spontaneous polarization has important theoretical and practical value.Barium titanate（BTO）is a hot topic in ferroelectric film research due to its high polarization strength,low coercive field,and excellent stability.In this paper,we use Laser Molecular Beam to prepare epitaxial ferroelectric oxide films.Taking BTO films as an example,we studied the regulation of spontaneous polarization of ferroelectric films.The main results and discussions of the paper are as follows:（1）We first obtained STO substrates with a single Ti O₂interface termination layer through wet chemistry treatment and heat treatment,and the single Sr O interface termination layer obtained by depositing a unit cell of SRO on the treated substrate.（2）Using laser molecular beam epitaxy technology,we prepared BTO thin films with different interface termination layers by using chemically treated Nb:STO（wt=0.5%）substrate.We verified the role of interface termination layers and flexoelectric effect in regulating the polarization of ferroelectric films.The polarization direction of BTO thin films was measured using piezoresponse force microscopy（PFM）.We found that the spontaneous polarization direction of BTO films with a thickness of 4.8 nm and a Ti O₂ interface termination layer was downward,while that of BTO films with a Sr O interface termination layer was upward.In addition,we observed that as the thickness increased,the BTO thin film with a Ti O₂interface termination layer would reversed from downward to upward.The BTO thin film with a Sr O interface termination layer always had an upward spontaneous polarization direction.（3）Using SRO（～3.93?）as the bottom electrode,we inserted LAO or LTO into the BTO/SRO heterojunction interface.Due to the Thomas-Fermi screening effect,an artificial dipole would be formed at the interface.We studied the effect of artificial dipoles on the polarization regulation of ferroelectric films.We found that the polarization state of BTO thin films could be regulated by inserting artificial dipoles.Similarly,as the thickness of the film increased,the flexoelectric effect would dominate in the regulation of the polarization of ferroelectric films.Currently,there are few reports on the regulation of ferroelectric film polarization by artificial dipoles.（4）BTO lattice constant（～4.00?）,using LBSO（～4.11?）with a larger lattice constant as another type of bottom electrode,studying the effects of different lattice constant bottom electrodes on the polarization control of BTO thin film ferroelectricity.As the thickness increases,the flexoelectric effect dominates the polarization control of the ferroelectric thin film.We found that when using LBSO as the bottom electrode,the polarization direction of the 30 nm BTO film with a Sr O interface termination layer is downward.In summary,this paper has demonstrated the significant influence of various factors on the polarization state of ferroelectric thin films,including interface termination layers,artificially inserted interface dipoles,film thickness,and different bottom electrodes.To summarize,we have investigated the correlation between interface effects,flexoelectric effects,and ferroelectric polarization.Our research findings are expected to provide a universal non-electric field approach for controlling ferroelectric polarization.