Ferroelectric Phase Engineering Of PbTiO₃ Thin Film By Thickness And Freestanding Structure

Ferroelectricity has been widely studied and applied due to stable spontaneous polarization without any external field.Recently,novel topological domain structure has been observed in superlattice structure and multilayer structure.However,their application prospect has been limited due to the complex superlattice structure and their incompatibility with silicon substrate.Therefore,research related to ferroelectric phase transition and possible topological domain structure in PbTiO₃（PTO）single composition is of significance for theoretical research and device applications.This work has grown high quality PTO thin film and related freestanding structures by oxide molecular beam epitaxy（OMBE）.Then we designed heating stage which is compatible with X-ray diffraction（XRD）and atomic force microscopy（AFM）.Finally,we carried out systematical exploration on the dependence of PTO lattice structure and domain structure with temperature.Our work can be classified as follows:1)Design and fabrication of compatible heating stageIt is meaningful to characterize the dependence of ferroelectric domain structure with temperature in various methods.However,recent commercial heating stage equipped on various instruments are not compatible with each other and are difficult to do characterization on several scales.This work designed heating stage which is compatible with XRD and AFM and applied the heating stage to in-situ annealing experiment.This heating stage is flexible and can be easily attached.Also,it can realize quick and accurate control of temperature.Based on our design and fabrication,we realized in-situ observation of the complex domain structure in PTO.2)Thickness engineering of ferroelectric phase in PTOWe grow a set of high-quality PTO thin film with different thickness using Oxide-MBE.XRD results show that the c/a ratio of PTO thin film increases as the sample thickness increases due to the decrease of depolarization field.Then we apply our designed heating stage to XRD to do in-situ annealing experiment.From the annealing experiment we observed the relationship between T_C and PTO sample thickness.Under the condition of ultrathin film,T_Cdecreases due to the existence of depolarization field.Finally,we observed that extra phase with large out-of-plane lattice parameter can effectively increase T_C.3)Domain structure engineering in freestanding PTO thin filmWe grow Sr₃Al₂O₆（SAO）sacrificial layer on STO substrate in order to transfer freestanding PTO film with different thickness.Then do in-situ annealing experiment to characterize domain structure evolution during the heating and cooling process.The results show that PTO freestanding film is c-domain dominant before annealing.However,after the annealing process our freestanding PTO thin film has turned to a-domain dominant.Our results clarify that the c-domain in transferred samples are metastable state.Moreover,we apply miscut angle to break in-plane symmetry.After the annealing process,the sample also show a-domain dominant.In the range of our work,Tc is irrelevant with miscut angle and thickness and a-domain is energetically lower than c-domain in freestanding PTO thin film.This work provides new idea for research in manipulating domain structure in PTO film.