Strain-engineered BiFeO₃ Thin Films And The Growth Of BiFeO₃ Ultra-thin Films

Multiferroic materials are defined as materials which have two or more ferroic properties such as ferroelectric,ferromagnetic and ferroelastic,orderly coexist.As the interaction between the ordered parameter such as ferroelectric and ferromagnetic coupling as well as other physical characteristics,there are rich application prospects.BiFeO₃?BFO?is a kind of multiferroic material under room temperature,which has become a remarkable issue due to its excellent ferroelectric properties,ferroelectric and ferromagnetic coupling and photovoltaic effects.The domains and physical properties of BFO thin films are closely related to the strain state,orientation and thickness.In this work,we fabricated BFO thin films with opposite strain state and different crystal orientations,besides that,atomically flat BFO ultra-thin films and nano stripe structure were also deposited by Laser-MBE.After that,we studied the domain structure and relating mechanisms of BFO thin films.The main results of this thesis are listed below:1.BFO thin films with different strain states were fabricated by Laser-MBE technique and we investigated the effect of buffer lay and strain state on the structure and properties.Using SrRuO3?SRO?as bottom electrode,we choose two kind of perovskite material with different lattice constant,one is BaTiO3?BTO?while the other one SrTiO3?STO?,as buffer layer to deposited BFO / BTO / SRO / STO and BFO / STO / SRO / STO heteroepitaxy thin films.XRD measurements show that BFO epitaxy thin film suffer from tensile strain when deposited on BTO buffer layer while suffer from compressive strain when deposited on STO buffer layer.By adding buffer layers,we fabricated BFO thin films with tensile or compressive strain.The results show that the buffer layer can be used to adjust the ferroelectric domain structures,leakage current,remanent polarization and magnetic properties of BFO films and the strain state was the main factor affecting their structure and properties.2.The effect of crystal orientation on the domain structure and properties of BFO thin films was investigated in this work.BFO film were grown on?100?,?110?and?111?STO single crystal substrates.It was found that the orientation of the films had an important influence on the domain structure and ferroelectric polarization.The PFM results show that the?100?epitaxial BFO thin film have obvious multi-domain structure,and down toward domain increase obviously in?110?epitaxial BFO thin film,while the?111?epitaxial BFO thin film only had a single domain structure.At the same time,?111?epitaxial BFO thin film had the largest polarization and coercive field,which had the switchable diode retification effect in the virgin state.3.The growth of BFO ultra-thin films was studied.BFO ultra-thin films with finger like nano stripe structure and atomically flat surface were obtained,and we analyzed the growth mechanism and ferroelectric domain structure.Primary treated STO substrates with unit cell steps were used as substrates,10 nm thick SRO conducting electrode and 2 nm 12 nm BFO nominal thickness epitaxial thin film were deposited.By studying the morphologies and ferroelectric domain using PFM,we found periodic finger like nano stripe structure on 2 nm thick BFO ultra-thin film.The nano stripes disappeared with thickness increasing and when the thickness was 8 nm,the BFO film surface exhibit atomically flat.The periodicity of BFO finger like nano stripe structure was consistent with the unit cell step structure on STO single crystal substrate while the surface of SRO epitaxial layer had a step structure consistent with the STO substrate.Therefore,the formation of the BFO finger like nano stripe structure was closely related to the step structure of the single crystal substrate.In additional,all of the BFO ultra-thin films had good ferroelectric properties;with the thickness increasing,the original reverse stripe periodic domain structure of BFO ultra-thin film gradually evolved into a random structure.And the original polarization orientation was determined by the termination of SRO.