Study On The Interface Structrure And Interface Control Of Typical Ferromagnetic And Ferroelectric Oxide Thin Films

Owing to their versatile ferroelectric and magnetic function and wide application,the oxidethin films with peroviskite and spinel structure have attracted much attention,and became thekey point in the electronic material studies recently.In the growth and application of thecomplex oxide thin films,it is inevitable that the surface and interface affect the electronicsystems.Due to the limit of the theoretical and experimental condition,the related theory andmechanism of the interface is still lacking including the growth mechanism and dynamics ofinterface and the control of the interface microstructure.Especially when the thickness of thefilms reaches the nanometer level,the unclear problems has become noticeable,which hasstrongly prevented the development of these functional oxide films.In this dissertation,basedon the epitaxial growth mechanism and dynamic of oxide thin film and the method of theinterface control,the interface microstructure and its effect on the physical properties offerroelectric and ferromagnetic films has been systematically studied by experiments with theLaser Molecular Beam Epitaxy techniques (LMBE),including the interface strain modulation,growth orientation and their effects on the electric and magnetic properties.Firstly,the growth behavior and interface microstrucutre of CFO thin film grown byLMBE with an in-situ reflective high energy electron diffraction (RHEED) were systematicallystudied.It was found that the layer-by-layer,island and Strnaski-Krastanov (layer-by-layer plusisland) growth modes were determined by the growth temperature and deposition rate.Therefore,the growth modes of CFO could be precisely controlled by varying the depositionparameters and hence the growth mode map of the CFO/STO was obtained as a function of thesubstrate temperature for various laser repetition rates.On the other hand,the process ofinterface relaxation was analysised according to the RHEED oscillation and the activity energyfor the particle migration both at surface and interface of the CFO films were calculated to be0.6 eV and 1.6 eV by the Arrhenius theory,respectively.It proves that the particles at interfaceneed more activity energy for migration than these at surface due to the high lattice distortionbarrier (Es) induced by the lattice mismatch.In order to reveal the difference of interfacestructure between the films with island or layer-by-layer growth mode,the TEM technique wasemployed to investigate the cross-sectional characteristic of the CFO films.It was found that theCFO epitaxially grown on STO substrate at the temperature of 650℃and deposition rate of1.25(?)/sec exhibited island growth mode.The misfit dislocations with Burgers vector a<010> occured when the CFO thickness exceeded the critical thickness about 5nm.The misfit densityof the dislocation could be measured to be 1.25×10⁸m^-1,which was less than the value of2×10⁸m^-1 by theoretical calculation.Actually,part of lattice mismatch stress was released bylattice strain;while the CFO grown on STO substrate at the same temperature and depositionrate of 0.5(?)/sec exhibited layer-by-layer growth mode.It was noted that any misfit dislocationcould be observed even in the CFO film with thickness beyond 15nm,which suggested that thelattice mismatch stress in CFO film was released by lattice strain.Therefore,the interfacemicrostructure of CFO film was mainly determined by the lattice strain and dislocation energy.Based on the study of the interface microstructure,we extensively studied the influence ofthe interface-stress modulation on the microstructure and physical properties of theseheteroepitaxial CFO thin films.First,the stress relaxation and its effects on the magnetism ofthe CFO were systematically studied in the CFO thin films epitaxially grown under the tensileand compressive stress induced by substrates.An asymmetrical growth behavior due to interfacestress was found during the heteroepitaxial growth of the CFO,which could be described thatthe tensile stress was relaxed by changing the lattice constant,while the compressive stress wasrelaxed by changing the interface structure.By stress modulation,a strong magnetic anisotropywas obtained in the compressive strained CFO heteroepitaxial film.The remanent magnetism(Mr) in the vertical ([001]) and parallel ([100]/[010]) direction of compressive strained CFOfilm was measured to be 190 emu/cm³ and 150 emu/cm³,respectively.Besides,the coercivemagnetic field (Hc) in the vertical direction was three times that in parallel direction.Furthermore,we systematically studied the influence of stress modulation on the microstructureand the physical properties including ferroelectric and magnetic properties in heteroeptaxiallygrown BaTiO₃(BTO)/CFO/Nb-STO films.The lattice parameters were precisely obtained byX-ray Reciprocal Space Mapping and the residual stress was calculated using the latticeparameter method.A residual stress theoretical model about the relaxation mechanisms of stressin complex oxide heteroepitaxial films with ordered structure was put forward for bettercomprehension of the dependence between thickness and stress.Furthermore,the electricalmeasurement results show that the ferroelectric properties were enhanced with the relaxation ofthe tensile stress in BTO/CFO/STO heteroepitaxial system.Secondly,based on the principle of interface thermodynamics,the oriented growthbehavior of the Pb(Zr_1-xTi_x)O₃ (PZT) films epitaxially grown by pulsed laser depositiontechnique were systematically studied.According to bicrystalline epitaxial growth mechanism atinterface,the (110)-oriented PZT ferroelectric films were epitaxially grown on MgO(100) substrates.On the other hand,the hetero-nuclear PZT polycrystalline film with (111)-texturecould be obtained by precisely controlling the heating rate.Therefore,the textured growth mapof the polycrystalline PZT was obtained as a function of the substrate temperature for vaviousheating rates.The electrical properties of the texture PZT films were also measured.Finally,the c-oriented BTO ferroelectric thin films were successfully epitaxially grown onGaAs substrate by using LMBE technique via STO as buffer layer.The interface control methodwas extensively studied in the large lattice mismatch system.According to the NearlyCoincidence Site Lattice (NSCL) theory,the lattice mismatch was reduced by the 45°rotation ofthe STO lattice.The interface was investigated by TEM technique.It was found that theinterface with high density of misfit dislocation in noncoincidence-site growth caused thedegradation of the electrical properties.By inserting a strained interfacial buffer layer of STObelow the critical thickness,a well ordered interface at atomic scale could be obtained in thestrained BTO/STO/GaAs heterostructure.The favor thickness of STO buffer should becontrolled between 12-16 monolayers.The BTO was grown on STO buffer in layer-by-layergrowth mode with an in-plane compressive strain induced by the STO.Compared with thesimilar heterostructure grown on GaAs,The strained Pt/BTO/STO/GaAs heterostructre exhibitsexcellent electrical performance with a 2Pr of 6.5μC/cm² and leakage current below4×10^-6A/cm² at the voltage bias of±5V,which is suitable for device application.