Preparation And Physical Property Investigation On Perovskite Superconductor/Ferroelectrics/Ferromagnet Films And Heterojunctions

In this thesis，a series of highly c-oriented perovskite oxide thin films andheterostructures which are composed by high-Tcsuperconductor, ferroelectric andcolossal magnetoresistance material were fabricated by pulsed laser deposition （PLD）method, and their properties of structure, surface, interface and transport wereinvestigated. The structure of thin films was characterized by X-ray diffraction （XRD）and single phase of the films were obtained. The interfaces of these heterostructureshave been detected by Transmission electron microscopy （TEM）. Atomic forcemicroscopy （AFM） and Scanning electron microscopy （SEM） were used to dectect thesurface morphology. AFM and SEM results showed that thin films with smoothlysurface were obtained. The flat surfaces are suitable for the preparation of multilayerheterostructures. The resistivities and carrier densities of LCCO and LSMO are all in104Ωcm and1021cm3orders, respectively. The IV curves and transport properties ofthese heterostructures were measured and studied by Physical Property MeasurementSystem （PPMS）. The main results are listed as follows.1. La_1.89Ce_0.11CuO₃/Ba_0.5Sr_0.5TiO₃/La_0.67Sr_0.33MnO₃heterostructureP-type CMR La_0.67Sr_0.33MnO₃（LSMO）, insulator Ba_0.5Sr_0.5TiO₃（BST）, andn-type high-TcLa_1.89Ce_0.11CuO₃（LCCO） thin films were deposited on SrTiO₃substrates by using PLD method. Several kinds of perovskite heterostructures havebeen studied, such as LCCO/LSMO, LCCO/BST, BST/LSMO, and LCCO/BST/LSMO.The structure and surface properties were investigated carefully by XRD, TEM, SEMand AFM. The I-V characteristics of these junctions were also investigated. The majorresults are in the following:（1） LCCO/LSMO heterostructure has no rectification,because both of them have the high carrier densities.（2） The I-V curves ofLCCO/BST, BST/LSMO heterostructures have weak asymmetry under different bias.The BST layer plays a key role in the rectification effect.（3） The significantrectification effect was obtained in LCCO/BST/LSMO p-i-n heterostructure. Therectification effect was explained by diffusion theory, energy band theory anddepletion layer theory. The rectification can be attributed to the depletion layer andcarrier density.2. La_1.89Ce_0.11CuO₃/SrTiO₃/La_0.67Sr_0.33MnO₃heterostructureIn order to clarity the role of ferroelectric property in BST layer,La_1.89Ce_0.11CuO₃/SrTiO₃/La_0.67Sr_0.33MnO₃trilayer heterostructures were prepared with different SrTiO₃thicknesses from15nm to30nm. The I-V curves of theseheterostructures showed asymmetric at room temperature. The best rectification effectwas obtained when the thickness equal to20nm. The best thickness value calculatedby diffusion theory is21nm, which is consistent with the experimental result. Besides,the experimental results were also analyzed by the energy band theory, and itindicated that the ferroelectric property of BST is not necessary for the rectification.3. La1-xSrxMnO₃/Nb-SrTiO₃heterostructureThe La1-xSrxMnO₃（x=0.1-0.6） thin films were deposited on0.7wt%Nb dopedSrTiO₃substrates by PLD method. A series of heterostructures were fabricated bymicrofabrication techniques. The results showed that the threshold voltage wasincreased with the decrease of the temperature. The leakage current at interface candamage the rectification. The I-V characteristics under ac voltage source weremeasured at room temperature and low temperature. The I-V curves of theseheterostructures had strong asymmetry when the voltage source at low frequency, butthe rectification would be destroyed when the frequency larger than1KHz. Thecapacitance of the heterostucture was analyzed by using equivalent circuit.