Preparation And Properties Of Ferroelectric （Dielectric）/YBA₂CU₃O_7-x Heterostructures

Due to the unique electrical properties of YBa₂Cu₃O_7-x（YBCO）superconductor and its similar crystal structure and lattice constant to most of the ferroelectric and dielectric materials with perovskite structure,novel physical phenomenas will maybe occur when preparing a superconducting heterostructure combining YBCO and them.Further study of these physical phenomenas will not only be helpful to the deep understanding of the high-temperature superconducting mechanism,but may also lead to potential development of new superconducting electronic devices.To realize functional applications,YBCO films and its heterostructures need usually to be processed into specialized fine-patterns.However,limited by intrinsic characteristics of YBCO,the present fine-patterns processing methods can easily cause the degradation of the superconductivity and are difficult to prepare YBCO fine-patterns with a large area.Aiming at the above problems,the main work of this paper is as follows.A sol-gel process has been developed to prepare YBCO films with good superconducting properties.YBCO photosensitive sol was synthesized by adding a chemical modifier,benzoylacetone（BzAcH）,into YBCO sol;the preparation technology of YBCO fine-patterns with larger area using this photosensitive sol was studied（photosensitive sol-gel process）.For the first time,YBCO superconducting delay line with good superconducting and delay properties was fabricated by this method.The fine-pattern process of LaNiO₃（LNO）film electrodes was further investigated and LNO electric pole arrays with 200 μm diameter were deposited on the SrTiO₃/LaNiO₃（STO/LNO）structure to form a LNO/STO/LNO tri-layer film with symmetric structure displaying good dielectric tunability.Pb(Zr_0.52Ti_0.48)O3/YBa₂Cu₃O_7-x（PZT/YBCO）heterostructure with interfacial coherent structure characteristics was prepared and the ferroelectric hysteresis loops of the Pt/PZT/YBCO structure was measured within the temperature range between 50 and 300 K.It was found that the remnant polarization of PZT remained almost constant as the temperature decrease,while the coercive field increased obviously.By the photosensitive sol-gel fine-pattern process,a PZT/YBCO patterned structure was fabricated and the influence of PZT polarization on the resistance-temperature（R-T）curve of YBCO film was studied with this structure.Results indicated that the R-T curve of YBCO shifted upwards when the PZT was positively polarized（YBCO was of higher potential）,while the opposite occurred for negative polarization（YBCO was of lower potential）.It was further found that the magnetization intensity of YBCO decreased as the remnant polarization of PZT increased.The study of the current transport properties of Pt/PZT/YBCO structure indicate that a trun-on voltage（Vt）exists in the current density-voltage（J-V）curve.When the applied voltage was lower than the Vt,the leakage current of PZT was relatively low and the current transport was mainly determined by its own thermally activated carriers inside the materials.When the applied voltage was higher than the Vt,the leakage current increased rapidly and the current transport was in accordance with the Schottky emission mechanism.Pt/STO/YBCO heterostructure was grown on LaAlO₃ substrate and its dielectric properties were studied.With the decrease of the temperature and test frequency,the relative dielectric constant of the structure increased while the dielectric loss decreased.The investigation of the J-V characteristic curve of the Pt/STO/YBCO heterostructure showed good rectifying properties within 50 and 300 K.Furthermore,for the positive bias voltage smaller than 3.7 V,the leakage current of the STO/YBCO heterostructure decreased with the decrease of the temperature;when the positive bias voltage was larger than 3.7 V,the leakage current increased with the decrease of the temperature.This is caused by the change in current transport characteristics of Pt/STO/YBCO before and after reaching 3.7 V.The current transport characteristics showed a Fowler-Nordheim tunneling mechanism（FN）when the voltage was lower than 3.7 V（2³.7 V）,while it turned to Schottky emission mechanism（SE）as the voltage was higher than 3.7V.The fact that the current density decreased with the decrease of temperature at negative voltage was mainly ascribed to Ohmic conduction mechanism（0³.5 V）and space charge limited current mechanism（SCLC,3.5⁸ V）.The deviation of the turn-on voltage（Vt）change with temperature from the original law was also observed along with the superconducting transition of YBCO,and the deviation value of Vt before and after superconducting transition could well characterize the size of YBCO superconducting energy gap.Ba_0.09Sr_0.91TiO₃/YBa₂Cu₃O_7-x（BST/YBCO）heterostructure with a high c-axis orientation was prepared via a sol-gel process,the BST film prepared on YBCO as the bottom electrode demonstrated an improved dielectric tunability compared to the BST film on LNO bottom electrode,while possessing a higher dielectric loss（tanδ）than the BST film on LNO bottom electrode.As the temperature was decreased,the dielectric tunability of the Pt/BST/YBCO structure increased and the tanδ decreased.Under the testing conditions of 100 kHz and 83 K,the tunability and the tanδ of Pt/BST/YBCO resulted about 58%and 0.029,respectively.By the photosensitive sol-gel process,YBCO electric pole arrays were fabricated on Nb:SrTiO₃（Nb:STO）substrate.The Nb:STO/YBCO heterostructure was formed,exhibiting good electrical resistance switching characteristics（RS）.Ultraviolet irradiation experiment was conducted on the above-mentioned heterostructure,indicating that Nb:STO/YBCO showed a good photovoltaic effect at both room and low temperature.The open-circuit voltage was about 0.82 V at 300 K and it increased with the decrease of the temperature to reach about 1.2 V at 80 K.