Electromagnetic Properties Of BiFeO₃/ZnO Composite Films By Chemical Solution Deposition

The heterostructures of ferroelectric/semiconductor which show the switchingof resistivity can be made into ferroelectric random access memories(FRAM). Theferroelectrics made in FRAM must have high Curie temperature and residualpolarization. BiFeO₃(BFO) is a material wihich coexists ferroelectricity andferromagnetism at room temperature. There are few kinds of magnetic ferroelectrics.Its crystal is contorted perovskite, whose space group is R3c, Curie temperature is1103K and residual polarization is 90μC/cm², which is close to the value of PZTthin films. Hence BFO which has no lead can be used into FRAM. However, Fe3+changes to Fe²⁺, result in the change of the concentration of oxygen vacancy. Hence,the BFO thin films always have large leakage current, which confine the applicationof BFO thin films. In this paper, the electromagnetic characteristic of theheterojunction of ZnO/BFO and ZnO/ZNA/BFO via sol-gel method has beeninvestigated.First, pure BFO thin film is produced on n-Si substrate by sol-gel method,whose raw materials are ferric nitrate and bismuth nitrate. The I-V curves andhysteresis loop are obtained. It is found the switching of resistivity and its residualpolarization is 0.79μC/cm²when the electric field strength is 16.67kV/cm.Then ZnO and ZnO nanorod array(ZNA) is prepared by sol-gel andhydrothermal method, respectively. The raw materials are zinc nitrate,diethanolamine, methanol. Then ZNA which are single crystal preferred in thedirection of C-axis is grown uprightly on the surface of ZnO films via hydrothermalmethod in ZnNO₃/KOH aqueous solution. AFM (Atomic Force Microscope) is usedto research the modulus of elasticity of ZNA.At last, the BFO thin film is manufactured on the ZNA through the method ofsol-gel. By contrast, heterojunction of ZnO/BFO and BFO/ZnO is researched. When electric field strength is 10kV/cm, the residual polarization is 80.9μC/cm2,which is extreme saturated. Besides, since there are the different coefficients ofthermal expansion between the substrates and wet film, there is residual stress in thecomposite films, hence Zinc Oxide has residual polarization which result in theresidual polarization of BFO thin film when the external electric field is zero.Magnetic testing found that composite films when the temperature is 5K show veryweak magnetic while at room temperature the magnetic hystereses loop is linear.