Investigation Of Multiferroic BiFeO₃ And Derived Thin Films

BiFeO3 and BiFe0.95Mn0.05O3 thin film have been successfully prepared by sol-gel method, and the related ferroelectric capacitors with Pt electrodes are fabricated using magnetron sputtering method with a shadow mask. The microstructure and morphology of the thin films as well as the ferroelectric and magnetic properties of the capacitors have been investigated using various techniques, such as x-ray diffraction (XRD), atomic force microscopy (AFM), ferroelectric tester (Precision LC Unit) and vibrating sample magnetometer (VSM). The main results are as follows:1. Pt/BiFeO3/Pt ferroelectric capacitors have been fabricated on Pt/Ti/SiO2/Si substrates via a sol-gel method. The x-ray diffraction (XRD) measurement indicates that BiFeO3 film is well-crystalized. The atomic force microscopy (AFM) image indicates BiFeO3 film is dense with uniform grains. Hysteresis loop of Pt/BiFeO3/Pt capacitor, measured at 5 V, is very symmetric. Relation of dielectric constant with applied voltage shows a "butterfly" pattern. Leakage current density of Pt/BiFeO3/Pt capacitor measured at 5 V is less than 10-4 A/cm2, agreeing well with the theory of space-charge-limited current.2. BiFeo.95Mn0.05O3 powders under different annealing temperature have been sintered and the thermo-gravimetric-differential thermal analyzer (TG-DTA) analysis indicates the optimized pyrolyzing temperature is 300℃. Effects of annealing time on the electrical properties of Pt/BiFe0.95Mn0.05O3/Pt capacitors have been studied, it is found that the optimized annealing time for 550℃anneal is 20 min.3. Pt/BiFeo.95Mno.o503/Pt capacitor was well fabricated by sol-gel process. The x-ray diffraction measurement indicates that BiFe0.95Mn0.05O3 film is polycrystalline. Pt/BiFeo.95Mno.o503/Pt capacitor with good retention characteristic shows a saturated hysteresis loop and a large remnant polarization about 52μC/cm2 at 1 MV/cm, the leakage current density is 3.6x 10-4 A/cm2.4. Multilayered metal-insulator-metal structures with BiFe0.95Mn0.05O3 and Ba0.6Sr0.4TiO3 thin films were fabricated to reduce the leakage current through the capacitor stack. Compared to BiFe0.95Mn0.05O3 in the heterostructures, the leakage current density was significantly reduced by two orders of magnitude and the BST/BFMO/BST capacitor has good retention property.