| A ferroelectric material exhibits reversible spontaneous polarization in which direction can be switched by an applied electrical field. The switchable polarization in two "up" and "down" states has achieved commercial application in FeRAMs. In this thesis, the ferroelectric thin films of bismuth ferrite and Poly(vinylidene fluoride-trifluoroethylene) were fabricated. The former has a large prospect in the application of high-density memory, and the latter can be used in low-cost RFID chips. To address their obstacle driven by rapidly growing application, we emphasized on the enhancement of retention time of bismuth ferrite thin films with preferred domain orientations and the improvement of writing/reading performance of P(VDF-TrFE) copolymer thin films.I. The oxygen vacancy-defected bismuth ferrite films deposited by PLD were systematically characterized by atomic force microscopy and pulsed voltage measurement. We can justify the domain switching current from the leakage of the films. The main results are summarized as following.Firstly, Atomic force microscopy micrograph of bismuth ferrite films with Fe excess deposited on bottom oxide electrodes reveals the flat film surface of square-like grains. Piezoresponse microscope images illuminated the domain polarization relaxation properties of different components of thin film, in consistence with the macroscopic electrical measurements.Secondly, we developed a technique to transfer ferroelectric domain switching currents under the pulses into polarization-voltage (P-V) hysteresis loops. With this transformation, it is doable to derive the remanent polarization and coercive voltage from domain switching current after the shortest imprint and retention time of35ns. After an identification of domain switching component from film leakage current, we measured the P-V hysteresis loop in a semiconducting BiFeO3leaky thin film, where the apparent coercive field highly reaches320kV/cm, implying a domain switching mechanism different from other insulators.Thirdly, in order to assess the leakage current effects on the electrical performance of the films and to improve the retention property of highly strained films, three Fe-enriched films were fabricated. Charge injection proved to be an effective way to enhance the retention time, which paves a way to expand the application for ultra-thin films in ferroelectric devices. The results present that the retention worsens when the film thickness decreases due to a strong depolarization field. The field weakens when the Fe content increases. Under a negative field stressing, the polarization was enhanced approaching up to a theoretical value via charge injection, which supplies an effective way to symmetrize the P-E loop of a highly strained ferroelectric thin film.Ⅱ. In order to improve the writing/reading performance of the devices, the P (VDF-TrFE) films with various thicknesses prepared by spin coating were modeled using the equivalent in-series capacitance to analyze the electrical domain switching current. The main results are presented.Firstly, the results display the high coercive voltage mainly due to the interfacial layer and non-ferroelectric phase in the P(VDF-TrFE) films. Through an electrical equivalent in-series capacitance circuit, the intrinsic capacitance and coercive voltage can be derived from either domain switching current transient or voltage dependence of the switched polarization. Interestingly, the non-ferroelectric capacitance reduces with the enhancement of domain switching speed while the continuous reduction of the remanent polarization, which suggests the thickening of the preceding capacitive layers with enhanced domain switching speed.Secondly, on a basis of the equivalent-circuit description of the films and parallel plate capacitor model, we extract the intrinsic coercive field across the ferroelectric layer in the films with unknown thickness of the ferroelectric layer. The derived intrinsic coercive field of different thickness P(VDF-TrFE) thin-films consists with the theoretical prediction reported previously.The equivalent in-series capacitance model offers an effective way to quantify the non-ferroelectric capacitor of interfacial layers and its effects on the ferroelectric properties of both organic and inorganic ferroelectric thin-films. |
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