| Layered bismuth-based Aurivillius compounds with extensive physical and chem-ical properties, incolud electrical, magnetic, optical and catalytic etc., have been at-tracted broad attention. In recent years, it is found that higher perovskite-like layers of Aurivillius compounds (Bi2O2)2+(Bin-1Fen-3Ti3O3n+1)2-(n≥ 4) are possi-ble multiferroic materials that coexistence of ferroelectric and ferromagnetic at room temperature or above together, which has a great potential application value in the field of high density storage, sensor, spin electronic devices. Due to the ferroelectric prop-erties is the basic attributes of the Aurivillius compounds, research in the number of n^4 mainly concentrated in the magnetic and dielectric properties, but less attention to the ferroelectric properties. As a potential room temperature multiferroic materials, systematic investigations and optimization of the ferroelectric properties of the paren-t compound are very important before doping magnetic species at the Ti(or Fe) sites to explore single-phase multiferroics. In addition, the miniaturization, lightweight and integration are required with the development of microelectronic and optoelectronic devices, which need high performance of thin film.In this dissertation, the thin film with Bi6Fe2Ti3O18 (n= 5) were prepared by chemical solution deposition method and the ferroelectric properties were studied sys-tematically. The main research contents include:1) Bi6Fe2Ti3O18 film thickness effec-t; 2) Bi6Fe2Ti3O18 film annealing effect; 3) Bi6Fe2Ti3O18 film fatigue and retention properties; 4) Bi6Fe2Ti3O18 ultra thin film and its ferroelectric and magnetic properties. The main results of thoes experiments are as follows:Chapter 1:A brief introduction of the ferroelectrics and ferroelectricity, as well as the detail description of Aurivlillius bismuth-layered perovskite oxide materials and its research history. The current research situation of Bi6Fe2Ti3O18 and the possible problems are also emphasized. Preparing ferroelectric thin films by chemical solution deposition method and their main features are quick overview. Base on the previous investigation, the main topics of this thesis are issued.Chapter 2:We prepared Bi6Fe2Ti3O18 thin films on Pt/Ti/SiO2/Si (100) sub-strates with thickness ranging from 300 to 900 nm by using a chemical solution de-position route and investigated the impacts of film thickness on the microstructural, morphologies, dielectric, leakage and ferroelectric properties in this chapter. Increas-ing thickness improves the surface morphology, dielectric, and leakage properties of derived thin films and a well-defined ferroelectric hysteresis loops can form for the thin films with the thickness above 400 nm. Moreover, the thickness dependence of satu- ration polarization is insignificant, whereas the remnant polarization decreases slightly with increasing thickness and it possesses a maximal value of 20 μC/cm2 for the 500 nm-thick thin films.Chapter 3:We prepared Bi6Fe2Ti3018 thin films on Pt/Ti/SiO2/Si (100) sub-strates by chemical solution deposition using different annealing processes and found that a well-defined ferroelectric hysteresis loop with a rather large remnant polarization (Pr≈26 μC/cm2) occurs in Bi6Fe2Ti3O18 thin films prepared by rapid thermal anneal-ing in air. This value of remnant polarization is superior to those of other ferroelectric Aurivillius compounds such as Bi2WO6, Bi3TiNbO9, Bi4Ti3O12, Bi5FeTi3O15, and Bi6Fe2Ti3O18 thin films prepared by chemical solution deposition and pulsed laser de-position. The results of X-ray diffraction, high-resolution transmission electron mi-croscopy, X-ray photoelectron spectroscopy and ferroelectric-hysteresis loops re-vealed that the grain size, surface morphologies, oxygen vacancies and lattice distortion play very important roles in the determination of the remnant polarization. The present study provides an effective route to prepare layered Aurivillius thin films with a large remnant polarization by chemical solution deposition.Chapter 4:Ferroelectric Bi6Fe2Ti3O18 thin film was prepared on platinum coat-ed silicon substrate by chemical solution deposition and the ferroelectric and reten-tion properties were investigated. Larger remnant polarization (Pr) and excellent re-tention properties are observed in Bi6Fe2Ti3O18 thin film. The value of 2Pr in the Bi6Fe2Ti3O18 film is more than 50μC/cm2, which almost an order of magnitude higher than that of the bulks (～4μC/cm2). Moreover, the well-defined ferroelectric hysteresis loop can persist in the measurement frequency of 100 kHz indicate ultra-fast switching speed of such domain. The frequency dependence of coercive field obeys a power-law form Ec(f)-sfd/α, indicate that the domain switching kinetics is influenced by domain wall motion. The polarization of Bi6Fe2Ti3O18 thin film essentially remains constant at a waiting time up to 2 × 104 s and is independent on the applied electric field. The switchable polarization loss is 32% after 9 ×1010 read/write switching cycles at a fre-quency of 1 MHz. The change in the switchable polarization is discussed in terms of the aggregation of oxygen vacancies in the film. These results provide important contri-butions to understand the Aurivillius-type ferroelectrics with higher n and provides the basis factor for enhance multiferroic properties in single-phase Aurivillius-type multi-ferroics.Chapter 5:The achievement of Pb-free ferroelectric thin films with thickness down to sub-100 nm and low-voltage driving are very desirable in ferroelectric mem-ory applications. Here, we first report thickness dependent ferroelectric polarization in Pb-free Aurivillius Bi6Fe2Ti3O18 thin films on Pt/Ti/SiO2/Si substrates with thickness down to sub-100 nm and low-voltage driving by chemical solution deposition. It is ob-served that considerable remnant polarization (～15μ.C/cm2 in room temperature) at 5 V is obtained in Bi6Fe2Ti3O18 thin film with thickness of 85 nm. The polarization both after 107 switching cycles and retention time of 104 s show no obvious degrada-tion, suggesting good fatigue-resistant properties. The results suggest Bi6Fe2Ti3O18 thin films can be considered as a type of Pb-free low-voltage driving ferroelectric thin films using low-cost processing. The temperature dependence of measurements show that Bi6Fe2Ti3O18 curves exhibit a characterization of paramagnetic (PM)-like state. But the nonlinear hysteresis loops at low temperature is an evidence of the weak ferro-magnetic or antiferromagnetic interaction.Chapter 6:Deliver the summary of this thesis and an outlook for the related future work. |
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