Polarization Switching And Imprint Behaviors Of P(VDF-TrFE) Ferroelectric Ultrathin Films

Compared with the inorganic materials, poly(vinylidene fluoride)(PVDF) and its copolymers, poly(vinylidene fluoride-trifluoroethylene)(P(VDF-Tr FE)), exhibit a series of unique advantages, including high flexibility, excellent polarization stability, low costs, outstanding chemical stability and ease fabrication process. The polarization switching can occur at low voltage, especially when the size of ferroelectric copolymer materials is reduced to below 100 nm. These properties are indispensable for practical application in micro-sensors, ferroelectric random access memory, electronics, and photonic. Therefore, it is considered as one of the most promising organic ferroelectric materials. However, as the thickness of the film is reduced to below 100 nm, a large drop of the polarization level and the imprint effect are observed, after more than 10 million cycles switching. The polarization fatigue and imprint effect would lead to failure of digital information stored in the devices. Therefore, it is essential to improve the polarization property and imprint effect of P(VDF-TrFE) ferroelectric ultrathin films.In this thesis, the P(VDF-TrFE) copolymer ultrathin films are deposited on titanium-metallized silicon wafer by spin coating technique. The influence of humidity, temperature, re-annealing process, and thickness on polarization switching and imprint effect in ferroelectric P(VDF-Tr FE) copolymer ultrathin film capacitors have been investigated. In addition, we focus on the imprint and switching behaviors at high temperature for P(VDF-TrFE) ultrathin films with different electroactive interlayers. The main contents and results of this thesis are as follows:(1) The effects of different annealing temperature and relative environment humidities on polarization behavior/fatigue property are achieved. Then the microstructure of films is revealed using X-ray diffraction(XRD) and scanning electron microscope(SEM) analysis. It is obvious that the degree of crystallinity in these films is affected significantly by the annealing temperature. The crystallinity in the copolymer ultrathin films can be improved due to the increased of annealing temperature. A more appropriate heat treatment temperature has been found. The results also indicate that the polarization properties depend on the relative humidity during the film preparation process, and the polarization fatigue can be further enhanced with increased relative environment humidities.(2) The correlation between temperature/annealing process and the imprint of P(VDF-TrFE) copolymer ultrathin film capacitors have been discussed. Furthermore, the influence of the re-annealing process on the fatigue properties and ―inherent‖ imprint of the films is also studied. Meanwhile, the crystallinity of the samples before and after re-annealing are measured by the technique of Fourier transform infrared spectroscopy(FTIR). It is observed that the temperature-induced shift of the imprint rate is limited. The polarization properties and imprint behaviors can be improved obviously by a re-annealing process, and the possible origins have been discussed. The analyses also show that the imprint as well as the disturbance time are strongly responsible for the polarization switching properties of P(VDF-TrFE) ultrathin films.(3) The thickness dependence of ferroelectric switching behavior of the films with PEDOT-PSSH as electroactive interlayer is analyzed. It is found that the polarization properties, i.e., coercive field and leakage current are increase gradually as the film thickness is reduced. Moreover, the polarization fatigue can be improved by the interlayer even at high temperature for a given film thickness.(4) The imprint and switching behaviors at high temperature for P(VDF-TrFE) ultrathin films with different electroactive interlayers have been systematically investigated. It is suggested that the lower imprint rate and the faster switching speed can be achieved through the introduction of appropriate interlayer materials. Combined with the results for the imprint and switching behaviors at room temperature, the temperature-dependent imprint and switching mechanisms for different electroactive interlayers in this system are proposed.