Study of ferroelectric switching and fatigue behaviors in poly(vinylidene fluoride-trifluoroethylene) copolymer nano-films

The nano-films of poly (vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer, with mole ratio of VDFTrFE 70/30, are deposited on titanium-metallized silicon wafer by spin coating technique. Annealing temperature and humidity dependence of polarization switching and fatigue babivors in ferroelectric P(VDF-TrFE) copolymer thin film capacitors have been investigated. Firstly, the effect of different annealing temperature on polarization behavior is revealed. It is found that the polarization of the film is improved by increasing annealing temperatures. When the annealing temperature is higher than 100℃, with increasing switching cycles, the ferroelectric polarization characteristics exhibit a trend of increasing firstly and then decreasing, a top value appears at the number of cycles near 104. A more appropriate heat treatment temperature is 130℃. Further analyses on the crystalline structures with X-ray diffraction show that the degree of crystallinity of the films is strongly dependent on the annealing temperature. It can be seen that the diffraction peak of the ferroelectric phase ( phase) becomes very strong and sharp with increasing annealing temperatre. It is demonstrated that the effect of annealing temperature on ferroelectric properties could be explained by the changes of the degree of crystallinity in these films from the results of X-ray and the polarization behaviors. Meanwhile, the microstructure of the 140 nm film annealed at 130℃ is obtained by using scanning electron microscope, which shows that the film exhibits a worm-like, dense, well-crystallized microstructure. Secondly, for the capacitor P(VDF-TrFE) films with a thickness of 140 nm, the ferroelectric polarization hysteresis loops as functions of electric field for the films at different relative humidities are achieved. It is obvious that the polarization properties depend on the relative humidity during the film preparation process, the polarizaiton fatigue can be further enhanced through a higher relative humidity during the sample preparation. In addition, one of the most important features for ferroelectric material to be used as an alternative FeRAM is the low leakage current density. Therefore, the descriptions of the leakage current density versus different relative humidities are given. It is observed that the voltage behavior of the leakage current has a minor dependence on relative humidity. In a word, these results illustrate that the polarization properties are strongly dependent not only on the annealing temperature, but also the relative humidity in a process for the preparation of the nano-films. Furthermore, according to a re-annealing treatment to improve the crystalline degree of the ferroelectric phase, the influence of the re-annealing process on the fatigue properties of the films is also studied. The polarization fatigue can be improved obviously by a re-annealing process, and the possible origins have been discussed. To further understand the variation of crystallization properties of the samples before and after re-annealing, the crystallinity of the film are studied by the technique of Fourier transform infrared spectroscopy. It is indicated that the crystallinity of the films can partly be recovered through re-annealing treatment. These results are very helpful and provide an available way to improve the ferroelectric polarization and fatigue properties of the ferroelectric nano-films.