Fabrication And Ferroelectric Properties Of Organic Ferroelectric P(VDF-TrFE) Copolymer Thin Films

Ferroelectric memories with high speed, high density, low cost and low operating voltage have been intensively studied. With the development of modern science and technology, the ever-increasing demands for nonvolatile memories such as ferroelectric field effect transistor (FeFET) memory, have led to the intensive search for a new kind of material. At present the materials used to prepare ferroelectric thin films for FeFET memory application are mainly PZT series, they have some favorable ferroelectric properties, such as large remnant polarization value and low processing temperature, and some serious drawbacks such as poor anti-fatigue performance and toxicity. A new kind of ferroelectric material needs to be found to replace PZT. Organic ferroelectric vinylidene fluoride-trifluoroethylene copolymer (P(VDF-TrFE)) thin film has some favorable properties such as low crystallization temperature, low leakage,and high fatigue endurance large remanent polarization, It may be used for FeFET memory appliction. In this master thesis, organic ferroelectric P(VDF-TrFE) thin film were deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by a spin-coating method; And their ferroelectric properties, leakage and dielectric properties were studied in detail.In chapter 1 of the master thesis, the general situation and potential application of ferroelectric thin film, especially for organic ferroelectric thin film, preparation methods and the research status for ferroelectric thin film are reviewed. On the basis of the review, the selectness of materials, aim and content of this thesis are proposed. In chapter 2, the materials and method of our experiment are introduced.In chapter 3 and chapter 4, the effect of the thickness of P(VDF-TrFE) thin film and the content of Diethyl phthalate (DEP) on P(VDF-TrFE) thin films on the microstructure, ferroelectrical and electrical properties of P(VDF-TrFE) films were studied. The results show that the ferroelectric properties increase with increasing the thickness of P(VDF-TrFE). The XRD results indicate that the degree of crystallinity is increase with increasing the thickness of P(VDF-TrFE).When the thickness of P(VDF-TrFE) film is under 100nm, the degree of crystallinity of P(VDF-TrFE) will decrease rapidly;and the ferroelectric properties also decrease fleetly. By adding DEP, other different types of molecules are introduced into the copolymer molecular structure. These inhomogeneous molecules disarrange and reduce the regularity of the copolymer chains. Hence, the degree of crystallinity is reduced. The experimental results show that the addition of diethyl phthalate significantly improves the polarization properties and reduces the leakage current density.The ferroelectrical and electrical properties of the DEP-doped P(VDF-TrFE) films are not a monotonous function of the content of DEP, but decrease as the content increases after an optimal the content of DEP. The results show that the optimal content of DEP is 15%.In chapter 5, Nd³⁺substituted bismuth titanate Bi_3.5Nd_0.5Ti₃O₁₂ (BNT) thin films and the P(VDF-TrFE)/BNT bilayer composites ferroelectric thin films were fabricated. The composites films were deposited on platinized silicon substrates using two successive spin coatings. The surface morphologies examined by scanning electron microscope show the pores in BNT films were effectively suppressed by the presence of P(VDF-TrFE) copolymer films. The ferroelectric, leakage and dielectric properties of bilayer composites with different thickness ratio of P(VDF-TrFE) and BNT films were measured. With increasing the thickness of P(VDF-TrFE), the remnant polarization, coercive electric field, leakage current density and dielectric constant of bilayer composites films all decreased. With increasing the thickness of P(VDF-TrFE), the remnant polarization of the hysteresis loops of the bi-layer composites ferroelectric thin films decrease because of the remnant polarization of P(VDF-TrFE) is lower that that of BNT. With increasing the thickness of P(VDF-TrFE), the mobile charges in bi-layer composites ferroelectric thin films is suppressed. So the leakage current density will decrease. For metal- P(VDF-TrFE)/BNT-metal capacitor device, high frequency capacitance C is given by the series combination. So with increasing the thickness of P(VDF-TrFE), the dielectric constant of the bi-layer composites ferroelectric thin films decrease as the dielectric constant of P(VDF-TrFE) film is lower that that of BNT film.