Domain Structure And Domain Switching Of The Bi_3.15 Nd_0.85 Ti₃ O₁₂ Ferroelectric Thin Film

Ferroelectric thin films have attracted a great deal of attention for applications in non-volatile ferroelectric random access memories (FeRAMs). Bi3.15Nd0.85Ti3O12 (BNT) thin film is one of the most promising ferroelectric for FeRAM because of its excellent fatigue-free property, low processing temperature, high Curie temperature, large remnant polarization and lead-free chemical composition. The domain is the physics foundation of the properties and applications for ferroelectric films. Therefore, it is necessary for us to investigate the domain of BNT ferroelectric film. In this thesis, BNT thin films were fabricated by different preparation methods and techniques firstly, and then their domain structures and domain switching were observed. In addition, the effects of annealing temperature and annealing atmosphere on domain structure and domain switching of BNT film were also studied. The main research contents and obtained results in this thesis are summarized as follows:1. Preparation, domain structure and domain switching of BNT thin filmsBNT thin films were prepared by sol-gel and pulsed laser deposition (PLD) methods, respectively. X-ray diffraction (XRD), atomic force microscopy (AFM), ferroelectric tester and piezoresponse force microscopy (PFM) were used to study the microstructures, ferroelectric properties, domain structures and domain switchings of the prepared films. The results show that the BNT film prepared by sol-gel presents strong (117) peak while the BNT film prepared by PLD presents strong (00l) peaks. The clear domain structures and domain switching are observed in all the obtained BNT films. The domain structure of BNT film prepared by PLD is clearer than that of BNT film prepared by sol-gel. Through PFM study on the PLD-prepared film, it is found that the domain switching of BNT film is restricted by both grain boundary and internal field.2. The effect of annealing temperature on domain structure and domain switching of BNT thin filmsBNT thin films were prepared at different annealing temperatures by sol-gel, and the effect of annealing temperature on domain structure and domain switching of BNT film was investigated by PFM. The results show that with the increase of annealing temperature, the density of domain walls decreases, the piezoresponse signal changes more uniform, the phase contrast becomes stronger, and the switching voltage changes smaller. By analysis the results of XRD and AFM, those phenomena can be attributed suggested that the crystallinity of BNT film becomes higher with the increase of annealing temperature, and thus leads to the improvement of properties.3. The effect of annealing atmosphere on domain structure and domain switching of BNT thin filmsBNT thin films were prepared at different annealing atmospheres by sol-gel, and the effect of annealing atmosphere on domain structure and domain switching of BNT film was studied by PFM. The XRD and AFM results show that the crystallinity, grain size and remanent polarization increase with the increment of oxygen content in annealing atmosphere. The PFM results show that the piezoresponse signal changes stronger and more uniform, and the domain switching becomes more complete with the increment of oxygen content in annealing atmosphere. Results suggest that oxygen vacancy concentration plays a significant role in the grain size and domain switching, and low oxygen vacancy concentration is propitious to domain switching of BNT film.