Fabrication And Surface Nanoscale Electrical Properties Of Perovskite Ferroelectric Thin Films

High-density memories based on modifying the ferroelectric domains are considered as promising memories in the future because of their nonvolatile, anti-irradiation and anti-interference features. The ferroelectric storage based on scanning probe force microscope (SPM) technique has been widely investigated due to its near to molecular level data density and the subnanosecond domain switching speed, and is considered as a promising model for the next ultra-high density memories. In ferroelectric memories, the logic value of a bit is represented by the polarization states of ferroelectric domains. So, the domain patterns and the polarization reversal properties play an important role in the performance of ferroelectric memories. Up to now, the domain polarization reversal has been deeply investigated based on SPM and the most researches mainly focus on single-crystal films and the epitaxial growth (or preferred orientation) thin films. Polycrystal ferroelectric thin films prossess more interesting characteristics of polarization reversal due to their complex crystallite orientations and grain boundaries. During the domain polarization reversal induced by the tip, the poling bias can also lead to the change of other surface nanoscale electrical properties of ferroelectric thin films which, in return, may disturb the domain polarization. In this paper, we focus on the surface nanoscale electrical properties of perovskite ferroelectric polycrystal thin film during the domain polarization reversal induced by the tip.In the chapter 1, we briefly describe the basic principle of the ferroelectric memories and review the present states. We find out the problems in ferroelectric memories. Therefore we get the research objectives and the main research contents in our future works.In the chapter 2, the LaNiO₃ thin films were synthesized by a simple chemical solution coating process, which were used as the bottom electrode in our paper. The crystal structure and the surface image were investigated by XRD and DFM.During poling a high-density array by the tip, the corresponding injected-charge array can be created which, in return, must affect the domain polarization. In the chapters 3 and 4, we investigate the surface charging characteristics of BaTiO₃ thin film by Kelvin probe force microscopy (KFM) after poled by DC. After poled by small DC, the polarization charges are observed on the surface. With the increase of poling bias, the free charges are gradually injected and shield the polarization charges. The in-built electric field between the bottom electrode LaNiO₃ and BaTiO₃ leads to the injected charge asymmetry under the opposite poling bias. The injected charge dependence on poling scanning speed demonstrates that the injected charges will increase rapidly and the charging area will become larger with decreasing the scanning speed. With the elapse of time, the injected charges will decrease rapidly, but the trend became weaker. The injected charges can be neutralized partly by the grounded tip and erased completely by the opposite poling bias. During the KFM imaging, the electric field between the film and the tip induced by the injected charges can make the injected charges drift or diffuse with the tip moving.In the chapter 4, a nine-dot array was constructed by SPM. Anomalous charges are observed in the moving paths of the SPM tip. The results indicate that the anomalous charges are dependent on the moving paths of the tip at the constant injected charges, and if the path of the tip is fixed, these anomalous charges are related to the injected charges. They exhibit path-related behavior. We suggest that these path-related anomalous charges are induced by the collective effects of the injection charges in the array. The injected charges in these dots produce an anti-parallel electric field between the film and the tip during the tip moving to next dot, which leads to the opposite polarity charges injected. The path-related behavior is attributed to the different potential distributions of these injected charges in different moving paths. The calculation of potential distributions in the moving paths confirmed our suggestion. The collective effects of the injected charges not only cause the opposite polarity charges injected, but also can lead to the anti-parallel domain switching. Obviously, the collective effects of injection charges must be taken into account during the scan-writing process in this kind of high-density memory. Moreover, the finding of path-related anomalous injection charges will help us to minimize or avoid the collective effects, increase the memory density by designing the best paths of the tip.In the chapter 5, a series of PZT grain thin films with different Zr/Ti were synthesized by a Sol-gel coating process. The results show that all PZT films are the tetragonal polycrystal structure with no evidence of preferential orientation. The size of grains became smaller and the corresponding domain-structure also changes gradually from predominantly multidomain to predominantly single-domain with the increase of Zr element. The PFM results about the PbTiO₃ thin film with complex multidomain show that the increase of PFM AC voltage not only can enhance the resolution capability, but also cause the domain switching. The switching indicates that the PbTiO₃ thin films hold theμs or below domain switching speed, and exhibit the forward domain growth mechanism and a preferential down domain polarization state.An anti-parallel poling reversal phenomenon was observed in certain grains with complex multidomain pattern in polycrystalline tetragonal PbTiO₃ film during poling by PFM. The observed effect is an intrinsic characteristic of grains with complex multidomain pattern, thus, is a more common poling reversal phenomenon than previous reported by external factor. We suggest that the observed anti-parallel domain switching is principally caused by the transient internal stress inside the grains created by the complex domain wall moving during field-induced polarization switching.The easy fatigue and toxicity of PZT thin film drive scientific worker to develop new ferroelectric storage materials. BiFeO₃ thin films are considered as the promising ferroelectric storage materials. In Chapter 6, BiFeO₃ thin films were synthesized by a simple Sol-gel coating process. The results of XRD and Raman show that the crystal structure of BiFeO₃ thin films is different with the bulk BiFeO₃. The results of XPS show that the Fe element in is present in Fe³⁺ form and changes gradually in Fe²⁺ form with the increasing of layer number. The results of PFM and KFM show that the 3-layer thin films have good ferroelectric properties and can be used in ferroelectric memory. Most of the grains of as-prepared BiFeO₃ thin films show the single-domain pattern.In the chapter 7, we make a conclusion of the researches in this paper, point out the originality and something to be desired. We expect the future investigations on the surface nanoscale electrical properties of perovskite ferroelectric thin film.