| Recently, owing to ferroelectric materials and its products containing much lead, it can cause environmental pollution and damage people's health during manufacture and use. In order to protect our environment from lead pollution, the scientists all over the world are engaged in lead-free or low lead content piezoelectric and ferroelectric materials. Among all these materials, bismuth layer-structured ferroelectric (BLSF) thin film have the characteristics of large remanent polarization, high fatigue resistance, good retention, lead-free chemical composition. It has been studied intensively for potential applications in ferroelectric random access memories (FeRAM). In present work, we study the properties of BLSF thin film by experimental and theoretical calculation approach.In the experimental field, thin films of Nd3+/V5+-cosubstituted bismuth titanate, (Bi4-xNdx)(Ti3-yVy)O12 (BNTV), were fabricated by chemical solution deposition (CSD) technique. The effect of annealing temperature and V content on the ferroelectric and electrical properties were studied, respectively. The crystallized phase and microstructure were investigated using X-ray diffraction (XRD) and scanning electron microscope (SEM). At mean time, polarization hysteresis loops (P-E), leakage current-voltage (I-V), and capacitance-voltage loops(C-V), fatigue resistance and the dependence of hysteresis loops upon frequency of the BNTV thin film were measured. As a result, the V5+ substitution of Ti-site had improved the optimal annealing temperature of BNT thin film to 800°C. It seems that the effect of V-doped to restrain Bi loss is obvious. Compared with the BNT thin film, it also found that (Bi3.15Nd0.85)(Ti2.91V0.09)O12 thin film have the lower leakage current (5.99×10-9 A @ 3V), better fatigue resistance. The change of V contents in BNTV thin film may result in oxygen vacancies, space trapped charge and lattice defect. This suggests that the desired ferroelectric thin film with good electrical properties would be obtained if the substituted V5+ contents are well controlled.At the theoretical field, The Preisach model was improved through an approximate integral of the statistic distribution function of the dipoles in thin film. Then, the defect of the hysteresis loops, which caused by approximation of integral, was corrected by superposition method. The simulated hysteresis loops show full, smooth and symmetric shape. The improved model is able to describe the unsaturated loops due to its history-dependent electric field effect. It is available in the small signal ferroelectric capacitor model. A methodology proposed by Furukawa has been employed to describe the butterfly loops. The simulated loops'singularity at zero bias is much higher than the curve by the definition of permittivityε= dP/dE. This case is in good agreement with the experimental result. Thus, the present model is fast, accurate and few undesirable parameters, and therefore suitable for use in circuit simulators and theoretical research in ferroelectric memory and tunable device.Additionally, based on above improved model, we extend single interface layer model to investigate imprint behaviors by establishing two interface layers model. The electrical conductivity of top/bottom layer is used as medi-parameter to correlate the cause and phenomenon of imprint failure. It can explain various phenomenon including the change of coercive field, size dependence of shift effect and swelled or shrinking shape at the lower part of hysteresis loop. The simulated hysteresis loops with bias shift, titled loop, variational coercive field and anamorphic shape had been reproduced by the two interface layers model, which agree well with the experiment. As a result, the presented model may give a theoretical guidance to resolve the imprint problem: forming complementary top and bottom interface layers. Hence the model could provide an insight into the possible mechanism and means of reducing the imprint failure. |
PDF Full Text Request