Study On The Na_0.5Bi_0.5TiO₃ Ferroelectric Thin Films Doped With Strontium And Calcium

As new materials of environmental harmony, sodium bismuth titanate (Na_0.5Bi_0.5TiO₃) ferroelectric thin films have attracted considerable attention for their good ferroelectric, dielectric, pyroelectric and piezoelectric properties. There are more and more studies about them. But Na_0.5Bi_0.5TiO₃ thin films prepared by current motheds cann't show properties good enough for their realistic use. Enlightened by the ion substituent of Na_0.5Bi_0.5TiO₃ ceramics, people begin to focus on the substituent of NBT thin films with low-lead or lead-free components and try to find a prepareing technic that can work in a lower tempreture. Curently the Sol-Gel method is used the most. In this work, we used the metalorganic solution deposition method, which is similar to Sol-Gel method but without a process of gelling, tried to simplize the preparing process and studied the influence t on films'properties, including ferroelectric and dielectric properties.The work systemically reports the preparing process of Na_0.5Bi_0.5Ti03 thin films by metalorganic solution deposition method. According to the results of thermal analysis and infrared spectrum analysis, the preparing technic was improved in the aspects of components rate, thermal treating condition and annealing tempretatures. The prepared (1-x)Bi_0.5Na_0.5TiO₃-xSrTiO₃ and (1-x)Bi_0.5Na_0.5TiO₃-xCaTiO₃ (x=0.13, 0.15, 0.18, 0.20) thin films'crystallization, ferroelectric and dielectric properties were analysed.(1-x)Na_0.5Bi_0.5TiO_3-xSrTiO₃ thin films'crystallization process was analysized first by the thermogravimetry/differential thermal analysis and infrared spectrum analysis of 0.87NBT-0.13ST powders dried at different temperatures. And it was found that the film's crystallization was generated via multiple steps of evaporation of water and solvent, and decomposition and polycondensations of organics. With the component rate x increasing, the films'crystallization temperatures were decreased gradually for about 50 oC. So we chose 400 oC and 550 oC 700 oC as the thermal treating temperature and annealing temperature range. The XRD amalysis result of prepared thin films by spin-coating showed that 650 oC is proper as the main annealing temperature. Then the electric properties of films were characterized by TF Analyzer 2000 and LF Impedance Analyzer. NBT-ST thin films prepred by metalorganic solution deposition method showed good ferroelectric properties and resistance against imprinting failure. The values of remanent polarizations decreased with x. With annealing temperatures increasing, the remanent polarizations didn't show an obvious heighten, while the property of resistance against imprinting failure was enhanced. The four NBT-ST films were referred to as polarization-type switching that is the desired mode for memory operation.Preparation and properties of (1-x)Bi_0.5Na_0.5TiO_3-xCaTiO₃ thin films were also studied systemically. First the precursor powders dried at defferent temperatures were analysized by thermal analysis and infrared spectrum analysis. It wsa found that with the component rate x increasing, crystallization temperatures ranges of the films were lowered from 650℃ 600℃to 600℃ 500℃. So according to that, we chose 400 oC and 550℃ 650℃as the thermal treating temperature and annealing temperature range. The XRD amalysis result of prepared thin films by spin-coating showed that compared with that in NBT-ST thin films, the second phase in NBT-CT thin films was much reduced with only one noticeable peak at 30°. Results of hysteresis loops and fatigue curves showed that NBT-CT thin films show good ferroelectric properties. Four thin films annealed at 650 oC had the similar complete P-E loops. When annealed at 600 oC, NBT-CT thin films'P-E loops showed strong incompleteness, except 0.85NBT-0.15CT thin film. When annealed at 550 oC, the four films'P-E loops were characterized with high values of remanent polarizations, high Ec and strong incompleteness. NBT-CT films (when x= 0.13, 0.15, 0.18) were referred to as polarization-type switching. Compared with NBT-ST films, NBT-CT films'Cmax and Cmin were higher, while the memory windows were smaller.(1-x)Bi_0.5Na_0.5TiO_3-xSrTiO₃ and (1-x)Bi_0.5Na_0.5TiO_3-xCaTiO₃ thin films can be prepared by metalorganic solution deposition method at low temperatures. And the prepared thin films showed low coercive fields, good switching endurance under bipolar pressure up to 10⁹ switching cycles and referred to as polarization-type switching that is desirable for memeory operation.