Investigation On The Dielectric Tunability Of Na_0.5Bi_0.5TiO₃-based Thin Films

Over the past decades, there has been an increasing interest in the development of lead-free ferroelectric materials replacing lead-based perovskite ferroelectric composites. As a potential candidate, sodium bismuth titanate Na_0.5Bi_0.5TiO₃?NBT? is one of the environmentally friendly ferroelectric materials, which has gained much attention and been widely investigated. In consideration of the nonlinear dielectric property, which is the dependence of dielectric constant??r? on bias voltage?V?, NBT-based thin films can be used for tunable devices such as phase shifters, tunable oscillators, tunable filters and varactors. In such devices, it is desirable to have a low leakage current density?J?, modest ?r, low dissipation factor?tan?? and large tunability as well as figure of merit?FOM?. However, it is difficult to obtain desired electrical properties for pure NBT thin film due to its high conductivity. They suffer from nonstoichiometric compositions as well as the generation of oxygen vacancy? OV???, which mainly arise from the high volatility of A-site elements and the valence transfer of the B-site element during thermal treatment. In order to further improve the performance of NBT films, great efforts have been tried in a variety of methods. The effects of buffer layer, epitaxial or oriented growth, formation of multilayered structure and forming solid solution with other component on the ferroelectric and dielectric properties of NBT thin film were investigated. Besides, to compensate the oxygen vacancies, acceptor dopants which can occupy the B-site of the ABO3 perovskite structure, are used to reduce the J because of the formation of defect complex.Among all the methods of preparing metal oxide thin film, the metal organic decomposition process is widely applied because of the simple preparation as well as the precise control of chemical stoichiometry. In this study, we prepared Zn-doped NBT thin films by metal organic decomposition combined with sequential layer annealing. The influences of Zn²⁺ doping content and parameters of preparing process?i.e. annealing temperature, single-coated layer thickness and atmosphere? on the crystallization and electrical performance?i.e. insulating, ferroelectric and dielectric tunability? of all samples were examined and discussed. The working contents are shown in the following statements:1. The Zn²⁺ doped Na_0.5Bi_0.5TiO₃?NBTZnx, x=0⁴ at.%? thin films were prepared. The microstructure and electrical properties are sensitive to the Zn²⁺ content. Compared with other films, NBTZn_0.01 thin film shows enhanced insulating characteristic with lower leakage current, batter ferroelectric property. At 300 k V/cm the J is about 10-4 A/cm2, the largest Pr is about 24.8 ?C/cm2. The tunability of NBTZn_0.01 thin film at 300 k V/cm and 100 k Hz is 43.6%.2. NBTZn_0.01 thin films were annealed at various temperatures from 450 to 600 °C. XRD measurement reveals that the film can be crystallized into single perovskite at an annealing temperature as low as 500 °C. The tunability of NBTZn_0.01 thin film annealed at 550 °C under 300 k V/cm and 100 k Hz is 43.6%.3. NBTZn_0.01 thin films with various single-coated layer thicknesses?35, 25, 20 and 15 nm? annealed at 550 °C were prepared. The highly preferred?110?-orientation in the film of 20 nm per layer?nm/l? is consistent with the typical columnar structure observed from cross-sectional morphology. Also, for the film with 20 nm/l, it exhibits the higher tunability of 46.5% and FOM of 8.8 at 300 k V/cm and 100 k Hz due to well-crystallization and lower content of defects.4. NBTZn_0.01 thin films were deposited on the indium tin oxide?ITO?/glass substrates annealed under different atmospheres?air, N₂, O₂, O₂/N₂?. For the film annealed in O₂/N₂ atmosphere, the lower leakage current density and higher dielectric tunability can be obtained, which may be ascribed to the improvement of crystallization with more homogeneous grain sizes and dense microstructure. A higher tunability of 47.6% and FOM of 7.9 at 300 k V/cm and 100 k Hz for this film can be observed.