| Ferroelectric materials with the effects of ferroelectricity and piezoelectricity have been widely applied in the fields of memory, piezoelectric transducer and actuator. At present, lead-based perovskite compounds are key members in the commercial market of the electronic industry. However, these compounds contain>60 wt.% toxic lead, which leads to serious environmental problem during the producing, using and waste treatment processes.Na0.5Bi0.5TiO3 (NBT) with perovskite structure acts as one of the potential candidates to replace lead-based compounds. Nevertheless, NBT, especially in the form of thin film suffers from volatilization of Na or Bi during annealing process at high temperature. This may cause nonstoichiometric problem, bringing the secondary phase. On the other hand, the generation of oxygen vacancies may deteriorate the insulating performance of NBT thin film. Therefore, the NBT thin film is broken down before the applied voltage reaches the saturated value. For promoting performance of NBT thin film, a lot of approaches have been carried out, such as (i) formation of NBT-based solid solutions with other perovskites (BiFeO3 or BaTiO3), (ii) preparation of thin film with orientation, (iii) construction of heterogeneous multilayer structure, (iii) doping with single ion or co-doping with various ions at A/B site and so on. Among all, doping technique based on site engineering is considered to be effective. For Ti4+ located in B-site of NBT, its deflection to the center ion in octahedral Ti-O leads to the generation of electrostatic dipole moment and contributes to the polarization. On the other hand, Ti4+ is active to transform to Ti3+, generating free electron and increasing the leakage current. Therefore, doping NBT thin film with B-site ions attracts our attention.NBT thin film is an A-site complex ionic compound and ion doping makes it more difficult for the uniformity of thin film. The chemical solution decomposition (CSD) based on precursors tends to be more potential in promoting the uniform dispersion of ions, even in a molecular lever. Therefore, in this paper, we prepared a series of NBT-based thin films doped with Fe3+, Zr44+ and Nb5+, respectively using CSD combined with sequential annealing process. The effects of parameters for preparing process (i.e. annealing temperature and atmosphere) and ion-doping content on crystalline and performance (i.e. insulating, ferroelectric, dielectric and piezoelectric properties) were investigated. The working contents are shown in the following statements:1. By promoting the precursors (e.g. modifying the solute, solvent, especially PEG additive), phase-pure NBT-based thin films [e.g. Nao sBi0.5(Ti,Zr)O3 and Na0.5Bi0.5(Ti,Fe)O3] are obtained on single crystal Si and LaNiO3(100)/Si, respectively. Although the lattice mismatch between Si and NBT-based thin film reaches as high as 40%, the impurity phase of Bi2Ti2O7 is restrained to be formed. Meanwhile, by promoting the densification and homogeneity of Nao sBi0.5(Ti,Fe)O3 thin film, the remanent polarization (Pr) is increased by 44.1%, to 21.9μC/cm2. Also, the dielectric constant versus frequency for the film shows a small dispersion tendency. At 100kHz, the dielectric constant (εr) is 411, which is increased by 52.8%, and the dissipation factor (tanδ) is 0.15, which is decreased by 33.3%.2. The Na0.5Bi0.5(Tio.99Fe0.01)O3 thin film deposited on LaNiO3(100)/Si exhibits optimizing dielectric tunability. It varies with changing the measuring voltage and frequency, and can reaches the maximum value of 29.36%. At 100kHz, the εr and tanδ are 400 and 0.12, respectively and the FOM is 4.78. The crystalline and electrical properties of Na0.5Bi0.5(Ti0.98Fe0.02)O3 thin film is sensitive to annealing temperature. At the temperature of 500℃, the film exhibits the biggest average grain size and surface with high uniformity. This favors the improvement of ferroelectric and dielectric properties. The Pr, and εr at 100kHz reach the maximum with the values of 27.6μC/cm2 and 485, respectively. Meanwhile, the film exhibits self-polarization with the piezoelectric coefficient (d33) of about 19.6pm/V.3. The crystalline and electrical properties of Na0.5Bi0.5(Ti1-xZrx)O3 (x=0,0.01,0.02,0.04) thin film deposited on FTO/glass are sensitive to annealing atmospheres and Zr4+ doping content. Under N2, the Na0.5Bi0.5(Ti0.98Zr0.02)O3 thin film possesses the best insulating and ferroelectric properties with Pr of 16.4μC/cm2. This may be attributed to the appropriate content of oxygen vacancies with the favorable effects of grain growth, making the high densification dominated factor on performance of thin film. Also, the dielectric constant versus frequency for the film shows a small dispersion tendency with er and tanδ of 410 and 0.03 at 100kHz, respectively.4. On ITO/glass, by doping Nb5+ into NBT thin film, the ferroelectricity is obviously improved. This can be attributed to the introduction of defects with positive charge [(Nb5+Ti4+)·], leading to the reduce of leakage current. Also, doping Nb5+ with larger radius brings in distortion of TiO6, which promotes the domain motion, increasing the remanent polarization. Especially, compared with Na0.5Bi0.5(Ti0.98Nb0.02)O3 thin film annealed under O2, the one under N2 possesses the bigger Pr value of 10.7μC/cm2. Also, the film shows the better dielectric properties with a small dispersion tendency. At 100kHz, the εx and tanδ are 402 and 0.07, respectively. This can be ascribed to the better crystalline. |
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