Investigation On The Preparation And Properties Of Mn Doped Na_0.5Bi_0.5TiO₃ Thin Films

In recent decades, ferroelectric materials with ferroelectric, piezoelectric and pyroelectric properties have been widely applied in microelectronic devices. At present, lead-based perovskite compounds are key members in the market. But, there are lead volatilization in the preparation and using of lead-based materials, which will lead to serious environmental problem and threat to human health.Na_0.5Bi_0.5TiO₃?NBT?, as one of perovskite structure ferroelectrics, is considered as one of potential candidates to replace lead-based compounds. However, for NBT, especially in the form of thin film, the high leakage current lead to the intrinsic properties is difficult to exhibit. This is mainly because A-site elements volatilization introduce many oxygen vacancies in thin film, then promote the conductivity of film. On the other hand, A-site will simultaneously cause nonstoichiometric problem. For the above high leakage current problem, a lot of approaches have been carried out to promote performance of NBT thin film. Such as, ion doping, formation of NBT-based solid solutions with other ferroelectric materials and introduction of buffer layers. Among all, doping technique is considered as one of most effective ways.As an A-site complex ionic compound, ion doping makes the components of NBT thin film more complicated and the uniformity of thin film become more important. The chemical solution decomposition, as a simple and easy way, can bring the precursors solution to a molecular level uniform, and ensure the uniformity of thin film. Therefore, in this paper, we prepared a series of Mn-doping NBT-based thin film on ITO/glass using chemical solution decomposition combined with sequential annealing process. The effect of Mn-doping content, annealing temperature, annealing atmosphere on Mn-doping NBT thin film were investigated. We fabricated Mn-doping NBT-based thin film on different substrates?Pt/TiO₂/SiO₂/Si and La NiO₃/Si?, and the effects of substrate on film were investigated. A film with sandwich structure?NBTMn/NBTNb/NBTMn? was prepared and the feasibility of combination among different methods for performance improvement is explored.The main research contents are as follows:1. We prepared a series of Mn-doping NBT-based thin film on ITO/glass. The effect of Mn-doping content, annealing temperature, annealing atmosphere on NBT-based thin film were investigated. The results shown that:?1? The Mn-doping can reduce effectively the leakage current in a certain range of doping?0-2mol%?, then enhance the electrical properties of thin film. However, continuing to increase the Mn-doping content to 4mol%, introducing too much impurity ions in thin film deteriorates the performance of films.?2? 500? is the lowest crystallization temperature Mn-doping NBT thin film. The density of thin film gradually increases with the increasing of annealing temperature. The enhanced crystallization promote the decrease of leakage current, thus, improving the ferroelectric property of thin film. However, when the annealing temperature up to 600?, thin film deteriorated in performance due to the appearing of recrystallization.?3? Mn-doping NBT thin film with O₂ shown the reduced content of oxygen vacancy. The oxygen in environment compensates the oxygen vacancies which is caused by the volatilization of A-site elements. The Mn-doping NBT thin film annealed at oxygen shows the obviously enhanced electrical properties.Comprehensive the above experiment, the optimal doping content of Mn is 2mol% and the best annealing temperature and atmosphere are 550°C andO₂, respectively. The Mn-doping NBT thin film with the optimal process parameters shows the optimal electrical properties, including the lowest leakage current density?<10-5A/cm²?, the highest remanent polarization?38?C/cm²?, high relative dielectric constant?501? and low dielectric loss?0.04?.2 The Mn-doping NBT thin films deposited on ITO/glass ? Pt/TiO₂/SiO₂/Si and La NiO₃/Si substrates were fabricated. The effect of substrate on Mn-doping NBT thin film was investigated. The experimental results indicated that the Mn-doping NBT thin film deposited on La NiO₃/Si shows the preferred orientation with?l00?. Compared with film on Pt/TiO₂/SiO₂/Si, Mn-doping NBT thin film fabricated on LaNiO₃/Si substrate shows the much dense microstructure due to the presence of the epitaxial growth. The enhance crystallization promote the decrease of leakage current and the improving of ferroelectric property?remanent polarization Pr=10.2?C/cm²?.3 The sandwich structure NBTMn/NBTNb/NBTMn film was prepared. Compared with the NBTNb thin film, the sandwich structure film shown the obviously improved electrical properties with the Pr of 25?C/cm². This is mainly due to the good leakage current characteristic of NBTMn thin film.