面向NvFeRAM应用的低退火温度SrBi2(Ta,Nb)2O9薄膜的增强铁电性能
Enhanced ferroelectric properties of low-annealed SrBi2(Ta,Nb)2O9 thin films for NvFeRAM applications
AI总结：
本研究通过Nb取代调控SBTN薄膜结构，在700℃低温退火下实现剩余极化强度3倍提升与钙钛矿相占比提高，为高性能无铅NvFeRAM用铁电薄膜制备提供了可行路径。
摘要：
本研究采用显微拉曼光谱和X射线衍射技术，探究了在700℃低温退火的Nb取代SrBi2(Ta1-xNbx)2O9（SBTN，铌掺杂钽酸锶铋）晶体薄膜的晶格动力学特性。研究发现，当x=0.1-0.4时，SrBi2(Ta1-xNbx)2O9薄膜由细晶球形结构组成；而当x=0.5时，由于薄膜钙钛矿结构发生应力诱导相变，会形成棒状晶粒。此外，研究还揭示，在Nb阳离子取代过程中，SBTN薄膜内的Aurivillius相（奥里维里斯相）形成得到增强并成为主导相，而萤石/焦绿石相的形成受到高度抑制。我们认为这些变化是由铁性钙钛矿中发生的铁畸变导致的，即当x=0.2-0.5时，(Ta,Nb)O6八面体发生倾斜畸变。八面体倾斜畸变会改变A位阳离子的配位环境，同时由于B位上Ta和Nb的离子半径与质量存在差异，会降低SBTN的对称性，这可能导致SBTN晶体结构发生显著变化。研究还观察到，SBTN薄膜中铁电钙钛矿相的占比和剩余极化强度随Nb含量的变化呈现出相同的非单调趋势。当Nb原子取代浓度为10%-20%时，剩余极化强度可提升至原来的3倍，钙钛矿相占比从66%提升至87%。因此，所得结果可用于在低退火温度下制备具有高剩余极化强度的无铅薄膜，在先进非易失性铁电随机存取存储器（NvFeRAM）应用中颇具前景。
英文摘要：
Micro-Raman spectroscopy and X-ray diffraction have been used to explore the lattice dynamics of Nb-substituted SrBi2(Ta1-xNbx)2O9 (SBTN) crystalline thin films annealed at low temperature, 700oC. It turned out that SrBi2(Ta1-xNbx)2O9 films consist of fine-grained spherical structures for x=0.1-0.4, while the formation of rod-like grains occurs for x=0.5 due to the stress-induced transformation of the thin film perovskite structure. Moreover, it was revealed that during Nb cationic substitution Aurivillius phase formation was enhanced and become dominated in SBTN thin films and fluorite/pyrochlore phase formation was highly suppressed. We assume that these changes are conditioned by the ferrodistortion occurring in ferroic perovskites, namely by the tilting distortion of (Ta,Nb)O6 octahedra for x=0.2-0.5. The octahedral tilting distortion can change the coordination environment of the A-cite cation, as well as it lowers the SBTN symmetry due to the differences in ionic radius and mass between Ta and Nb in the B-sites, that can lead to significant changes of the SBTN crystal structure. The same nonmonotonic trends were observed for the ferroelectric perovskite phase fraction and remanent polarization on Nb content in SBTN films. The substituting Nb atoms with a concentration of 10-20 % made it possible to increase the remanent polarization in 3 times and raise the perovskite phase fraction from 66% to 87%. Therefore, obtained results can be used for the production of lead-free thin films with a high remanent polarization under low annealing temperature, being promising for advanced nonvolatile random access ferroelectric memory (NvFeRAM) applications. Keywords: ferroelectric, sol-gel method, perovskite structure, crystallization annealing, defects, remanent polarization, SBTN films, Raman spectroscopy, Landau-Devonshire approach.
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