Enhanced ferroelectric properties of low-annealed SrBi₂(Ta,Nb)₂O₉ thin films for NvFeRAM applications

Micro-Raman spectroscopy and X-ray diffraction have been used to explore the lattice dynamics of Nb-substituted SrBi₂(Ta_1-xNb_x)₂O₉ (SBTN) crystalline thin films annealed at low temperature, 700 °C. It turned out that SrBi₂(Ta_1-xNb_x)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 became 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.