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(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.