Preparation of Ferroelectric Thin Films of Bismuth Layer Structured Compounds

Ferroelectric thin films of bismuth layer structured compounds, SrBi₂Ta₂O₉, SrBi₂Nb₂O₉, SrBi₄Ti₄O₁₅ and their solid solutions, were formed onto a sputtered platinum layer on a silicon substrate using spin-on technique and metal-organic decomposition (MOD) method. X-ray diffraction (XRD) analysis and some electrical measurements were performed on the prepared thin films. XRD results of SrBi₂(Ta_1-x, Nb_x)₂O₉ films (0≤x≤1) showed that niobium ions substitute for tantalum ions in an arbitrary ratio without any change of the layer structure and lattice constants. Furthermore, XRD results of SrBi_2xTa₂O₉ films (0≤x≤1.5) indicated that the formation of the bismuth layer structure does not always require an accurate bismuth content. The layer structure was formed above 50% of the stoichiometric bismuth content in the general formula. SrBi₂(Ta_1-x, Nb_x)₂O₉ films with various Ta/Nb ratios have large enough remanent polarization for nonvolatile memory application and have shown high fatigue resistance against 10¹¹ cycles of full switching of the remanent polarization. Mixture films of the three compounds were also investigated.