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(Ta1- 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 SrBi2 x Ta2O9 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(Ta1- 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.

We have developed the series of thin-film bismuth layer structured ferroelectric (BLSF) materials such as SrBi2Ta2O9, SrBi2Nb2O9, SrBi4Ti4O15 and their solid solutions using metallo-organic-decomposition (MOD) spin-on coating techniques. We found that SrBi2Ta2O9 is one of the best potential candidate materials for ferroelectric nonvolatile memories. The SrBi2Ta2O9 thin-film capacitor had the remanent polarization (P r+-P r-) of 20 µ C/cm2, coercive field of 35 kV/cm and dielectric constant of 250. SrBi2Ta2O9 thin film on platinum electrode has fatigue-free characteristics for up to 2×1011 cycles without requiring any complicated electrode system such as conductive oxide. Moreover, SrBi2Ta2O9 thin film has many advantages, e.g., high signal/noise ratio of 8 at 1.2 V, low-voltage operation at as low as 1 V, long data-retention, little surface effect, superior imprint properties and low leakage current. We considered that these advantages are due to (1) less space charge and (2) the inherent domain motion.

Studies of depolarization characteristics have been carried out on so-gel lead zirconate titanate (PZT) ferroelectric thin-film capacitors by electrical evaluation of the pulse switching response employing a capacitor-loaded circuit. Four consecutive positive pulses were used to evaluate the depolarization effect after negative writing, instead of conventional positive and negative double pulses. The extended evaluations were carried out for, e.g., pulse height, pulse width, temperature dependence, and space charge effect. The amount of depolarization strongly depended only upon the value of load capacitance. The transition and hysteresis loops from pulse switching clearly showed that the depolarization occurred due to the depoling (partial switching) of domains by an opposite-directional depolarization field which arose from accumulated switched charge on the load capacitor. We considered that depolarization occurs as a result of both external depolarization field from an external load capacitor and internal depolarization field from interfacial layers within ferroelectric material.

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We evaluated depolarization characteristics of sol-gel Pb(Zr0.4Ti0.6)O3 ferroelectric thin-film capacitors. The depolarization characteristics were strongly dependent upon the capacitance of the load capacitor. The newly obtained hysteresis loop during the pulse response indicates that significant voltage of opposite direction is generated on the ferroelectric capacitor at the moment the read pulse becomes zero. We experimentally found that the mechanism of depolarization is based on depoling (or partial switching) by an opposite-direction voltage arising from accumulated switching charge on a load capacitor. This mechanism can be easily extended to the internal depolarization effect due to the interfacial linear capacitor between the ferroelectric region and electrode.