Heteroepitaxial BaTiO3 films of various thicknesses ranging from 12 nm to 79 nm were prepared on SrRuO3/SrTiO3 substrates by radio-frequency magnetron sputtering employing a two-step deposition technique, and the crystallographic and ferroelectric properties of the heteroepitaxial films were evaluated. A ferroelectric hysteresis loop was clearly observed in the heteroepitaxial BaTiO3 films even when the thickness was reduced to 12 nm, probably due to improved crystallinity at the interface between the ferroelectric film and the electrodes through optimization of the preparation technique. The observation of hysteresis at the temperature of 200°C was explained in terms of modification of the Curie temperature from the inherent 130°C to far above 200°C by lattice misfit strain. A possible relationship between the artificially raised Curie temperature and the limited thickness dependence of ferroelectricity was discussed.

Induced ferroelectricity was observed in epitaxially grown BaxSr1−xTiO3 (BST) thin films with thicknesses of about 220 nm. The BST films with various Ba content were deposited at 600 °C on Pt/MgO substrates, by radio frequency magnetron sputtering with double targets, BaTiO3 and SrTiO3. The epitaxial growth of the BST films was confirmed with x-ray-diffraction and reflection high-energy electron-diffraction analyses. The D–E hysteresis curve and the Curie temperature shift confirmed the BST films with Ba content x≥0.44 had ferroelectricity at room temperature. The mechanism of the ferroelectricity appearance was discussed in relation to the elongation of the c axis in the thickness direction, caused by lattice mismatch between Pt and BST.

Asymmetric ferroelectricity and conduction of anomalous leakage current were observed in heteroepitaxial BaTiO3 thin films grown by rf magnetron sputtering at 600° C on three different electrode/substrate combinations: SrRuO3/SrTiO3, Pt/MgO and Nb doped SrTiO3. The voltage shift of hysteresis loops of the capacitance was a linear function of the thickness of the BaTiO3 films and became as large as 10 V in the film with a thickness of 410 nm. The asymmetry did not disappear even after a heat treatment carried out at 800° C in air. On the other hand, a steep increase in the leakage current was observed in the heteroepitaxial films when a positive voltage was applied. The origin of both the asymmetric hysteresis and the anomalous conduction is discussed in terms of asymmetric crystal structure caused by misfit dislocations introduced in heteroepitaxial growth.

Voltage shift phenomena of the hysteresis loop were characterized for a c-axis oriented heteroepitaxial BaTiO3 film by means of switching current measurements using various types of pulse sequences. During application of voltage, the hysteresis loop gradually shifted along the voltage axis according to the polarity of the voltage. Even after the application of voltage, while the top and bottom electrodes were short-circuited, the hysteresis loop continued to move. Under certain conditions, a part of the hysteresis loop shifted back, whereas the rest shifted forward. These results were explained, assuming that there is a nonswitching layer between the ferroelectric layer and the bottom electrode, and that the discontinuity of polarization can be compensated by injection of negative charges from the electrode. It was suggested that the nonswitching layer is possibly formed by relaxation of lattice misfit strain in the heteroepitaxial ferroelectric thin film.

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