Naoko Yanase

Thickness Dependence of Ferroelectricity in Heteroepitaxial BaTiO3 Thin Film Capacitors

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.

Asymmetric Ferroelectricity and Anomalous Current Conduction in Heteroepitaxial BaTiO3 Thin Films

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 in a heteroepitaxial BaTiO3 thin film capacitor

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.

Ferroelectric Properties of SrRuO3/(Ba, Sr)TiO3/SrRuO3 Epitaxial Capacitor

Ferroelectric properties were investigated using SrRuO3/(Bax, Sr1-x)TiO3(BSTO)/SrRuO3 all-perovskite heteroepitaxial capacitors on strontium titanate substrates. The dielectric film had a composition with a Ba content of x=0 to 1, i.e., the whole composition of the SrTiO3–BaTiO3 solid-solution system. The c-axis of the BSTO film was elongated depending on the Ba content, and reached 0.441 nm at the BaTiO3 composition, which was 8.6% larger than the bulk c-axis. A peak relative dielectric constant of 870 and maximum remanent polarization (2Pr) of 58 µC/cm2 were obtained depending on the Ba content at a dielectric thickness of 20 nm. These are the highest storage capacitance capacitor and the thinnest ferroelectric capacitor reported so far. Possibilities for deep submicron ferroelectric and dielectric memory applications are also discussed.

Modification of ferroelectricity in heteroepitaxial (Ba, Sr)TiO3 films for non-volatile memory applications

Abstract Modification of ferroelectricity in heteroepitaxial Ba x Sr1-x TiO3(BST) films by making use of lattice misfit was investigated. Theoretical calculation suggested that application of two-dimensional compressive stress of 3 GPa to the ferroelectric crystal would result in Curie temperature increase by 270°C. This theoretical prediction was experimentally confirmed. Heteroepitaxial Ba0.6Sr0.4TiO3 films grown on SrRuO3/SrTiO3 substrates exhibited ferroelectric hysteresis at 200°C, even though the inherent Curie temperature is known to be 0°C. The possibility of heteroepitaxial BST films for ferroelectric non-volatile memory applications and their advantages were pointed out.

Ferroelectric Properties in Heteroepitaxial Ba0.6Sr0.4TiO3 Thin Films on SrRuO3/SrTiO3 Substrates

Heteroepitaxial Ba0.6Sr0.4TiO3 thin films with thicknesses from 26 nm to 97 nm were grown on SrRuO3 bottom electrode films on SrTiO3 single crystal substrates. Although the films have a paraelectric composition, ferroelectric hysteresis was observed in displacement versus electric field (D–E) loops. The ferroelectricity is considered to be developed as a result of bi-axial stress, caused by lattice mismatch between the dielectric films and the bottom electrodes. The maximum difference in the displacement was over 0.4 C/m2, even when the thickness of the film was reduced to 26 nm. The hysteresis loops showed an asymmetric characteristic at a polarity of applied voltage. The asymmetry was discussed in terms of an asymmetrical deformation of the crystal structure, which is probably introduced during the heteroepitaxial growth.

P2G-4 Suppression of Acoustic Energy Leakage in FBARs with Al Bottom Electrode: FEM Simulation and Experimental Results

One of the most challenging issues in designing film bulk acoustic wave resonators (FBARs) is how to realize high-Q resonators. According to our experimental results, an acoustic leakage is the dominant loss factor at antiresonance frequency for FBARs with an aluminum bottom electrode. In this paper, we report simulation results obtained using the 2-dimensional finite element method (2D FEM), which was employed in order to confirm the above-mentioned acoustic loss mechanisms and optimize the design parameters of the resonator. As a result, optimizing the aluminum bottom electrode thickness and properly designing an attenuation structure that reflects the laterally propagating Lamb waves inside the resonator areas suppress the acoustical leakage significantly. Comparisons between FEM simulation and measured results in terms of the relationship between the Q-factors at antiresonance frequency and the structural parameters of the resonators are shown.

Nonswitching Layer Model for Voltage Shift Phenomena in Heteroepitaxial Barium Titanate Thin Films

We have introduced a nonswitching layer model to explain voltage shift phenomena of hysteresis loops in a heteroepitaxial barium titanate thin film capacitor. A nonswitching layer, which has irreversible spontaneous polarization, was assumed to be present between the ferroelectric layer and the bottom electrode layer. Calculation based on the model demonstrated that the presence of the nonswitching layer would cause an inherent voltage shift of the hysteresis loop from the origin along the voltage axis. If free charges accumulate at the interface between the ferroelectric layer and the nonswitching layer to compensate the discontinuity of the polarization, the hysteresis loop would inversely move toward the opposite direction. A series of experimental results observed for a c-axis oriented barium titanate thin film seemed consistent with the model. It was suggested that the nonswitching layer may be formed in accordance with relaxation of lattice misfit stress in the early stage of the heteroepitaxial growth.

Epitaxial Growth of BaTiO3 Thin Films by High Gas Pressure Sputtering

Heteroepitaxial BaTiO3 thin films with lattice-misfit strain were prepared on SrRuO3/SrTiO3 substrates by rf magnetron sputtering under high gas pressure, in order to realize large area deposi-tion with uniform ferroelectric properties. Lattice constants, composition and ferroelectric properties of the epitaxial BaTiO3 films were characterized as a function of pressure as well as the incident angle of sputtered particles, under the assumption that the sputtered particles were eradiated from the eroded area of the target surface. Although ferroelectric properties of films sputtered under low gas pressure had remarkable place dependability, they have been drastically improved by high gas pressure sputtering. X-ray diffraction analyses revealed that the BaTiO3 film deposited with a pressure of 6.3 Pa had a large lattice strain of 5%, even though the substrate was placed at a position directly facing the erosion area of the target, where a number of high-energy ions such as oxygen ions are thought to collide against the film surface if the pressure is low. The BaTiO3 film exhibited a ferroelectric hysteresis loop with remnant polarization of 43 µC/cm2. These results indicate that high gas pressure sputtering is effective for achieving large area uniformity of ferroelectric properties in heteroepitaxial BaTiO3 thin films.

Asymmetric Switching of Ferroelectric Polarization in a Heteroepitaxial BaTiO3 Thin Film Capacitor

Asymmetric switching of ferroelectric polarization was measured and analyzed for a heteroepitaxial BaTiO3 thin film of 58 nm thickness. The BaTiO3 film prepared on a SrRuO3/SrTiO3 substrate by radio-frequency magnetron sputtering has a c-axis oriented normal to the surface which is 3% longer than that of the bulk due to lattice misfit between SrRuO3 and BaTiO3. When positive and negative voltage pulses of the same amplitude were sequentially applied, asymmetric responses were observed in the transient current. Although the switching charge densities Qsw were the same for both the polarities, the switching time ts was longer and the peak current imax was smaller for the positive response. From the analyses of the transient current, the asymmetric switching of polarization was attributed to the discrepancy in coercive voltages Vc between polarities.