Kenya Sano

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.

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.

Heteroepitaxial TiN Film Growth on Si(111) by Low Energy Reactive Ion Beam Epitaxy.

Epitaxial TiN(111) films were grown on Si(111) substrates by low energy reactive ion beam deposition using N2 ion beam irradiation and Ti evaporation. TiN epitaxial film formation was investigated as a function of the substrate temperature (T s) and the kinetic energy of the ion beam (E b). Using X-ray diffraction (XRD) and in-situ reflection high-energy electron diffraction (RHEED), the epitaxial relationship was found to be TiN(111)|| Si(111), TiN || Si above substrate temperature of 400° C. Cross-sectional bright-field transmission electron microscope (TEM) observation has shown that the interface is quite flat and sharp and that four TiN lattice spacings match three spacings of the Si lattice. A lowest resistivity value of 18 µΩ cm (T s=600° C, E b=100 eV) was obtained.

Lattice deformation and magnetic properties in epitaxial thin films of Sr1−xBaxRuO3

Crystal structure and magnetic properties in epitaxially grown Sr1−xBaxRuO3 on SrTiO3 substrates were determined. Epitaxial Sr1−xBaxRuO3 exhibits a simple perovskite structure in the whole region of the Ba/Sr ratio, in contrast to the complex hexagonal layered perovskite of Ba-rich bulk Sr1−xBaxRuO3, which has plane-sharing oxygen octahedra. Tetragonal deformation was enhanced from pseudocubic in SrRuO3 to a highly distorted tetragonal lattice in BaRuO3. Electronic properties such as conductivity and magnetization were examined. A metal–insulator transition was not observed in this system, and metallic conductivity was maintained in the whole region of Ba concentration. Ferromagnetic ordering at 160 K seen in bulk SrRuO3 was observed to be suppressed in the Sr1−xBaxRuO3 films with increasing tetragonal deformation and Curie temperatures decreased to 50 K in BaRuO3.

Epitaxial Growth of Ti1¡xAlxN Buffer Layer for a Ferroelectric (Ba, Sr)TiO3 Capacitor on Si Substrate

Epitaxial titanium aluminum nitride, Ti1-xAlxN(100) (0x //Si by X-ray diffraction (XRD) and in situ reflection high-energy electron diffraction (RHEED). Although the electrical resistivity increased exponentially with x, the values remained on the order of 10-4 Ωcm. Using the Ti1-xAlxN buffer layer, we have successfully fabricated a capacitor with an epitaxial (Ba, Sr)TiO3 film on a Si substrate.

Electrical Properties and Thermodynamic Stability of Sr(Ti1-x,Rux)O3 Thin Films Deposited by Inductive-Coupling-Plasma-Induced RF Magnetron Sputtering

Sr(Ti1-x,Rux)O3 (STRO) epitaxial thin films were deposited on single-crystal SrTiO3(100) substrates using the inductive-coupling-plasma-induced RF magnetron sputtering method without oxygen. The electrical conductivity of STRO films increases with Ru concentration and levels of the Ru 4d states are observed in the band gap of SrTiO3 by X-ray photoelectron spectroscopy (XPS) analysis. These results are consistent with those obtained by first-principles calculations. Thermodynamic stability increases with the decrease of Ru concentration, and STRO (x<0.50) is free from degradation under annealing H2 atmosphere at 600°C. This high resistance against reductive processes indicates that STRO (x<0.50) is one of the most suitable candidates for conductive oxide electrodes of oxide capacitors.

Ferroelectric properties of epitaxial barium titanate thin film capacitors on silicon substrate

Abstract A heteroepitaxial BaTiO3 film with a thickness of 65 nm was prepared on SrRuO3/Pt/TiAlN/Si. The Ti0.91Al0.09N film was grown on Si substrate by DC sputtering in Ar/N2 ambient at 700°C. The BaTiO3, SrRuO3 and Pt films were deposited by radio-frequency magnetron sputtering at substrate temperatures ranging from 450°C to 600°C. The BaTiO3 film has a c-axis 1.6% longer (0.410 nm) than that of bulk, due to lattice misfit between SrRuO3 and BiTiO3. When positive and negative voltage pulses with an amplitude of 5 V were applied, a switching charge density, Q sw, of 40 μC/cm2 was obtained. No fatigue was observed after 1011 cycles of polarization switching even when an amplitude of 7 V was applied. Remanence of polarization for 4 hours was confirmed in a retention test. These results indicate that the heteroepitaxial BaTiO3 film capacitors on Si substrates can be used for ferroelectric nonvolatile memory applications.