Takeshi Kanashima

Giant ferroelectric polarization beyond 150 μC/cm2 in BiFeO3 thin film

Ferroelectric BiFeO3 thin films have been deposited on Pt/TiO2/SiO2/Si substrate by pulsed-laser deposition. From the X-ray diffraction analysis, the BiFeO3 thin film consists of perovskite single-phase, and the crystal structure shows the tetragonal structure (c/a = 1.018) with a space group P4mm. It is obtained that the BiFeO3 thin film shows a well-saturated remarkably giant saturation polarization of 158 µC/cm2 and a remanent polarization of 146 µC/cm2 for a maximum applied voltage of 20 V at 90 K. These values of polarization are largest ever-measured in ferroelectrics.

Enhancement of electrical properties in polycrystalline BiFeO3 thin films

Ferroelectric BiFeO3 thin films were grown on Pt∕TiO2∕SiO2∕Si substrates by pulsed-laser deposition. From the x-ray diffraction analysis, the BiFeO3 thin films consist of perovskite single phase, and the crystal structure shows the tetragonal structure with a space group P4mm. The BiFeO3 thin films show enhanced electrical properties with low leakage current density value of ∼10−4A∕cm2 at a maximum applied voltage of 31V. This enhanced electrical resistivity allowed the authors to obtain giant ferroelectric polarization values such as saturation polarizations of 110 and 166μC∕cm2 at room temperature and 80K, respectively.

Analyses of High Frequency Capacitance-Voltage Characteristics of Metal-Ferroelectrics-Insulator-Silicon Structure.

Capacitance–voltage (C–V) characteristics of the metal-ferroelectrics-insulator-silicon (MFIS) structure have been calculated by the conventional metal-oxide-silicon (MOS) analysis method. The potential profile is obtained by using the Poisson equation of D–E hysteresis, and its capacitance is estimated as combination of SiO2-Si capacitance and ferroelectric film capacitance obtained from dielectric constant corresponding to the internal field. PbZrxTi1-xO3 (PZT) and SrBi2Ta2O9 (SBT) ferroelectrics, SiO2, MgO, SrTiO3 (STO) and CeO2 insulators, and silicon semiconductor, were used as independent parameters for the analyses. The dependence of MFIS C–V characteristics on ferroelectric properties, film thickness and impurity concentration of the semiconductor is calculated. Film thicknesses of ferroelectrics and insulators affect the memory window width markedly, and have to be optimized in order to obtain a large memory window width. On the other hand, MFIS C–V characteristics and memory window width are not influenced by Pr because the effective Pr is small in the MFIS structure.

Preparation and Characterization of High-k Praseodymium and Lanthanoid Oxide Thin Films Prepared by Pulsed Laser Deposition

Several lanthanoid oxide thin films such as those of PrOx, Sm2O3, Tb4O7, Er2O3 and Yb2O3 have been prepared on Si(100) wafers by the pulsed laser deposition method (PLD). PrOx film shows thin SiO2-equivalent oxide thickness (EOT) and low leakage current simultaneously. On the other hand, SmOx thin film does not show good properties. It is revealed by XPS spectra of the PrOx film that the deposition in O2 ambient of 0.2 Torr produces an interfacial SiO2 or silicate layer. The sample deposited in a high vacuum at RT has only an ultra-thin interfacial layer, but hysteresis in the C–V characteristic and leakage current are large. Other techniques have been carried out to reduce the energy of ablated particles in order to prevent the growth of an interfacial layer. In the deposition method using a shadow mask, very flat thin films were obtained. However, the deposition rate was very low, and growth of the interfacial layer could not be prevented. By enlarging the distance between substrate and target, the smallest EOT with the PrOx film in our study has been obtained by the reduction of the energy of ablated particles.

Photoreflectance characterization of the plasma-induced damage in Si substrate

Si surface damage induced during Ar and CHF3/CF4/Ar plasma processing has been characterized by photoreflectance spectroscopy (PRS). The photoreflectance (PR) signal intensity decreases drastically as the rf input power increases in the Si substrate exposed to the plasma. The recovery of the plasma-induced damage is confirmed by PRS after subsequent annealing over 500 °C. We found, from the shift of the PR spectra, the strain at the Si surface induced by the plasma treatment might be due to the introduction of the ions, C, F, or H, not to those of Ar. The depth profile of the defect density in the Si substrate is estimated quantitatively by analyzing the depth profile of the PR signal intensities.

Structural and ferroelectric properties of epitaxial Bi5Ti3FeO15 and natural-superlattice-structured Bi4Ti3O12–Bi5Ti3FeO15 thin films

Bismuth-layer-structured multiferroic Bi5Ti3FeO15 (BTFO15) (number of pseudoperovskite blocks, m=4) and natural-superlattice-structured Bi4Ti3O12–Bi5Ti3FeO15 (BIT-BTFO15) (m=3 or 4) thin films were prepared on (001)-oriented and (110)-oriented La-doped (3.73 wt %) SrTiO3 (La-STO) single-crystal substrates using pulsed laser deposition. X-ray diffraction patterns of these thin films on (001) La-STO single crystals indicated that the obtained thin films were single-phase (00l)-oriented layered perovskite, and that the BIT-BTFO15 (m=3 or 4) natural-superlattice structure was also obtained. On (110) La-STO single crystal, layered perovskite (11l)-oriented thin films were also obtained. Moreover, cross-sectional transmission electron microscope images of these thin films revealed four pseudoperovskite blocks (m=4) in the BTFO15 thin films, and an intergrowth structure of three and four pseudoperovskite blocks (m=3 or 4) in BIT-BTFO15 sandwiched by two (Bi2O2)2+ layers. In ferroelectric D-E hysteresis loop meas...

A pseudo-single-crystalline germanium film for flexible electronics

We demonstrate large-area (∼600 μm), (111)-oriented, and high-crystallinity, i.e., pseudo-single-crystalline, germanium (Ge) films at 275 °C, where the temperature is lower than the softening temperature of a flexible substrate. A modulated gold-induced layer exchange crystallization method with an atomic-layer deposited Al2O3 barrier and amorphous-Ge/Au multilayers is established. From the Raman measurements, we can judge that the crystallinity of the obtained Ge films is higher than those grown by aluminum-induced-crystallization methods. Even on a flexible substrate, the pseudo-single-crystalline Ge films for the circuit with thin-film transistor arrays can be achieved, leading to high-performance flexible electronics based on an inorganic-semiconductor channel.

Pulsed Laser Deposition and Characterization of Sr and Zn Co-Substituted BiFeO3 Thin Films

Bi1.1SrxFe0.9Zn0.1O3 (x=0.05,0.1) (BSFZO), Bi1.1SrxFe0.9O3 (x=0.05,0.1) (BSFO), and Bi1.1Fe0.9Zn0.1O3 (BFZO) thin films have been prepared on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition, and their ferroelectric and ferromagnetic properties have been characterized. X-ray diffraction (XRD) patterns of BSFO and BFZO thin films show a peroveskite single phase and BSFO thin films show shifting and splitting of peaks with increasing fraction of Sr. The BFZO thin film shows a lower leakage current than the BFO thin film and their P–E and M–H hysteresis loops are obtained. The XRD peaks of BSFZO thin films also shift to an angle lower than that of BFO thin films. The P–E hysteresis loop is obtained at 80 K and the reamanent polarizations of the Bi1.1SrxFe0.9Zn0.1O3 thin film are 79 and 58 µC/cm2 for x=0.05 and 0.1, respectively. The M–H hysteresis loop is also obtained at 80 K and the reamanent magnetization and coercive field are 1.1 and 6 emu/cm3, and 222 and 792 Oe at a maximum magnetic field of 10 kOe for x=0.05 and 0.1, respectively.

Soft X-ray photochemistry beamline at SPring-8.

Design and construction of a soft X-ray beamline at SPring-8 is reported. The beamline utilizes high-quality linearly polarized soft X-rays obtainable from a figure-8 undulator for the study of photophysical and photochemical processes of atoms, molecules and surfaces in the inner-shell excitation region. It consists of two experimental stations, a photochemistry station and a chemical vapour deposition (CVD) station. A high-resolution grating monochromator is installed at the photochemistry station, while the intense undispersed undulator radiation is used at the CVD station. Unique features of the experimental chambers and of the analysis and characterization systems are described along with those of the monochromator.

Tactle array sensor with inclined chromium/silicon piezoresistive cantilevers embedded in elastomer

Tactile array sensor of the micro-cantilevers embedded in the elastomer has been fabricated to detect normal and 2-axes shear stresses. It is demonstrated that tactile array sensor with 3 × 3 detective elements can be fabricated with excellent yield. The sensor element is sensitive to both normal and shear stresses applied on entire sensor surface. Moreover, it has good directional characteristics so that it is shown that magnitude and direction of shear stress can be obtained by difference of output voltages of adjacent sensor cantilevers. The sensor output has good reproducibility for multiple measurement. The output from sensor element drastically changes with shifting applying position of force. It is considered that we can obtain pressed position from distribution of output from sensor element array.