Masatoshi Ishii

Chemisorption of water and carbon dioxide on nanostructured BaTiO3–SrTiO3(001) surfaces

The interaction of water and carbon dioxide with nanostructured epitaxial (Ba,Sr)TiO3(001) thin film and bulk single crystal SrTiO3(001) surfaces was studied using x-ray photoemission spectroscopy (XPS), thermal desorption spectroscopy (TDS), and density functional theory (DFT). On both surfaces, XPS and TDS indicate D2O and CO2 chemisorb at room temperature with broad thermal desorption peaks (423–723 K) and a peak desorption temperature near 573 K. A comparison of thermal desorption Redhead activation energies to adsorption energies calculated using DFT indicates that defect surface sites are important for the observed strong adsorbate-surface reactivity. Numerical calculations of the competetive adsorption/desorption equilibria for H2O and CO2 on SrTiO3(001) surfaces show that for typical atmospheric concentrations of 0.038% carbon dioxide and 0.247% water vapor the surfaces are covered to a large extent with both adsorbates. The high desorption temperature indicates that these adsorbates have the pote...

Electrooptic planar deflector switches with thin-film PLZT active elements

First prototypes of electrooptic (EO) planar deflector switches (PDSs) are fabricated with hybrid integration on Si substrates. Planar optical modules, made in silica-on-silicon technology, consist of input and output (I/O) waveguide microlenses facing each other and slab waveguides in between. The modules interconnect the I/O fibers with laterally collimated light beams less than 400 /spl mu/m in width at distances up to 100 mm with losses lower than 3 dB. Thin lead lanthanum zirconium titanate (PLZT) films with prism-shaped electrodes grown on SrTiO/sub 3/ substrates form the deflector elements. The PLZT films are more than 10 /spl mu/m thick with EO coefficients about 40 pm/V. The deflector assembly technology provides chip vertical positioning accuracy better than 1 /spl mu/m. The deflector chips are attached to the optical substrates with thermo-compression flip-chip bonding. The optical power losses of the modules with test silica chips can be as low as 3.6 dB. However, the lowest module losses achieved with PLZT are about 10 dB. The channel-to-channel switching operations are demonstrated at about 40 V and switching times less than 500 ns.

Effect of Y doping and composition-dependent elastic strain on the electrical properties of (Ba,Sr)TiO3 thin films deposited at 520 °C

We demonstrate that large and simultaneous improvements in permittivity, tunability, and leakage current density of (Ba,Sr)TiO3 (BST)-based thin-film capacitors can be achieved by yttrium doping. We have found that, for a low deposition temperature (520 °C) sputtering process, Y-doped BST capacitors exhibit tenfold lower leakage current density (<10−9A∕cm2 at 100KV∕cm) and 70% higher permittivity than nominally undoped BST-based capacitors. Furthermore, this work suggests an intriguing correlation between dopant concentration-dependent elastic strain in the films and their enhanced dielectric properties.

Band offsets at the epitaxial SrTiO3/SrZrO3 (0 0 1) heterojunction

The interface formation between SrTiO3 and SrZrO3 has been studied using in situ photoelectron spectroscopy. Epitaxial SrZrO3 thin films were grown on (0?0?1)-oriented SrTiO3?:?Nb single crystals via pulsed-laser deposition. The epitaxial SrZrO3 growth was verified via x-ray diffraction and Cs-corrected high angle annular dark field scanning transmission electron microscopy. A type I straddling configuration has been found for the interface analysed with a valence band and conduction band offset of 0.5?eV and 1.9?eV, respectively.

Crystal orientation dependence of the electro-optic effect in epitaxial lanthanum-modified lead zirconate titanate films

Lanthanum-modified lead zirconate titanate epitaxial films with (100), (101), and (111) orientations were grown on (100), (101), and (111) niobium lending conductivity strontium titanate using chemical solution deposition, respectively. We investigated the changes in the refractive index induced by the electric field in these films using the prism coupling method. Birefringence was observed in the (101)- and (111)-oriented epitaxial films induced by the electric field. However, no birefringence was observed in the (100) epitaxial film. This crystal orientation dependence of the electrooptic effect can be explained by the rotation of the refractive indicatrix that accompanies the switching of the polar clusters.

Electrooptic Properties of Epitaxial Lead Zirconate Titanate Films on Silicon Substrates

Electrooptic (EO) properties and propagation losses of the lead zirconate titanate (PZT) films grown on silicon (Si) substrates have been investigated. PZT films were prepared on Si substrates by chemical solution method. Refractive index changes and propagation losses of PZT films were evaluated by prism coupling method. A (100)-oriented 8.9-µm-thick epitaxial PZT film grown on a Si substrate with strontium ruthenium oxide/ceria/yttria-stabilized zirconia (SRO/CeO2/YSZ) epitaxial buffer layer was found to have large EO effect and very low propagation loss. Propagation of an infrared light with a wavelength of 1550 nm into a PZT film on Si substrate was successfully confirmed.

Electronic transport behavior of off-stoichiometric La and Nb doped SrxTiyO3−δ epitaxial thin films and donor doped single-crystalline SrTiO3

Above room temperature electronic transport properties of SrxTiyO3−δ films with cation A/B = (La + Sr/Nb + Ti) ratios of 0.9 to 1.2 are compared to STO single crystals with combined Hall carrier densities of 3 × 1016 cm−3 ≤ nH ≤ 1022 cm−3. In contrast to Hall mobility which is single crystal-like (μH ≈ 6 cm2/Vs) only near A/B = 1, the Seebeck coefficient (S) is single crystal-like over a range of nonstoichiometry. For nH   1021 cm−3, S is metallic-like with m∗/mo ∼ 8. No marked increase in m∗ with decreasing nH owing to a carrier filling dependence is observed.

Electrooptic properties of lead zirconate titanate films prepared on silicon substrate

The electrooptic (EO) properties of light propagated through lead zirconate titanate (PZT) films prepared on silicon (Si) substrates were successfully evaluated. Polycrystalline, {101}-oriented and {100}-oriented epitaxial PZT films with a thickness of approximately 2 µm were prepared on various types of Si substrate by chemical solution deposition. The anisotropic EO effect was observed in the polycrystalline and {101}-oriented PZT films. The in-plane and out-of-plane refractive indices of the {100}-oriented epitaxial PZT films changed with the type of substrate. The EO effect of the {100}-oriented epitaxial PZT film grown on a Si substrate was found to be more isotropic than those of the polycrystalline and {101}-oriented PZT films. The largest EO coefficients (re=54 pm/V and rc=60 pm/V) were obtained in the {101}-oriented PZT film on the Si substrate.

Dielectric constant dispersion of yttrium-doped (Ba,Sr)TiO3 films in the high-frequency (10kHz–67GHz) domain

The frequency dispersion of the dielectric constant of yttrium (Y)-doped (Ba,Sr)TiO3 thin films (Y-BST) in the high-frequency domain (10kHz–67GHz) was investigated. In order to remove the substantial parasitic capacitances, inductances, and resistances from the measured impedance data, test samples, short-circuit standard, and open-circuit standard structures were fabricated and their frequency response was measured. Before removing parasitic components, the measured dielectric response showed a rolloff at approximately 4GHz. However, after circuit calibration, the dielectric constant was almost constant up to 40GHz where another rolloff was observed. However, this rolloff was due to the uncompensated small parasitic components. Therefore, the dielectric constant of the Y-BST films (170 with a film thickness of 30nm) showed small frequency dispersion corresponding to the Curie–von Schweidler dispersion, of which the exponent is −0.0131, up to 40GHz. Furthermore, the decrease of the capacitance was 17% in ...

Thermopower in quantum confined La-doped SrTiO3 epitaxial heterostructures

We characterize effects of dimensionality reduction and electron confinement on thermopower in La-doped SrTiO3 (LSTO) thin films using hetero-epitaxial capping layers with known conduction band offsets. Hall measurements on SrZrO3/LSTO/(La0.3Sr0.7)(Al0.65Ta0.35)O3 quantum well structures indicate carriers are homogeneously confined in the LSTO film, leading to an unambiguous well width. We do not observe a substantial enhancement of the Seebeck coefficient (S) for confinement widths as thin as 8 A. Recent work towards elucidating the role of thickness scaling on S in complex oxides and implications of our results for using 2D electron confinement to develop advanced thermoelectric materials are discussed.