Yoshiki Iwazaki

Electric field induced piezoelectric resonance in the micrometer to millimeter waveband in a thin film SrTiO3 capacitor

Thin film SrTiO3 metal–insulator–metal (MIM) capacitors were fabricated in order to characterize the piezoelectric resonance in the micrometer to millimeter waveband arising from electric field induced ferroelectricity in the SrTiO3 film. The specimens showed a second-order phase transition, and the piezoelectric resonance appeared when an electric field of 250 kV/cm was applied at room temperature. Finite element method (FEM) analysis was used to interpret the piezoelectric resonance observed in the capacitors. The FEM analysis data basically agrees well with the experimental data, and the few differences between the theoretical and experimental data are interpreted mainly as artifacts caused by overlapping of resonant/antiresonant peaks during the measurement. The piezoelectric resonance of thin film MIM capacitors is strongly influenced by the longitudinal stacked structure of the MIM.

Negatively charged hydrogen at oxygen-vacancy sites in BaTiO3: Density-functional calculation

With a specific focus on defects formed from H atoms and oxygen vacancies (VO) in perovskite-type oxide, stability and stable valence states of complex defects formation processes within BaTiO3 are studied by using first-principle density-functional theory calculations. In our findings, H atoms diffuse as protons (H+) into interstitial sites in BaTiO3, whereas these atoms when trapped at centers of VO sites convert to negatively ionized states (H−). We also find that H atom trapping at VO sites occurs only in n-type carrier-rich environments without carrier compensation of VO2+ and H+. If carrier electrons are compensated, H+ ions are excluded from VO2+ sites due to the repulsive Coulomb potential existing between the H+ ion and the positively charged VO2+ site. Difficulties in the calculation of the diffusion-energy diagram for H atoms, involving essential changes in the stable valence states during the diffusion process, are discussed and a practical solution is presented.

Artificial SrTiO3/SrO Superlattices by Pulsed Laser Deposition

Artificial SrTiO3/SrO superlattice thin films epitaxially grown on SrTiO3 substrates were fabricated by pulsed laser deposition. The artificial superlattices were designed and fabricated by introducing a Ruddlesden-Popper (RP) layer periodically. The intensity of reflection high-energy electron diffraction (RHEED) patterns during the growth of the superlattices increased at the deposition of SrTiO3 layers and decreased at the deposition of the SrO monolayer, alternately. The gradual intensity decrease of RHEED reflection during SrO layer deposition is attributable to the island growth of SrO. The different type of RP layers normal to the film surface, observed in a high-temperature deposited superlattice, may originate from the higher diffusion condition and island growth of SrO.

Defect Structure of Heteroepitaxial SnO2 Thin Films Grown on TiO2 Substrates

Heteroepitaxal SnO2 thin films were grown on rutile (100) TiO2 single crystal substrates by pulsed laser deposition. The defect structure and surface morphology were investigated by transmission electron microscopy, reflection high-energy electron diffraction, and atomic force microscopy. The misfit-induced large biaxial compressive stress in the 200-nm-thick SnO2 thin film was almost fully relaxed to yield the high-density interfacial misfit dislocations and related planar defects. The misfit dislocation network on the hetero-interface consisted of two types of partial edge dislocations with Burgers vectors of 1/2 and 1/2 , which involved {101} and {010} planar defects formed in the film, respectively. The evolution of the surface morphology and cross-sectional structure for thinner films suggested that the introduction of such defects occurs at the early growth stage, due to large lattice misfit, and is strongly affected by point defect condensation.

Diversity of hydrogen configuration and its roles in SrTiO3−δ

As a source of carrier electron, various configurations of hydrogen in SrTiO3 are searched by using first-principles calculations. The most stable form of hydrogen is found to be H−, where doubly charged oxygen vacancy VO2+ changes into singly charged HO+. Most importantly, an additional H− is found to be weakly trapped by HO+, which completely neutralizes carrier electrons by forming (2H)O0. These unexpected behaviors of hydrogen, which can explain reported experimental results, expand the role of the hydrogen in carrier-control technology in transition-metal oxides.

Thermal Stability of Artificial SrTiO3/SrO Superlattice Epitaxially Grown on SrTiO3 Single Crystal

A post-annealed artificial (SrTiO3)5/SrO superlattice grown on a (100) SrTiO3 single crystal substrate was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), to investigate the thermal stability of inserted rock-salt layers of SrO, the so-called Ruddlesden-Popper (RP) layers, in the epitaxial thin film. The intensities of XRD satellite peaks stemming from the superstructure decreased with increasing annealing temperature and almost disappeared at 1000°C. The high-resolution cross-sectional TEM image of the superlattice after annealing at 1000°C showed that RP layers perpendicular to the film surface appeared, which was predominant over in-plane RP layers. The predominant formation of the vertical RP layers may be attributed to the strain release of RP layers at the film surface.

X-ray absorption fine structure analysis of molybdenum added to BaTiO3-based ceramics used for multilayer ceramic capacitors

The effect of slight molybdenum doping of perovskite-type BaTiO3-based ceramics on the reliability of a multilayer ceramic capacitor (MLCC) and on the valence state of molybdenum in the BaTiO3-based ceramics has been investigated by highly accelerated lifetime tests and X-ray absorption fine structure analysis. The molybdenum added to the BaTiO3-based ceramics is located at Ti sites and improves the highly accelerated lifetime and lowers the initial dielectric resistivity in MLCCs. Through sintering in a reducing atmosphere, which is an important process in the fabrication of BaTiO3-based MLCCs, the oxidation state of the molybdenum added could be adjusted from +6 to a value close to +4.

Integrated (Ba,Sr)TiO3 Thin-Film Technology for RF and Microwave Applications

The microwave tunable capability and its related material optimization of (Ba,Sr)TiO3 thin films in the parallel-plate capacitor form is discussed in terms of the dependence of barium concentration, acceptor doping, and in-plane film stress, based on the present broadband microwave characterization technique under various bias fields. The barium-content dependence indicates the tradeoff between tunability and dielectric loss, and the notable field-induced loss in SrTiO3 is confirmed as an intrinsic quasi-Debye contribution. The Mg dopant incorporated into a perovskite lattice shows almost no effectiveness on tunable device performance, except for enhanced insulation as an electron acceptor, while the low bias-field dependence of the dielectric loss suggests the possibility of the partial occupation of the alkaline-earth-ion site by Mg. The reduction of in-plane thermal stress controlled by the pressure during sputtering deposition leads to higher permittivity and tunability while degrading the film crystallinity by ion bombardment. The low-frequency loss tends to increase with crystal damage; however, the microwave loss remains unchanged, revealing the applicability of sputtering stress control to real microwave devices. In addition, we demonstrate the operation of an analog phase shifter using parallel-plate ferroelectric tunable capacitors and its application to a phased array antenna monolithically integrated on a silicon substrate.