Shiro Takeno

Structural modulation and superconducting properties in Bi2-xPbxSr2CaCu2O8+d and Bi2-yPbySr2YCu2O8+d

Abstract Changes in the structural modulation and the superconducting properties were examined in Bi 2- x Pb x Sr 2 CaCu 2 O 8+ d and Bi 2- y Pb y Sr 2 YCu 2 O 8+ d systems. In both systems, the b -axis decreased and the periodicity of the structur al modulation along the b -axis increased with increasing Pb content, and the structural modulation disappeared in a high Pb concentration region. These changes in structural modulation were explained by the decrease in additional oxygen in the Bi-O layer. Besides this relaxation of the structural modulation, the superconducting transition became sharper, and a slight increase in T c was observed in the Bi 2- x Pb x Sr 2 CaCu 2 O 8+ d system.

Electrical properties and microstructures of Pt/Ba0.5Sr0.5TiO3/SrRuO3 capacitors

Thin film polycrystalline Ba0.5Sr0.5TiO3 capacitors employing conductive perovskite oxide SrRuO3 as a bottom electrode were fabricated on Si substrates by rf-magnetron sputtering. Dielectric constants of these capacitors were observed to be 206, 214, and 146 for 60, 40, and 20 nm Ba0.5Sr0.5TiO3 (BSTO) thicknesses, respectively. The lowest SiO2 equivalent thickness of 0.54 nm was obtained for 20-nm-thick BSTO capacitors; their leakage current density was less than 1×10−7 A/cm2 for ±1.8 V bias. The cross-sectional transmission electron microscope observation revealed that BSTO and SrRuO3 (SRO) formed continuous columnar grains and showed epitaxial growth at the interface of BSTO/SRO within each column; this microstructure of the capacitors was tentatively designated “local epitaxial film.”

Suppression of the floating-body effect in SOI MOSFET's by the bandgap engineering method using a Si/sub 1-x/Ge/sub x/ source structure

The bandgap engineering method using a SiGe source structure is presented as a means to suppress the floating-body effect in SOI MOSFETs. Experiments using Ge implantation are carried out to form a narrow-bandgapped SiGe layer in the source region. It has been confirmed that Ge-implanted SIMOX exhibited a 0.1 eV bandgap narrowing with a relatively low Ge-dosage of 10/sup 16/ cm/sup -2/. The fabricated N-type SOI-MOSFETs exhibited suppressed parasitic bipolar effects, such as improvement of the drain breakdown voltage or latch voltage, and suppression of abnormal subthreshold slope. Advantages over other conventional methods are also discussed, indicating that the bandgap engineering provides a practical method to suppress the floating-body effect.

Evaluation of two-dimensional strain distribution by STEM/NBD.

We proposed a strain mapping technique by Nano Beam electron Diffraction (NBD) combined with an energy filter (EF) and a scanning transmission electron microscopy (STEM) function. The STEM function improves the accuracy of a position where a diffraction pattern is acquired. The EF excludes inelastic scattering and enables novel numerical processing for the appropriate measurement of distances between diffraction disks. Using this technique, strain distributions were measured for two different types of p-MOSFETs, i.e., source/drain regions of each MOSFET is composed of different types of silicide, and the difference of their strain distributions in the channel region was confirmed. The proposed method was able to clarify that the strain distributions are quite different depending on the silicide materials even if the exterior of the MOSFETs was almost identical.

Depth resolution improvement in secondary ion mass spectrometry analysis using metal cluster complex ion bombardment

Secondary ion mass spectrometry analyses were carried out using a metal cluster complex ion of Ir4(CO)7+ as a primary ion beam. Depth resolution was evaluated as a function of primary ion species, energy, and incident angle. The depth resolution obtained using cluster ion bombardment was considerably better than that obtained by oxygen ion bombardment under the same experimental condition due to reduction of atomic mixing in the depth. The authors obtained a depth resolution of ∼1nm under 5keV, 45° condition. Depth resolution was degraded by ion-bombardment-induced surface roughness at 5keV with higher incident angles.

Effect of Fe Doping of Thin (Ba,Sr)TiO3 Films on Increase in Dielectric Constant

Barium strontium titanate [(Ba,Sr)TiO3, BST] is an attractive material with a very high dielectric constant. However, the dielectric constant of BST films decreases with film thickness, which has become a serious problem for dynamic random access memory (DRAM) application. We found Fe doping of BST films to be remarkably effective for suppressing the decrease in dielectric constant down to the very thin film regime. As a result, a marked increase in the dielectric constant for thin BST films is achieved. Furthermore, we identify the site where the doped Fe ions are substituted in the crystal by means of electron standing wave method using SrTiO3 single crystal. The result shows that most of the Fe ions are substituted at B sites (Ti sites), which suggests that Fe ions as acceptors compensate donor electrons and reduce band bending. This is thought to be the most likely mechanism of the increase in dielectric constant caused by Fe doping.

A high-resolution electron microscopy study on epitaxial infinite- layer SrCuO2 thin films grown on a SrTiO3 (100) substrate

Abstract The authors investigated defect structures introduced into so-called infinite-layer SrCuO 2 thin films sputtered on a SrTiO 3 (100) substrate. By means of both cross-sectional and plan-view high-resolution electron microscopy (HREM), it has been revealed that the films were epitaxially grown with their c -axis normal to the substrate surface, and several types of defect structures have been estimated. They were essentially composed of double Sr-O planes that make {100} atomic walls. It is remarkable that such walls tended to arrange as loop-like configurations, and also could produce a different ordered structure, i.e., a Sr 2 CuO 3 -like structure in local regions. The authors propose defect structural models in the epitaxial infinite-layer SrCuO 2 thin films in detail.

Periodicity Change in Structural Modulation in Bi2Sr2Ca1-yREyCu2O8+δ (RE=Y, Nd) System

Changes in structural modulation for the Bi2Sr2Ca1-yREyCu2O8+δ system (RE=Y, Nd), accompanied with the transition from the superconductor to the antiferromagnetic insulator, were examined. In the Bi2Sr2Ca1-yYyCu2O8+δ system, the modulation periodicity continuously decreased, from about 5.0 times to 4.0 times the length of the subcell along the b axis, with increasing Y content substituted. The modulation periodicity for Bi2Sr2NdCu2O8+δ was about 4.0 times the length of the subcell along the b axis, the same as in the Bi2Sr2YCu2O8+δ. This decrease in the modulation periodicity is attributed to the valence change of Y3+ for Ca2+.

Preparation of Epitaxial SrCuOX Thin Films with an Infinite-Layer Structure

The authors successfully prepared epitaxially-grown SrCuOX films with an infinite-layer structure by stacking atomic layers of SrOX and CuOX alternately, using a shutter-controlled sputtering method. High-resolution electron microscopy (HREM) observation indicated that the films had an infinite-layer structure and were epitaxially grown. It was considered that in order to prepare epitaxial SrCuOX films with an infinite-layer structure, the CuOX layers had to be CuO, not Cu2O or Cu.

Defect structure study of YBa2Cu3O7−x epitaxial thin films on SrTiO3(100) and (110) substrates

Abstract The authors investigated the micro-defect structures in YBa 2 Cu 3 O 7− x (YBCO) superconducting thin films sputtered on SrTiO 3 (100) and SrTiO 3 (110) substrates without post-deposition annealing. HREM observations and RHEED studies were performed. Near the surface regions of YBCO films on SrTiO 3 (100) were oriented mainly with the a axis normal to the substrate, and the film showed many {103} domain boundaries with a zigzag trace. On the other hand, the YBCO films on SrTiO 3 (100 ) were oriented with the [110] axis normal to the substrate, and the films often showed out-of-phase boundaries along the c direction. These out-of-phase boundaries can be originated by the introduction of misfit dislocations with extra planes in the substrate crystal. The localization defect structures in YBCO thin films are proposed.