Noburu Fukushima

Thickness Dependence of Ferroelectricity in Heteroepitaxial BaTiO3 Thin Film Capacitors

Heteroepitaxial BaTiO3 films of various thicknesses ranging from 12 nm to 79 nm were prepared on SrRuO3/SrTiO3 substrates by radio-frequency magnetron sputtering employing a two-step deposition technique, and the crystallographic and ferroelectric properties of the heteroepitaxial films were evaluated. A ferroelectric hysteresis loop was clearly observed in the heteroepitaxial BaTiO3 films even when the thickness was reduced to 12 nm, probably due to improved crystallinity at the interface between the ferroelectric film and the electrodes through optimization of the preparation technique. The observation of hysteresis at the temperature of 200°C was explained in terms of modification of the Curie temperature from the inherent 130°C to far above 200°C by lattice misfit strain. A possible relationship between the artificially raised Curie temperature and the limited thickness dependence of ferroelectricity was discussed.

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.

Interfacial properties of single-crystalline CeO2 high-k gate dielectrics directly grown on Si (111)

Interfacial properties of single-crystalline CeO2 high-k dielectrics directly grown on Si (111) were investigated by comparing metal–insulator–semiconductor field-effect transistors (MISFETs) without any interfacial layer [(w/o-IL); direct growth of CeO2 on Si] and those with an interfacial layer (w-IL). FET characteristics, such as the drain current and the S factor, for the w/o-IL MISFET were much worse than those for the w-IL MISFET. The in-gap states attributed to the oxygen defects were detected in the CeO2 directly grown on Si by x-ray photoelectron spectroscopy measurements. The large interface state density induced by the oxygen defects at the CeO2/Si interface may deteriorate the w/o-IL MISFET performances.

Critical current density of wire type Y-Ba-Cu oxide superconductor

We have fabricated wire type Y-Ba-Cu oxide superconductor by powder metallurgy technique. A large critical current density Jc of 7.25 × 102 A/cm2 at 77 K and 0 T and the resistive Tc transition of 89 K to 87 K were obtained for the wire type conductor heat treated without Cu sheath. However, the Jc(77 K) drastically decreased by a factor of 200, applying a magnetic field of 1 T. On the contrary, Jc(4.2 K) little changed; 1.63 × 102 A/cm2 at 1 T and 1.33 × 102 A/cm2 at 12 T. Based on the magnetic field dependence of Jc, we estimate upper critical field Bc2 (i.e. corresponding to Bc2 defined by Tc (offset)) and GL coherence length ζ.

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.”

Electrical Properties of Single Crystalline CeO2 High-k Gate Dielectrics Directly Grown on Si (111)

We have grown epitaxial single crystalline CeO2 directly on Si (111) by molecular beam epitaxy. Metal-insulator-semiconductor (MIS) capacitors were fabricated to evaluate electrical properties of CeO2 films. By eliminating an interfacial layer, equivalent oxide thickness (EOT) as small as 0.38 nm was obtained for a CeO2 physical thickness of 5 nm. The dielectric constant (e) was calculated to be e~52, which is two times larger than the value reported for bulk (polycrystalline) CeO2 (e~26). Enhancement of e is probably attributable to the anisotropy of the single crystalline CeO2 and/or the lattice distortion of CeO2 directly grown on Si. A leakage current density of ~ 1 A/cm2 was obtained at an equivalent electrical field of 5 MV/cm for the CeO2 MIS capacitors, which is much smaller than the expected value for SiO2 capacitors with the same EOT. It is revealed that single crystalline CeO2 is promising as an alternative gate dielectric.

Electrical Properties of All-Perovskite Oxide (SrRuO3/BaxSr1-xTiO3/SrRuO3) Capacitors

Thin film capacitors consisting totally of perovskite oxides (SrRuO 3 /Ba x Sr 1-x TiO 3 (20nm)/SrRu03) were fabricated on Si and SrTiO 3 substrates by rf magnetron sputtering. The dielectric constants for polycrystalline and single-crystal epitaxial films were observed to be 274 and 681, respectively. The lowest SiO 2 equivalent thickness for the single-crystal epitaxial capacitor was 0.11 nm. The leakage current density was less than 1 x 10 -7 A/cm 2 for a bias of ±1.2V. The electrical properties of all-perovskite oxide capacitors were compared with those of samples with Pt top electrodes, and large differences in the values of the dielectric constant were observed. These are explained in terms of differences in the interface between the dielectric and the top electrode and the possible existence of a low-e layer at the interface between the Pt top electrode and Ba x Sr 1-x TiO 3 . In addition, the origin of the large difference in the dielectric constants for polycrystalline and single-crystal epitaxial capacitors are discussed in terms of the lattice distortion in Ba x Sr 1-x TiO 3 .

Effect of Oxygen Content and Sr/Ca Ratio on Superconducting Properties in Bi2Sr2-xCa1+xCu2O8+δ

Superconducting properties of the 80 K class superconductor in the Bi-Sr-Ca-Cu-O system were examined from the viewpoints of the oxygen content and the Sr/Ca ratio. The Tc of Bi2Sr2CaCu2O8+δ was reduced with decreasing δ. The solid solution region of Bi2Sr2-xCa1+xCu2O8+δ was confirmed for x values between -0.25 and 1.0. The Tc changed slightly in the solid solution region. The transition to insulator, however, was not observed by changing the oxygen content or the Sr/Ca ratio.

LaAlO3 gate dielectric with ultrathin equivalent oxide thickness and ultralow leakage current directly deposited on Si substrate

By a careful choice of film deposition conditions, LaAlO3 (LAO) gate dielectric film with equivalent oxide thickness (EOT) of 0.31nm and gate leakage current density (Jg) of 0.1A∕cm2 (at Vfb+1V) has been successfully demonstrated. Elimination of interfacial low-k layer at LAO/Si and reduction of defect density in LAO has been realized through both LAO film deposition at high-temperature (700°C) and subsequent low-temperature (200°C) annealing. By using thermal desorption spectroscopy technique, we find that our process reduces remnant H2O or OH− species in the LAO film, which are responsible for the degradation of EOT and Jg.

Electrical and Magnetic Properties in Bi2Sr2Ca1-xYxCu2O8+δ

Electrical and magnetic properties were examined on Bi2Sr2Ca1-xYxCu2O8+δ. A solid solution was confirmed in this system for x between 0 and 1. With increasing Y, the superconducting transition shifted toward lower temperatures, and a transition to an antiferromagnetic insulator was observed. Since the oxygen content was not influenced by Y substitution, the decrease in Tc and the transition to AFI seemed to be attributed to the decrease in the hole concentration in the Cu-O level.