Yorinobu Yoshisato

Fabrication and characterization of Ba1−xKxBiO3/Nb-doped SrTiO3 all-oxide-type Schottky junctions

A reproducible process for fabricating Ba1−xKxBiO3/Nb-doped SrTiO3 (BKBO/STNO) all-oxide-type Schottky junctions has been established, and the electrical properties of the junctions have been investigated at room temperature. Improving the STNO surface conditions by high-temperature oxygen-annealing made it possible to fabricate junctions with good rectification properties. The current–voltage characteristics of the junctions were explained by conventional thermionic emission theory. Anomalous capacitance–voltage characteristics of the junctions were observed and were analyzed with a model taking into account the electric-field-dependent permittivity of STNO and the presence of the interfacial layer. The potential barrier profile of the BKBO/STNO interface was calculated and it can be concluded that this model quantitatively explains the capacitance–voltage characteristics.

Effect of the Field Dependent Permittivity and Interfacial Layer on Ba1-xKxBiO3/Nb-Doped SrTiO3 Schottky Junctions

The electrical properties of Ba1-x Kx BiO3/Nb-doped SrTiO3 all-oxide-type Schottky junctions have been investigated by measuring their current-voltage and capacitance-voltage ( C-V ) characteristics at room temperature. The relative permittivity e r of Nb-doped SrTiO3 could be approximated as e r(E)=b/√ a+E2, where E is the electric field, a and b are constants. Taking into account the field dependent permittivity and interfacial layer, we carried out a quantitative analysis of the potential barrier and 1/C2-V characteristics. The non-linear 1/C2-V characteristics could be quantitatively explained by considering the field dependent permittivity and the bias dependence of the barrier height due to the presence of the interfacial layer.

Property Control for High-Quality Ba1-xKxBiO3 Epitaxial Thin Films Prepared by High-Pressure Reactive rf-Magnetron Sputtering

The electrical properties of Ba1-xKxBiO3(BKBO) epitaxial thin films on SrTiO3(110) substrates have been successfully controlled using high-pressure reactive rf-magnetron sputtering under an 80-Pa discharge gas. We investigated the relationship between the target composition and the electrical properties of BKBO films. The zero resistance temperature (Tce) and the temperature dependence of resistivity above the superconducting onset temperature (Tco) were found to be primarily influenced by the ratio of Ba/K and the Bi content of the target composition, respectively. Electrical measurements showed that the highest Tce of as-grown BKBO epitaxial film was 28 K.

Microwave Detector Using Granular-Type YBCO Superconductors

A highly sensitive microwave detector has been developed for the first time by granular-type bulk YBCO superconductors with fine particles. Particles with a size of about 1 µm, having a SNS-like grain boundary, were controlled by a coprecipitation method, followed by a sintering process. A new fabrication method for bulk superconductors was also developed to produce a precise microbridge-type structure. Sensitivity to microwave power below -40 dBm was obtained, and a direct mixing effect was obtained at a frequency of 8.6 GHz at the temperature of 77 K. It was confirmed that the Josephson effect of the detector at the SNS grain boundary formed in the sintering process for ceramics is the most responsive for microwave detection.

Junction Characteristics of an Au/Ba1-xKxBiO3/Niobium-Doped SrTiO3 Structure

Au/Ba1-xKxBiO3 and Ba1-xKxBiO3/niobium-doped SrTiO3, two types of junctions which correspond to the emitter base and base collector in superconducting base transistors, respectively, have been fabricated and studied. The highest Tc end point (Tc0) of 28 K was obtained of thin film Ba1-xKxBiO3 by rf-sputtering method. The Au/Ba1-xKxBiO3 tunnel spectrum showed a clear gap structure and was very close to the ideal Bardeen Cooper Schrieffer theorys (BCS) form with Δ(0)=3.19 meV. The Ba1-xKxBiO3/SrTiO3(Nb) junction with heavy niobium doping (0.1 wt%) showed tunnellike I-V characteristics. The dI/dV spectrum for a high-Tc superconductor/semiconductor junction displayed the energy gap structure of a superconductor.

Characterization of a Natural Barrier in an Au/Ba1-XKXBiO3 Junction

X-ray photoelectron spectroscopy (XPS) has revealed that the “natural barrier” on a Ba1-XKXBiO3 (BKBO) thin-film surface is composed of a K-compound layer and a nonstoichiometric layer resulting from Ba depletion. An Au/BKBO junction was fabricated at various temperatures, and its electrical properties were investigated. Junction resistivity could be reproducibly controlled between 10-1 and 10-5 Ω·cm2 by varying the temperature of Au deposition between room temperature (RT) and 300°C. The decrease in junction resistivity with increasing Au-deposition temperature was found to be a result of decomposition of the K-compound layer and rearrangement of the nonstoichiometric layer by annealing the BKBO film.

Preparation and Characterization of Nb/MgO/Bi2Sr2CaCu2Ox Junctions Using the Cleavage Planes of a Single-Crystal Superconductor

Nb/MgO/Bi-Sr-Ca-Cu-O junctions using the cleavage planes of Bi2Sr2CaCu2Ox single crystals have been fabricated using an artificial barrier for the first time. A large step, in the order of several hundred nm on the cleavage plane, proved to be closely related to the anisotropic behavior of tunneling spectroscopy through the surface of a Bi2Sr2CaCu2Ox single crystal. This effect for the results of quasi-particle tunneling in the Nb/MgO/Bi-Sr-Ca-Cu-O junctions was in good agreement with calculated results for the local density of states in the superconductor/normal-conductor superstructure model.

Epitaxial growth of Ba1-xKxBiO3 thin films by high-pressure reactive RF-magnetron sputtering

Epitaxial growth of Ba1-XKXBiO3 (110) thin films has been carried out on SrTiO3 (110) substrates using reactive rf-magnetron sputtering for the first time under a 80 Pa discharge gas which is of a rather higher pressure than is usual. Electrical measurements showed that Tc of as-grown films was 16 K. A preferred orientation of (110) and epitaxial growth of the Ba-K-Bi-O film on the substrates were confirmed by XRD (X-ray diffraction) and RHEED (reflection high-energy electron diffraction), respectively.

Study of the Tunneling Characteristics of Ba1-xKxBiO3 Superconducting Thin Films Based on the Gap-Energy Distribution Model

The broadening of the gap edge in the conductance-voltage ( dI/ dV–V) characteristics for native-barrier Ba1-x Kx BiO3-gold (BKBO//Au) and MgO-barrier Ba1-x Kx BiO3-gold (BKBO/MgO/Au) junctions was investigated by using a gap-energy distribution model where the inhomogeneity of the energy gap Δ at the junction interface was taken into consideration. It was demonstrated that the broadening of the dI/ dV–V curves and their temperature dependence (T<2T C/3) in both junctions could be explained by assuming the Gaussian distribution of gap energies (at T=0 K) with Δ0(0)=3.1 meV, δΔ=0.9 meV for the BKBO//Au junction and Δ0(0)=3.5 meV, δΔ=1.1 meV for the BKBO/MgO/Au junction, where Δ0(0) and δΔ represent the center point and the width of the distribution, respectively. It was found that the value δΔ could be regarded as a criterion for the quality of the tunneling junction.

YBCO-BiO Composite Superconductor for Granular-Type Josephson Microwave Detector

We have developed a new YBCO-BiO composite superconductor for granular-type Josephson microwave detectors. This composite superconductor was prepared by doping YBa2Cu3Ox with Bi2O3 and it was found that the normal state resistance increased with the amount of Bi2O3 doping up to 15 wt.%, without deterioration at the zero-resistance temperature. BaBiO3 was segregated at the interface of the YBCO particles during sintering at a temperature of about 940°C. This seemed to enhance the resistance of the grain boundary and improve the microwave detection responsivity. Practical responsivity (RMv) of 180 V/W and noise equivalent power (NEP) of about 1.4×10-14 W/√Hz were obtained using a YBCO-BiO detector with 15 wt.% Bi2O3 doping.