Hideki Yamawaki

Bi-Sr-Ca-Cu-O film on sapphire grown by plasma-enhanced halide CVD

Plasma-enhanced halide chemical vapor deposition (CVD) for Bi-Sr-Ca-Cu-O (BSCCO) thin film has been developed. Superconducting BSCCO films were fabricated on 3-in-diameter sapphire substrates without postannealing. The CVD apparatus has four source-gas generation cells in which source materials (BiCl/sub 3/, SrI/sub 2/, CaI/sub 2/, and CuI) are evaporated or sublimated by heaters. Source gases are carried to the deposition chamber with helium. Oxidizing gases are O/sub 2/ and/or H/sub 2/O. The total pressure in the deposition chamber was 0.1 torr, and the O/sub 2/ partial pressure was 0.01 torr. Deposition was at 2 AA/min. It was found that the superconducting BSCCO film could be deposited on sapphire substrates at less than 700 degrees C without a solid-phase reaction between the film and substrate and that plasma-enhanced CVD controlled the BSCCO phases even at 580 degrees C. RF-plasma enhancement resulted in as-deposited superconducting BSCCO films. The c-axis orientation of the films was perpendicular to the sapphires (1102)-plane. The 700-AA-thick (2212)-phase BSCCO film showed that the resistive transition started at about 100 K and that the zero-resistivity temperature was 70 K. The critical current density was about 2.5*10/sup 6/ A/cm/sup 2/ at 10 K.

Boron out diffusion from Si substrates in various ambients

Abstract We have investigated out diffusion of boron under various ambient conditions. Boron out diffusion is significant in an H 2 ambient, while the out diffusion is negligible in an N 2 or He ambient. Comparing the analytical model to our experimental data, we identified the diffusion coefficient of boron in an H 2 ambient, and found that it is smaller than that in an N 2 ambient. The significant out diffusion in H 2 ambient is attributed to the enhancement of the boron transport coefficient at the Si surface. Negligible out diffusion in an N 2 or He ambient is attributed to the negligible transport coefficient at the surface.

Study of the microstructure of CVD-grown Bi-Sr-Ca-Cu-O thin films on (001) MgO substrates by high-resolution transmission electron microscopy

Abstract We have studied the atomic structure of hetero-interface and microstructural defects in Bi-Sr-Ca-Cu-O thin films by using cross-sectional high resolution transmission electron microscopy. The films were grown on (001) MgO substrates by chemical vapor deposition. From cross-sectional observation, it is proved that the interface between the thin film and the substrate is atomically very abrupt and no clear evidence of a reaction between the thin film and the substrate is found. A high degree of atomic order at the interface indicates that a Bi-O layer is initially grown on the substrates. We have also found that although multi-steps (1–2 nm) exist on the substrate surface, no anomalous growth or generation of defects have been observed at these steps. In very rare cases, facets inclined 10–20° to the substrate are observed. In this particular situation, we have discovered that the thin film is grown in such a way that the c -axis is exactly normal to one of these highly misoriented facets, indicating a rather weak epitaxial relationship in this system. In thin films, 90° twist boundaries or 90° grain boundaries are often generated. The surface region of the MgO substrate occasionally exhibits residual damage induced during the polishing process. Dislocation networks are often observed in the substrate, suggesting that it contains small angle tilt boundaries.

Growth of Epitaxial CeO2 Films on (1012) Sapphire by Halide Source Plasma Enhanced Chemical Vapor Deposition.

We grew epitaxial CeO2 films on (012) sapphire by halide source plasma enhanced chemical vapor deposition (halide-PECVD) using CeBr3 and O2 as source materials. We obtained amorphous films at substrate temperatures of 620° C, polycrystalline films at 680° C, and highly aligned films at 720°C. X-ray diffraction patterns show that (001) CeO2 is parallel to (012) sapphire, and electron channeling patterns show [110] CeO2 aligned with [201] sapphire. AFM shows very smooth films with a maximum variation of ± 2 nm. We grew YBCO film on CeO2/sapphire with a T c of 83 K.

Structural evaluation of CVD-grown Bi-Sr-Ca-Cu-O thin films on MgO substrates by transmission election microscopy

Abstract Bi-Sr-Ca-Cu-O thin films grown on (001) MgO substrates by chemical vapor deposition with T c at 110 K have been evaluated by transmission electron microscopy. From plan-view observations, the films are found to consist of large domains oriented along the c axis, with diameters of 15-100 μm. These domains exhibit incommensurate superstructures along the b axis. In addition, we have observed similar superstructure spots elongating in two equivalent directions normal to each other. Cross-sectional observation has indicated that the interface between the Bi-Sr-Ca-Cu-O thin film and the substrate is very abrupt. Five different perovskite-related layers along the c axis, with different thickness are successfully observed. In the thin film, 80 K phases with c = 3.0 nm are dominant and 110 K phases with c = 3.6 nm are less likely, which is very consistent with the results from X-ray diffraction. Three other phases are very rarely observed. Furthermore, boundaries where the layered structure is different on both sides are often found.

Direct deposition of SrBi2Ta2O9 Film on IrO2 electrode using liquid source CVD method

Abstract SrBi2Ta2O9 (SBT) thin films were directly deposited on IrO2 electrode using liquid source CVD (LS-CVD) method. Bi deficient region in the SBT films, which existed near the conventional Pt bottom electrode, was successfully eliminated using IrO2 bottom electrode. X-ray diffraction analysis showed our SBT films on IrO2 have good crystalline quality. Satisfactory electric properties (6 μC/cm2 of 2Pr and 10−6 A/cm2 of leakage current) were obtained

S/N heterostructure of BiSrCaCuO/BiSrCuO grown by halide CVD

Abstract Thin film S/N heterostructures of superconducting BiSrCaCuO and metallic BiSrCuO with a good surface morphology, crystallinity, and superconducting properties were formed on (100) MgO substrates by halide CVD. Critical temperatures (zero resistance at Tc) and critical current densities (Jc) of the BiSrCaCuO films on MgO substrate were Tc = 75 to 80 K and Jc = 1×10 6 A/cm 2 at 60 K. Electrical resistance of the metallic BiSrCuO films was 1 to 2 ×10 −3 ohm-cm from 10 to 300 K. High resolution transmission electron microscopy (HRTEM) images showed the interface between the BiSrCaCuO and BiSrCuO films to be atomically very abrupt. The critical temperatures of S/N-multilayer samples was about 75 K. In this work, we fabricated S/N-heterostructures of bismuth-compound crystals grown by halide CVD and studied the film quality and superconducting properties.

The Role of Grain-Boundaries in the Transport Properties of CVD-Grown Bi-Sr-Ca-Cu-O Films

The effect on the temperature(T) dependence of electrical resistivity, ρ(T), in the a-b plane of grains/grain-boundaries is studied in Bi-Sr-Ca-Cu-O films grown by CVD. ρ(T) is found to be different in three types of films: (1)ρ(T)=cT, ρb(T)/ρa (T) ~2, for a single grain, (2)the resistivity perpendicular to the grain-boundaries, ρ⊥GB(T)=c1T+c2/T and parallel to the grain-boundaries, ρ//GB(T)=cT, in a sample which contains a few grainboundaries, and (3)ρ (T) =cT and has no anisotropy for many small grains(≥10000).

The role of grain boundary in the normal-state transport properties of CVD-grown Bi-Sr-Ca-Cu-O films

The effect on the a-b plain normal transport property of the grain boundaries is studied on Bi-Sr-Ca-Cu-O c-axis oriented films grown by Chemical Vapor Deposition (CVD). The electric resistance behavior is found to be characteristically different depending whether the current direction is either perpendicular or parallel to the grain boundary.

CVD of Bi-Sr-Ca-Cu-O Thin Films

We developed a chemical vapor deposition (CVD) method for high-Tc superconducting Bi-Sr-Ca-Cu-O thin films, and obtained a high-Tc phase (about 110 K) without postannealing in an atmosphere containing oxygen. Films were deposited on (001) MgO substrates in He in an open-tube reactor with O2 and/or H2O oxidizing agents. The source materials --anhydrous metal halides such as BiCl3, SrI2, CaI2, and CuI -– were evaporated in the CVD system to be used as source gases. The deposition temperature was between 700°C and 850°C. The average deposition rate was about 1.5 nm/min and the average film thickness was about 0.1 μm. Films were a mixture of low-Tc (about 80 K) and high-Tc (about 110 K) phases. Their C-axis orientation was perpendicular to the substrate. Their electrical resistance decreased abruptly below 115 K and the zero-resistance temperature was around 98 K. A critical current density of about 1.7 x 104 A/cm2 was obtained at 4.2 K. The O2 in the CVD atmosphere played an important role in determining the phase of films at higher temperature depositions.