Single-crystal films of high-TC superconductors are of immense importance for fundamental physical studies and the device applications. The in-situ growth of the single-crystal films of YBa2Cu3O7-x (YBCO) at relatively low temperatures is reviewed. The (001)-oriented single-crystal films of good quality in crystallinity have been prepared by activated reactive evaporation and they exhibit the superior superconducting properties. The orientation of the film grown on SrTiO3 (110) face was discussed in relation to the lattice mismatch between the substrate and the film. It is also discussed that the crystal transformation of YBCO with oxidation had the effect on the microstructure and the oxidation degree of the film. The in-situ reflection high energy electron diffraction observation during depositon on MgO (100) suggests that the deposition occurs in the layer-by-layer growth manner. Some of the superconducting and physical properties measured using our films are reported.
The conditions necessary for synthesizing thin oxide superconductor films in vacuum deposition systems are described. It is shown that the oxygen partial pressure on the substrate kept at the crystallization temperature must be higher than the oxygen dissociation pressure of the multicomponent oxide films. This condition can be satisfied for the YBa2Cu3O7-δ-O2 system when the oxygen partial pressure lies in the range 10-110-2mb at the substrate temperatures of 800-900K. For the Bi-Sr-Ca-Cu oxide system, the number of CuO layer in a unit cell can be controlled by utilizing the multilayer deposition technique. The multilayer films thus obtained have smooth surface and contain no grain boundaries or precipitates. The large solid-solubility found in these multilayered films shows the possibility of obtaining films consisting of non-equilibrium phase.
We have obtained a REBa2Cu3O7-y (RE=rare earth element) superconducting single-crystal thin film on a MgO (100) substrate using a magnetron sputtering method. For a HoBa2Cu3O7-y thin film, we obtained a Jc of more than 3×106A/cm2 at the temperature of liquid nitrogen, and the deterioration of the magnetic field was far less than that for sintered bulk material. Through X-ray diffraction, it was confirmed that the growth of the c axis was perpendicular to the substrate. In addition, reflection high energy electron diffraction and observation with a high resolution scanning electron microscope confirmed that the surface was extremely smooth and flat. This is a great improvement in comparison with the poly-crystal film (with a Jc of 104 to 105A/cm2) which was previously reported, and it is expected that this can open the way for applications in electronics fields which require a Jc of 106A/cm2 or higher.
Basic thin film deposition processes for the high-Tc superconductors of Bi-systems are described in relation to their structure and superconducting properties. For the rare-earth high-Tc superconductors YBC the thin film deposition processes are clasified into three processes: (i) deposition below crystallizing temperature followed by annealing [process (1)], (ii) deposition above crystallizing temperatme followed by annealing [process (2)], (iii) deposition above crystallizing temperature without postannealing [process (3)]. For the Bi-system there appear several superconducting phases including the low-Tc phase Bi2Sr2Ca2Cu2Ox and the high-Tc phase Bi2Sr2Ca2Cu3Ox. Thin films with these superconducting phases are synthesized by a selection of the substrate temperature Ts during the deposition in the process (1) and/or (2): The high-Tc phase with Tc ≈=110K is synthesized at Ts>800°C; the low-Tc phase with Tc=80K, at Ts<600°C. However, these films often comprise intergrowth structure between the differrent superconducting phases. The close control of the supercondurting phase has been achieved by the layer-by-layer deposition in the atomic layer epitaxy process.
This review contains recent information on the properties of Tl-Ca-Ba-Cu-O thin films. The conditions for the Tl-Ca-Ba-Cu-O film growth by sputtering and by molecular beam deposition are described. The highest zero resistance temperature is 116K. The as-deposited films are insulators, but they become superconductive after being annealed in oxygen at 880930°C for 10min. After post-annealing, the films on MgO and SrTiO3 substrates exhibit zero resistivity at 116K and 80K, respectively. The crystal structures of the films are tetragonal, so that the surface morphologies are essentially twin free. Using a SQUID magnetization measuring technique in a parallel field, it was found the randomly oriented polycrystalline Tl-Ca-Ba-Cu-O thin film shows a critical current density (Jc) in excess of 1.5×106A/cm2 at 50K. This large Jc value indicates the presence of surface pinning, which is much stronger than any bulk pinning observed by a measurement in a perpendicular field. It was also found that the microbridge DC-SQUID's fabricated from the Tl-Ca-Ba-Cu-O films patterned by maskless etching using focused ion beam (FIB) processes, operates up to 95K.
The quality of epitaxial film significantly depends on the substrate material. The oxide superconducting film growth is also the case. In the present work, Ln1Ba2Cu3Oy (Ln=Y, Nd, Er) films were grown by the reactive rf-magnetron sputtering on (100), (110)-SrTiO3 and (100)-MgO single crystal substrates. To get more information on the growth mechanism, the initial growth stage (growth thickness of 1-3nm) was closely studied. MgO served as an excellent substrate on which even two monolayers of YBaCuO grew epitaxially with very smooth surface. The films grown on both (100) and (110)-SrTiO3 required thickness more than 10nm to obtain epitaxial-like RHEED pattern. The RHEED pattern itself still looked spotty, showing the surface roughness existing even for 100nm thick film grown on the SrTiO3 substrate. The heteroepitaxial and double-heter-oepitaxial (superconbuctor/insulator/superconductor) technologies are also demonstrated in the text.
In this review, the potentiality of the OMCVD method for a high-Tc thin film formation technique is described. In the beginning of 1988, Naval Research Institute, Tohoku University and Oki Electric have independently succeeded in the high-Tc ceramics film formation by the OMCVD method. Within less than one year Y-Ba-Cu-O films could be prepared by the method with a good quality of Tc_≥90K and Jc=1.9×106A/cm2 at 77K, which is comparable to that obtained by sputtering or MBE methods. Now, more than ten research groups have joined in this promissing field.
The present status of Josephson effect and tunnel effect of high Tc oxide superconductors is reviewed. In the beginning, the concepts for Josephson and tunnel effects are introduced. With respect to the Josephson effect, the properties of grain boundary junctions are discussed in detail, especially based on the observed current-voltage characteristics and the quantum interference effect. As for the tunnel experiment, a variety of data are presented and discussed in connection with the proposed theoretical models. Finally, the problems on the formation of an ideal Josephson tunnel junction are discussed.
Superconducting transistors, especially those comprised of high-critical temperature (high-Tc) superconductors, are reviewed. There are three basic types of superconducting transistors, a quasiparticle injection device, a field effect and a superconductor base transistor. Characteristic and operating principles of these superconducting tcansistors are described. These transistors have mainly been fabricated using conventional metal superconductors. However, a few high-Tc superconducting transistors have been tried to fabricate to date. An operating temperature performances, characteristics, and peculiar properties concerning to device structure and fabrication methods expected for high-Tc superconducting transistors are also discussed in this report.
Advantages of superconducting interconnection in LSI circuit performance are discussed. Effective application areas of superconducting lines are signal lines for high drivability devices, power lines and chip to chip interconnection. Factors limiting the application such as cooling capacity and signal distortion due to multiple reflection are discussed. High frepuency characteristics of oxide superconductors and basic problems which should be overcome are also described.
A new fabrication process called the alginate method is outlined for the production of high-Tc superconducting oxide ceramic fibers. The process is based on the gelation of a sodium alginate aqueous solution, in which cations are exchanged with protons or multivalent metal ions. Dense and continuous Y-Ba-Cu-O superconducting fibers have been successfully fabricated by firing the Y-Ba-Cu-alginate fibers at temperatures above 900°C. The fibers, diameter of which is as thin as 100μm, have demonstrated at ensile strength of 192 MPa and a Tc of 85K.