T. H. Geballe

In situ grown YBa2Cu3O7−d thin films from single‐target magnetron sputtering

Using single‐target off‐axis sputter deposition, high quality superconducting films of YBa2Cu3O7−δ were made in situ. These films have properties which are distinctly different from those of bulk ceramics and of post‐deposition annealed films. Their superconducting resistive transitions remain sharp regardless of the value of Tc between 75 and 86 K. Normal‐state conductivities are as high or higher than the best single crystals. Critical current densities as high as 6×107 A/cm2 at 4.2 K. Tc (R=0) falls off with film thickness down to 10 K for 35–40 A films. All of the above properties are relatively insensitive to compositional variation. The results can be explained if the in situ growth results in well‐formed CuO2 planes with defects occurring elsewhere.

Deposition of in-plane textured MgO on amorphous Si3N4 substrates by ion-beam-assisted deposition and comparisons with ion-beam-assisted deposited yttria-stabilized-zirconia

We report the growth of in-plane textured (100) MgO on amorphous Si3N4 substrates by ion beam assisted deposition (IBAD). The textured MgO can be used as a structural template for subsequent epitaxial thin film deposition. The results are compared with IBAD of (100) and (111) yttria-stabilized-zirconia (YSZ). Based on transmission electron microscopy (TEM) and in situ reflection high energy-electron diffraction (RHEED), we find that MgO texturing is a nucleation-controlled process and the alignment is a function of nuclei size and density. This differs greatly from the evolutionary-type texturing process observed for IBAD (100) YSZ. Consequently, we are able to make 100 A thick MgO films with 7° in-plane alignment, whereas IBAD (100) YSZ films need to be thicker than 5000 A to achieve in-plane alignment better than 13°. This has important implications for the economical application of IBAD induced alignment in real manufacturing processes, including high Tc superconductor (i.e., YBCO) coated tapes, photov...

INTERCALATION COMPLEXES OF LEWIS BASES AND LAYERED SULFIDES: A LARGE CLASS OF NEW SUPERCONDUCTORS.

Exploration of the generality of the recently discovered reaction whereby certain organic molecules can be inserted between the metallic layers of the superconductors tantalum disulfide and niobium disulfide revealed that a large variety of organic and inorganic molecules can penetrate between the crystalline layers of a number of transition metal dichalcogenides and that the resulting complexes are superconducting if the layered chalcogenide from which they are formed is superconducting. The critical temperatures of the 50 new superconductors we report depend on the nature of the intercalate but are insensitive to a separation of the superconducting planes of up to 57 angstroms.

Epitaxial MgO on Si(001) for Y-Ba-Cu-O thin-film growth by pulsed laser deposition

Epitaxial MgO thin films were grown on Si(001) by pulsed laser deposition. In spite of a large (−22.5%) lattice mismatch, epitaxy occurs with alignment of all crystallographic axes. Epitaxial quality and deposition rate are both sensitive to temperature and oxygen pressure. We believe this is the first demonstration of epitaxial MgO on Si. We employ MgO intermediate layers for superconducting epitaxial YBa2Cu3O7−δ/BaTiO3 thin films on Si with a critical current density of 6.7×105 A/cm2 at 77 K.

High critical currents in strained epitaxial YBa2Cu3O7-δ on Si

Epitaxial YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (YBCO) films were grown on Si (100) using an intermediate buffer layer of yttria-stabilized zirconia. Both layers are grown via an entirely {ital in} {ital situ} process by pulsed laser deposition. All films consist of {ital c}-axis oriented grains as measured by x-ray diffraction. Strain results from the large difference in thermal expansion coefficients between Si and YBCO. Thin ({lt}500 A) YBCO films are unrelaxed and under tensile strain with a distorted unit cell. Rutherford backscattering spectroscopy indicates a high degree of crystalline perfection with a channeling minimum yield for Ba as low as 12%. The normal-state resistivity is 280 {mu}{Omega} cm at 300 K; the critical temperature {ital T}{sub {ital c}} ({ital R}=0) is 86--88 K with a transition width ({Delta}{Tc}) of 1 K. Critical current densities of 2{times}10{sup 7} A/cm{sup 2} at 4.2 K and 2.2{times}10{sup 6} A/cm{sup 2} at 77 K have been achieved.Epitaxial YBa2Cu3O7−δ (YBCO) films were grown on Si (100) using an intermediate buffer layer of yttria‐stabilized zirconia. Both layers are grown via an entirely in situ process by pulsed laser deposition. All films consist of c‐axis oriented grains as measured by x‐ray diffraction. Strain results from the large difference in thermal expansion coefficients between Si and YBCO. Thin (<500 A) YBCO films are unrelaxed and under tensile strain with a distorted unit cell. Rutherford backscattering spectroscopy indicates a high degree of crystalline perfection with a channeling minimum yield for Ba as low as 12%. The normal‐state resistivity is 280 μΩ cm at 300 K; the critical temperature Tc (R=0) is 86–88 K with a transition width (ΔTc) of 1 K. Critical current densities of 2×107 A/cm2 at 4.2 K and 2.2×106 A/cm2 at 77 K have been achieved.

Epitaxial yttria‐stabilized zirconia on hydrogen‐terminated Si by pulsed laser deposition

Epitaxial yttria‐stabilized zirconia films were grown on Si (100) and Si (111) by pulsed laser deposition. Rutherford backscattering spectroscopy indicates a high degree of crystalline perfection with a channeling minimum yield of 5.3%. A necessary predeposition process is removal of native silicon oxide from the Si prior to film growth. This is done outside the deposition chamber at 23 °C using a wet‐chemical hydrogen‐termination procedure. Epitaxial YBa2Cu3O7−δ films have been grown on these films.

Critical current densities and transport in superconducting YBa2Cu3O7−δ films made by electron beam coevaporation

Thin films of the high Tc superconductor YBa2Cu3O7−δ were prepared and characterized. The films made on SrTiO3 showed epitaxial growth and high critical current densities in excess of 9×104 at 78 K and 2×106 at 4.2 K. Also, surface pinning in the parallel direction was found to be very high. The measurements put a lower limit on the depairing critical current density of 5×107 A/cm2.

Synthesis and properties of YBa2Cu3O7 thin films grown in situ by 90° off-axis single magnetron sputtering

Abstract High quality superconducting films of YBa 2 Cu 3 O 7− x were deposited in situ using single target 90° off-axis sputtering. We have investigated their superconducting DC and RF properties, their normal state properties, and their microstructures. These films are distinctly different from bulk crystals and post-deposition annealed films. Sharp superconducting transition temperatures can be reproducibly obtained by control of deposition parameters. The T c can be varied from 75 to 89 K. The optimization of properties other than T c and the control of film texture occur under conditions different from those for which the highest T c is obtained. Normal state conductivities are as high as or higher than those of single crystals. Critical current densities reach 6 × 10 7 A/cm 2 at 4.2 K. All the above properties are relatively insensitive to compositional variations. The T c s have a much weaker dependence on the c -axis lattice parameters than do those of bulk samples. The measured low-temperature penetration depth is 1400 A and surface resistance at 4.2 K and 10 GHz is as low as 16 μΩ. Microstructural studies show sharp interfaces between films and their substrates and a variety of defect structures. Many of the properties of in situ films can be explained by clean grain boundaries and the characteristics of the surface growth occuring during in situ deposition.

Epitaxial and Smooth Films of a-Axis YBa2Cu3O7

YBa2Cu3O7 films have been grown epitaxially on SrTiO3 (100) and LaAlO3 (100) substrates with nearly pure a-axis orientation and with transition temperature Tc (R = 0) of 85 K. A unique feature of these films is their smooth surface. These smooth surfaces enable the growth of short-period superlattices with well-defined modulations. The films are untwinned and the grains grow with their c-axis along one of two perpendicular directions on the substrate ([100] or [010]). The fabrication of sandwich-type Josephson junctions with good characteristics may now be possible because unlike c-axis—oriented films, the superconducting coherence length of these smooth films is appreciably large perpendicular to their surfaces.

Thin-film synthesis of the high- T c oxide superconductor YBa 2 Cu 3 O 7 by electron-beam codeposition

The successful synthesis of high- T c YBa 2 Cu 3 O 7 films by means of electron-beam codeposition are reported. Several important growth parameters have been surveyed in a preliminary way. The substrates investigated include Al 2 O 3 , ZrO 2 , MgO, and SrTiO 3 , The films were characterized by resistivity measurements, x-ray diffraction, microprobe, and Rutherford backscattering analysis. Some TEM and critical current density studies were also carried out. The best results to date have been obtained on SrTiO 3 substrates with which polycrystalline epitaxial growth has been achieved. Resistive superconducting transitions with zero resistance at 89.5 K and a 2 K width have been observed in these films.