R. C. Taber

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.

Single source metalorganic chemical vapor deposition of low microwave surface resistance YBa2Cu3O7

YBa2Cu3O7 films 2000–7500 A thick were deposited onto [100] LaAlO3 substrates using a novel single source metalorganic chemical vapor deposition technique. We have so far observed critical currents as high as 4×106 A/cm2 at 77 K, transition temperatures (Tc at R=0) as high as 91 K, and the 10 GHz microwave surface resistance Rs as low as 40 μΩ at 4.2 K and 400 μΩ at 77 K.

YBa2Cu3O7-δ superconducting films with low microwave surface resistance over large areas

We demonstrate that in situ off‐axis sputtering from a composite target can reproducibly fabricate very high quality YBa2Cu3O7−δ films over large areas of deposition. A significant reduction in the microwave surface resistance, Rs, compared to many previously reported values is found. Rs values at 10 GHz were found to be as low as 20 μΩ at 4.2 K and 450 μΩ at 77 K. A systematic variation of several properties including the transition temperature, critical current density at high magnetic fields, and the c‐axis lattice parameter across the 3.8 cm×3.8 cm (14.5 cm2) deposition area was found.

Microwave surface resistance of epitaxial YBa2Cu3O7 thin films on sapphire

Microwave surface resistance data were measured on pairs of YBa2Cu3O7 thin films on Al2O3 {11_02} substrates by a parallel plate resonator technique. The surface resistance Rs at 10 GHz was 65 μΩ at 10 K and 850 μΩ at 77 K. These epitaxial YBa2Cu3O7 films were grown on 500‐A‐thick buffer layers of SrTiO3. X‐ray diffraction data showed that the YBCO thin films with the SrTiO3 buffer layers have better in‐plane epitaxy than those without such buffer layers. Critical current density of 2×106 A/cm2 at 74 K was measured by the ac mutual inductance response of the films. The improved microwave surface resistance and the higher critical current density are believed to be the results of better in‐plane epitaxy.

Microwave surface resistance of epitaxial YBa sub 2 Cu sub 3 O sub 7 thin films on sapphire

Microwave surface resistance data were measured on pairs of YBa2Cu3O7 thin films on Al2O3 {11_02} substrates by a parallel plate resonator technique. The surface resistance Rs at 10 GHz was 65 μΩ at 10 K and 850 μΩ at 77 K. These epitaxial YBa2Cu3O7 films were grown on 500‐A‐thick buffer layers of SrTiO3. X‐ray diffraction data showed that the YBCO thin films with the SrTiO3 buffer layers have better in‐plane epitaxy than those without such buffer layers. Critical current density of 2×106 A/cm2 at 74 K was measured by the ac mutual inductance response of the films. The improved microwave surface resistance and the higher critical current density are believed to be the results of better in‐plane epitaxy.

Properties of epitaxial YBa2Cu3O7 thin films on Al2O3 {1̄012}

Epitaxial YBa2Cu3O7 films were grown on Al2O3 {1012} by a laser ablation technique. X‐ray diffraction shows that films are epitaxial with the c axis perpendicular to the substrate surface and ‘‘123’’ [110] aligned with sapphire [1011], although the full width at half maximum of the rocking curve is larger than those of epitaxial films on SrTiO3. Typical Tc’s are between 85 and 88 K with transition widths between 0.5 and 3 K. The normal‐state resistivity is 270 μΩ cm at 300 K and extrapolates to zero at zero temperature while the magnetization Jc is as high as 5×106 A/cm2 at 4.2 K. High‐frequency loss measurements show that 2000‐A‐thick epitaxial films on Al2O3 {1012} have a surface impedance about 1 mΩ at 13 GHz at 4.2 K.

Large‐area, two‐sided superconducting YBa2Cu3O7−x films deposited by pulsed laser deposition

The in situ excimer‐laser deposition process is demonstrated, for the first time, to be an effective technique for producing superconducting films on both sides of substrate wafers of up to 5 cm in diameter. By exploiting the directed nature of the laser produced plume and controlling its off‐axis composition, thickness and stoichiometric variations of less than ±5% have been obtained over the entire surface. Coatings on 5‐cm‐diam LaAlO3 substrates have uniform transition temperatures of ≥ (R18)90 K on both sides, with a critical current density at 77 K and zero field of 2.5×106 A/cm2. The 10 GHz surface resistance for samples cut from a 5 cm wafer is 40–60 μΩ at 4 K.

Measurements of the magnetic penetration depth in YBa2Cu3O7−δ thin films by the microstrip resonator technique

We have utilized the superconducting microstrip resonator technique to measure the magnetic penetration depth in high Tc oxide thin films in the 1–25 GHz regime. This technique is particularly well suited for thin films, where the absolute value of the penetration depth can be accurately determined. Results for high Tc superconducting thin films show that the value of the penetration depth is sensitive to the preparation conditions of the film, and the temperature dependence is that expected of conventional superconductors.

Surface resistance of epitaxial YBa2Cu3O7 thin films on CeO2 diffusion barriers on sapphire

Epitaxial thin films of YBa2Cu3O7 (YBCO) have been deposited by off‐axis sputtering onto substrates of r‐plane sapphire coated with a CeO2 diffusion barrier which had been previously deposited by metalorganic chemical vapor deposition process. The YBCO films display excellent superconducting properties, with critical current densities and microwave surface resistances comparable to state of the art YBCO films deposited directly onto LaAlO3 or MgO substrates. The volume fraction of YBCO with large‐angle in‐plane misalignment relative to the underlying layers is comparable to, or smaller than that obtained for YBCO directly deposited onto MgO substrates.

Properties of epitaxial YBa sub 2 Cu sub 3 O sub 7 thin films on Al sub 2 O sub 3 l brace 1 012 r brace

Epitaxial YBa2Cu3O7 films were grown on Al2O3 {1012} by a laser ablation technique. X‐ray diffraction shows that films are epitaxial with the c axis perpendicular to the substrate surface and ‘‘123’’ [110] aligned with sapphire [1011], although the full width at half maximum of the rocking curve is larger than those of epitaxial films on SrTiO3. Typical Tc’s are between 85 and 88 K with transition widths between 0.5 and 3 K. The normal‐state resistivity is 270 μΩ cm at 300 K and extrapolates to zero at zero temperature while the magnetization Jc is as high as 5×106 A/cm2 at 4.2 K. High‐frequency loss measurements show that 2000‐A‐thick epitaxial films on Al2O3 {1012} have a surface impedance about 1 mΩ at 13 GHz at 4.2 K.