S. S. Laderman

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.

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.

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.

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.

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.

Limited reaction processing: Growth of Si1−xGex/Si for heterojunction bipolar transistor applications

Abstract Limited reaction processing (LRP) of silicon-based materials is reviewed as an alternative growth method to molecular beam epitaxy (MBE). LRP is a chemical vapor deposition technique which uses wafer temperature, rather than gas flow switching, to initiate and terminate growth. Processing takes place within a cold-wall, quartz reaction chamber, and gases are changed between successive lamp-heated growth cycles. In addition to minimizing thermal exposure, the technique allows individual layers in a multi-layer structure to be deposited at their optimum growth temperature. LRP is particularly well suited to the growth and processing of metastable layers such as strained Si 1− x Ge x on silicon. Several properties of LRP-grown Si 1− x Ge x are shown to be similar to those reported for MBE material, including qualitative islanding behavior and quantitative measurement of the onset of misfit dislocation formation. However, a direct comparison of thermal stability reveals larger numbers of misfit dislocations in MBE-grown films upon annealing. The electrical behavior of misfit dislocations in heterojunction diodes, and the growth and analysis of high-quality Si/Si 1− x Ge x /Si heterojunction bipolar transistors are also discussed.

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.