Post-annealing Step Research Articles

Comparison of YBCO-films prepared by laser ablation and sputtering

Two different preparation techniques, i.e. laser ablation and dc-sputtering have been applied for the production of thin films from YBa 2Cu 3O 7 − x. Both techniques do not require postannealing steps, as the condensation of the superconducting film is done onto a hot substrate (600–780 °C) at high oxygen pressures (1–6 mbar). Besides the superconducting properties ( T c ( R = 0) ≥ 90 K, j c (77 K) > 4 × 10 6 A/cm 2) also the structural properties are very similar (single crystalline, c-axis oriented) as measured by ion channeling ( χ min ≈ 5.5 %), x-ray scattering and TEM. The preparation methods require quite different process times, < 10 min for laser ablation compared to about one hour for sputtering. They yield different SEM-appearances of the surface: the sputtered surface is clean and smooth, but the laser oblated film shows in addition balls of 200 nm dia. The properties of films made by both techniques are compared and discussed in detail. In addition results on various buffer layers between YBCO and Si are presented. The interdiffusion of the various components have been analysed by RBS. Promising results have been obtained for buffer layers from ZrO 2, where only little interdiffusion could be observed, resulting in good superconducting YBCO-overlayers.

Significant improvement in the surface smoothness of post-annealed Y-Ba-Cu-O thin films upon silver addition

We report the effect of silver addition on the surface quality of rf-sputtered Y-Ba-Cu-O thin films. The Y-Ba-Cu-O-Ag films appear to have very smooth surfaces even if the post-annealing step is employed. The surface roughness of our ex situ grown Y-Ba-Cu-O-Ag film, having a 10.3 wt. % of silver, is within ±1% of the film thickness, which is more than 10 times better than the value of a typical post-annealed high-temperature superconducting film without silver.

Macroscopic Persistent Currents in Laser Ablated YBa2Cu3O7-X Films

AbstractThin films of YBa2Cu3O7-x have been produced in situ by laser ablation. As the condensation of the superconducting film is done onto hot substrates (600 - 780°C) at high oxygen pressures (~ 1 mbar), this techniques does not require any special postannealing step. Thus the total process time amounts to less than 10 min. Films were found to be polycrystalline with the c-axis preferentially oriented normal to the substrate plane on single crystalline (001) SrTiO3 and (random) ZrO2, as measured by ion channeling (Xmin = 5.5 % on SrTiO3), x-ray scattering and TEM. Complete superconducting transitions above 90 K with transition widths of about 1 K have been observed even on YSZ substrates. Critical current densities, jc (77 K), of more than 2 x 106 A/cm2 could be obtained on SrTiO3-substrates. The high quality of the thin films over large areas was demonstrated by the measurement of persistent superconducting current densities at 77 K in the order of 105 A/cm2 in a 1.5 mm broad ring of 9 mm diameter. No dissipation of this persistent current could be observed during one hour. Measurements of the same film after 8 months in normal air showed a 40 % decrease of jc and a stronger decrease of jc with an externed magnetic field, suggesting that in the meantime the macroscopic ring current density is dominated by intergranular weak links.

High T c YBa2Cu3O7−x thin films on Si substrates by dc magnetron sputtering from a stoichiometric oxide target

Thin films of YBa2Cu3O7−x were deposited on Si substrates at 600–700 °C by dc magnetron sputtering from a stoichiometric oxide target. Resistivity measurement results indicate that these films are superconducting with a zero resistance Tc as high as 76 K, without further high-temperature post-annealing treatments. These films give both core and valence-band x-ray photoemission, and x-ray diffraction spectra similar to those for superconducting films prepared with a high-temperature post-annealing step. No significant diffusion of Si from the substrate into the film was detected for the films deposited at 650 °C or lower, according to depth profiles obtained using secondary ion mass spectrometry.

Improved Room-Temperature Laser Performance in GaAs Diffused-Junction Diodes

Significant reduction in room-temperature laser threshold and stimulated emission delay is obtained in diodes made from zinc-diffused wafers (850°C—3 hours in Zn–As atmosphere) if the single-diffusion process is modified with multiple diffusions, or, more controllably, by the addition of a post-annealing step (∼900°C in As atmosphere) following the initial diffusion. Incremental sheet-conductivity measurements show the zinc concentration gradient is reduced and etching studies show a lowered dislocation density near the junction after annealing. Without an annealing step, diffused-junction diodes show consistently larger emission delays (observed above ∼200°K) than epitaxial-junction diodes made from the same n-type substrates. The deliberate in-diffusion of high copper concentrations causes large delays even at 77°K. However, copper is probably not the center causing delays near room temperature because a mass spectrographic analysis did not reveal higher copper concentrations in diffused-junction wafers than in epitaxial-junction wafers when there was no deliberate copper doping.