J. L. Skov

Smooth YBa2Cu3O7−x thin films prepared by pulsed laser deposition in O2/Ar atmosphere

We report on pulsed laser deposition of YBa2Cu3O7−x in a diluted O2/Ar gas resulting in thin epitaxial films which are almost outgrowth‐free. Films were deposited on SrTiO3 or MgO substrates around 800 °C at a total chamber pressure of 1.0 mbar, varying the argon partial pressure from 0 to 0.6 mbar. The density of boulders and outgrowths usual for laser deposited films varies strongly with Ar pressure: the outgrowth density is reduced from 1.4×107 to 4.5×105 cm−2 with increasing Ar partial pressure, maintaining a critical temperature Tc,zero≊90 K and a transport critical current density Jc(77 K)≥106 A/cm2 by extended oxygenation time during cool down.

Improved current transport properties of post annealed Y1Ba2Cu3O7-x thin films using Ag doping

The influence of Ag doping on the transport properties of Y1Ba2Cu3O7−x thin films prepared by Y, BaF2, and Cu co‐evaporation and optimized ex situ post annealing has been investigated. Both undoped and Ag doped films have values of Tc above 90 K, but Jc (77 K) is highly dependent on the nominal thickness (tnom) of the as‐deposited film. For undoped films with tnom≤300 nm Jc (77 K) (≫106 A/cm2) decreases monotonically with increasing film thickness. Above 300 nm Jc (77 K) decreases rapidly to values below 5×105 A/cm2. Ag doped films with tnom≥200 nm have higher Jc (77 K) values than those of undoped films. Ag doped films have a maximum in Jc (77 K) around 250 nm. As for the undoped films, there is a large decrease in Jc (77 K) for Ag doped films with tnom≥300 nm. It was found that the higher values of Jc (77 K) for the Ag doped films were due to a better epitaxial growth of the YBCO compound. The low values of Jc (77 K) for both undoped and Ag doped single layer films with tnom≥300 nm were found to be due ...

Andreev reflections at interfaces between δ‐doped GaAs and superconducting Al films

By placing several Si δ‐doped layers close to the surface of a GaAs molecular beam epitaxy–grown crystal, we achieve a compensation of the Schottky barrier and obtain a good Ohmic contact between an in situ deposited (without breaking the vacuum) Al metallization layer and a highly modulation doped (n++) conduction layer embedded below the δ‐doped layers in the GaAs crystal. When cooled to below the critical temperature (≊1.2 K) of Al, superconductivity is induced in the conductive layer of the semiconductor. We have studied the current voltage (I–V) characteristics in a planar geometry where the Al has been removed in a thin stripe. We find a manifestation of the superconducting energy gap and a rich fine structure at injection energies both below and above the gap.

Double-sided YBa2Cu3O7-x thin films for microwave applications

YBa2Cu3O7-x(YBCO) thin films with superconducting transition temperatures above 91 K were deposited on both sides of double-sided, polished LaAlO3(100) substrates. Using ex situ furnace annealing, the substrates could be kept at 45 degrees C during the codeposition of Y, Cu and BaF2. No O2 was necessary in the deposition chamber. The furnace annealing was carried out in a mixture of Ar, O2 and H2O vapour. The gas flow was 1000 sccm at an O2 partial pressure of 1.0 mbar. The maximum annealing temperature was 810 degrees C which was maintained for 1 h. Double-sided YBCO superconducting thin films may provide a good base material for microwave devices in which one superconducting side acts as a low-loss ground plane.

Comparison of high-pressure dc-sputtering and pulsed laser deposition of superconducting YBa2Cu3Ox thin films

Superconducting YBa2Cu3Ox thin films were deposited on NdGaO3 (110) substrates using two different techniques: dc sputtering at high oxygen pressure and pulsed laser deposition. The structure, electrical properties, and surface morphology of the obtained films were compared. The superior crystal quality of dc-sputtered films fabricated at the same temperature and at oxygen pressure of the same range as for laser-deposited films can be explained by a lower deposition rate providing time for recrystallization processes. The re-evaporation becomes significant for dc sputtering at high deposition temperatures and results in Badeficient films. The high mobility of atoms on the surface of the growing film during laser deposition helps in the formation of smoothc-oriented areas of the film.

Pulsed laser deposition of high-Tc superconducting YBa2Cu3Ox thin films in Ar/O2 atmosphere

Electrical and structural properties of superconducting YBa2Cu3Ox thin films, obtained with pulsed laser deposition in an Ar/O2 mixture at different oxygen partial pressures and constant total pressure, were studied. NdGaO3 (110) and LaAlO3 (001) substrates were used. The crystal quality of the films was found to correlate to the contents of inclusions with the axis oriented normal to the substrate plane. The increase of argon partial pressure resulted in decrease of particle density on the film surface to less than 106 cm−2, accompanied by an increase of a-oriented phase content and a rapid fall of the critical temperature from 91 K to 80 K

Diffusive electronic transport in superconductor-semiconductor-superconductor junctions of Al or Nb on δ-doped GaAs

We report measurements on planar superconductor-semiconductor-superconductor (S−Sm−S) junctions consisting of a n++ modulation doped conduction layer in MBE grown GaAs with superconducting contacts of Al or Nb. At distances between the two superconducting banks below ≈3.5μm we observe a coupling between the two superconductors, due to multiple Andreev reflections at the S−Sm interfaces.