Superconducting YBa2Cu3O7 Thin Films Research Articles

Measurement of Electrical Properties of Superconducting YBCO Thin Films in the VHF Range.

A new measurement technique of electrical parameters of superconducting thin films at the Very High Frequency (VHF) range is described, based on resonators with microstrip (MS) structures. The design of an optimal resonator was achieved, based on a thorough theoretical analysis, which is required for derivation of the exact configuration of the MS. A theoretical model is presented, from which an expression for the attenuation of a MS line can be derived. Accordingly, simulations were performed, and an optimal resonator for the VHF range was designed and implemented. Production constraints of YBa2Cu3O7 (YBCO) limited the diameter of the sapphire substrate to 3″. Therefore, a meander configuration was formed to fit the long λ/4 MS line on the wafer. By measuring the complex input reflection coefficients of a λ/4 resonator, we extracted the quality factor, which is mainly affected by the dielectric and conductor attenuations. The experimental results are well fitted by the theoretical model. The dielectric attenuation was calculated using the quasi-static analysis of the MS line. An identical copper resonator was produced and measured to compare the properties of the YBCO resonator in reference to the copper one. A quality factor of ~ was calculated for the YBCO resonator, three orders of magnitude larger than that of the copper resonator. The attenuation per unit length of the YBCO layer was smaller by more than five orders of magnitude than that of the copper.

Vortex activation energy in the AC magnetic response of superconducting YBa2Cu3O7 thin films with complex pinning structures

The vortex activation energy UAC in the AC magnetic response of superconductors exhibits a logarithmic variation with the screening current density J (regardless of the pinning structure details), and takes surprisingly high values in the vicinity of the DC irreversibility line, especially at low external DC magnetic fields, as often reported. This is essentially different from the behaviour of the vortex-creep activation energy at long relaxation time scales in DC magnetic measurements, and is not completely understood. We investigated the DC relaxation and the AC response for YBa2Cu3O7 films containing nanorods and nanoparticles, with the DC and AC fields oriented perpendicular to the film surface. It is shown that the large UAC values in the vicinity of the DC irreversibility line, where the critical-state-related AC signal occurs, are generated by a non-diffusive vortex motion during the AC cycle, with the mean vortex hopping length longer than the average distance between the pinning centres. In these conditions, the smearing of the vortex pinning potential by thermally induced vortex fluctuations is weak, and UAC mainly results from the strong influence of the pinning-enhanced viscous drag on the vortex hopping process. The logarithmic UAC(J) dependence is consistent with a high UAC.

Smooth Stress Relief of Trifluoroacetate Metal-Organic Solutions for YBa2Cu3O7 Film Growth

The mechanisms controlling stress relief during the drying and pyrolysis processes of trifluoroacetate metal-organic precursors have been elucidated in view of high rate fabrication of epitaxial superconducting YBa2Cu3O7 thin films. Combining FTIR, TGA, and film thickness evolution measurements, we conclude that gel drying and sintering occurs below T ∼ 250 °C while film densification, due to metal-organic pyrolysis, occurs in the range 250−310 °C. Stress relief is the driving force leading to smooth or structurally inhomogeneous films, displaying buckling or macrocracks, depending on the rate of film transformation. High heating rates can be used during the gel drying and sintering process preserving the film homogeneity because the developed stress is below that required to generate buckling, as demonstrated by nanoindentation experiments. Within the temperature window where the metal-organic decomposition process occurs, the gas flow and the temperature ramp can be tuned to achieve fast pyrolysis prese...

An investigation of superconducting YBCO thin films grown on different substrates

In this work, superconducting YBa2Cu3O7 (YBCO) thin films were grown on different substrates under the same conditions and their behaviours were investigated. At a higher substrate temperature, because of the occurrence of a chemical reaction between the substrate and film, the film quality was impaired, especially in case of the Al2O3 (sapphire) substrate. Films were also deposited onto other substrates, SrTiO3, MgO and ZrO2, and the effects of these substrates on the film quality were compared by x-ray diffraction.

Topotactic reactions of superconducting YBa2Cu3O7 thin films with water vapour

The mechanism of the topotactic reaction of superconducting YBa 2 Cu 3 O 7 thin films with water vapour is shown to be strongly influenced by the reaction kinetics. At elevated temperatures a quasi-continuous transition is observed while at lower temperatures a discontinuous transformation occurs with two phase regions and the appearance of a new superconducting metastable intermediate. The latter can be described as a second stage compound. Models are discussed — based primarily on diffraction and spectroscopic data — for the changes observed in structure and stoichiometry in order to understand the complex reaction mechanism of this low temperature solid state process and the effects on the electronic properties of the thin layer material.

Modification of the adhesion and contact resistance of the Ag/YBa2Cu3O7 interface with keV electron irradiation

The effect of 3.5 keV electron irradiation on adhesion and contact resistivity of the Ag/YBa2Cu3O7 interface has been studied using an evaporated silver layer on c-axis oriented superconducting YBa2Cu3O7 thin films. Electron doses ranged between 1016 and 1018 electrons/cm2. The Q-tip method of adhesion testing showed that even at the lowest electron dose adhesion is significantly improved. The contact resistivity of the interface was measured using a cross-junction four-point probe. Contact resistivity was unchanged at the lowest electron dose but increased as the electron dose increased. A theoretical model involving an electron irradiation damaged layer at the interface has been developed to explain measured contact resistivity changes.

Dc SQUIDs based upon YBa2Cu3O7 nanobridges

Superconducting quantum interference devices (dc SQUIDs) were fabricated from high-temperature superconducting YBa2Cu3O7 thin films, using nanobridges as weak links. The weak link behavior in these devices caused by coherent motion of vortices was observed for bridges with dimensions smaller than the effective penetration depth, which was approximately 400–600 nm for the films. The dc SQUIDs consisted of two parallel nanobridges with widths of 100–500 nm. The size of the SQUID hole ranged from 2×2 to 2×10 μm2. The SQUIDs were operated in the temperature range from liquid helium up to 85 K and showed voltage modulations as a function of an applied magnetic field. At 75 K peak-to-peak values of 15 μV were observed. The flux noise at 4.2 K was determined to be smaller than 8×10−5 φ0 /√Hz.

Excellent thermal stability of superconducting properties in YBa2Cu3O7 films irradiated by pulsed excimer lasers

We have investigated the microstructure and superconducting properties of YBa2Cu3O7 films irradiated by nanosecond pulsed excimer lasers. The superconducting YBa2Cu3O7 thin films were deposited in situ on (100) LaAlO3 substrates using the pulsed laser evaporation technique. The virgin unirradiated YBa2Cu3O7 films exhibited excellent superconducting properties with Tco (temperature for zero resistance) of 90 K and critical current densities Jc (at 77 K and zero magnetic fields) of approximately 5.0 × 106 A/cm2. The films were further irradiated by a pulsed excimer XeCl laser (λ=308 nm, τ=45×10−9 s) with energy density varying from 30 to 300 mJ/cm2. Excellent thermal stability was observed for YBa2Cu3O7 films on LaAlO3 substrate for laser irradiation at all energy densities between 30 and 300 mJ/cm2, with no deterioration in the Tco values (90±0.3 K). An improvement in the Jc of the films was observed at low-energy density irradiation, however, for energy densities above the melt threshold the Jc values decreased sharply. The Raman measurements for laser irradiated films showed improvement in the crystallinity of the film with stronger Raman peaks at 140 and 340 cm−1, thus correlating the improvement in Jc to better crystallinity of the films.

Ion beam sputtering of i n s i t u superconducting Y-Ba-Cu-O films

Oriented superconducting YBa2Cu3O7 thin films were deposited on yttria stabilized zirconia and SrTiO3 substrates by ion-beam sputtering of a nonstoichiometric oxide target. The films exhibited zero-resistance critical temperatures as high as 83.5 K without post-deposition anneals. Both the deposition rate and the c-lattice parameter data displayed two distinct regimes of dependence on the beam power of the ion source. Low-power sputtering yielded films with large c-dimensions and low Tc’s. Higher-power sputtering produced a continuous decrease in the c-lattice parameter and increase in critical temperature. Films having the smaller c-lattice parameters were Cu rich. The Cu content of films deposited at beam voltages of 800 V and above increased with increasing beam power.

Ion beam deposition of i n s i t u superconducting Y-Ba-Cu-O films

Oriented superconducting YBa2Cu3O7 thin films were deposited on yttria-stabilized zirconia substrates by ion beam sputtering of a nonstoichiometric oxide target. The films exhibited zero-resistance critical temperatures as high as 80.5 K without post-deposition anneals. Both the deposition rate and the c lattice parameter data displayed two distinct regimes of dependence on the beam power of the ion source. Low-power sputtering yielded films with large c dimensions and low Tc’s. Higher power sputtering produced a continuous decrease in the c lattice parameter and an increase in critical temperatures.

In-situ superconducting YBa2Cu3O7 thin films grown by ion beam co-deposition

We present superconducting YBa2C3O7 (YBCO) thin films grown in-situ by three-ion-beam sputtering. Y, Y2O3, Cu, Cu2O, BaF2 and BaCO3 sputter targets have been investigated. The highest quality films were prepared using a BaCO3 target. Auger analysis of films grown using a BaCO3 target show no carbon content. Y2O3 and Cu2O are more suitable than the native metals as sputter targets for YBCO growth as they are much less prone to sputter rate variations with oxygen partial pressure. They also supply oxygen to the growing film. As-deposited YBCO films are metallic (resistivity 240 μΩ cm at 100 K), reflective, and of highly homogeneous composition with TCO transition temperatures of 73 K and transition widths of 15 K. Post-annealing in flowing oxygen improves TCO's to 82 K. Critical currents are in excess of 105 A cm-2 at 77 K. Films are textured with c-axis orientation perpendicular to the (100) SrTiO3 substrate surface. As-deposited superconducting YBCO films have also been prepared on SiO2 and Y2O3 buffer layers on Si wafers.

Twinning characteristics in YBaCuO superconducting thin films

Abstract We have analyzed the nature, distribution and number density of twins as a function of orientation of superconducting YBa2Cu3O7 thin films deposited on strontium titanate (100), magnesium oxide (100) and ytria-stabilized zirconia substrates. The twinning characteristics in these high T c superconducting thin films have been found to be different from those in bulk materials as a results of texturing, interfacial stresses and constraints during crystal growth. The free surface has been found to play an important role in the relaxation of stresses introduced in the film during tetragonal-to-orthorhombic phase transformation. Detailed electron microscopy studies were performed on superconducting thin films deposited by evaporation of bulk YBa2Cu3O7 targets using a pulsed excimer laser ( λ = 308 nm; τ p = 45 × 10 −9 s ). Calculations of stresses in thin films indicated that higher shear stresses and strain energies were introduced in films with the c axis normal to subtrate than in those parallel to the substrate, thereby resulting in a higher number density of twins in the first case. The electron diffraction patterns of twinned crystals were simulated to understand the crystallography of the twinning process, and high resolution transmission electron microscopy studies were performed to determine the bending of planes across the twin boundary. Good agreement between experimental observations and theoretical models was obtained.

In-Situ Pulsed Laser Deposition of High-TC Yba2Cu3O7 Superconducting Thin Films on (100) LaAlO3 Substrates

AbstractThe formation of superconducting thin films on lanthanum aluminate substrates is very important for high‐frequency applications. In this paper, we discuss the fabrication of epitaxial superconducting YBa2Cu3O7 thin films on (100)LaAlO3 substrates, which exhibit excellent dielectric properties required for high frequency applications. The films were deposited by the biased pulsed laser evaporation technique (PLE) at substrate temperatures between 500‐650°C and exhibit excellent crystallinity with best minimum ion channeling yields corresponding to approximately 3%. The superconducting transition temperatures varied from 88‐92 K with critical current densities at 77K and zero magnetic field greater than 4 ‐ 5 x 106 Amps/cm2.

Effect of noble metal buffer layers on superconducting YBa2Cu3O7 thin films

Superconducting YBa2Cu3O7 thin films have been prepared by using a magnetron sputtering system in the single-source mode. Samples deposited on [100] single-crystal MgO with and without a Au buffer layer all show high transition temperatures (82–87 K). The use of a Au buffer layer significantly improves the superconducting properties, particularly the Meissner effect and critical current density (3.3×106 A/cm2 at T=2 K and 3.5×104 A/cm2 at T=77 K). The Au films remain metallic after high-temperature annealing in an oxygen atmosphere. We propose to use Au buffer layers as current shunts to protect superconducting films and devices.