Molecular beam epitaxial growth of high-Tc Bi-Sr-Ca-Cu-O Films

Molecular beam epitaxy (MBE) has been used to grow high-temperature superconducting Bi-Sr-Ca-Cu-O films with adequate control over growth of number of unit layers. Oxide sources of Sr and Ca used for electron beam evaporation have been found to be useful for epitaxial growth of films. Deposited films show superconducting properties comparable to films deposited by using pure metals with a complicatedin situ oxidation technique. Optimum deposition and annealing conditions have been obtained to growc-axis-oriented 2212 phase BSCCO film.In situ reflection high-energy electron diffraction (RHEED) study of the films has revealed the growth of epitaxial films with atomically smooth surfaces.     

Microstructural study of YBa2Cu3O7/SrTiO3/YBa2Cu3O7 heteroepitaxial trilayer films grown on (100) SrTiO3 substrates

Detailed transmission electron microscopic study has been carried out on heteroepitaxial YBa₂Cu₃O₇/SrTiO₃/YBa₂Cu₃O₇ trilayer thin films grown on (100)SrTiO₃ substrates prepared by DC and RF magnetron sputtering. The microstructural results showed the existence of somea-axis-oriented YBCO grains 20–90 nm wide in thec-axis-oriented YBCO matrix. Some of thea-axis grains in the lower YBCO thin film layer have protruded into the above SrTiO₃ layer, which may cause short circuit between the two YBCO superconducting layers. This is unsuitable for the application of trilayer thin films for microelectronic devices. The defects on the surface of the substrates would also influence the growth quality of the YBCO thin films.     

Effect of PrBa2Cu3O7?x buffer layer thickness on the properties of YBa2Cu3O7?x thin films grown on sapphire by laser ablation

Superconducting YBa₂Cu₃O_7–x (YBCO) thin films were deposited onr-plane A1₂O₃ substrates with PrBa₂Cu₃O_7–x (PBCO) buffer layer by XeCl excimer laser ablation. The thickness of PBCO buffer layer was systematically changed to investigate the superconducting properties of YBCO thin films on sapphire. The structure and surface morphology of the films were characterized by X-ray diffraction and scanning electron microscopy (SEM). Superconducting transition temperatures were varied depending on the buffer layer thickness. Interdiffusion between laser-ablated YBCO thin films and A1₂O₃ substrates had been studied by Auger electron spectroscopy (AES). The results of this study show that diffusion does not occur between the YBCO thin film and the substrate even with 20 Å thick PBCO buffer layer.     

Processing and characterization of high temperature superconductor films formed by spin-on coating and rapid thermal annealing

A simple method has been developed to fabricate oxide high temperature superconductor (HTS) films of Y---Ba---Cu---O (YBCO) and Bi---Sr---Ca---Cu---O (BSCCO) compounds. Using spin-on coating and rapid thermal annealing (RTA), high T_c films on MgO and copper substrates have been demonstrated. Of the various solvents employed, we have achieved the best results utilizing nitric acid. The use of this solvent coupled with annealing in oxygen at 960–990 °C for 30–60 s by RTA has produced polycrystalline films with grain sizes of 5–10 μm, as indicated by scanning electron microscopy. Auger electron spectroscopy of the films shows that the stoichiometric ratio has been maintained during the transition from precursor to film. X-ray diffraction patterns of YBCO films show that the films mainly contain 123 phase. Superconductivity with zero resistance temperatures above 77 K was observed from both YBCO and BSCCO films. When copper substrates are used, a buffer layer of silver is coated between the substrate and the film so as to prevent interface reaction. The simple spin-on method combined with the short and intense heating of the RTA process may prove to be a simple and rapid method of manufacturing HTS films for large-scale electrical applications.     

Growth and Characterization of YBCO Thin Films by Sequential Deposition and Annealing

A novel thin film growth procedure, sequential deposition and annealing (SDA), which contains the advantages of both in situ and ex situ procedures, was proposed. Y₁Ba₂Cu₃O_{7 – x} (YBCO) high temperature superconducting thin films were grown and characterized by the SDA procedure. Purely c-axis-oriented YBCO thin films with no foreign phases and other oriented grains were successfully prepared. The superconducting transition properties of SDA-grown YBCO thin films were measured by measurement of inductance and resistance. The inductance measurements gave a T _c ^onset of 85 K and a T _c of 5 K. The resistance measurements gave a T _c ^onset of 90 K and a T _c of 5 K. Atomic force microscopy studies showed that SDA-grown YBCO thin films had micrometer-size grains surrounded by many nanometer-size grains. The nanometer-size grains in SDA-grown YBCO thin films are responsible for degradation of superconducting transition properties.     

A Proposed New Measurement of the Superconducting Gap Around $\bar{M}$ -point in Bi2Sr2CaCu2O8

In previous publications, Zhao et al. have presented first-principle calculations of the electronic structure, T _c, and the superconducting energy gaps on the Fermi surfaces of YBa₂Cu₃O₇ (YBCO). However, experimental measurements of the superconducting gaps on the Fermi surfaces of YBCO still face some challenges due to surface problems for YBCO samples. Considering the similarities between the crystal structure, electronic properties, and features of the Fermi surfaces of YBCO and those of Bi₂Sr₂CaCu₂O₈ (BSCCO), we discuss the need to measure the superconducting gap on the small sheet of the Fermi surface around the [`(M)]\bar{M} -point in BSCCO. This gap may be similar to the one of YBCO around the S-point. The superconducting gap on this small sheet of the Fermi surface around the [`(M)]\bar{M} -point in BSCCO is expected to show minor variations from about 18 to 25 meV, as we found on the small sheet of the Fermi surface around the S-point of YBCO. There is no node on the superconducting gap on this small sheet of the Fermi surface around the [`(M)]\bar{M} -point of BSCCO.     

Effect of PrBa2Cu3O7−x buffer layer thickness on the properties of YBa2Cu3O7−x thin films grown on sapphire by laser ablation

Superconducting YBa₂Cu₃O_7?x (YBCO) thin films were deposited onr-plane A1₂O₃ substrates with PrBa₂Cu₃O_7?x (PBCO) buffer layer by XeCl excimer laser ablation. The thickness of PBCO buffer layer was systematically changed to investigate the superconducting properties of YBCO thin films on sapphire. The structure and surface morphology of the films were characterized by X-ray diffraction and scanning electron microscopy (SEM). Superconducting transition temperatures were varied depending on the buffer layer thickness. Interdiffusion between laser-ablated YBCO thin films and A1₂O₃ substrates had been studied by Auger electron spectroscopy (AES). The results of this study show that diffusion does not occur between the YBCO thin film and the substrate even with 20 Å thick PBCO buffer layer.     

Geometrical model and experimental evidence for the growth of Ag-doped YBa2Cu3O7?x thin films

The improvement in the properties of laser-ablated Ag-doped YBa₂Cu₃O_7–x (YBCO) thin films in both normal and superconducting states has been interpreted using a growth model. The model is simple and is based on the widely accepted characteristics of Ag such as its flux action at high temperature and nonreactivity with YBCO phase.     

Phase-transition toughening of high-T c superconducting ceramics

The effect of the superconducting transition (SCT) on the plane-strain fracture toughness K _1c of YBCO and BSCCO is studied by the method of three-point bend loading at room temperature and at 77 K. Toughening is observed and related to variation of the Gibbs free energy through the SCT, and hence to the condensation energy of a Cooper-pair gas. This is explained by the additional energy needed to excite superconducting electrons to normal ones in crack surfaces owing to magnetic field penetration. The superconducting gap parameters of YBCO and BSCCO are estimated and the results are reasonable.     

YBa2Cu3O7−x Thin Films by Citrate-Based Non-fluorine Precursor

A new fluorine-free chemical solution deposition route has been developed for YBa₂Cu₃O_7−x (YBCO) thin films. The precursor was formed by dissolving metal nitrates and citric acid in ethylene glycol. Polyethyleneimine was used to modify the solution. The preparation of YBCO thin films consisted of repeated dip coating, drying, heat-treating in air and annealing in flowing argon with a small addition of oxygen and with final treatment in pure oxygen. The films were characterized by XRD and SEM and show dominating and well-textured YBCO phase. The AC susceptibility and magnetization measurements reveal a critical temperature about 90 K and a high critical current density, comparable with that of pulse laser deposited films. The simplicity and high performance of the method may be attractive for large-scale superconducting applications.     

Influence of Interfacial LSMO Nanoparticles/Layer on the Vortex Pinning Properties of YBCO Thin Film

YBa₂Cu₃O_7?δ (YBCO) thin films have been deposited on bare and La_0.67Sr_0.33MnO₃ (LSMO) modified single crystal SrTiO₃ (STO) substrates. The effect of randomly distributed ferromagnetic LSMO nanoparticles and a complete LSMO layer, present at STO/YBCO interface, on the superconducting properties of YBCO thin films has been investigated by temperature dependent magnetization studies. The YBCO thin film on LSMO nanoparticles decorated STO substrate shows significant improvement in the critical current density and pinning force density as compared to the YBCO thin film deposited on bare STO substrate and this improvement is more significant at higher applied magnetic field. However, the LSMO/YBCO bilayer showed the improved flux pinning properties only up to a magnetic field of 1.5 T above which it deteriorates. In the case of LSMO/YBCO bilayer, the underlying LSMO layer gives rise to magnetic inhomogeneities due to domain structure, which leads to improved flux pinning properties limited to lower field. However, in the case of LSMO nanoparticles decorated substrate, the presence of LSMO nanoparticles at YBCO/STO interface seems to introduce magnetic inhomogeneities as well as structural defects, which might be acting as correlated pinning sites leading to improved flux pinning properties of the YBCO thin film over a wide range of applied magnetic field.     

Microstructural investigation of Bi–Sr–Ca–Cu–oxide thick films on alumina substrates

The microstructure of Bi–Sr–Ca–Cu–oxide (BSCCO) thick films on alumina substrates has been characterized using a combination of X-ray diffractometry, scanning electron microscopy, transmission electron microscopy of sections across the film/substrate interface and energy-dispersive X-ray spectrometry. A reaction layer formed between the BSCCO films and the alumina substrates. This chemical interaction is largely responsible for off-stoichiometry of the films and is more significant after partial melting of the films. A new phase with f c c structure, lattice parameter a = 2.45 nm and approximate composition Al₃Sr₂CaBi₂CuO _x has been identified as reaction product between BSCCO and Al₂O₃. This revised version was published online in November 2006 with corrections to the Cover Date.     

High-temperature superconducting dual-mode resonator on (100) MgO substrate

We report, for the first time to our knowledge, a clear resonant peak split in the range of 7.7–9.7 GHz in a perturbed dual-mode disk-type resonator (DMDR) made of YBa₂Cu₃O_7–x (YBCO) superconducting thin film on MgO substrate. Epitaxial YBCO superconducting thin films were grown on (100) MgO substrates by pulsed laser deposition technique. The critical temperature of superconducting thin film on MgO substrate was 85 K. Superconducting dualmode disk resonators were designed by microwave design software, EEsof, and patterned by photolithography and a wet-etch process. The unloaded quality factor (Q_UL) of the superconducting DMDR was found to be 1,312 at 77 K. We believe this type of DMDR can be utilized for dual-mode resonator-based filters for satellite communications.     

Superconducting behavior of high-Tc YBa2Cu3O7 thin films with BaO impurity produced by pulsed laser deposition

We have investigated the superconducting behavior of high-T _c YBa₂Cu₃O₇ (YBCO) thin films containing BaO impure phase produced by pulsed laser deposition. The thin films were characterized by the standard four-probe method, X-ray diffraction (XRD), and scanning electron microscopy (SEM). XRD showed that all these thin films contained BaO impurity, with thec-axis normal to the surface of the substrates. The presence of impurity existed from substrate temperatureT _s of 727 to 796°C. When these thin films with BaO impurity were measured under the magnetic fields, it was found that the critical current densityJ _c increased slightly with increase in magnetic fieldB within the range ofB500 G, in the case ofB perpendicular to thec-axis of the film.     

Effect of post-annealing on thin YBCO films deposited from a nanocrystalline target

Pulsed laser deposition is used to ablate thin superconducting YBCO films on SrTiO ₃ substrates. The most important parameters of thin superconducting films are high critical current density, ability to stand magnetic fields and smoothness of surfaces. Smoothness is important in fabrication of layered structures and for research of basic properties of thin superconducting structures. The target sintered from YBCO nanopowder is a promising material for making films which meet most of the requirements above. Investigations by AFM show that our target has grains about one order of magnitude smaller than usual grain size of commercial targets. At optimal deposition parameters, the oxygen pressure of 0.4 torr in the chamber and the substrate temperature 725°C, films with T _c = 90 K, J _c =8 × 10⁶ A/cm ² (77 K) and RMS surface roughness = 1.5 nm are obtained. Thermal annealing of the deposited films for 18 h at 900°C further increases the value of J _c .     

Nano Techniques for Enhancing Critical Current in?Superconducting YBCO Films

A range of nano techniques is explored in order to increase the critical current in pulsed laser deposited (PLD) superconducting YBa₂Cu₃O _x (YBCO) thin films. The structural measurements are linked with magnetic and transport measurements of the films. The effectiveness of PLD techniques is analyzed from the point of view of the dimensionality of nanostructures formed prior to and during the film growth. It is shown that a combination of two-dimensional substrate decoration with nanoparticles before the film deposition and one-dimensional growth of external phase nanorods during the deposition offers a high critical current in magnetic field both along the c-axis and in the ab-plane of YBCO.     

Comparison of Flux Pinning Mechanism in Laser Ablated YBCO and YBCO:BaZrO3 Nanocomposite Thin Films

Thin films of YBCO and YBCO:BaZrO₃ (BZO) nanocomposite have been deposited using the pulsed laser deposition technique. Substantial increase in critical current density (J _C ) and pinning force density (F _p ) of the nanocomposite thin films was observed. The possible pinning mechanism in YBCO:BZO nanocomposite thin films has been explored and compared with the pinning mechanism in pure YBCO thin film by studying the variation of J _C with magnetic field (B) and temperature. In the intermediate field regime (0.1–1 T), J _C follows B ^−α with nearly similar values of α for YBCO and YBCO:BZO nanocomposite thin films indicating similar pinning mechanism in both thin films. The variation of J _C with reduced temperature (t=T/T _C ) has been studied for both the films and it was observed that the mechanism of pinning in both YBCO and YBCO:BZO thin films is similar (δT _C pinning). The observed enhanced values of J _C and F _p of the nanocomposite thin film is attributed to the presence of BZO nanoparticles, which induces more defects due to lattice mismatch between YBCO and BZO leading to improved flux pinning properties of the nanocomposite thin film.     

Effect of Melt Processing Programme on Microstructure and Properties of YBCO Thick Films

The screen printing technique was used in the fabrication of YBa₂Cu₃O_7−σ (YBCO) superconducting thick films on yttria stabilised zirconia (YSZ) substrates. Different slow cooling rates were used in the preparation of YBCO thick films after fast cooling from the melt processing temperature. The effects of the melt processing programme on texturing, microstructure and superconducting properties of the melt processed YBCO films were studied. Slow cooling rates between 1005 and 990 °C were effective in increasing the interaction of viscous molten with reduced film/substrate, and hence a relatively large grain size has been obtained. Moreover, different c-axis texturing ratios and grain morphologies were observed.     

Influence of Deposition Pressure (O2) on the YBCO (Y123) Thin Films Prepared by Pulsed Laser Deposition

In this paper, the influence of oxygen pressure on film quality and superconducting properties of YBa₂Cu₃O_7?δ (YBCO-Y123) thin films prepared by Pulsed Laser Deposition (PLD) was investigated. For this purpose, YBCO thin films were deposited on polished LaAlO₃ (l00) (LAO) substrates at three different oxygen pressures (150, 200, and 250 mTorr). X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques were used to make comparative studies of film microstructure. Except for oxygen pressures, all other variables such as number of pulses, repetition rate, deposition temperature, heating and cooling rate, target-substrate distance, laser excitation energy, annealing temperature, and annealing pressure were fixed. For this fixed set of parameters, SEM, XRD analysis, and AC susceptibility measurements of these films revealed that the crystal structure quality and superconducting properties of YBCO thin films are optimum at the oxygen deposition pressure of 150 mTorr. As the deposition pressure increased, Y₂BaCuO₅ (Y211) phase peaks were seen in XRD patterns. The reason for this was believed to be caused by decreased concentration of CuO and BaO as determined by Energy Dispersive X-Ray Spectroscopy (EDX) of thin films.