YBa2Cu3O7 Films Research Articles

Liquid phase epitaxy processed coated conductors on metal tapes

Influence of the buffer material addition to the liquid for the YBa 2Cu 3O y (Y123) growth in liquid phase epitaxy (LPE) process and the suitable constructions of the LPE coated conductors were investigated for both MgO- and NiO-saturated systems. In the MgO-saturated system, influence of MgO addition to the liquid was negligible in Y-solubility in liquid and the peritectic temperature ( T p) value. The double layered LPE films of YBa 2Cu 3O y layer on YBa 2(Cu,Mg) 3O y layer, which were grown from MgO-saturated liquid, was grown on the Hastelloy tape with an MgO buffer layer. However, the growth of the second layer was unstable. It was found that the partial melting back of the first layer during dipping of the second layer growth is the reason for the unstable growth of the second layer. The problem was solved by lowering the growth temperature for the second layer than that of first layer due to selection of RE in RE123. On the other hand, NiO-addition made T p of the Y123 phase decrease and the crystallization temperature increase. This means that the possible temperature range for stable growth of the YBa 2(Cu,Ni) 3O y phase becomes narrow due to NiO addition. This problem was also solved by the selection of RE element in RE123 with high T p. The double layered construction was confirmed using Y123(LPE)/Sm123-Ni(LPE)/NiO/Ni. Consequently, the suitable constructions for each system were developed.

Josephson modulations of critical current above 77 K in YBa 2Cu 3O 7–Au thin film composites grown on LaAlO 3

Composite films of YBa 2Cu 3O 7–Au were grown on LaAlO 3 substrates using a sequential bilayer deposition process with different thickness of the Au layer. A reproducible and clear Josephson effect was observed only when the composite exhibited a labyrinth-like microstructure. Magnetic field modulation of the critical current was obtained at temperatures above 77 K. In this case gold-rich islands constrained weak-link zones, which conduced to a quasi-controlled array of Josephson junctions in the composite.

Collective pinning of a frozen vortex liquid in ultrathin superconducting YBa2Cu3O7 films.

The linear dynamic response of the two-dimensional (2D) vortex medium in ultrathin YBa2Cu3O7 films was studied by measuring their ac sheet impedance Z over a broad range of frequencies omega. With decreasing temperature the dissipative component of Z exhibits, at a temperature T*(omega) well above the melting temperature of a 2D vortex crystal, a crossover from a thermally activated regime involving single vortices to a regime where the response has features consistent with a description in terms of a collectively pinned vortex manifold. This suggests the idea of a vortex liquid which, below T*(omega), appears to be frozen at the time scales 1/omega of the experiments.

Near-field scanning optical microscopy studies of electronic and photonic materials and devices

Probing optical properties of materials and optical characterization of crystallographic defects at the nanometer scale have been inaccessible until recently due to the diffraction limit of light. With the invention of near-field scanning optical microscopy (NSOM), resolution at the 50–100 nm level using visible or near infrared light is now practical. In addition to describing the NSOM technique, this review focuses on the application of NSOM to the characterization of electronic and photonic materials and devices, with particular emphasis on defects. The unique capability of NSOM to simultaneously measure surface topography and local optoelectronic properties, thereby eliminating the need to perform cross correlation analysis on results obtained using different techniques, is particularly useful in this area. Several examples are discussed. By performing near-field photocurrent (NPC) measurements, NSOM is used to probe electrical activities associated with individual threading dislocations and dislocation networks in strain relaxed, compositionally graded GeSi films. The non-destructive nature of NSOM helps elucidate how microstructural defects in the SrTiO3 bicrystal substrates affect YBa2Cu3O7 film growth and GBJJ performance. Characterization of III–V and II–VI semiconductors, guantum dots grown by strain epitaxy, laser diodes, waveguides, and photonic crystals is also included. The advantages and disadvantages of NSOM in each application will be outlined. Throughout the review, emphasis is placed on how NSOM complements existing materials characterization techniques, as well as how quantitative results can be obtained from NSOM measurements.

Low-loss YBa2Cu3O7 films on flexible, polycrystalline-yttria-stabilized zirconia tapes for cryoelectronic applications

High-temperature superconducting films on flexible, low-thermal conductivity, low-loss substrates offer a unique base for the development of cryoelectronic digital interconnects. Using an ion-beam-assisted pulsed-laser-deposition technique, we developed biaxially textured YBa2Cu3O7 (YBCO) films on flexible polycrystalline-yttria-stabilized zirconia (YSZ) substrates with the following materials properties: (i) in-plane x-ray Φ-scan full width at half maximum of ∼7°; (ii) transition temperatures (Tc) in the range of 88–89 K with transition widths (ΔTc) of ∼0.5 K; (iii) critical current densities (Jc) in the range 1.5–2×106 A/cm2 at 77 K, zero field; (iv) magnetic penetration depth (λ) of 284 nm at 77 K; and (v) surface resistance (Rs) of 700 μΩ at 77 K, 10 GHz. The low-microwave loss, biaxilly textured YBCO films combined with the low-thermal conductivity YSZ substrate could facilitate a variety of RF cryoelectronic applications.

Vortex channeling and the voltage–current characteristics of YBa2Cu3O7 low-angle grain boundaries

We have performed (voltage–current) V–I measurements on a thin film YBa2Cu3O7 4° [001] tilt low-angle grain boundary over an extensive range of temperatures and fields, verifying the presence of a linear characteristic. We report on the occurrence of the linear characteristic in its basic form and on the observation of V–I kinking into several, and in some cases numerous, linear segments. We interpret these findings in terms of a variation in the dissipative width at the grain boundary. Kinking from one linear V–I section to another of different gradient is described in terms of a change in the number of vortex rows being viscously channeled along the boundary.

Microstructure and superconducting properties of ultrasonically spray pyrolysed YBa 2Cu 3O 7− x films

An ultrasonic mist pyrolysis system has been used to deposit in situ thick films of YBa 2Cu 3O 7− x , on single crystal LaAlO 3. Growth temperatures of ∼900°C have been utilised to spray nitrate precursors at rates varying between 0.7 and 1 μm min −1. Epitaxial growth of biaxially aligned (<3°), 3 μm thick, films has been achieved, with typical T c endpoint∼85 K and T c onset∼91 K, with J cs of >10 4 A cm −2 1 T, 77 K. The film microstructure is very dense and grain connectivity is good. The main objective of this research has been to obtain high T c values and to further investigate the limitations of this very scalable route for conductor fabrication by studying film properties in relation to nitrate concentration and growth rate.

Crystalline growth rate and microstructure in YBaCuO thin films

The influence of the anisotropy of the crystalline growth rate on the microstructure and morphology of YBaCuO thin film deposited by laser ablation is studied by means of transmission electron microscopy. The results obtained from high resolution lattice imaging and large angle convergent beam diffraction, together with the investigations of the thin film surface morphology show the strong influence of the different crystalline growth rates on both the internal stress present in the YBaCuO film and the roughness of the outer surface. These results should be of prime importance for the superconducting properties. This is an essential in tailoring these thin films for device applications.

Resonators and filters made of YBaCuO thin films on sapphire wafers

300–500 nm thick YBaCuO thin films with c-axis orientation were simultaneously deposited by sputtering of sintered hollow cylinders on both sides of CeO 2 buffered r-cut sapphire wafers with a diameter of 3 inch. Microstrip resonators and filters were manufactured to study the microwave properties of the films at 77 K in the frequency band from 2 to 6 GHz which is relevant to satellite communication. At linear response the resonance curve of the transmission resonators could perfectly be described by a Lorentz function. Very high values of the unloaded quality factor, Q 0, of up to 32 000 were reached at 4 GHz. At nonlinear response Q 0 decreased due to the power dependence of the surface resistance of the YBaCuO films which steeply increased at an oscillating power of several watts corresponding to a critical surface magnetic field of about 10 mT. The seven- and three-pole bandpass filters with center frequencies of 6 and 2 GHz, respectively, showed a low attenuation of less than −0.3 dB in the pass band. The results manifest that sputtering is a qualified and reliable method for large-area deposition of YBaCuO thin films which are suitable for microwave applications.

YBaCuO deposition by MOCVD on metallic substrates: a comparative study on buffer layers

YBaCuO films of about 1 /spl mu/m thickness have been deposited by metal organic chemical vapor deposition (MOCVD) on biaxially textured Ni-based substrates. Different buffer layers (MgO, YSZ, CeO/sub 2/), and also NiO epitaxially grown on Ni, have been tested with subsequent YBCO deposition in the same reactor. When using NiO as first buffer layer, Ni-based substrates were oxidized in a rapid thermal processing system or in a conventional furnace previous to deposition of the second buffer layer. In the present work, we study different conductor architectures from the point of view of T/sub c/, J/sub c/ (77K), epitaxial growth, and chemical stability of the structure.

Y-Ba-Cu-O thin films as active high power switches

The authors have studied the active switching of nonshunted YBaCuO films. Therefore films of 10-300 nm thickness were deposited on substrates by thermal co-evaporation. Bridges, 5 mm/spl times/26 mm and 10 mm/spl times/42 mm, were structured by standard photolithography. No topcoat of gold was used to retain higher switching powers. The active switching of the DC-biased YBaCuO bridges was triggered by heat pulses or radio frequency. For the thermal trigger, a resistive thin film heater was evaporated on the back side of the substrate. The samples were biased by currents up to 20A and triggered by heat pulses (up to 500 W) at 10-70 K. A resonant circuit was used in the case of the RF-trigger. The pancake coil of this circuit was placed 2 mm above the 10 mm wide YBaCuO bridge. At 77 K the stripes were biased by currents up to 40A and triggered with RF-pulses of >1 ms duration and 3-40 W (10 MHz). Active switching was observed in both cases. In comparison to the thermal trigger, power for RF induced switching power is very low.

Temperature dependence of the critical current in high-Tc superconductors with low-angle boundaries between crystalline blocks

A model for the limiting of the critical current in rather perfect high-Tc superconducting crystals and epitaxial films with a block structure with small angles of misorientation θ of the crystalline blocks is considered for the case when the distance d between edge dislocations along the boundary between blocks is greater than the coherence length ξ(T). It is shown that under these conditions the transparency of low-angle boundaries for the superconducting current carriers near the critical temperature Tc is practically independent of θ and T. As a result, the only factor governing the temperature dependence of the critical current density jc(T) remains the depairing current j0(T)∝(1−T/Tc)3/2. Near Tc, when ξ(T)&amp;gt;d, a transition from the dependence jc(T)∼(1−T/Tc)3/2 to a dependence jc(T)∼(1−T/Tc)2 occurs. This behavior of jc(T) is in good agreement with the results of experimental measurements of the critical currents in thin epitaxial films of YBa2Cu3O7−δ.

AFM, SEM, EDX and HRTEM study of the crystalline growth rate anisotropy-induced internal stress and surface roughness of YBaCuO thin film

The influence of the anisotropy of the crystalline growth rate on the microstructure and morphology of YBaCuO thin film deposited by laser ablation has been studied by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM), atomic force microscopy (AFM) and energy dispersive X-ray (EDX) analysis. The results obtained from high resolution lattice imaging and large angle convergent beam electron diffraction (LACBED), together with the investigations of the thin film surface morphology, show the strong influence of the different crystalline growth rates on both the internal stress present in the YBaCuO film and the roughness of the outer surface. These results should be of prime importance for the superconducting properties. This is essential in tailoring these thin films for device applications.

Spontaneous and light-induced resistive transitions in superconducting thin film bridges

In order to study dynamically the resistive states which appear as jumps in the I–V curves of narrow bridges, we have submitted YBaCuO and Nb films to nanosecond current steps and laser pulses simultaneously. In constant current feed, the distinction between hot spots (HS) and phase-slip centers (PSCs) is unambiguous, since HS are compelled to grow or decay, at variance with the stable-in-time PSCs. Thanks to the transient method we show that, even if Joule dissipation associated to a current I is virtually sufficient to sustain the film above Tc, the zero resistance state remains metastable: a hot spot does not arise unless initiated by a PSC, which fact was not reported before. The domains of occurrence of HS and PSCs were then organized in a current–temperature plane. Finally, the theoretical problem of the HS velocity of growth is given an exact solution, in semiquantitative agreement with experiment.

Transport properties of Cr-patterned Yba2Cu3O7 thin films

ABSTRACTEpitaxial thin films of Yba2Cu3O7 (YBCO) have been deposited and patterned on (100) SrTiO3 and (110) NdGaO3 substrates. Wet etchants used in conventional photolithography react too quickly with YBCO thin films, resulting in undercutting of patterned structures. We have implemented a technique to pattern thin lines from films without wet or dry etching. Chromium is evaporated onto the substrates and subsequently patterned. YBCO is then deposited by pulsed laser deposition, which results in an epitaxial YBCO film on patterned areas and randomly oriented YBCO deposited on chromium. An annealing step is required for the chromium to diffuse through the undesired YBCO in order to render those sections insulating. YBCO lines patterned with this technique exhibit resistivity and critical temperatures similar to those of the unpatterned films. As we decrease the width of the YBCO lines, the effect of the edges, where there is chromium-reacted material, on transport becomes more pronounced. The resolution of the patterning technique and the profile of superconducting versus non-superconducting materials at the edges is presented.

Correlation between the morphology of Ag and the contact resistivity of the Ag/YBa2Cu3O7-δ thin film contact

The morphology of the silver films deposited and annealed on laser ablated YBa2Cu3O7−δ thin films and the corresponding contact resistivity have been systematically investigated. A minimum contact resistivity of 6 × 10−8 Ω cm2 was reached at 77 K by annealing Ag/YBa2Cu3O7−δ contact at the optimum temperature. The effect of the annealing temperature on the contact resistivity was explained by considering the morphology of the silver films and the diffusion of silver into YBa2Cu3O7−δ film, etc. The difference of the contact resistivity for Ag contact to polycrystalline, single crystal and thin film of YBa2Cu3O7−δ were also explained.

Field Deployable Microwave Filters Made from Superconductive Thick Films

Radio frequency (RF) resonators are made by depositing thick films of YBa2Cu3O7−δ onto three-dimensional substrates. The superconducting resonators are then coupled together to form high performance RF filters. The topologies and frequency responses of some of these devices are briefly discussed prior to beginning a more detailed discussion aimed at addressing some of the common misconceptions of high temperature superconductive (HTS) thick film devices, namely the nonlinearity and the size. The intermodulation distortion (IMD) is shown to be below the noise floor in most practical applications, and a mathematical model that uses the empirical surface impedance data to predict the behavior of the IMD is described.

Optical-microwave interaction modeling in high-temperature superconducting films

Optical-microwave interaction in high-T/sub c/ superconductors is investigated for performing basic optoelectronic functions in cryogenic environment. A fast bolometric photoresponse in conjunction with the unique electrical properties of high-temperature superconductor (HTS) materials allows us to explore a series of novel optoelectronic devices with low-noise/low-power and high-speed/high-frequency characteristics. After reviewing the HTS photoresponse categories, we describe the fast bolometric photoresponse and its condition with heat transfer analysis. The analytical solution of heat diffusion equation for an HTS strip mounted on a substrate will be presented for three different type of optical sources. Then the effect of optical irradiation will be incorporated in the two-fluid model by thermomodulation concept in order to model the interaction of optical radiation with electrical signal. The current-field relationship and the supercurrent response time are evaluated in the presence of both optical and electrical signals. Our numerical simulations for YBaCuO film demonstrate the possibility of RF harmonic generation when the laser beam is modulated by the RF signal in the presence of dc bias current and RF signal mixing when the HTS film is fed by a time-harmonic microwave source. The developed model can also be used to study optical control and tunability techniques for HTS microwave devices for analog signal processing.

Muon Spin Rotation and Relaxation Experiments on Thin Films

The recent development at the Paul Scherrer Institute of a beam of low energy muons allows depth dependent muon spin rotation and relaxation investigations in thin samples, multilayers and near surface regions (low energy μSR, LE-μSR). After a brief overview of the LE-μSR method, some representative experiments performed with this technique will be presented. The first direct determination of the field profile just below the surface of a high-temperature superconductor in the Meissner phase illustrates the power and sensitivity of low energy muons as near-surface probe and is an example of general application to depth profiling of magnetic fields. The evolution of the flux line lattice distribution across the surface of a YBa2Cu3O7 film in the vortex phase has been investigated by implanting muons on both sides of a normal-superconducting boundary. A determination of the relaxation time and energy barrier to thermal activation in iron nanoclusters, embedded in a silver thin film matrix (500nm), demonstrates the use of slow muons to measure the properties of samples that cannot be made thick enough for the use of conventional μSR. Other experiments investigated the magnetic properties of thin Cr(001) layers at thicknesses above and below the collapse of the spin density wave.

Spatial Distribution Analyses of Superconducting Transition Temperature in Epitaxial YBa2Cu3O7 Film Using Variable Temperature Scanning Laser Microscopy

ABSTRACTThe spatial distribution of superconducting properties using variable temperature scanning laser microscope (VTSLM) has been investigated. The superconducting thin film used in this study is an epitaxial YBa2Cu3O7 film photolithographically patterned to a 300 μm-wide bridge. Since the voltage response, δV(x,y) is proportional to dR/dT(x,y), the spatial distribution of superconducting transition can be obtained in VTSLM images. In the resistive transition region, there is a strong correlation between the VTSLM images and the resistance of the sample. With decreasing resistance, the area with large δV(x,y) shifts toward the ends of the bridge. This indicates that the resistive transition is not uniform and both ends of the bridge have lower transition temperature, Tc. Different currents or different output power of lasers do not affect the images. VTSLM technique is a powerful tool to image the local superconducting properties and to identify the weaker superconducting areas.