Epitaxial YBCO Thin Films Research Articles

Nd:YAG infrared laser as a viable alternative to excimer laser: YBCO case study

We report on the growth and characterization of epitaxial YBa_{2}Cu_{3}O_{7-delta } (YBCO) complex oxide thin films and related heterostructures exclusively by Pulsed Laser Deposition (PLD) and using first harmonic Nd:Y_{3}Al_{5}O_{12} (Nd:YAG) pulsed laser source (lambda = 1064 nm). High-quality epitaxial YBCO thin film heterostructures display superconducting properties with transition temperature sim 80 K. Compared with the excimer lasers, when using Nd:YAG lasers, the optimal growth conditions are achieved at a large target-to-substrate distance d. These results clearly demonstrate the potential use of the first harmonic Nd:YAG laser source as an alternative to the excimer lasers for the PLD thin film community. Its compactness as well as the absence of any safety issues related to poisonous gas represent a major breakthrough in the deposition of complex multi-element compounds in form of thin films.

High Performance of YBCO Films Prepared by Fluorine-Free MOD Method and a Direct Annealing Process

Chemical solution-deposited (CSD) is a cost-effective non-vacuum method for YBCO coated conductor fabrication. We developed a new fluorine-free metal-organic deposition (MOD) method with metal acetates, propionic acid and xylene as the starting materials. Using this non-fluorine MOD method, we were able to get high performance YBCO superconducting films within a shortened heat treatment time, which was reduced by at Least 5 h in comparison with that for pyrolysis-annealing non-fluorine MOD process. Superconducting property, with a critial current density (Jc) over 0.55MA/cm2at 77K, self-field has been obtained for 380 nm epitaxial YBCO thin films on (00l) LaAlO3(LAO) single crystal substrate. Owing to the low price of starting materials and shorter heat treatment time, fluorine-free MOD method is a very effective and cost-cutting process.

Microstructure of epitaxial YBa2Cu3O7−δ thin films grown on Pb(Mg1/3Nb2/3)0.7Ti0.3O3 substrates

The microstructure of epitaxial c-axis oriented YBa2Cu3O7−δ (YBCO) thin films grown on piezoelectric/ferroelectric (001) Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) substrates by pulsed laser deposition has been studied using transmission electron microscopy. It is shown that major defects in the films are interfacial misfit dislocations and planar defects. Moreover, a small amount of a-axis oriented domains with inclined domain walls are observed in the YBCO films and the formation mechanism of the inclined domain walls is discussed. In addition, tiny precipitates near the interface are observed in some cases. All these defects and the domain structure contribute to the strain relaxation of the epitaxial YBCO thin films. These findings may be helpful for future research on correlation of microstructure and physical properties in the high-Tc superconductor–ferroelectric heterostructures.

Fabrication of 5 cm long epitaxial Sm0.2Ce0.8O1.9−x single buffer layer on textured Ni–5%W substrate for YBCO coated conductors via dip-coating PACSD method

Five centimeters long epitaxial Sm0.2Ce0.8O1.9–x (SCO) single buffer layer for YBCO coated conductors was deposited via dip-coating polymer-assisted chemical solution deposition (PACSD) approach on bi-axially textured Ni–5%W (200) alloy substrate. The film formation and texture evolution were investigated using X-ray diffraction and scanning electron microscopy. Flat, dense and crack-free SCO films with sharp (200) c-axis texture were obtained by way of carefully controlling the concentration of precursor solution, withdrawing speed, annealing temperature and dwelling time. On consideration of both microstructure and texture, epitaxial SCO single buffer layers were fabricated using precursor solution of 0.3M cationic concentration, the withdrawing speed of 10mm/min and heat treatment at 1100°C in Ar–5%H2 mixture gas for 0.5h. Epitaxial YBCO thin films with a homogeneous surface microstructure were deposited on the SCO-buffered NiW substrate via dip-coating PACSD approach. The PACSD approach was a promising way to fabricate long and low-cost YBCO coated conductors.

Effect of Deposition Temperature on the Epitaxial Growth of YBCO Thin Films on RABiTS Substrates by Pulsed Laser Deposition Method

Epitaxial YBCO thin films were grown on CeO2/YSZ/CeO2 buffered RABiTS substrates by PLD method and the effects of deposition temperature on their microstructure and the critical current were studied. YBCO thin films were prepared with substrate temperature from 700 to 820°C. The YBCO grown at below 740°C showed mixed a-axis and c-axis orientation, and the film grown at higher temperature showed high c-axis orientation. The (001) preferred orientation of the YBCO films was improved with increasing the temperature. While the critical current of the YBCO thin film increased firstly with increasing substrate temperature and had a maximum value at 770°C. The Ic of the YBCO film with 0.2 ?m thickness was 66 A (Jc = 3.3 MA/cm2) at 77 K and 0 T external field.

Preparation and structural properties of YBCO films grown on GaN/ c-sapphire hexagonal substrate

Epitaxial YBCO thin films have been grown on hexagonal GaN/ c-sapphire substrates using DC magnetron sputtering and pulsed laser deposition. An MgO buffer layer has been inserted between the substrate and the YBCO film as a diffusion barrier. X-ray diffraction analysis indicates a c-axis oriented growth of the YBCO films. Φ-scan shows surprisingly twelve maxima. Transmission electron microscopy analyses confirm an epitaxial growth of the YBCO blocks with a superposition of three a– b YBCO planes rotated by 120° to each other. Auger electron spectroscopy and X-ray photoelectron spectroscopy reveal no surface contamination with Ga even if a maximum substrate temperature of 700 °C is applied.

Chemically processed BaZrO3 nanopowders as artificial pinning centres

This work reports on the preparation of a BaZrO3 (BZO) nanopowder by the chemical decomposition of an oxalate precursor starting from barium acetate and zirconium oxychloride. The X-ray peak profile analyses and the scanning electron microscopy (SEM) have indicated that the mean powder size ranges from 30nm to 340 nm for the thermal treatment temperature varying from 700°C to 1200°C, respectivelly. The as prepared powder has been used to manufacture YBCO-5 mol. % BZO composite targets from which epitaxial YBCO thin films on (100)SrTiO3 substrate were ablated. The X-ray analysis have revealed that the BZO nanoislands are epitaxially grown throughout the YBCO film, having the same epitaxial relationship as YBCO. Moreover, as revealed by SEM, the presence of BZO considerably improves the morphology of the YBCO film. The critical temperature (Tc) registered a slight decrease with the BZO concentration. Nevertheless, the BZO addition resulted in an improvement of the critical current density, Jc. The JcvsB curve exihibits a large plateau extended up to about 2.5 Tesla. The YBCO-BZO composite films showed pinning forces with a c-axis correlated character, as revealed by the angular behaviour of Jc.

A BILAYER BUFFER USING 214T Eu2CuO4 AND CUBIC YSZ FOR GROWING YBa2Cu3Oy THIN FILMS ON Si

Highly epitaxial thin films of YBa 2 Cu 3 O y (YBCO) have been successfully grown on Si using a double buffer of Eu 2 CuO 4 (ECO)/(yttrium-stabilized zirconia) YSZ. The severe reaction between Si and YBCO is blocked by YSZ, and the crystallinity and superconductivity of the grown YBCO due to the lattice mismatch between YBCO and YSZ are improved by the ECO layer. The grown films were characterized by high-resolution X-ray diffraction, grazing incidence X-ray reflection, scanning electron microscopy (SEM), and Doppler broadened annihilation radiation spectroscopy, respectively. Very clear interfaces were found at YBCO/ECO/YSZ boundaries without any intermediate layer. The YBCO film surface was more smooth and stable. The results obtained indicate that highly epitaxial YBCO thin films can be successfully grown on Si wafers, demonstrating advantages of such a double buffer structure.

SUPERCONDUCTIVITY AND CRYSTALLINITY IN EPITAXIAL THIN FILMS OF YBa2Cu3Oy GROWN ON Eu2CuO4/YSZ DOUBLE BUFFERED SILICON

A double buffer of EU 2 CuO 3/ YSZ has been used to grow highly epitaxial thin films of YBa 2 Cu 3 O y (YBCO) on Si . These films showed enhanced superconductivity and improved crystallinity in comparing with that of films grown on Si directly. Well defined interfaces with no immediate layers were found. The YBCO film surface was more smooth and stable. The results obtained indicate that highly epitaxial YBCO thin films can be successfully grown on Si wafers, demonstrating advantages of such a double buffer structure.

Ac susceptibility of bicrystal-like type-II superconducting films

The ac susceptibility of a type-II superconducting thin film divided into two halves by a narrow mid region with a smaller value of critical-current density is calculated by a model based on Bean’s critical-state model and energy minimization. Results are compared to measurements of ac susceptibility as a function of ac-field amplitude of artificial bicrystal-like films prepared by photolithography patterning a gridline of holes on epitaxial YBCO thin films. A qualitative agreement is found between calculated and experimental results.

Microwave Response of Perfect YBa2Cu3O7−x Thin Films Deposited on CeO2-Buffered Saphire: A Probe for Pairing Symmetry

Microwave surface impedance, Z s(T), of epitaxial YBCO thin films deposited on CeO2-buffered sapphire substrates, was measured at several discrete frequencies within the range 5–134 GHz by use of coplanar resonator and end-plate cavity resonator techniques. The main features of obtained experimental results are as follows: (i) surface resistance R s(T) at low temperatures obeys the exponential law: R s(T) = R res+R 0⋅exp [−δ/T] with a small gap δ value (δ≈ 0.7 T c); (ii) the most perfect quasi-single-crystalline films reveal a distinct two-peak structure of R s(T) dependence, which is not observable in films with a less ordered crystal structure. These features are believed to reveal some intrinsic electron properties of such films, namely: (i) mixed (d+is) type symmetry of electron pairing, and (ii) dominant role of extended c-oriented defects (e.g., edge dislocation arrays or twin planes) in quasiparticles scattering for the most perfect films, which demonstrate the two-peak anomalous R s(T) behavior.

Spectroscopic and X-ray diffraction study of high Tc epitaxial YBCO thin films obtained by pulsed laser deposition

We report spectroscopic characterization of epitaxial YBCO thin films grown on LaAlO 3 by pulsed laser deposition. Raman spectroscopy and spectroscopic ellipsometry were used for film characterization and the results were correlated with X-ray diffraction measurements. The mentioned techniques allowed us to analyze crystallographic, micro-structural, and morphological properties of YBCO thin films. We also demonstrated that relatively low resolution Raman spectroscopy and spectroscopic ellipsometry are reliable techniques for a rapid and non-destructive characterization of epitaxial YBCO thin films.

Improvement of epitaxy and crystallinity in YBa 2Cu 3O y thin films grown on silicon with double buffer of ECO/YSZ

A novel double buffer of Eu 2CuO 4 (ECO)/YSZ (yttrium-stabilized zirconia) was developed for growing YBa 2Cu 3O y (YBCO) thin films on Si substrates. In these films, the severe reaction between Si and YBCO is blocked by the first YSZ layer, whereas, the degradation of crystallinity and superconductivity in the grown YBCO is greatly improved by the second ECO layer. Such an ECO material possesses a very stable 214-T' structure and excellent compatibilities with YBCO and YSZ. The result shows that the epitaxy and crystallinity of YBCO deposited on Si could be considerably enhanced by using the ECO/YSZ double buffer. The grown films are characterized by high-resolution X-ray diffraction, grazing incidence X-ray reflection, and transmission electron microscopy (TEM), respectively. It is found that well defined interfaces are formed at YBCO/ECO/YSZ boundaries. No immediate layer could be seen. The defect density in all grown layers is kept at a lower level. The YBCO film surface turns out to be very smooth. These films have full superconducting transitions above 88 K and high current carrying capacity at 77 K. The successful growth of highly epitaxial YBCO thin films on silicon with ECO/YSZ buffer, demonstrate the advantages of such a double buffer structure.

In situ grown epitaxial YBa2Cu3O7−x thin films by pulsed laser deposition under reduced oxygen pressure during cool-down time

We report novel pulsed laser deposition conditions that were used to obtain superconducting epitaxial YBCO thin films, grown in situ using an oxygen pressure lower than the usual one during the cool-down time. We studied the influence of the PLD conditions as substrate temperature, oxygen pressure, laser fluence, and number of laser pulses on the crystallographic and morphological features, and on the superconducting properties of the films. Good superconducting properties were obtained without a high temperature post-deposition annealing process. A maximum critical temperature of 88.6 K was obtained.

Correlation of structural and linear electromagnetic properties ofhigh-Tc thin films with their nonlinear microwave performance

A comprehensive study of the correlation between various structural(Δω°,c-axis),electrical (Tc,ΔT) and linearmicrowave properties (Rs(T0), λ(T0) andσ1(T0)) of epitaxial YBCOthin films on MgO and LaAlO3 substrates, and also their nonlinear microwave properties(Hrf*,d Rs/d Hrf,Hrf _ TOI2,m-slopeand Hrf _ 30 dB2) has been performed. It was found that among the numerous parameters from the first group, thenonlinear microwave properties appear to correlate well only with the low-temperature penetration depthλ(T0), residual quasiparticleconductivity σ1(T0) and the c-axis lattice parameter. Here, among those three parametersthe best correlation of the nonlinear properties is observed withλ(T0). The correlation of the other parameters from the first group with the nonlinear microwaveperformance of the films is either not evident, because of the wide spread of the data, or absent.There is also seen a correlation within the group of linear microwave parameters (such asRs(T0),σ1(T0) and λ(T0)) and partial correlation within the group of the nonlinear parameters (such asHrf*,d Rs/d Hrf and Hrf _ TOI2). However, all the aforementioned correlations seem to be dependent on many conditions(such as film growth technique and substrate), and are by no means quantitative, but onlyqualitative.

Characterization of MgO substrates for growth of epitaxial YBCO thin films

YBCO films were grown on magnesium oxide (MgO) substrates for fabricatingstep-edge junction SQUIDs and other Josephson junction-based devices. In-plane45° grain misorientation was frequently observed in films grown on degraded orcontaminated MgO substrates. The appearance of these misoriented grains results in adecrease of the thin-film critical-current density and reduces the device yield.In this work, we investigated the chemical properties of MgO substrates withvarious surface conditions due to different substrate preparation methods andenvironmental degradation, by using x-ray photoelectron spectroscopy (XPS). The XPScharacteristics of the surface are compared before and after a thermal annealing at760 °C resembling the thin-film deposition heating cycle. The MgO substrates, after lithographicprocessing or only weeks of exposure to the laboratory environment, showed surfacedegradation characterized by the presence of hydroxyl groups, carbonate, and other possiblecarbon compounds such as bicarbonate, alcohols and carboxyl. Heating of the substrates to760 °C improves the surface quality to a certain degree with the removal of some of the abovecontaminants, but is not sufficient to recover the MgO surfaces. A final Ar ion-beam etchcleaning process at low ion energy proved to be very effective in refreshing the MgOsubstrate surface that had been degraded due to lithographic processing or storage. Filmsgrown on MgO with this pre-treatment showed perfect grain alignment and highcritical-current densities.

Twinning and domain structure of epitaxial YBa 2Cu 3O x films studies by X-ray diffraction methods

Structure, orientational features, and twinning of epitaxial superconductive YBa 2Cu 3O x (YBCO) thin films and YBCO/CeO 2 heterostructures on (1 1 0) NdGaO 3 (NGO) and tilted-axes NdGaO 3 substrates with an inclination of normal of the substrate from the [1 1 0] axis were investigated by X-ray diffraction methods. Orthorhombic structure of NdGaO 3 results in an increase of the angle between (1 1 0) and (1 1 0) twinning planes in YBCO films to 90.20° and in a difference in volume of two twin domain systems. The orientation of epitaxial YBCO thin films was shown to be influenced by the deposition rate and the presence of symmetrical-equivalent directions [1 1 0] and [1 1 0] in the substrate and [1 0 0], [0 1 0], and [0 0 1] in the CeO 2 layer. Domain structure of the YBCO thin film changes with an increase in the inclination angle. The YBCO thin films on the tilted-axes substrates are twinned in the same way as on (1 1 0) NGO substrates. However, formation of one or both twinning complexes is suppressed with an increase of the inclination angle.

Vortex motion in YBa2Cu3O7−δ twinned single crystals and epitaxial films

The transport properties of superconductive YBa2Cu3O7−δ (YBCO) twinned single crystals and epitaxial thin films are similar in the flux creep regime. In particular, the activation energy of the individual vortices is proportionnal to the temperature in the low temperature range. As a matter of fact, data from the literature show that this energy takes the form Ea=4πνkBT, where ν is an integer. To investigate this behavior, the current–voltage characteristics of epitaxial YBCO thin film microbridges were measured. The activation energy of these samples has proved to take the form given above. This common behavior of single crystals and thin films is interpreted in the framework of a critical state model according to which the vortices move along the sample twin boundaries that behave as arrays of weak links.

Structural characterization of epitaxial YBCO thin films prepared by a fluorine-free sol–gel method for coated conductors

Using a fluorine-free sol–gel method, both c- and a-axis textured YBCO thin films were synthesized in low oxygen partial pressure. The experimental results of both x-ray diffraction 2θ scan and pole figure showed well epitaxial grown films on LaAlO3 (LAO) single crystal substrates. When the processing condition was altered, the scanning electron microscopy indicated a large fraction of a-axis oriented grains in the film. High-resolution transmission electron microscopy was performed on both types of films and revealed interface lattice structural characteristics. A classic nucleation and growth model was used to explain the texturing mechanism. Other possible mechanisms for the a-axis oriented grains are also discussed.

Formation of epitaxial YBCO thin films by ex-situ processing of a polymerized complex

High T/sub c/ and J/sub c/ YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films have been formed by ex-situ processing of a spin coated organic sol-gel precursor based on a polymerized complex. To form the precursor we have reacted metal cations with a chelating organic acid, then formed polyesterified complexes through reaction with a polyhydroxy alcohol. This method permits a simple, cheap and reliable route to the fabrication of YBCO films. The value of J/sub c/ achieved for the film deposited onto a LaAlO/sub 3/ [100] substrate at 77 K under a zero field is routinely as high as 0.25 MA/cm/sup 2/ with T/sub c,zero/ of 90.6 K.