Increase of achievable film thickness by UV-lamp irradiation in a fluorine-free metal-organic deposition process of YBa2Cu3O7

We have studied the effects of UV-lamp irradiation in a fluorine-free metal-organic deposition (FF-MOD) process for epitaxial YBa₂Cu₃O₇ (YBCO) films. We found that the irradiation dramatically improves the elemental homogeneity in a precursor film, and increases the thickness of epitaxial growth by twofold with respect to the same process without irradiation. We obtained a 1.05-μm-thick epitaxial YBCO film, which is thickest for FF-MOD processes. A possible reason for the improved element uniformity is simultaneous decompositions of three metal-organic ingredients by photons, which strongly reduce grain growths in a precursor film that have been known as an unavoidable feature in the conventional all-pyrolytic MOD.     

Evaporated YBa2Cu3O7 thin films and device technology

Epitaxial thin films of YBa₂Cu₃O₇ were grown by evaporation of the metals from electron-guns in the presence of atomic oxygen. The films were characterized structurally by X-ray diffraction and electrically by measurements of d.c. magnetization and flux creep. The progress in device technology is discussed. Results are presented on Josephson junctions, and a superconducting microwave filter with the ground plane grown on the back.     

Process monitoring during post-annealing of YBa2Cu3O7 thin films containing fluorine

The post-annealing method of producing thin films of YBa₂Cu₃O₇ (YBCO) has taken on a new impetus due to the recent work showing that films of the highest quality can be made by using low partial pressures of oxygen during the annealing cycle. Here it is shown that for films produced by using BaF₂ as a source material, the post-annealing procedure can be closely controlled by monitoring the F that evolves due to the water vapor reaction with BaF₂. The use of an ion-sensitive electrode allowed small F evolution rates (about 1 ng s^–1) to be detected above background, sufficient to measure the F evolution rate from even the smallest samples used. The time interval during which F evolves was found to increase with increasing YBCO film area being annealed.     

Thickness dependence of microcrack formation in YBa2Cu3O6 + x thin films on NdGaO3 (001) substrates

Thickness dependence of parallel microcrack formation in YBa₂Cu₃O_6 + x thin films prepared by pulsed laser deposition from nano- (n) and microcrystalline (μ) targets on NdGaO₃ (001) is systematically investigated. Atomic force microscope and x-ray diffraction measurements show parallel microcracks normal to uniaxial twin boundaries. The amount of microcracks increases with film thickness. Superconducting properties of the films decrease very strongly with film thickness as a result of microcrack formation. The n-films have more rigid lattice and thus show more extensive cracking than μ-films. It is found that the μ-films have a thickness threshold (∼ 70 nm) where the first signs of cracking appear.     

Twinning of YBa2Cu3O7 thin films on different substrates

The twinning of YBa₂Cu₃O₇ (YBCO) thin films with c-axis orientation on (001) MgO, (001) SrTiO₃, (012) LaAlO₃, (110) NdGaO₃ and (001) NdGaO₃ substrates, prepared by laser ablation, has been examined using a combination of and θ/2θ scans at a four-circle diffractometer. On all substrates, except for (001) NdGaO₃, the tetragonal to orthorhombal phase transition results in four different orientations of YBCO twins relating to the substrate. On (001) NdGaO₃ only two different twin orientations, accompanied by a slight lattice monoclinization, has been observed.     

Testing field and annealing temperature dependence of leakage properties in Bi3.25La0.75Ti3O12 thin films

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ thin films annealed at different temperatures were prepared on Pt/Ti/SiO₂/Si substrates by metalorganic decomposition method. The leakage current behavior and the current conduction mechanism were investigated. For all films, the leakage current density − electric field (J−E) characteristic is confined within a “triangle” in the log (J)− log (E) plane bounded by three limiting curves: Ohm's law (J ∝ E), trap-filled-limit (J ∝ E^a, a > 1), and Child's law (J ∝ E²). At high field region, Bi_3.25La_0.75Ti₃O₁₂ thin films with grains of rod-like show higher leakage current, while films with grains of spherical- or planar-like exhibit lower leakage current.     

Characterization of Al2O3/ZrO2 nano multilayer thin films prepared by pulsed laser deposition

Microstructural characterization of pulsed laser deposited Al₂O₃/ZrO₂ multilayers on Si (1 0 0) substrates at an optimized oxygen partial pressure of 3 × 10⁻² mbar and at room temperature (298 K) has been carried out. A nanolaminate structure consisting of alternate layers of ZrO₂ and Al₂O₃ with 40 bi-layers was fabricated at different zirconia layer thicknesses (20, 15 and 10 nm). The objective of the work is to study the effect of ZrO₂ layer thickness on the stabilization of tetragonal ZrO₂ phase for a constant Al₂O₃ layer thickness of 5 nm. The Al₂O₃/ZrO₂ multilayer films were characterized using high temperature X-ray diffraction (HTXRD) in the temperature range 298–1473 K. The studies showed that the thickness of the zirconia layer has a profound influence on the crystallization temperature for the formation of tetragonal zirconia phase. The tetragonal phase content increased with the decrease of ZrO₂ layer thickness. The cross-sectional transmission electron microscope (XTEM) investigations were carried out on a multilayer thin films deposited at room temperature. The XTEM studies showed the formation of uniform thickness layers with higher fraction of monoclinic and small fraction of tetragonal phases of zirconia and amorphous alumina.     

Current transport across grain boundaries in superconducting YBa2Cu3O7 films

Superconducting transport properties of YBa₂Cu₃O₇ grain boundaries have been investigated. The results of these experiments are summarized.     

Optically induced changes and long-term relaxations of resistivity and critical temperature in He irradiated YBa2Cu3Ox

Thin films of YBa₂Cu₃O_x with superconducting critical temperatures (T_c) ranging from 58 K to 72 K were prepared by pulsed-laser deposition and subsequent irradiation with 75 keV He⁺ ions. Optical excitation of the films with a He-Ne laser resulted in a small resistivity reduction, when the experiment was carried out a short period of time after the ion irradiation. The value of T_c was essentially unchanged by the optical treatment. Further light excitation experiments did not show a significant effect on the resistivity of the samples. We conclude that ion-irradiated metallic YBa₂Cu₃O_x does not exhibit the persistent photoconductivity observed in oxygen-depleted samples with comparable critical temperatures. On the other hand we have observed a long-term relaxation of T_c towards larger values that takes place even when the samples are kept in the dark. Our results indicate that T_c may increase after ion irradiation to an asmptotic value about 20% over the initial value on a typical time scale of years. This observation may be important for possible applications of light-ion irradiated YBa₂Cu₃O_x.     

High-resolution and analytical transmission electron microscopy of epitaxial YBa2Cu3O7−x thin films and YBa2Cu3O7/PrBa2Cu3O7 superlattices

Transmission electron microscopy (TEM) studies of epitaxial YBa₂Cu₃O_7−x thin films and YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices are summarized. High-resolution imaging of cross-sections and plan views and energy-dispersive X-ray microanalysis and electron energy loss spectroscopy in the transmission electron microscope were the methods applied. In the first section results on YBa₂Cu₃O_7−x thin films With varying oxygen stoichiometry deposited onto SrTiO₃ are discussed. Then, YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices deposited onto SrTiO₃ and MgO are investigated. Finally, an interface analysis of high-quality YBa₂Cu₃O_7−x thin films deposited onto sapphire with yttrium-stabilized zirconia buffer layers is presented.     

Temperature and spectral dependences of persistent photoconductivity in YBa2Cu3Ox thin films

Temperature and spectral dependences of photoinduced changes of resistance were measured in YBa₂Cu₃O _x thin films with oxygen content ranging as 6.35 <x < 6.75. The absolute value of efficiency of initiation of photoinduced changes decreases with increase in oxygen content, but the position of peaks in the spectral dependence does not change with a change ofx. Temperature dependences of efficiency have an anomaly atT220 K, which is present in all the samples studied, and correlates with anomalies observed by other experimental techniques. Qualitatively similar temperature and spectral dependences of efficiency for the samples in both the insulating and metallic phases may be considered as an indication that the persistent photoconductivity effect in YBCO on both sides of the metal-insulator transition has a common origin.     

Degradation of thin films of YBa2Cu3O7 by annealing in air and in vacuum

The degradation of epitaxial thin films of YBa₂Cu₃O₇ has been studied as a function of annealing temperature in air and in vacuum; some samples had an evaporated overlayer of CaF₂. Degradation was monitored by the measurement of electrical properties after consecutive 30-min annealing treatments. The room-temperature resistance registered significant increases for all samples after annealing at temperatures above about 200°C; the critical current density at 77 K was degraded for annealing temperatures 400°C in air, and 200–250°C in vacuum. By annealing in oxygen at 550°C, electrical properties were restored in degraded bare YBCO samples annealed in vacuum, but not for those annealed in air.     

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.     

Optimization of annealing temperature for YBa2Cu3O7 thin films post-annealed at a low oxygen partial pressure

Post-annealing of YBa₂Cu₃O₇ (YBCO) thin films is usually performed at 850–900°C in atmospheric-pressure oxygen. In this study, coevaporated YBCO films on LaAlO₃ were post-annealed in an oxygen partial pressure of 29 Pa at temperatures in the range 700–825°C. Zero resistance transition temperatures were 89–90 K. Both d.c. (room-temperature resistance and critical-current density) and a.c. parameters (extracted from eddy-current response measurements at 25 MHz) were monitored. The optimum temperature is close to 750°C, which is on the YBCO thermodynamic stability line at this low oxygen partial pressure.     

Characterization of YBa2Cu3O7/PrBa2Cu3O7 superlattices

We investigated the structural and superconducting properties ofc-axis oriented (YBa₂Cu₃O₇) _nY /(PrBa₂Cu₃O₇) _npr superlattices with thicknesses of the individual layers down to one unit cell (10nY1; 18>nPr 1). By transmission electron microscopy and X-ray diffraction we find an excellent structural quality of the samples, though the quantitative analysis shows the existence of defects. In superlattices with decoupled YBa₂Cu₃O₇ layers of two unit cell thickness we find a highT _c value of 75 K. We probed the flux line structure in the superlattices by measurements of the critical current density in magnetic fields. The experiments show that the flux-line dynamics is dominated by the movement of pancake vortices.     

Specific heat of YBa2Cu3O7

Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearT _c , on a number of YBa₂Cu₃O₇ samples have revealed several correlations among strongly sample-dependent parameters. These correlations suggest that the sample dependence of the parameters reflects a sample dependence of the volume fraction of superconductivity, which is in turn correlated with a low concentration of Cu²⁺ moments. The correlations give a criterion for recognizing the values of the parameters characteristic of the fully superconducting material. Preliminary results on the effects of sample heat treatment are reported. New data on the linear term is presented and discussed.     

Polycrystalline ZnO-In2O3 thin films as gas sensors

Polycrystalline ZnO-In₂O₃ thin films were prepared by thermal oxidation in air of metallic Zn-In films deposited onto glass substrates by thermal evaporation under vacuum. Different oxidation conditions (oxidation temperature, oxidation time, heating rate) were used in order to prepare homogeneous films that can be used as gas sensors. Polycrystalline structure of the as-obtained films was confirmed by X-ray and electron diffraction investigations. The electrical conductivity of various thin film samples ranged between 0.84 and 6.44 (Ω cm)^− 1.Gas sensitivity to six different gasses (ammonia, methane, LPG, acetone, ethanol and formaldehyde) was evaluated and it was found that the highest sensitivity was obtained for ammonia.     

Critical current density and microstructure of YBa2Cu3O7 thin films post-annealed at a low oxygen partial pressure

Post-annealing of thin films of YBa₂Cu₃O₇ (YBCO) has been performed at 29 Pa and 750°C. For films 0.6 m thick, a critical current density >1 MA cm^–2 is obtained at 77 K, with a sharp eddy current response at 25 MHz. Microstructural investigation of these films by crosssectional and planar transmission electron microscopy reveals that the YBCO film has thec-axis normal to the plane of the substrate in a continuous sheet of varying thickness, frequently covering the entire thickness of the film. Mutually perpendicular rods with thec-axis in the plane of the LaAlO₃ substrate are also seen. The microstructure and critical current density of these films are compared with those of previously reported films post-annealed in atmosphericpressure oxygen.     

Scanning tunneling microscopy of laser-deposited YBa2Cu3O7 thin films on Y-stabilized zirconia substrate

The surface morphology and growth mechanism ofc-oriented YBa₂Cu₃O₇ thin films deposited on Y-stabilized zirconia substrate by laser deposition has been investigated with scanning tunneling microscopy. Both screw dislocation and layered island growth processes took place. Surface features such as screw dislocation, steps, subgrain boundaries, holes, and troughs were present, suggesting that they may act as pinning sites for flux lines. A thin layer of nonsuperconducting material with a thickness of a few angstroms is seen on the surface of the film.     

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.