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.     

Preparation of high-T c superconducting film in the system of Bi-Sr-Ca-Cu-O from sol-gel process and its properties

High-T _c superconducting Bi₂Sr₂CaCu₂O_x films withT _c ^off =80 K were prepared by the dipping method of sol-gel processing using inorganic salts. The influence of the preparation conditions on the superconducting properties of the derived material is reviewed. Bi, Sr, Ca and Cu nitrates were used as raw materials. Glycerol was used as solvent. The thickness of films made by the dip method was about 0.5 m. The films were crystallized by heat-treatment at 830°C for 10 min.T _c ^off of films was 80 K andJ _c at 77 K was more than 8 kA cm^–2. Synthesis of high-T _c superconducting films was very easy and the crystallization of films was possible with a relatively low heat-treatment temperature.     

Superconducting energy gaps and transition temperatures of quench-condensed cadmium and zinc films

The superconducting transition temperatures of Cd and Zn films deposited upon a liquid³He-cooled substrate increase compared to the bulk values. In disordered Cd, values of around 0.9 K are obtained, while correspondingT _c for Zn is about 1.4 K. These values agree well with theoretically predicted ones. For a Cd_0.9Ge_0.1, probably amorphous, film,T _c is further increased to about 1.6 K in approximate agreement with theory. The ratio between the energy gap andT _c , 2(0)/k _B T _c , remains considerably lower than 3.53 in all the disordered films. This, as well as fairly broad transition ranges and the shape of the fluctuation pairing contribution to the conductance, is taken as an indication of structural inhomogeneities of the films. In pure, quenchcondensed Mg films, no superconductivity is detected above 0.35 K.     

Superconducting beryllium films

Resistance-vs.-temperature measurements were made on a series of beryllium films condensed on liquid-helium cooled surfaces. Two vapor sources designed to reduce contamination were used and films were prepared on both crystalline quartz and glass substrates. The samples were superconducting with a transition temperature ofT _c =9.6±0.1 K as indicated by a sharp falloff of resistance on cooling. This qualitatively confirms earlier reports of the superconductivity of quenched beryllium films. The transition curves were, however, appreciably sharper and the transition temperature about a degree higher than previously reported. Good agreement found from sample to sample indicates that the residual impurity concentration was small enough to be unimportant and that the observed transition temperature is characteristic of pure beryllium. The phase of beryllium responsible for the highT _c value disappeared on annealing in the range 40–60 K. No indication was found of a reportedT _c 6-K phase. Beryllium films thicker than about 750 Å broke up during deposition, indicating the presence of large stresses.The work at Karlsruhe was supported by the Deutsche Forschungsgemeinschaft, Germany.     

High-pressure dc sputtering of High-Tc BPSCCO thin films withTc(0) above 100 K

High-T _c Bi(Pb)-Sr-Ca-Cu-O thin films have been made on single-crystal MgO substrates using high-pressure dc sputtering technique. X-ray studies confirm the crystallinity and highly oriented structure withc-axis perpendicular to the substrate. By optimizing the annealing schedule the formation of the high-T _c phase is stabilized. The best film exhibited superconducting transition temperature with zero-resistance temperature,T _c(0), as high as 101 K. Temperature dependence ofJ _c indicates the presence of Josephson-type weak links.     

Superconductivity in transparent zinc-doped In2O3 films having low carrier density

Abstract

Thin polycrystalline zinc-doped indium oxide (In₂O₃–ZnO) films were prepared by post-annealing amorphous films with various weight concentrations x of ZnO in the range 0x 0.06. We have studied the dependences of the resistivity ρ and Hall coefficient on temperature T and magnetic field H in the range 0.5T 300 K, H6 Tfor 350 nm films annealed in air. Films with 0x0.03 show the superconducting resistive transition. The transition temperature T_c is below 3.3 K and the carrier density n is about 10²⁵–10²⁶ m^?3. The annealed In₂O₃–ZnO films were examined by transmission electron microscopy and x-ray diffraction analysis revealing that the crystallinity of the films depends on the annealing time. We studied the upper critical magnetic field H_c2 (T) for the film with x = 0.01. From the slope of dH_c2 /dT, we obtain the coherence length ξ (0) ≈ 10 nm at T = 0 K and a coefficient of electronic heat capacity that is small compared with those of other oxide materials.     

Effect of Firing Temperature on the Structure and Superconducting Properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> Films

YBa₂Cu₃O_7−x (YBCO) films were prepared on LaAlO₃ single crystal substrate under various firing temperatures (750–800 °C) in the crystallization process by metalorganic deposition (MOD) method. The coating solution was made by mixing the fluorine-free precursor solution containing Y and Cu with Ba–fluorine precursor solution (Ba-TFA). The effect of firing temperature on the structure and superconducting properties of YBCO films was systematically investigated. The results indicated that YBCO-films were smooth, crack-free, exhibited good textures and retain high oxygen content according to the XRD and SEM images. Sample of YBCO-film fired at 780 °C showed highest superconducting properties including high critical transition temperature T _c=89 K, sharp transition temperature ΔT _c<1 K, and critical current density J _c=2.8 MA cm⁻², which are attributable to excellent in-plane textures and dense microstructures with good connectivity between the grains.     

Surface Superconducting State of Heavy-Fermion Superconductor CeIrIn5 Probed by CeIrIn5-Ag Junctions

The CeIrIn₅-Ag junctions of about 2×10^–9 cm^–2 area have been made using microfabrication techniques, and the surface superconducting state of CeIrIn₅, which has two characteristic temperatures T ₀ and T _c, has been investigated, where T ₀ and T _c are the transition temperature to zero-resistivity state and the bulk, thermodynamic transition temperature, respectively. The temperature, below which superconducting anomalies are observed, varies from junction to junction, and yet it is always well above T _c=0.4 K. This result, together with no indication of transition at T _c, suggests that at least the surface of CeIrIn₅ is in the superconducting state above T _c. The data on the critical current I _c in superconducting anomalies point to the possibility to define a local transition temperature for each junction.     

The Transport Properties of Bi-Riched Bi2Sr2CuO6+δ Single Crystal

As-grown superconducting Bi-riched Bi₂Sr₂CuO₆₊ single crystals have been grown by the traveling solvent floating zone technique. The superconducting transition temperature T _c was about 6 K and the room temperature resistivity was about 2×10^–3 Ohm-cm. Transport properties, such as resistivity, magnetoresistance and Hall effect were measured from overdoped to underdoped samples annealed in inert atmosphere at 650°C. The transition temperature can be raised to 12 K after post annealing. The Hall measurement shows that the hole carrier density decrease after annealing. The temperature dependence of Hall angle is T ^1.5, not quadratic as observed for most high-T _c superconducting oxides such as YBa₂Cu₃O₇. The variation of onset T _c with different external magnetic field is very different from high-T _c superconductors. The in-plane conductivity shows the dependence of ln T and can be explained by weak localization theory.     

Study of Josephson effects in microbridges of Bi-Sr-Ca-Cu-O films made by spray pyrolysis

Superconducting Bi-Sr-Ca-Cu-O (2212) films were prepared by spraying stoichiometric aqueous solutions of nitrates of bismuth, strontium, calcium and copper on heated MgO (100) substrates and subsequent annealing in air. TheR-T curves of the films show metallic behaviour above the superconducting transition temperature.T _c (R=0) is observed between 80 and 85 K. Annealing temperature has a profound effect onT _c (R=0) and on the orientation of the film. Annealing in air in near-melting region yields highly oriented films withc-axis perpendicular to the substrate. These films show a sharp superconducting transition with zero resistance at 85 K. Microbridges of the dimensions of 50 μm × 50 μm have been patterned photolithographically followed with chemical etching. The 1 V characteristics of the microbridges show Josephson effects due to the presence of grain boundary weaklinks. The temperature dependence of the critical current for these microbridges suggest formation of superconductor-normal-superconductor type weaklinks.     

DC-sputtered Bi(Pb)SrCaCuO films on single (MgO) and polycrystalline (MgO,CuO) substrates

Bi(Pb)SrCaCuO films have been deposited on single (MgO) and polycrystalline (poly MgO, CuO) substrates by DC sputtering. All the films became superconducting after post-annealing them in air. The films on single crystal MgO showedT _c onset at 120 K andT _c at 92 K. However, the extrapolated zero resistivity is obtained at 106 K. The ac susceptibility showed an onset at 106 K and a sharp transition at 85 K. The films on poly MgO and poly CuO had aT _c=72 K. The preparation, X-ray diffraction and morphology of these samples are presented.     

In situ grown superconducting YBCO films on buffered silicon substrates for device applications

Thin films of YBa₂Cu₃O_7– (YBCO) have been grownin situ on silicon single-crystal (100) substrate by using SrTiO₃ as a buffer layer. The deposition has been carried out by on-axis rf magnetron sputtering method. The deposition condition have been optimized by studying the plasma characteristics and correlating them with the superconducting performance of the film. Films deposited at substrate temperature in the range of 680–700°C from stoichiometric YBCO targets in an argon + oxygen mixture (31) are superconducting and showc-axis epitaxy. Compositional confirmation has been carried out using Rutherford backscattering. Scanning tunneling microscopy of the films reveal formation of well-defined layered structure with some defects in the initial stages ofin situ growth of the films. Films grown on SrTiO₃ substrates have excellent crystalline quality (XRD), transition temperatureT _c0=81 K and the critical current densityJ _c >2×10⁵ A/cm² for unpatterned films at 77 K. On silicon substrates using buffer layers thein situ deposited YBCO films shows a higher transition width andT _c0 is also slightly less (71 K).     

MOCVD-grown homoepitaxial YBa2Cu3Ox thin films and its multilayers

YBa₂Cu₃O _x (YBCO) films, Zn-doped YBCO (YBCO : Zn) films, and their bilayers have been epitaxially grown on SrTiO₃(100) and single-crystal YBCO(001) substrates by metalorganic chemical vapour deposition. The YBCO(001) films homoepitaxially grown on YBCO(001) substrates have flat surfaces on an atomic scale, and interfaces free from crystalline defects. We can systematically reduce the superconducting transition temperature (T _c) of YBCO : Zn films from 90 K to 37 K by increasing Zn concentration. The bilayers have a sharp distribution of Zn as evaluated fromT _c measurements of the upper YBCO films and depth profiles of secondary ion mass spectrometer, suggesting the possibility to form the homoepitaxial SNS (S, superconductor; N, normal metal) junction operatable between 40 K and 90 K.     

Some properties of electron beam evaporated amorphous Mo-N films

A series of amorphous Mo-N films was prepared by electron beam evaporation of molybdenum in varying partial pressures of nitrogen and deposited onto substrates cooled to about 80 K. The alloy films were characterized by X-ray diffraction, the superconducting transition temperature,T _c, and the crystallization temperature,T _x. The maximumT _c (8.3 K) and sharpest transition occurred with the minimum nitrogen pressure necessary to form the amorphous structure, as revealed by X-ray diffraction. After annealing the as-deposited films, both bcc and fcc phases were found with the bcc/fcc ratio decreasing with increasing nitrogen partial pressure. Differential scanning calorimetry (DSC) measurements showed significant differences in the shape of peaks associated with either bcc, bcc+fcc, or fcc phases. The temperature,T _x, associated with the fcc crystallization increased with nitrogen content. Heats of crystallization had an average value of about 63 J g^?1. Changes in position of the first amorphous X-ray diffraction peak showed that the amorphous structure was expanded by increasing nitrogen content.     

Improvement of the High-Magnetic-Field Critical Current Density of the <Emphasis Type="Italic">Ex-Situ</Emphasis> Annealed MgB<Subscript>2</Subscript> Thick Films by Oxygen Doping

This paper presents a very simple way to synthesis MgB₂ thick films with high critical current density in a magnetic field by ex-situ annealing precursor B films in air with excessive Mg in a sealed quartz tube. The films show a significant improvement of critical current density in a magnetic field compared to the high purity films annealed in vacuum, while its zero-resistance transition temperature T _c ^zero and normal state resistivity still maintain about 38 K and 17 μΩcm. The results demonstrate MgB₂ thick films have great potential applications in superconducting coated conductors.      

Proton-induced reduction ofR s,Jc, andT c in YBa2Cu3O7−δ thin films

We have explored the effect of 2-MeVH⁺ irradiation on the superconducting transport properties of thin films of YBa₂Cu₃O_7?δ [T _c, J_c(B=0; 77 K, 4.2 K), andR _s(36 GHz;T)]. The inductively measured critical temperatureT _c changed slowly and uniformly (～2 K per 10¹⁶/cm²) for fluences less than ～3×10¹⁶/cm². Beginning at ～3–4×10¹⁶/cm², the superconducting transition broadened and dropped more quickly with fluence. The critical current density measured at 77 and 4.2 K changed roughly linearly with fluence. The microwaveT _c (as defined by the sharp transition inR _s as a function of temperature) resembled the low-frequency inductiveT _c measurement at low fluences but was depressed more strongly for large fluences. The residual surface resistance (～6–10 mΩ) was not affected for fluences up to 5×10¹⁶/cm². We have interpreted the sudden and reproducible reduction in the microwaveT _c transition as a sensitive indicator of disruption in the copper-oxygen chain sublattice and compared the proton-induced change to that observed in oxygen gettering studies of bulk materials.     

Low Field Magnetoristance and Inhomogenieties in La2/3Ca1/3MnO3 Films

La_2/3Ca_1/3MnO₃ films were prepared using a conventional paint-on method on alumina and monocrystalline TiSrO₃ substrates. A significant difference between the peak resistivity temperature T _p=178 K and the magnetic transition temperature T _c=240 K was found in contrast to single crystal samples. The behavior of the magnetoresistance with temperature also differs from that found in bulk samples. Although the films show significant granular morphology differences, their transport and magnetoresistive properties are very similar. Maximum magnetoresistance ratios of 16% and 10% at 0.3 T and 77 K were obtained for films deposited on alumina and TiSrO₃ substrates, respectively. Magnetization measurements above T _c show a nonlinear behavior, consistent with the presence of ferromagnetic inhomogeneities within the paramagnetic phase. These results suggest that, in addition to the importance of grain boundary scattering in determining the magnetoresistance at low fields, microscopic inhomogeneity plays a role in determining magnetotransport properties.     

Universal behavior of3He-4He mixture films near the Kosterlitz-Thouless transition

We report measurements of the convective thermal conductance of³He-⁴He mixture films near the Kosterlitz-Thouless transition. The thickness of our⁴He films is 14.7 and 19.1 Å above the inert layer and the³He concentration ranged from 0.033 to almost 2%. The thermal response is tested for the critical behavior as observed in pure films, and it is found to be preserved in the mixture films case. However, the parametersb, D/a ² andT _c exhibit a strong dependence on the concentration. The mixture film conductance at fixedT-T _c is found to decrease upon addition of³He implying a decrease in the 2D correlation length. Mixture films thus exhibit 2D behavior over a narrower temperature range than pure films. Further, for temperaturesT _c , the largest measurable conductance decreases sharply with the addition of³He. We attribute this behavior to a³He-⁴He scattering mechanism and a³He induced free-vortex density.     

Electrical conductivity and domain structure in ferroelastic Tl2SeO4

We have carried out the optical observation, electrical conductivity and ²⁰⁵Tl NMR measurements, and subsequently investigated the origin of the large conductivity above ferroelastic phase transition temperature T_c (=661 K) on the basis of the domain structure and the crystal structure. Electrical conductivity exhibits the discontinuous increase around T_c with increasing temperature and becomes approximately 5 × 10⁻³ S m⁻¹ above T_c. Moreover, from the ²⁰⁵Tl NMR measurements, it is found that mobile Tl ions exist above T_c. Furthermore, from the analysis of the domain structure based on the crystal structure in the low-temperature ferroelastic phase, it is also found that the anomalously large fluctuations of SeO₄ tetrahedrons exist above T_c. It is deduced from these results that the high electrical conductivity above T_c is caused by the mobile Tl ions closely related to the anomalously large fluctuations of SeO₄ tetrahedrons accompanied by the ferroelastic phase transition.