Epitaxial Growth of High-J c NdBa2Cu3O7−δ Films on RABiTS by Pulsed Laser Deposition

NdBa₂Cu₃O_7−δ (NdBCO) films were grown on rolling-assisted biaxially textured substrates (RABiTS) via pulsed laser deposition. c-Axis-oriented epitaxial NdBCO films with high performance were obtained under optimal deposition conditions. Transmission electron microscopy analysis shows that the NdBCO film grown on RABiTS has a clear interface with a CeO₂ cap layer and a nearly perfect lattice structure. The NdBCO film exhibits higher T _c of 93.7 K and better in-field J _c in magnetic fields and at all field orientations, compared to pure YBCO films.     

Microwave properties of epitaxial large-area Ca-doped YBa2Cu3O7−δ thin films on r-plane sapphire

Ca doping of YBa₂Cu₃O_7−δ (YBCO) is well known to enhance the critical current density in large-angle grain boundaries for example of bicrystals. However, up to now no data are available on microwave properties of epitaxial Ca-doped YBa₂Cu₃O_7−δ thin films on r-plane sapphire with CeO₂ buffer layer.Therefore, first results are presented for large-area pulsed laser deposition (PLD) grown Ca_xY_1−xBa₂Cu₃O_7−δ films on 3-in. diameter sapphire wafers. The PLD process is optimised for undoped YBCO thin films and shows high reproducibility for YBCO. The microwave surface resistance R_s at 8.5 GHz of Ca-doped YBCO (x=0.1) thin films shows clear reduction (up to 20%) with respect to that of YBCO for temperatures from about 20–50 K. In addition, microwave surface resistance R_s of Ca-doped YBCO is lower than that of YBCO even for enhanced microwave surface magnetic field up to about 20 mT for temperatures 20 and 40 K.     

Microstructures and electrical properties of SrRuO3 thin films on LaAIO3 substrates

Conductive SrRuO₃ thin films have been deposited using pulsed laser deposition on LaA10₃ substrates at different substrate temperatures. Structural and microstructural properties of the SrRuO₃/LaAlO₃ system have been studied using x-ray diffraction, scanning electron microscopy, and scanning tunneling microscopy. Electrical properties of SrRuO₃ thin films have been measured. It was found that the film deposited at 250°C is amorphous, showing semiconductor-like temperature dependence of electrical conductivity. The film deposited at 425°C is crystalline with very fine grain size (100∼200?), showing both metallic and semiconductor-like temperature dependence of electrical conductivity in different temperature regions. The film deposited at 775°C shows a resistivity of 280 μΩ.cm at room temperature and a residual resistivity ratio of 8.4. Optimized deposition conditions to grow SrRuO₃ thin films on LaA10₃ substrates have been found. Possible engineering applications of SrRuO₃ thin films deposited at different temperatures are discussed. Bulk and surface electronic structures of SrRuO₃ are calculated using a semi-empirical valence electron linear combination of atomic orbitals approach. The theoretical calculation results are employed to understand the electrical properties of SrRuO₃ thin films.     

Fracture toughness of Cu-Sn intermetallic thin films

Intermetallic compounds (IMCs) are formed as a result of interaction between solder and metallization to form joints in electronic packaging. These joints provide mechanical and electrical contacts between components. The knowledge of fracture strength of the IMCs will facilitate predicting the overall joint property, as it is more disposed to failure at the joint compared to the solder because of its brittle characteristics. The salient feature of this paper is the measurement of the fracture toughness and the critical energy-release rate of Cu₃Sn and Cu₆Sn₅ intermetallic thin films, which is the result of the interaction between Sn from the solder and Cu from the metallization. To achieve the objective, a controlled buckling test was used. A buckling test in the current work refers to one that displays large transverse displacement caused by axial compressive loading on a slender beam. The stress and strain along the beam can be easily calculated by the applied displacement. Fracture-toughness values of Cu₃Sn and Cu₆Sn₅ are 2.85 MPa √m ± 0.17 MPa √m and 2.36 MPa √m ± 0.15 MPa √m, respectively. Corresponding critical energy-release rate values are 65.5 J/m² ± 8.0 J/m² and 55.9 J/m² ± 7.3 J/m², respectively. The values obtained were much higher than the ones measured in bulk intermetallic samples but correlated well with those values obtained from conventional fracture-toughness specimens when fracture was confined within the intermetallic layers. Hence, the controlled buckling test is a promising fast and effective way to elucidate mechanical properties of thin films.     

Study of high-quality epitaxial YBCO thin films grown directly on Y-Cut LiNbO3

Pulsed laser deposition was used to deposit high-quality YBa₂Cu₃O_7-δ (YBCO) thin films directly on y-cut LiNbO₃ substrates. The as-deposited YBCO films had a high degree of in-plane orientation and showed superconducting transition temperature (T_co) at 91K with a transition width of less than IK. Transport critical current densities were found to be ∼10⁶ A/cm² at 77K and zero field. An ion beam minimum channeling yield of 16% was obtained for YBCO films, indicating high crystallinity. High-resolution transmission electron microscopy studies showed that the interface between the film and the substrate was quite smooth and free from interfacial interdiffusion. The defects in thin films are also identified. The work showed that high-quality high T_c thin films can be deposited directly on LiNbO₃. Novel devices based on the properties of both YBCO and LiNbO₃ could be realized based on these results.     

Comparison of the surface morphology and microstructure of in situ and ex situ derived YBa2Cu3O7−x thin films

The surface morphology and microstructure of in situ and ex situ derived YBa₂Cu₃O_7−x (YBCO) thin films have been investigated. In situ films were deposited by single-target off-axis sputtering and three-target co-sputtering. Ex situ films were derived by metalorganic deposition (MOD) of trifluoroacetate precursors. Surface defects resulting from mixed a-axis and c-axis orientation as well as secondary phases have been identified in these films. Despite these defects, films with excellent electrical properties have been formed. However, defects interfere with film patterning and the fabrication of multi-layered structures. Several secondary phase precipitates have been identified, including CuO, Y₂O₃, Cu-Ba-O, and Y₂Cu₂O₅. Secondary phases resulting from a lack of stoichiometry can be eliminated by direct composition control in the MOD and three-target sputtering techniques, and by composition control through the application of an externally applied magnetic field in single-target off-axis sputtering. Secondary phases caused by contamination were also identified: Cr-Ba-O in off-axis sputtering, resulting from contamination by the oxidized heater block; and BaSO₄ in MOD, resulting from gas phase impurities. These results suggest that cation composition control is not sufficient to prevent the formation of secondary phases and that small levels of contamination may prevent phasepure material from being formed.     

Effect of thermal treatment on the properties of YBa2Cu3O7-x thin films with multilayer Al/Cr/Yb metals as ohmic contact electrodes

Detailed studies were conducted on YBa₂Cu₃O_7-x thin films with multilayer Al/Cr/Yb metals as ohmic contact electrodes. Ytterbium not only provided an excellent ohmic contact to YBa₂Cu₃O_7-x thin films, but also improved the zero resistance temperature after thermal treatment of the contacts. Superconducting thin films with zero resistance temperature of around 90 K with extremely sharp resistivity transitions, contact current density greater than 1.3 x 10³A/cm², and specific contact resistivity as low as 10^-9 ohm · cm² at 77 K were achieved on yttrium stabilized ZrO₂ substrates. This occurred after contact deposition and thermal treatment at 450° C in pure oxygen for 20 min.     

Superconducting Properties of (M x /YBa2Cu3O7−δy )N Multilayer Films with Variable Layer Thickness x

The superconducting properties of (M _x /YBa₂Cu₃O_7−δy )_N multilayer films were studied for varying layer thickness x. Different M phases were examined including green-phase Y₂BaCuO₅ (211), Y₂O₃, BaZrO₃, CeO₂, SmBa₂Cu₃O_7−δ (Sm123), brown-phase La₂BaCuO₅ (La211), and MgO. Multilayer (M _x /YBa₂ Cu₃O_7−δy )_N structures were grown by pulsed laser deposition onto SrTiO₃ or LaAlO₃ single-crystal substrates by alternate ablation of separate YBa₂Cu₃O_7−δ (123) and M targets, at temperatures of 750°C to 790°C. The x layer thickness was varied from 0.1 nm to 4.5 nm, and the y 123 layer thickness was kept constant within a given range of 10 to 25 nm. Different M phase and x layer thicknesses caused large variations of the microstructural and superconducting properties, including superconducting transition (T _c), critical current density as a function of applied magnetic field J _c(H), self-field J _c(77 K), and nanoparticle layer coverage. Strong flux-pinning enhancement up to 1 to 3x was observed to occur for M additions of 211 and BaZrO₃ at 65 to 77 K, Y₂O₃ at 65 K, and CeO₂ for H < 0.5 T. BaZrO₃ had a noticeably different epitaxy forming smaller size nanoparticles ∼8 nm with 3 to 4x higher areal surface particle densities than other M phases, reaching 5 × 10¹¹ nanoparticles cm⁻². To optimize flux pinning and J _c (65 to 77 K, H = 2 to 3 T), the M layer thickness had to be reduced below a critical value that correlated with a nanoparticle surface coverage <15% by area. Unusual effects were observed for poor pinning materials including Sm123 and La211, where properties such as self-field J _c unexpectedly increased with increasing x layer thickness.     

Microwave performance-property relationships of YBa2Cu3O7-δ thin films grown on LaAlO3 (001) substrate

We report correlations between growth parameters, structure and microwave performance for YBa₂Cu₃O7-δ (YBCO) thin films. Two series of YBCO thin films were grown byin-situ laser ablation. The first series were deposited at 100 mTorr oxygen pressure with a laser fluence of 2.9 Joule/cm², the second series were deposited at 600 mTorr of oxygen and a higher laser fluence of 5 Joule/cm². Microwave performance of these films was evaluated using a 5.6 GHz resonator. In both series, we found that films with higherT _c , narrower x-ray rocking curve width, and sharper electron channeling pattern tend to have higher microwave resonator Qs. Optimal film growth conditions were infered from this systematic study. Films grown at higher oxygen pressure have smoother surfaces and fewer second phases.     

YBa2Cu3-xZnxO7薄膜红外探测器的研究

在对红外探测器进行理论分析的基础上,设计并研制了液氮温度下的Yba2Cu3-xZnxO7薄膜红外探测器,系统地测试了器件的特征参数.最好的结果为:对于波长为10μm,调制频率为f=500Hz,带宽为Δf=1Hz的红外输入辐射Rv(500,10,1)=3587V/W,NEP(500,10,1)=6.5×10-12W/Hz1/2,D*(500,10,1)=7.2×1012cmHz1/2/W,τ(500,10,1)=1.2ms.     

Improved barium fluoride process YBa2Cu3)7-x films via post-anneal monitoring

The continuous monitoring of the conversion, of Y, Cu and BaF₂ precursors to form superconductor thin films has been achieved using a fluorine-specific-ion electrode immersed in an effluent gas-washing cell. High-quality thin films of YBa₂Cu₃O_7-x deposited on NdGaO₃, LaA10₃ and LaGaO₃, have been produced by limiting the wet oxygen annealing phase of the post-deposition anneal. When the films were over-annealed in humidified oxygen the superconducting transition temperature as measured by inductive methods and the crystal quality, determined by x-ray rocking curves were degraded.     

用溶胶-凝胶法制备的Cu2Bi2Cr2O8薄膜 及其光学和铁磁性能研究

以硝酸铜Cu(NO₃)₂·3H₂O、硝酸铬Cr(NO₃)₃·9H₂O、硝酸铋Bi(NO₃)₃·3H₂O和乙二醇为原料,利用溶胶-凝胶工艺在石英衬底上制备了纳米Cu₂Bi₂Cr₂O₈薄膜。通过X射线衍射(X-Ray Diffraction, XRD)和拉曼测试对样品进行了表征。结果表明,Cu₂Bi₂Cr₂O₈薄膜具有良好的光学特性,其禁带宽度为1.49 eV;在磁性测试方面,Cu₂Bi₂Cr₂O₈薄膜呈现出了良好的铁磁性。     

Large critical current densities in YBa2Cu3O7-x thin films formed by plasma-enhanced metalorganic chemical vapor deposition at reduced temperature

YBa₂Cu₃O_7-x (YBCO) superconducting thin films with a critical current density exceeding 1 x 10⁶ A/cm² at 77.7 K were prepared by a plasma-enhanced metalorganic chemical vapor deposition (PE-MOCVD) process. The thin films (130 nm thick) were formed in-situ on LaAlO₃ substrates at a temperature of 670‡ C in 2 Torr partial pressure of activated N₂O. Resistivity and magnetic susceptibility measurements of the as-deposited films show a sharp superconducting transition temperature of 89.8 K. Critical current densities were measured by the dc transport method with a patterned bridge of 120 × 40 Μm. Both x-ray diffraction and high resolution electron microscopy measurements indicate that films grew epitaxially with the c-axis perpendicular to the surface of the substrate.     

Synthesis and characterization of Pb(Zr0.54Ti0.46)O3 thin films on (100)Si using textured YBa2Cu3O7−δ and yttria-stabilized zirconia buffer layers by laser physical vapor deposition technique

We have fabricated high-quality <001> textured Pb(Zr_0.54Ti_0.46)O₃ (PZT) thin films on (00l)Si with interposing <001> textured YBa₂Cu₃O_7−δ (YBCO) and yttria-stabilized zirconia (YSZ) buffer layers using pulsed laser deposition (KrF excimer laser, λ, = 248 nm, τ = 20 nanosecs). The YBCO layer provides a seed for PZT growth and can also act as an electrode for the PZT films, whereas YSZ provides a diffusion barrier as well as a seed for the growth of YBCO films on (001)Si. These heterostructures were characterized using x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering techniques. The YSZ films were deposited in oxygen ambient (∼9 × 10⁻⁴ Torr) at 775°C on (001)Si substrate having <001>YSZ // <001>Si texture. The YBCO thin films were deposited in-situ in oxygen ambient (200 mTorr) at 650°C. The temperature and oxygen ambient for the PZT deposition were optimized to be 530°C and 0.4-0.6 Torr, respectively. The laser fluence to deposit this multilayer structure was 2.5-5.0 J/cm². The <001> textured perovskite PZT films showed a dielectric constant of 800-1000, a saturation polarization of 37.81 μC/cm², remnant polarization of 24.38 μC/cm² and a coercive field of 125 kV/cm. The effects of processing parameters on microstructure and ferroelectric properties of PZT films and device implications of these structures are discussed.     

Laser processing of TiSi2 and CoSi2 thin films on silicon (100) substrates

We have investigated the formation of TiSi₂ and CoSi₂ thin films on Si(100) substrates using laser (wave length 248 nm, pulse duration 40 ns and repetition rate 5 Hz) physical vapor deposition (LPVD). The films were deposited from solid targets of TiSi₂ and CoSi₂ in vacuum with the substrate temperature optimized at 600° C. The films were characterized using x-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and four point probe ac resistivity. The films were found to be polycrystalline with a texture. The room temperature resistivity was found to be 16 μΩ-@#@ cm and 23 μΩ-cm for TiSi₂ and CoSi₂ films, respectively. We optimized the processing parameters so as to get particulate free surface. TEM results show that the silicide/silicon interface is quite smooth and there is no perceptible interdiffusion across the interface.     

Enhanced superconductivity in Nd1+xBa2-xCu3O7+δby low oxygen partial pressure annealing

Previous work has shown that unlike YBa₂ Cu₃ O_7-δ (Y123), the Nd-Ba-Cu-O system exhibits a solid solution Nd_1+xBa_2-x Cu₃O_7+δ (Nd123ss) for 0.04≤ × ≤0.6.^1–3) An earlier paper showed that although the superconducting properties decrease nonlinearly for increasing x, T_c can be varied by increasing the annealing temperature without changing the low temperature oxygen soak.² The changes in microstructure and T_c with increasing x are analogous with Y123 with increasing δ except that the total hole concentration remained constant. T_c was modeled in terms of oxygen disorder resulting from Nd³⁺ atoms on the Ba sites relocating chain oxygens to anti-chain sites. The variability in T_c as a function of x and processing conditions can be explained by the number of fourfold coordinated coppers on the chain sites. In this paper, the model has been further substantiated by processing in 1% O₂. The annealing in a reduced oxygen partial pressure followed by a 450°C oxygen soak resulted in a marked increase in T_c compared to the 100% PO₂ anneal. The low PO₂ anneal favors pairing of Nd³⁺ substituting for Ba²⁺ to conserve oxygen ions, resulting in fewer disrupted fourfold-coordinated coppers thus increasing charge transfer from the planes to the chains.     

Nd substitution in y/ba sites in melt processed YBa2Cu3O7− δ through Nd2O3 additions

YBa₂Cu₃O_{7− δ} (Y123) samples with excess Nd₂O₃ and Y₂O₃ additions in the same molar ratios were melt textured in air. In the Nd-doped samples, in addition to Y ion site substitution, partial substitution into the Ba²⁺ sites is anticipated because of the similar ionic sizes of Nd³⁺ and Ba²⁺. The microstructure, T_c, and magnetic properties of Nd-doped samples were analyzed and compared with undoped Y123 and samples with excess Y₂O₃. The Nd₂O₃ additions lead to significant magnetization improvements, likely due to both rare earth- and Ba-site substitution by the doped Nd³⁺ ions, and to increases in T_c. Y₂O₃ additions resulted in no marked property enhancement.     

Interfacial reactions in the Sn-(Cu)/Ni,Sn-(Ni)/Cu,and Sn/(Cu,Ni) systems

Specimens with the Sn/Cu/Sn/Ni/Sn/Cu/Sn structure reacted at 200°C are prepared and examined. The Cu₆Sn₅ and Cu₃Sn phases are formed at the Sn/Cu interface, and the Cu₆Sn₅ and Ni₃Sn₄ phases at the Sn/Ni interface. The reaction path in the original Cu/Sn/Ni part of the specimen is Cu/Cu₃Sn/Cu₆Sn₅/Sn/Cu₆Sn₅/Ni₃Sn₄/Ni. The peculiar phenomenon of the Cu₆Sn₅ phase forming at both sides of the Sn phase is illustrated using the Sn-Cu-Ni phase diagram with a very wide compositional-homogeneity range of the Cu₆Sn₅ phase. Interfacial reactions at 240°C between pure Sn and (Cu,Ni) alloys of various compositions are determined. The Cu₆Sn₅ phase is formed when the NI content is less than 30 wt.%, and the Ni₃Sn₄ phase is formed when the Ni content is higher than 40 wt.%. When the Ni content is between 35 wt.% and 40 wt.%, both Cu₆Sn₅ and Ni₃Sn₄ phases are formed. It is also noticed that the formation of the Cu₃Sn phase at the Sn/(Cu,Ni) interface is suppressed with more than 1wt.%Ni addition in the substrate.     

The effect of O<Subscript>2</Subscript> ambient annealing on the microstructure of Cu(Mg) in the form of a Cu(Mg)/SiO<Subscript>2</Subscript>/Si multilayer

The effect of annealing in an O₂ ambient on Cu(Mg)/SiO₂/Si multilayer films was investigated. As-deposited Cu(Mg)/SiO₂/Si multilayer samples with film thicknesses in the 1,000–3,000 ? range were annealed for 30 min in oxygen ambients at pressures ranging from vacuum to 100 mtorr. The results showed that annealing in an 8-mtorr O₂ ambient significantly decreased the electrical resistivity of a 1,000 ? sample from 10.5 μΩ-cm to 3.7 μΩ-cm. Annealing in the O₂ ambient enhanced Mg diffusion to the surface in comparison to vacuum annealing. Furthermore, O₂ ambient annealing leads to excessive grain growth. However, the effect of O₂ ambient annealing on resistivity is less when the thickness of the film increases.     

Powder processing of high temperature ceramic superconductor Tl2Ba2Ca2Cu3O10

We have developed a technique to produce high quality Tl₂Ba₂Ca₂Cu₃O₁₀ powders used for making superconducting wire, tape, lead, shield, and other large scale bulk applications. Starting with T1₂O₃, BaO₂, CaO, and CuO, we mix and grind these chemicals with a machine ball mill and then press the ground mixture into pellets. The pellets are sintered at about 895‡C for at least 30 h in an oxygen atmosphere. The sintered material is mainly the Tl₂Ba₂Ca₂Cu₃O₁₀ compound. To get more homogeneous superconductor powders, we pulverize the sintered material and use a magnetic superconducting material selector to separate and grade the material. Finally, the top grade material has a phase purity of <98% and a T_c(r < 0) of 123–126K.