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.     

Internal friction of superconducting YBa2Cu3O6+x wires at 200 kHz under in situ heat treatment

Using piezoelectric LiNbO₃ crystal oscillator method at 200 kHz, we have measured internal friction and modulus of YBa₂Cu₃O_6+x short wires with 1 mm-diameter and about 5mm length under in situ cyclic heating-cooling treatment at temperatures of 300–1020K. Polycrystalline YBa₂Cu₃O_6+x wires show anelastic behavior as well as orthorhombic-to-tetragonal (O-T) phase transition within present temperature range. There appears a strong internal friction peak below the O-T transition temperature, corresponding to the atomic site relaxation between oxygen and a vacancy in the Cu-O plane of oxygen defective YBa₂Cu₃O_6+x. A novel peak is observed after the cyclic heat treatments at around 700K. This relaxation is attributed to the hopping movement of oxygen in the defective local structure of YBa₂Cu₃O_6+x.     

Processing of large YBa2 Cu3Ox domains for levitation applications by a Nd1+x Ba2−x Cu3 Oy-Seeded melt-growth technique

YBa₂Cu₃O_x domains for levitation applications have been produced by a seeding technology that includes Nd_1+x Ba_2−x Cu₃O_y seeds and melt-processing technologies such as conventional melt-textured growth, melt-texturing with PtO₂ and Y₂BaCuO₅ additions, and the new solid-liquid-melt-growth technology. Large domains (∼20 mm) with high levitation forces (F₁ up to 8.2 N) have been produced. The reproducibility of the results is good, and the capability of producing a large number of pellets in a single batch indicates good potential for the production of large amounts of this material.     

Dislocations and flux pinning in YBa2Cu3O7

The dislocation structures of bulk textured and epitaxial thin film YBa₂Cu₃O₇ superconductors are examined. Correlations between increases in flux pinning and dislocation densities are noted. A model for flux pinning by individual dislocations is presented. This gives a treatment of strain induced effects and effects of normal state region interactions. It is shown that the values of pinning predicted are in line with experimental observations.     

LaNiO3 and Cu3Ge contacts to YBa2Cu3O7-x films

Metallization of high-T_c superconductors using low resistivity metal oxides and Cu-Ge alloys has been investigated on high quality pulsed laser deposited epitaxial YBa₂Cu₃O_7-x (YBCO) films. Epitaxial LaNiO₃ (LNO) thin films have been grown on YBCO films at 700°C using pulsed laser deposition. The specific resistivity of LNO was measured to be 50 μΩ-cm at 300K which decreases to 19 μΩ-cm at 100K indicating good metallicity of the LNO films. The contact resistance of LNO-YBCO thin film interface was found to be reasonably low (of the order of 10^-4Ω-cm² at 77K) which suggests that the interface formed between the two films is quite clean and LNO can emerge as a promising metal electrode-material to YBCO films. A preliminary investigation related to the compatibility of Cu₃Ge alloy as a contact metallization material to YBCO films is discussed. The usage of other oxide based low resistivity materials such as SrRuO₃ (SRO) and SrVO₃ (SVO) for metallization of high-T_c YBCO superconductor films is also discussed.     

Characterization of the texturing of YBa2Cu3O7−δ in the presence of liquid phase additives

Grain growth and texturing of YBa₂Cu₃O_7−δ is greatly influenced by the presence of liquid phase additives during sintering. Oxides such as TiO₂, SiO₂, Bi₂O₃, and Pr₆O₁₁ were incorporated into the liquid phase during the sintering of YBa₂Cu₃O_7−δ (123) by use of grain boundary diffusion couples and the microstructure was analyzed using scanning electron microscopy/electron dispersive spectroscopy. Exaggerated grain growth and domain formation was observed in bulk specimens. Differential thermal analysis and real time dynamic x-ray diffraction were used to determine reaction sequencing. The ability and extent of domain formation was determined for 123 samples coupled with impurity oxides to be a function of sintering temperature (940–980°) and oxygen partial pressure. Enhanced texturing was observed at low PO₂ atmospheres. The addition of Bi₂O₃ and TiO₂ was shown to degrade dc magnetic susceptibility of 123 whereas SiO₂ and Pr₆O₁₁ enhanced it. The domain formation and texturing takes place in the bulk for 123 at temperatures of 980°C or below (i.e. well below the peritectic decomposition temperature) by the interaction of an impurity doped liquid phase followed by a precipitation and exaggerated grain growth.     

Influence of excess yttrium and platinum on the growth behavior of YBa2Cu3O7−δ and Y2BaCuO5

The high temperature (1100°C) coarsening of Y₂BaCuO₅ (211) and subsequent YBa₂Cu₃O_7−δ (123) growth kinetics using melt quenched and 123 precursor powders were examined via quenching. Fine scale excess yttrium addition by sol gel coating was employed up to 20 mol percent. X-ray diffraction identified 211 fraction between 123 (∼30 wt.%) and melt quenched (∼10 wt.%) in the precursor powders. The addition of yttrium was seen to shift the 211 weight fraction to higher levels for the MQ powders. Refinement of the 211 particle size was seen in the presence of Pt but not with yttrium addition. The coarsening behavior of 211 in either powder did not appear to significantly change with excess yttrium addition at 1100°C. Differential thermal analysis showed that the 123 phase solidification temperature increased in the presence of Pt and reduced with yttrium addition up to 10 mol %. Dilatometric measurements showed the influence of yttrium addition on the densification behavior of 123 due to the presence of low temperature liquid phase formation. Directly inserting samples at soak teperatures were seen to significantly alter the 211 weight fraction for 123 powder in contrast to slower thermal heating to temperature. However, this effect was not seen in the case of the melt quenched precursor powder.     

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.     

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.     

The growth of low dislocation density Sr1−x,Bax Nb2O6 crystals

The dislocation structures of both pure and Nd doped strontium barium niobate crystals, grown by the Czochralski method, were studied using an etch pit technique. It was determined that dislocations in the boule were being propagated from the seed and were confined to the center of the crystal. Typical dislocation density was 5x10⁴ cm⁻². Through the careful control of growth parameters and use of seed material cut from the dislocation free outer portion of a crystal, it was possible to grow crystals with very low dislocation densities, 1x10² cm⁻², and on occasion dislocation free crystals.     

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.     

Effects of platinum and oxygenation on microstructure in YBa2Cu3O7−δ/Y2BaCuO5 bulk materials

Transmission electron microscopy examinations have been conducted on undoped and PtO₂ doped YBa₂Cu₃O_7−δ, with excess Y₂BaCuO₅ (211) in the molar ratio of 5∶1 (123/211), processing using the solid liquid melt growth technique. Magnetization hysteresis suggests that addition of Pt strongly influences the pinning behavior. Considerable differences in dislocation and stacking fault densities were observed. Dislocation nets and tangles were commonly observed in the Pt doped samples. In both samples, stacking faults were observed near 211 precipitates, interplatelet boundaries, and dislocations. Dislocations appear to be formed during high temperature processing, while stacking faults appear to be generated during the final oxygenation step. The density and distribution of fine precipitates (∼25–100 nm) were comparable in both specimens suggesting that Pt additon affects the size and acicular morphology of only the coarser 211 (∼1–10 μm). It is proposed that the observed increase in J_c due to Pt addtion may be attributed to the increase in defect density rather than fine precipitates.     

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.     

Kinetic Studies of the Interfacial Reaction of the Ba2YCu3O6+x Superconductor with a CeO2 Buffer

Interfacial reactions between the Ba₂YCu₃O_6+x superconductor and the CeO₂ buffer layers employed in coated conductors have been modeled experimentally by investigating the kinetics of the reaction between Ba₂YCu₃O_6+x films and CeO₂ substrates. At 810°C, the Ba₂YCu₃O_6+x -CeO₂ join within the BaO-Y₂O₃-CeO₂-CuO _x quaternary system is nonbinary, thereby establishing the phase diagram topology that governs the Ba₂YCu₃O_6+x /CeO₂ reaction. At a mole ratio of Ba₂YCu₃O_6+x :CeO₂ of 40:60, a phase boundary was found to separate two four-phase regions. On the Ba₂YCu₃O_6+x -rich side of the join, the four-phase region consists of Ba₂YCu₃O_{6 +x} , Ba(Ce_1−z Y _z )O_3−x , BaY₂CuO₅, and CuO _x ; on the CeO₂ rich side, the four phases were determined to be Ba(Ce_1−z Y _z ) O_3−x , BaY₂CuO₅, CuO _x and CeO₂. The Ba₂YCu₃O_6+x /CeO₂ reaction is limited by solid-state diffusion, and the reaction kinetics obey the parabolic rule, x = Kt ^1/2, where x = thickness of the reaction layer, t = time, and K = a constant related to the rate constant; K was determined to be 1.6 × 10⁻³ μm/s^1/2 at 790°C and 4.7 × 10⁻³ μm/s^1/2 at 830°C. The activation energy for the reaction was determined to be E _act = 2.67 × 10⁵ J/mol using the Arrhenius equation.     

Fast growth of Cd1−xZnxTe single crystals by physical vapor transport

Good quality single crystals of Cd_1−xZn_xTe (x=0.04, 0.08, and 0.115) were obtained by the very fast vapor growth technique developed in this laboratory. The abundance of lamellar twins seems to be less than that in CdTe grown under the same conditions. Chemical etching results indicate a similar quality of the vapor grown crystals to those obtained from melt growth. The overall compositions of the crystals are uniform and approach those of the initial source material with increasing amount transported.     

Stress testing of a recrystallizing CaO-B2O3-SiO2 glass-ceramic with Ag electrodes for high frequency electronic packaging

CaO−B₂O₃−SiO₂ recrystallizing glass-ceramics are attractive for use in microelectronics packaging since they can be processed at low temperatures (<950°C) and the resultant material has a low loss tangent. This paper reports on the stability of the Ag electrodes with respect to electromigration. Planer multilayer recrystallizing glass-ceramic/Ag samples were prepared and fired at temperatures from 800–950°C. Buried capacitor structures with Ag plates 10 mm in diameter with a separation distance of 0.1 mm were created. These were stressed with voltages from 50 V to 3 kV at temperatures from 25°C to 200°C to induce failure. Failure modes observed ranged from increased, leakage to catastrophic events. A population was analyzed for mean time to failure characteristics. SEM and EDX were used to try to identify leakage paths and movement of the Ag through the glass ceramic. An activation energy was determined for the most common failure mode.     

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.     

Elaboration and characterization of a KCl single crystal doped with nanocrystalsof a Sb2O3 semiconductor

Undoped and doped KCl single crystals have been successfully elaborated via the Czochralski（Cz） method.The effects of dopant Sb₂O₃ nanocrystals on structural and optical properties were investigated by a number of techniques,including X-ray diffraction（XRD）,scanning electron microscopy（SEM）,energy dispersive X-ray（EDAX） analysis,UV-visible and photoluminescence（PL） spectrophotometers.An XRD pattern of KCl:Sb₂O₃ reveals that the Sb₂O₃ nanocrystals are in the well-crystalline orthorhombic phase.The broadening of diffraction peaks indicated the presence of a Sb₂O₃ semiconductor in the nanometer size regime.The shift of absorption and PL peaks is observed near 334 nm and 360 nm respectively due to the quantum confinement effect in Sb₂O₃ nanocrystals.Particle sizes calculated from XRD studies agree fairly well with those estimated from optical studies.An SEM image of the surface KCl:Sb₂O₃ single crystal shows large quasi-spherical of Sb₂O₃ crystallites scattered on the surface.The elemental analysis from EDAX demonstrates that the KCl:Sb₂O₃ single crystal is slightly rich in oxygen and a source of excessive quantities of oxygen is discussed.     

NMOSFET characteristics on a double hetero-epitaxial Si/γ-Al2O3/Si structure fabricated by a new MBE growth method

A new MBE growth method for the fabrication of a high-quality double hetero-epitaxial Si/γ-Al₂O₃/Si structure was recently developed. In the present work, characteristics of NMOSFETs fabricated on the Si/γ-Al₂O₃/Si structure were investigated, and compared with those on a Si/MgAl₂O₄/Si structure. A γ-Al₂O₃ layer was created from a MgAl₂O₄ layer by reaction with Si beams as follows: MgAl₂O₄ + Si → γ-Al₂O₃ + SiO ↑ + Mg ↑. The MBE growth of Si on the effectively restructured γ-Al₂O₃ layer was then performed at a substrate temperature of 700° C, 150° C lower than for the MBE growth of Si on a MgAl₂O₄/Si substrate. The electron field effect mobility and leakage current between source and drain for the NMOSFETs fabricated on Si/γ-Al₂O₃/Si structures were 660 cm²/V · s and 2.8 pA/μm respectively, and exhibited a higher level of performance than those on a Si/MgAl₂O₄/Si structure. In the Si/MgAl₂O₄/Si, SIMS measurements confirmed that autodoped Al and Mg atoms near the interface between the Si epi-layer and MgAl₂O₄/Si substrate diffused anomalously and accumulated at the surface during device fabrication processes. These autodoped Al and Mg atoms acted as ionized impurities during test operation. Suppression of autodoping from insulator layers during the MBE growth of Si was thus deemed essential to the improvement of NMOSFET characteristics. In the Si/γ-Al₂O₃/Si structure, autodoped atoms were scarcely detectable. It was therefore concluded that the Si/γ-Al₂O₃/Si structure under study was very promising for SOI device applications.     

Factors affecting Y2BaCuO5 precipitate size during melt processing of YBa2Cu3O7−x

In the microstructures of melt processed YBa₂Cu₃O_7−x (123) superconductors, often unconsumed Y₂BaCuO₅ (211) particles are observed. The 211 particle size and distribution depend upon i) processing parameters such as peak temperature, heating rate, residence time above 1010°C, starting 123 grain size, etc., ii) second phase additions, and iii) the processing route employed. 211 particle size control is of primary interest for enhancing 123 flux pinning, and fracture toughness. Factors which determine the 211 particle size are reviewed.