Relationships between dislocations,second phases,and pinning in Y-Ba-Cu-O

The pinning effects and the critical current density predicted to occur in bulk YBa₂Cu₃O₇ from second phase particles and from dislocations are examined. It is shown that core or strain pinning by dislocations provides the best explanation of pinning forces, and that strain pinning by dislocations or particle pinning of individual (but not multiple) flux lines best fit the pinning energy. An experiment in which critical current is increased when particles are coarsened by long molten holding times during melt processing is correlated with increased dislocation density, despite a drop in volume fraction and increase in size of the added normal state particles. The combined aspects of these suggest that strain induced pinning from dislocations may be the dominant mechanism in high critical current bulk materials.     

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.     

Quantitative Magneto-Optical Imaging for Superconducting Thick Films and Tapes

Practical superconducting thick films and tapes, manufactured in an industrial process, have microscopic inhomogeneities. Quantitative magneto-optical imaging (MOI) is one of the most desirable techniques and provides both local and global information on defects, flux pinning, critical current density J _c, and current distribution. We present, herein, a comparative magneto-optical imaging study of the flux profile in YBa₂Cu₃O_7−δ (YBCO) thick films prepared in two different processes, the laboratory versus the industrial scalable process. The remarkably different flux propagation patterns are the manifestation of their different defect landscapes within these films. A method of determining J _c near zero applied field is also given for locally nonuniform superconducting films, using the quantitative MOI technique.     

Effect of site-selective substitution in bulk superconducting Tl2Ba2CaCu2O8

Magnetization measurements were carried out on bulk Tl₂Ba₂CaCu₂O₈ (referred to as Tl-2212) and on various site-selective substituted Tl-2212 samples. At 5K between 0 and 4.5 T, the 5 at. % Mg-doped Tl-2212 (Tl,Mg-2212) samples displayed enhanced pinning as demonstrated by a field dependent increase of the magnetic critical-current density J_c by 18 to 25 percent over that of pristine Tl-2212. Excess Mg (10–15 at. %), however, is deleterious. Rietveld refinement of the x-ray diffraction pattern showed Mg on the Tl sites. Auger electron spectroscopy analysis showed part of the Mg on grain boundaries. The flux-creep activation energies are higher for flux expulsion than for flux penetration in both Tl-2212 and Tl,Mg-2212 samples; the latter displays higher individual values. Our results demonstrate an increase in the number density of flux lines as a result of increased density of atomic-size-structural, defects by Mg (5 at. %) doping. In the Tl_2−yBa₂(Ca_1−zY_z)Cu₂O_8−x(z=0–0.3; single phase; x and y represent oxygen and thallium vacancies) system also studied, the T_c decreases as z increases. At z=0.3, the sample becomes an antiferromagnetic semiconductor.     

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.     

Superconducting and Transport Properties of (Bi-Pb)-Sr-Ca-Cu-O with Nano-Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Additions

The effect of nano Cr₂O₃ additions in (Bi, Pb)-Sr-Ca-Cu-O superconductors using the coprecipitation method is reported. Nano Cr₂O₃ with 0.1, 0.3, 0.5, 0.7, and 1.0 wt.% were added to the (Bi, Pb)-Sr-Ca-Cu-O system. The critical temperature (T _c) and transport critical current density (J _c) were determined by the four-point probe technique. The phases in the samples were determined using the powder X-ray diffraction method. The microstructure was observed by a scanning electron microscope and the distribution of nano Cr₂O₃ was determined by energy-dispersive X-ray analysis (EDX). The maximum T _c and J _c were observed for the sample with 0.1 wt.% nano Cr₂O₃. The variation in the J _c of all the samples was explained by the effective flux pinning by nano Cr₂O₃ in the samples. Using the self-field approximation together with the dependence of J _c on temperature, the characteristic length (L _c) associated with the pinning force was estimated to be approximately the same as the average grain size in all the samples.     

Dislocation-Induced deep level states in In0.08Ga0.92As/GaAs heterostructures

We have performed luminescence experiments on In_0.08Ga_0.92As/GaAs heterointerfaces to explore the energy distribution of deep level states in the bandgap for two cases: (1) unrelaxed, pseudomorphic In_0.08Ga_0.92As films (200Å thick), which have few if any dislocations at the interface, and (2) partially relaxed In_0.08Ga_0.92As films (1000Å thick) which are expected to have a substantial interfacial dislocation density. A combined photoluminescence and cathodoluminescence technique is used which allows us to profile the sample luminescence through the buried interface region. Our results show the existence of deep level luminescent features characteristic of the GaAs substrate and features common to In_0.08Ga_0.92As and GaAs, as well as the existence of a deep level feature near 1 eV photon energy which undergoes a shift in energy depending upon the degree of strain relaxation in the In_0.08Ga_0.92As film. In addition, a deep level feature near 0.83 eV becomes prominent only in In_0.08Ga_0.92As films which have relaxed, and thus contain misfit dislocations at the interface. These deep level differences may be due to bandgap states associated with the intrinsic dislocation structure, impurities segregated at the dislocation, or bulk point defects, or threading dislocations generated during the strain relaxation. Previous work has determined that a deep level state 0.7 eV above the valence band edge would account for the electrical behavior of relaxed In_0.08Ga_0.92As/GaAs interfaces, which is in good agreement with the range of deep level transitions near 0.8 eV photon energy which we observe. These measurements suggest that photo- and cathodoluminescence measurements of deep level emission in these III-V semiconductors can provide a useful indicator of electrically active defect densities associated with misfit dislocations.     

Iron nitride mask and reactive ion etching of GaN films

One of the major GaN processing challenges is useful pattern transfer. Serious photoresist mask erosion and hardening are often observed in reactive ion etching of GaN. Fine pattern transfer to GaN films using photoresist masks and complete removal of remaining photoresist after etching are very difficult. By replacing the etch mask from conventional photoresist to a sputtered iron nitride (Fe-8% N) film, which is easily patterned by wet chemical etching and is very resistive to Cl based plasmas, GaN films can be finely patterned with vertical etched sidewalls. Successful pattern transfer is realized by reactive ion etching using Cl (H) containing plasmas. CHF₃/Ar, C₂ClF₅/Ar, C₂ClF₅/Ar/O₂, SiCl₄, and CHCl₃ plasmas were used to etch GaN. The GaN etch rate is dependent on the crystalline quality of GaN. Higher crystalline quality GaN films exhibit slower etch rates than GaN films with higher dislocation and stacking fault density.     

Structure of organometallic chemical vapor deposited BaTiO3 thin films on LaAIO3

BaTiO₃ thin films grown on LaA1O₃ by organometallic chemical vapor deposition were characterized with cross-sectional high resolution transmission electron microscopy. Epitaxy was confirmed for the films grown on (100) oriented substrates. The films displayed an aaxis orientation. The interface between the film and substrate was nearly atomically abrupt. Partial relaxation of the films was observed as a result of misfit dislocation formation.     

Material properties of Pb1-xSnxSe epilayers on Si and their correlation with the performance of infrared photodiodes

Hall mobilities and resistance area products R_oA of infrared diodes in epitaxial Pb_1-xSn_xSe layers on CaF₂ covered Si(111) substrates were correlated with threading dislocation densities p. The low temperature saturation Hall mobilities were entirely determined by p and proportional to their mean spacing 1/ √ρ. For the photodiodes, the R₀A values at low temperatures were inversely propor-tional to ρ. A model where each dislocation in the active area of the diodes causes a shunt resistance correctly describes the results, the value of this resistance for a single dislocation is 1.2 GΩ for PbSe at 85K. The dislocation densities were in the 2 × 10⁷ to 5 × 10⁸cm^-2 range for the 3-4 μm thick as-grown layers. Higher R₀A values are obtainable by lowering these densities by thermal annealing, which sweeps the threading ends of the misfit dislocations to the edges of the sample.     

Phase and magnetic properties of iron oxide clusters in 20CaO·20SiO2·7Fe2O3·6FeO glasses

The annealing of 20CaO·20SiO₂·7Fe₂O₃·6FeO glasses at 973K in vacuo produced clusters of iron oxide, the shape of which was nearly spherical and the diameter distributed in the narrow range 25–115Å. The phase of clusters was identified to be Fe³⁺(Fe³⁺ _{poststagger|1.30}Fe²⁺ _{poststagger|0.55}V_0.15)·O₄ in the inverse spinel structure based upon the Mössbauer spectra and x-ray diffraction profiles. The clusters exhibited superparamagnetism and their effective anisotropy energy constant was inversely proportional to the cluster diameter. The magnetization of the glasses measured by a vibrating sample magnetometer was 7.2 × 10^-6 Wbmkg^-1 at 10 kOe at room temperature and smaller than the value calculated assuming that the whole clusters have superparamagnetism. These results suggest the pinning of spins near the cluster surface.     

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.     

Characterization of Si homoepitaxial films grown with and without surface photoactivation by ArF excimer laser-induced photodissociation of Si2H6

Silicon homoepitaxial films have been grown by photodissociation of Si₂H₆ by the 193 nm line of an ArF excimer laser in an ultra-high vacuum system. Silicon epitaxy has been achieved in two ways: one, in which the laser shines into the chamber parallel to the substrate and another, in which the laser is directly incident on the substrate at grazing angles (87° with respect to the substrate normal). Controllable growth rates of 0.5–4Å/min have been achieved for crystalline films by the first method using substrate temperatures as low as 250° C, Si₂H₆ partial pressures of 20 mTorr and photon flux densities of 10¹⁶ photons/pulse.cm². In the second method, where the laser beam is directly incident on the wafer at grazing angles, very high growth rates of up to 80Å/min have been achieved at 300° C, 20 mTorr Si₂H₆ partial pressure and a photon flux density of 2 × 10¹⁵ photons/pulse.cm². A comparison of the microstructure of the films grown by the two methods is presented on the basis ofin situ reflection high energy electron diffraction (RHEED) analysis and selected area transmission electron microscopy (TEM) studies. In both cases, the growth rates are found to be linearly dependent on the photon flux density for the process parameter ranges studied.     

Fluxon pinning in the nodeless pairing state of superconducting YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript>

Positive-muon spin rotation (μ+SR) spectroscopy and magnetic moment measurements were used to probe fluxon (or vortex) formation in the superconducting mixed state of a high-purity YBa₂Cu₃O₇ crystal. Random potentials caused by crystal-lattice defects pin fluxons. A fluxon lattice forms in an external magnetic field, and changes of thermal activation lead to fluxon pinning and depinning. The root second moment of the local magnetic field distribution (σ) determined by μ+SR contains information on the magnetic penetration depth and the pinning. Fluxon pinning leads to temperature-dependent transverse displacements of the fluxons that decrease σ and also fluctuations in the separation between fluxons that tend to increase σ. By accounting for the field-dependent and temperature-activated fluxon disorder, it is found that the experimental results for the penetration depth are consistent with a supercon-ducting order parameter of a strong-coupling two-fluid model, confirming that the superconductivity is nodeless with s-wave superconducting pairing. Quantitative results for fluxon displacements are discussed within the context of the fluxon field-temperature phase diagram.     

LPE growth of crack-free PbSe layers on Si(100) using MBE-Grown PbSe/BaF2CaF2 buffer layers

Crack-free PbSe on (100)-oriented Si has been obtained by a combination of liquid phase epitaxy (LPE) and molecular beam epitaxy (MBE) techniques. MBE is employed first to grow a PbSe/BaF₂/CaF₂ buffer structure on the (100)-oriented Si. A 2.5 μm thick PbSe layer is then grown by LPE. The LPE-grown PbSe displays excellent surface morphology and is continuous over the entire 8×8 mm² area of growth. This result is surprising because of the large mismatch in thermal expansion coefficients between PbSe and Si. Previous attempts to grow crack-free PbSe by MBE alone using similar buffer structures on (100)-oriented Si have been unsuccessful. It is speculated that the large concentration of Se vacancies in the LPE-grown PbSe layer may allow dislocation climb along higher order slip planes, providing strain relaxation.     

Liquid phase epitaxial growth and assessment of Pb1−xSn x Te alloys

The liquid-phase epitaxial growth of Pb_1−xSn_x Te on PbTe (100) substrates has been investigated over a range of growth temperatures from 600-400°C, and has been found to produce material with good uniformity and reproducibility of carrier concen-tration and alloy composition. The assessment of the epitaxial layers by such techniques as x-ray diffraction, dislocation etching and thermo-electric power measurements is described. Various features of the epitaxial layers such as interface irregularity, dislocation and diffusion effects are discussed, and likely mechanisms for their existence are proposed. The hole concentrations of the epitaxial layers, obtained by thermoelectric power measurements, are shown to have a similar dependence on preparation temperature as for bulk annealed material, suggesting that native defects are the dominant source of carriers above~ 2×10* cm^-3.     

Ultraviolet laser-assisted surface treatment of InP with phosphine gas

InP surface has been treated with phosphine (PH₃) gas photodecomposed by ArF excimer laser at a temperature as low as 150° C. It is shown by Auger electron spectroscopy analysis that the photolytic process of PH₃ gas is capable of removing native oxide and depositing simultaneously amorphous P film on the InP surface. Moreover, hydrogenation occurs on and near the surface of InP. An enhancement of the barrier height up to 0.63 eV is demonstrated for MIS Schottky junctions with a thin P layer formed on the treated InP substrates. Furthermore, it is shown that the barrier height varies depending on work function of the Schottky metal on the treated InP. This suggests that the present process causes a reduction in the surface state density which permits weakening of the Fermi level pinning at the surface of InP.     

Effects of processing variables on the Y2BaCuO5 size and magnetic properties of melt-processed YBa2Cu3Ox

Samples of YBa₂Cu₃O_x (123) with excess Y₂BaCuO₅ (211) in the molar ratio of 5:1 ( 123/211) were processed using the “solid liquid melt growth” (SLMG) technique. The effect of hold time above the peritectic on the magnetic properties was examined. Extended hold times above the peritectic during processing degrade the magnetic properties of SLMG processed 123. In SLMG 123, the very fine (>100 nm) 211 particles produced by this processing route are the primarymagnetic flux pinners. Extended hold periods reduce the number and/or coarsen the average size of these fine precipitates, resulting in a reduced magnetization. These results were compared to undoped Y123 processed by the more traditional melt texture growth (MTG). In MTG processing, extended hold times above the peritectic are found to result in improved magnetic behavior because of increased defect densities.     

Errata

The liquid-phase epitaxial growth of Pb_1−xSn_x Te on PbTe (100) substrates has been investigated over a range of growth temperatures from 600-400°C, and has been found to produce material with good uniformity and reproducibility of carrier concen-tration and alloy composition. The assessment of the epitaxial layers by such techniques as x-ray diffraction, dislocation etching and thermo-electric power measurements is described. Various features of the epitaxial layers such as interface irregularity, dislocation and diffusion effects are discussed, and likely mechanisms for their existence are proposed. The hole concentrations of the epitaxial layers, obtained by thermoelectric power measurements, are shown to have a similar dependence on preparation temperature as for bulk annealed material, suggesting that native defects are the dominant source of carriers above~ 2×10* cm^-3. The online version of the original article can be found at     

Thermoelectric Properties of Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript> and Prospects for Other Oxide Thermoelectrics

We discuss the thermoelectric properties of Na _x CoO₂ using the electronic structure, as determined in first principles calculations, and Boltzmann kinetic transport theory. The Fermi energy lies near the top of a manifold of Co t _2g bands. These t _2g bands are separated by a large gap from the higher-lying e _g states. Although the large crystal-field splitting implies substantial Co–O hybridization, the bands are narrow. Application of standard Boltzmann transport theory to such a narrow band structure yields high thermopowers in accord with experimental observations, even for high metallic carrier densities. The high thermopowers observed for Na _x CoO₂ can therefore be explained by standard band theory and do not rely on low dimensionality or correlation effects specific to Co. We also present results for the cubic spinel structure ZnRh₂O₄. Like Na _x CoO₂, this compound has very narrow valence bands. We find that if it could be doped with mobile carriers, it would also have a high thermopower, comparable with that of Na _x CoO₂.