Chemistry of electron doped Ln2−xCexCuO4 superconductors

The electron doped Ln_2−xCe_xCuO₄ (Ln=lanthanide) oxides have intergrowth structures consisting of superconductively active CuO₂ sheets alternating with inactive (Ln, Ce)₂O₂ fluorite layers along the c-axis. Stabilization of such intergrowth structures requires bond length matching across the intergrowth interface. The bond length matching criterion causes a monotonic decrease in the Ce solubility limit from x=0.24 to x=0.15 as the size of Ln³⁺ decreased from Ln=La_0.5Nd_0.5 to Ln=Gd. Annealing in N₂ atm of Ln_2−xCe_xCuO₄ at temperatures above 900°C creates oxygen vacancies and the number of vacancies decreases with increasing Ce content. The value of x at which a semiconductor to superconductor transition occurs in Ln_2−xCe_xCuO₄ increases with decreasing size of Ln³⁺ due to an increasing Madelung energy caused by a decreasing Cu−O bond length.     

Synthesis of new copper oxide superconductors at high oxygen pressure

The stability of the copper ion in +1, +2, and +3 oxidation states for oxides synthesized at oxygen activity, 10⁻⁶<f(O₂)<10³ bar, makes possible formation of a wide variety of distinct structures since the copper ion in a particular oxidation state prefers a unique coordination to oxygen. Thus, by controlling the oxygen pressure during the synthesis and annealing, the distribution of metaland oxygen-ions can be modified on atomic scale to optimize the structural and electronic properties. We present several examples of compounds of which the critical structural elements for superconductivity, i.e. the perfectly ordered CuO₂-planes, have been obtained and doped with holes by means of high oxygen pressure. These materials, La₂CuO_4+δ, La^2-xCa_xCuO₄, La_2-yCu₂O_6+° and GaSr₂Y_1-xCa_xCu₂O₇, offer a unique opportunity to study the relationship between superconductivity and structural, magnetic, and chemical properties.     

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.     

Rare-earth-modified Sr0.5Ba0.5Nb2O6, ferroelectric crystals and their applications as infrared detectors

Rare-earth-modified ferroelectric crystals with the formula (Sr_1−xBa_x)1− 3y/2 R_yNb2₂O₆, where R = La, Nd, Sm, Gd, and Lu, have Been prepared and studied. When R = La, Nd, x ≃ .5 and y = 0.02, the modified material, at room temperature, exhibited twice the pyroelectric coefficient and four times the dielectric constant of the unmodified Sr_1−xBa_xNb₂O₆ (x ≃ .5). Curie temperatures decreased, dielectric constants increased, while loss factor and detector signal-to-noise ratios remained nearly the same with the addition of rare earth doping. The calculated response based on the measured properties agree with the measured response of actual detectors. These properties suggested that the modified SBN are good materials for small element or array pyroelectric infrared detector applications.     

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.     

Partial Fraction Decomposition and Correlation Sequence in 2D Systems

In this paper, results of the one-dimensional (1D) digital filtering are extended to the two-dimensional (2D) case. It introduces a technique and an algorithm for the computation of the product H(z₁,z₂)H(z₁⁻¹,z₂⁻¹). The technique is used to find a minimum phase transfer function of a 2D system such that the previous product matches a given correlation sequence. The algorithm requires less arithmetic operations than the traditional methods. The former is based on a matrix formulation of the product, which is used to investigate the 2D partial fraction decomposition (PFD) and stability.     

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.     

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.     

Large Thermoelectric Power of La2−x Sr x CuO4 Possessing Two-Dimensional Electronic Structure

In order to gain deep insight into the origin of the large thermoelectric power and metallic electrical conduction observed for La_2−x Sr _x CuO₄ possessing a two-dimensional layered structure and the consequently obtained two-dimensional electronic structure, we investigate the energy–momentum dispersion and the lifetime of quasiparticles by using high-resolution angle-resolved photoemission spectroscopy. By calculating the thermoelectric power using the Bloch–Boltzmann theory and the experimentally obtained information about the electronic structure and electron scatterings, we found that the large thermoelectric power of La_2−x Sr _x CuO₄ is mainly brought about by the anisotropic electron scattering caused by the strong electron correlation, and that the two-dimensional electronic structure makes almost no important contribution to the large S(T) except for rather minor effect of the van Hove singularity near the Fermi level.     

Thermal Stability and Phase Purity in Polycrystalline Ba<Subscript>8</Subscript>Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>46−<Emphasis Type="Italic">x</Emphasis></Subscript>

Polycrystalline Ba₈Ga _x Ge_46−x exhibits promising thermoelectric performance with the figure of merit ZT close to that of single crystals. Polycrystalline Ba₈Ga _x Ge_46−x is promising for applications, but reproducibility and thermal stability of thermoelectric properties need to be demonstrated. Polycrystalline samples of Ba_8+dGa _x Ge_46−x -type clathrates (15.0 ≤ x ≤ 16.8 with varied nominal Ga content and d = 0 or 0.2) were prepared by direct reaction of the elements, followed by ball milling and hot pressing. Trace Ge impurity was observed (<1.0 wt.%) depending on the synthesis method. The electrical resistivity was stable in measurements up to 1000 K, regardless of Ge impurity. However, measurements to 1050 K resulted in irreversible increase in carrier concentration while the carrier mobility remained unchanged.     

Thermoelectric Properties of NdGd<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript> Prepared by CS<Subscript>2</Subscript> Sulfurization

Ternary rare-earth sulfides NdGd_1+x S₃, where 0 ≤ x ≤ 0.08, were prepared by sulfurizing Ln₂O₃ (Ln = Nd, Gd) with CS₂ gas, followed by reaction sintering. The sintered samples have full density and homogeneous compositions. The Seebeck coefficient, electrical resistivity, and thermal conductivity were measured over the temperature range of 300 K to 950 K. All the sintered samples exhibit a negative Seebeck coefficient. The magnitude of the Seebeck coefficient and the electrical resistivity decrease systematically with increasing Gd content. The thermal conductivity of all the sintered samples is less than 1.9 W K⁻¹ m⁻¹. The highest figure of merit ZT of 0.51 was found in NdGd_1.02S₃ at 950 K.     

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.     

Preparation and Thermoelectric Properties of Ag<Subscript>0.5</Subscript>In<Subscript>0.5−<Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript>5</Subscript>Sn<Subscript>4</Subscript>Te<Subscript>10</Subscript>

A series of compounds with composition Ag_0.5In_0.5−x Pb₅Sn₄Te₁₀ (x = 0.05 to 0.20) were prepared by slowly cooling the melts of the corresponding elements, and the effect of In content on the thermoelectric transport properties of these compounds has been investigated. Results indicate that the compounds’ electronic structure is sensitive to In content, and that the carrier concentration of these compounds at room temperature increases from 4.86 × 10¹⁸ cm⁻³ to 3.85 × 10²¹ cm⁻³ as x increases from 0.05 to 0.20. For these compounds, electrical conductivity decreases and Seebeck coefficient increases with increasing In content. Ag_0.05In_0.03Pb_0.5Sn_0.4Te₁₀ shows very low lattice thermal conductivity, and has a maximum dimensionless figure of merit ZT of 1.2 at 800 K.     

Secondary ion mass spectrometry characterization of D2O and H2 18O steam oxidation of silicon

Silicon wafers were steam oxidized at temperatures of 550–1000°C and pressures of 0.05–8 atm with H₂ ¹⁶O, H₂ ¹⁸O, and D₂ ¹⁶O. Deuterium (D) and¹⁸O profiles in 100–200 nm thick oxide films were measured with Cs⁺ beam secondary ion mass spectroscopy (SIMS). The use of D and¹⁸O isotopes enabled analysis of these elements without interference from the sputtering ambient. Peaks in the D profiles near the interface are due predominantly to abrupt changes in the ion yield and charging conditions as the interface is approached. Although the D concentration is nearly constant at ∼ 1 × 10²⁰ cm⁻³ for 700–1000°C oxidations, it rises to a non-equilibrium value of 6 × 10²⁰ cm⁻³ at 600°C. Analysis of steam indiffusions below 800°C indicates that the OD concentration is proportional to the (steam pressure)^1/2, in agreement with earlier results on as-oxidized wet thermal oxides. The results of sequential isotope oxidations indicate that there is rapid exchange ofboth the hydrogen and oxygen species during transport of “water” to the SiO₂. Si interface. We examine these results in terms of the concentrations of the interacting species in the oxide films, and our results are compared to similar reported studies.     

Precipitation and pinning in Ca and Sr-Rich High-Tc superconducting “Bi2Sr2CaCu2O8” ceramics

From a consideration of the phase equilibrium diagram of the system Bi₂O₃-SrO-CaO-CuO, a simple annealing procedure was developed to precipitate Bi_2+xSr_2+xCuO_6+d, Sr₁₄Cu₂₄O_41−x, and Bi₂Sr₃O₆ in high-temperature superconducting Sr-rich “Bi₂Sr₂CaCu₂O₈” ceramics and Ca₂CuO₃ and a liquid in Ca-rich “Bi₂Sr₂CaCu₂O₈” ceramics. The transformation results in an increase of the critical current density of which is believed to express improved pinning properties of the superconducting crystals, in particular an increased pinning energy, which reduces the probability for thermally activated depinning. Possible pinning centers which were introduced during precipitation of the second phases are the surface of the precipitates.     

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.     

Phase relations and homogeneity region of the hightemperature superconducting phase (Bi,Pb)2Sr2Ca2Cu3O10+d

For the nominal composition of Bi_2.27−xPb_xSr₂Ca₂Cu₃O_10+d, the lead content was varied from x < 0.05 to 0.45. The compositions were examined between 800 and 890‡C which is supposed to be the temperatue range over which the so-called 2223 phase (Bi₂Sr₂Ca₂Cu₃O_10+d) is stable. Only compositions between x < 0.18 to 0.36 could be synthezised in a single phase state. For x <0.36, a lead-containing phase with a stoichiometry of Pb₄(Sr,Ca)₅CuO_d with a small solubiliy of Bi is formed, for x > 0.18 mainly Bi₂Sr₂CaCu₂O_8+d and cuprates are the equilibrium phases. The temperature range for the 2223 phase was found to be 800 to 890‡C but the 2223 phase has extremely varying cation ratios over this temperaure range. Former single phase 2223 samples turn to multiphase samples when annealed at slightly higher or lower temperaures. A decrease in the Pb solubility with increasing as well as decreasing temperature with a maximum at about 850‡C was found for the 2223 phase.     

Low temperature heteroepitaxial growth of Si1−xGexon-Si by photo-enhanced ultra high vacuum chemical vapor deposition using Si2H6 and Ge2H6

Strained-layer Si_1×Ge_x-on-Si heteroepitaxy has been achieved by photolytic decomposition of disilane (Si₂H₆) and digermane (G e₂H₆) in an ultra high vacuum (UHV) chamber at substrate temperatures as low as 275°C. An ArF excimer laser (193 nm) shining parallel to the Si substrate was used as the UV light source to avoid surface damage and substrate heating. The partial pressures of the source gases in the reactor were chosen to vary the Ge mole fraction x from 0.06 to 0.5 in the alloy. The Si₂H₆ partial pressure was kept at 10 mTorr and the Ge₂H₆ partial pressure was varied from 0.13 to 2 mTorr with the laser intensity fixed at 2.75 × 10¹⁵ photons/cm²·pulse. To fit the Si_1−xGe_x growth rate and Ge mole fraction data, the absorption cross section of Ge₂H₆ at 193 nm was set to 1 × 10⁻¹⁶ cm², which is 30 times larger than that of Si₂H₆ (3.4 × 10⁻¹⁸ cm2). For Si_1−xGe_x alloy growth, the deposition rate of Si increases with Ge mole fraction, resulting in increased Si_1−xGe_x alloy growth rates for higher Ge content. The increase of the Si growth rate was attributed to the enhanced adsorption rate of Si₂H₆ pyrolytically in the presence of Ge, rather than due to photolytic decomposition reaction. The Ge mole fraction in Si_1−xGe_x alloys can be predicted by a new model for Si and Ge pyrolytic and photolytic growth. The model describes the increased growth rate of Si_1−xGe_x alloys due to a Ge₂H₆ catalytic effect during photo-enhanced chemical vapor deposition.     

Chemical–Mechanical Polishing of CdTe and ZnxCd1?xTe Single Crystals by H2O2(HNO3)–HBr–Organic Solvent Etchant Compositions

Chemical–mechanical polishing of CdTe and Zn _x Cd_1−x Te single-crystal surfaces by bromine-evolving compositions based on aqueous solutions of H₂O₂(HNO₃)–HBr–solvent has been investigated. The dependences of the chemical–mechanical polishing rate on the dilution of the base polishing etchant for various organic components have been determined. The surface condition after such polishing has been investigated using profilometry. The polishing etchant compositions for CdTe and Zn _x Cd_1−x Te single-crystal surfaces and the chemical polishing conditions have been optimized.     

Research of SiO2/InP structure prepared by photo-CVD

The SiO₂ film as an insulator in InP MOS structure was grown by mercury-sensitized photo induced chemical-vapor deposition (photo-CVD) utilizing gaseous mixture of monosilane (SiH₄) and nitrous oxide (N₄O) under 253.7 nm ultraviolet light irradiation. The PHOTOX SiO₂ film (i.e., SiO₂ film prepared by photo-CVD system) deposited at 250° C has a refractive index of 1.46 and breakdown field strength of 7.0 MV/cm. The 1 MHz capacitance-voltage characteristics of the InP MOS diode was measured to study the interface state densities. The minimum value is 1.2 × 10¹¹ cm⁻²eV⁻¹ for the sample prepared at a substrate temperature of 250° C.