T(z) phase diagram and optical energy gaps in CuGa(Se z Te1−z )2 alloys

TheT(z) phase diagram of the system CuGa(Se _z Te_1-z )₂ was obtained from x-ray diffraction and differencial thermal analysis (DTA) measurements. DTA measurements were carried out on each sample and the transition temperatures were plotted as a function of alloy composition. Values of lattice parameter were determined in all cases. Values of the optical energy gap at 300 K were obtained from optical absorption measurements, and these were analysed by using a recently proposed model which explains the downward bowing observed in the I-III-VI₂ alloys.     

T(z) diagram and optical energy gap values of Cd1−zMnzGa2Se4 alloys

The T(z) diagram of the system Cd_1−zMn_zGa₂Se₄ was obtained from x-ray diffraction and differential thermal analysis measurements. It was found that at lower temperatures, a single phase solid solution occurs across the whole compositional range and values of lattice parameters were determined as a function of z. At higher temperatures, an order-disorder transition occurs, in the range 0 < z < 0.6 to a partially ordered tetragonal structure and for 0.6 < z < 1. 0 to a disordered defect zinc-blende structure. In the T(z) diagram, both the ordering boundary and the solidus curve appear to show discontinuities at z = 0.6, corresponding to the change in the disordered phase. It is suggested that the symmetries of the terminal compounds are different one from the other. Optical absorption measurements were made at 300 K to show the variation of the direct optical energy gap E_o with z, and again the values appear to divide into two parts below the above z = 0.6.     

Ternary phase relations of the chalcopyrite compound CuGaSe2

The ternary Cu-Ga-Se phase diagram has been determined by DTA and x-ray analysis. In addition to two ternary solid solutions based on the binary compounds Cu_2.xSe and Ga₂Se₃, only one ternary phase, the chalcopyrite CuGaSe₂ (peritectic m.p. 1030°C), was encountered. The liquidus contains two regions of liquid immiscibility, one which extends from the Cu-rich (Cu, Se) binary liquid immiscibility to the Ga-rich (Ga, Se) binary immiscibility, and the other which is a minor extension of the Se-rich (Cu, Se) binary liquid immiscibility. The liquidus maxima include those at the binary boundaries: Cu (1087°C), Cu₆₇Se₃₃ (1148°C), GaSe (960°C); and a ternary liquidus maximum at Cu₁₉Ga_28.5Se_52.5 (1112°C), the maximum melting point of a solid solution based on the defect-zincblende phase of Ga₂Se₃. The primary phase fields are identified and the crystal growth of CuGaSe₂ solid solutions from nonstoichiometric melts is discussed, especially the most satisfactory Bridgman growth from the Cu₂Se-CuGaSe₂ join. Subsolidus phase relations are also proposed for the Cu-Ga-Se system, and probable similaritics in all I-III-VI ternary diagrams are suggested.     

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.     

Impurity-induced disordering of AlxGa1−xAs-GaAs quantum well heterostructures with (Si2)x(GaAs)1−x barriers

Recent work indicates that the alloy (Si₂)_x(GaAs)_1−x can be formed within the GaAs quantum well of an Al_xGa_1−xAs-GaAs quantum well heterostructure (QWH) and results in a shift of laser operation to higher energy. In this paper we show, by SIMS and EDS measurements, that the Si concentration in the (Si₂)_x(GaAs)_1−x layer far exceeds typical “doping” levels. The stability of these QWHs has been investigated with respect to thermal annealing and Zn impurity-induced layer disordering (Zn-IILD). Data are presented showing that the (Si₂)_x(GaAs)_1−x alloy is stable against thermal annealing unless a rich source of Ga vacancies is provided, and that relatively low temperature Zn diffusion greatly enhances the disordering process of the alloy layer.     

Anomalous anisotropy in the absorption coefficient of vacancy-ordered Ga2Se3

The structural and optical properties of Ga₂Se₃ on (100)GaP and (100)GaAs prepared by molecular beam epitaxy have been investigated. The electron diffraction studies revealed that the superstructure was formed in [011] direction by the spontaneous ordering of native gallium-vacancies in the defect zinc blende structure under a selenium-rich growth condition, and very large absorption anisotropy (Δα>10⁴cm¹) was observed in the vacancy-ordered Ga₂Se₃. Furthermore, polarization dependence of photoconductivity due to the absorption anisotropy was observed in the vacancy-ordered Ga₂Se₃ on (100)GaAs.     

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.     

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.     

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.     

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.     

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.     

The effect of Tl2Te3 on the properties of the solid solution (Sb2Te3)0.75.(Bi2Te3)0.25

The effect on transport properties of the addition of 0.5-5% Tl₂Te₃ to p-type solid solutions of antimony and bismuth tellurides was studied. It was found that the addition of Tl₂Te₃ caused a lessening of the increase of hole concentration as low temperatures were approached, resulting in a slower decrease of the Seebeck coefficient with a decrease in temperature. In partial fulfillment of M.Sc. degree, Hebrew University, Jerusalem. Permanent address, Dept. of Inorganic and Analytical Chemistry, Hebrew University, Jerusalem.     

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.     

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.     

Phase equilibria in the Cu-Ga-S-Sn system

Solid-liquid equilibria in the Sn-rich region of quaternary Cu-Ga-S-Sn system have been investigated experimentally. Solidus compositions corresponding to Cu-Ga-S-Sn liquidus points were determined using the crystals self-nucleated from solutions composed of Sn solvent and Cu-Ga-S solute. It has been found that, in the isothermal liquidus surface at 780°C, there exist at least four regions corresponding to different crystalline solids, having compositions of CuGaS₂, SnS, CuGa₃S₅ and Ga₄SnS_x (x=7∼8), respectively. The conditions of the solution growth of single-phase CuGaS₂ crystals using Sn solvent have been also discussed.     

Cu-In-Ga-Se nanoparticle colloids as spray deposition precursors for Cu(In, Ga)Se2 solar cell materials

The use of nanoparticle colloids for spray deposition of Cu(In,Ga)Se₂ (CIGS) precursor films and subsequent fabrication of CIGS solar cells has been investigated. According to this approach, amorphous Cu-In-Ga-Se nanoparticle colloids were first prepared by reacting a mixture of CuI, InI₃, and GaI₃ in pyridine with Na₂Se in methanol at reduced temperature. Purified colloid was sprayed onto heated molybdenum-coated sodalime glass substrates to form Cu-In-Ga-Se precursor films. After thermal processing of the precursor films under a selenium ambient, CIGS solar cells were fabricated. Cu-In-Ga-Se colloids and films were characterized by inductively coupled plasma atomic emission spectroscopy, thermogravimetric analysis, transmission electron microscopy, x-ray diffraction, scanning electron microscopy, and Auger electron spectroscopy. Standard current-voltage characterization was performed on the CIGS solar cell devices with the best film exhibiting a solar conversion efficiency of 4.6%.     

process and mechanism of growth of YBa2Cu3O6+x single crystal by the pulling method

Continuous growth of YBa₂Cu₃O_6+x(Y123) single crystals was achieved by the modified pulling method1 using BaO+CuO(3:5) solvent and Y₂BaCuO₅(Y211) solute. Y211 solid was placed at the bottom of the crucible. Temperature was set ten degrees higher at the bottom than on the surface of the solution. The crystal rotation speed was 120 rpm at first; but as the crystal radius increased, it was reduced to prevent increasing the interface temperature. The maximum crystal growth rate was about 0.2 mm/h. The crystal growth direction was controlled by the seed crystal direction.2 Two-dimensional numerical simulation was performed using a finite-difference method by a supercomputer to investigate the solution flow and temperature distribution.     

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.     

Effect of titanium dioxide (TiO2) seed layer on the phase composition of 0.7Pb (Mg1/3Nb2/3)O3-0.3PbTiO3 film prepared by pulsed laser deposition

Lead-magnesium niobate-lead titanate (PMN-PT) thin films with and without the TiO₂ seed layer were deposited on Pt/Ti/SiO₂/Si substrates through pulsed laser deposition. The study aimed to characterize the effect of the TiO₂ seed layer on the phase composition and properties of PMN-PT film. Without the TiO₂ seed layer, the pure perovskite phase could be obtained in the thinner PMN-PT film while with the TiO₂ seed layer, the pure perovskite phase was formed in the thicker PMN-PT film. The ferroelectric properties of PMN-PT films with the TiO₂ seed layer were exhibited. As a result, the maximum amount of remnant polarization reached the amount of 32 μC/cm² for the PMN-PT thin film with the TiO₂ seed layer.     

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.