Nonstoichiometric Melts Research Articles

LATTICE PARAMETER AND MICROHARDNESS INVESTIGATIONS ON CDTE1-XSEX

CdTe1-xSex anion mixed crystals have been grown from non-stoichiometric melts and by means of a modified vertical travelling-heater method. Selenium has been found to have an essentially higher affinity for cadmium than tellurium. The cubic phase (zincblende structure) exists in the composition range 0≤xCdSe≤0.45, the hexagonal phase (wurtzite structure) in the range 0.65≤xCdSe≤1. In order to compare the lattice parameters of the cubic and hexagonal modifications, the mean distances between next-neighbouring anions and cations, da-c, were calculated from the lattice parameters. The relation between da-c values and composition x roughly obeys Vegard's rule. By means of Vickers indentation microhardness measurements, the solid-solution hardening (SSH) behaviour was investigated for the first time and has been compared with the SSH of II–VI cation solid solutions. Regarding the hexagonal CdTe1-xSex modifications, one has to pay attention to the large anisotropy of microhardness depending on the surface orientation. © 1998 Kluwer Academic Publishers

Growth of periodic Hg1−x MnxTe structures by liquid-phase epitaxy

A procedure for the calculation of phase equilibria in the tellurium corner of the phase diagram of the ternary system Mn-Hg-Te is described within the framework of the model of regular associated solutions. The initial concentrations of the nonstoichiometric fluxed melts and the temperature regimes of their cooling for the controlled liquid-phase epitaxial growth of inhomogeneous Hg1−xMnxTe structures with prescribed modulation of the composition along their thickness are determined by numerical solution of a nonlinear inverse Stefan problem.

Properties of tellurium-doped gallium antimonide single crystals grown from nonstoichiometric melt

The electric and luminescence properties of tellurium-doped gallium antimonide single crystals grown from gallium-enriched melt by Czochralski’s method have been investigated. It was determined that the crystals possess n-type conductivity and are strongly compensated. It was found that toward the end of the ingot the concentration of the impurity tellurium increases more rapidly than that of the compensating acceptors. The possibilities of obtaining the properties of GaSb single crystals by growing the crystals from nonstoichiometric melts followed by heat treatment of the material are discussed.

Czochralski growth of SrLaAlO 4 and SrLaGaO 4 single crystals and its implications for the crystal morphology

Single crystals of SrLaAlO 4 (SLA) and SrLaGaO 4 (SLG) have been grown from nonstoichiometric melts by the Czochralski method making use of different seed orientations, [100] (SLA, SLG), [110] (SLA) and [001] (SLG). The anisotropic properties of the crystal structure are reflected in the growth morphology of the crystals. In the case of (SLA) {101} facets appear at the crystal/melt interface and in that of [100] SLG {101} and {103} facets occur, while for [001] SLG {001} and {103} facets are present. SLA crystals are very similar to the theoretical growth forms computed according to the Hartman-Perdok theory for models with an effective charge on oxygen, q O −2| e|. However the habit of SLG crystals corresponds better to the theoretical growth forms based on an effective charge q O = −1.5| e| due to the presence of oxygen vacancies.

The crystal perfection depends on the superheating of the mother phase too — experimental facts and speculations on the “melt structure” of semiconductor compounds

Depending on the degree of superheating, the “structure” of the melt, especially of associated liquid semiconductors with a high ionic contribution to the bond energy (II–VI, IV–VI), may affect the nucleation process and growth kinetics, and, hence, the crystal quality very strongly. Superheating-supercooling experiments in Bridgman ampoules show a distinct increase in the degree of supercooling in stoichiometric CdTe and PbTe melts being superheated in excess of a critical value above the melting point. In contrast, small transient and drastically increased supercooling has been obtained in non-stoichiometric melts containing an excess of tellurium. Moreover, the holding time in the molten state may influence the supercooling, as has been found in PbTe. Obviously, the destruction of associated melt complexes requires a certain degree and time of superheating or dissolution by the excess component. Hence, an inadequate superheating of stoichiometric melts before the crystal growth process may preserve strongly associated growing-in units that affect the growth kinetics (nucleation mode, substructure, twinning) and growth orientation considerably.

Influence of the cation sublattice on the growth, structure and properties of single crystals of 123- and 2212-type high- Tc superconductors

Series of 123- and 2212-type crystals were grown from non-stoichiometric melts and were investigated by electron-probe microanalysis (EPMA), X-ray diffraction (XRD) and optical methods. Among doped 123-type crystals, a group of samples has been distinguished with ordered distribution of cations. The presence of impurities in Cu(1) sites and large rare-earth (RE) cations has been determined to cause the structure ordering. The 123-type crystals with random and ordered cation distribution correspond to two regions of superconductivity, when they are scaled with respect to deficiency of Cu sites. In the BiPbSrCa, Y, RECuO system, series of solid solutions have been distinguished in which chemical composition, lattice parameters and properties change continuously. An increase of carrier concentration at intergrowth of two isostructural phases and the presence of regions with large deviation from stoichiometry are discussed.

Compositional mapping of GaSb wafers from as-grown crystals and after post-growth annealing treatments

Spatial compositional analysis has been carried out on single and polycrystal wafers of GaSb grown from stoichiometric and non-stoichiometric melts. In crystals grown from stoichiometric melt, the ratio of Ga to Sb is slightly more and remains uniform throughout. At the grain boundaries in polycrystals, the Sb content is more than in the other regions of the crystal. Crystals grown from either Ga- or Sb-rich melts exhibit inclusions of the excess component. Post-growth annealing treatments in vacuum and Ga-rich atmospheres have been performed. Heat treatments in vacuum atmosphere produce very little effect on the local composition of the crystal. On the other hand, localized crystallization at grain boundaries and inclusions takes place in the presence of excess gallium. It has been shown that annealing treatments in Ga ambient can produce defect-free wafers with extremely homogeneous composition. It is concluded that the excess Sb which is liberated from the crystal during growth resides at the grain boundaries and other extended defect centers. The vacant Sb sites are then responsible for the formation of the native acceptor centers like V Ga and Ga Sb.

Structure analysis of a Ca [sbnd]Nb [sbnd]Ga garnet

Neutron powder-diffraction intensities were measured in the range from room temperature to 800°C for a Ca Nb Ga garnet (CNGG) grown from a non-stoichiometric melt. A single crystal of CNGG was also studied at room temperature by means of X-ray diffraction. The space group I2 13 (No. 199) was tentatively assigned to CNGG based both on Rietveld analysis and on X-ray single-crystal structure analysis. There are some vacancies at one of the Ca 2+ sites and partial substitutions of Ca 2+ and Ga 3+ for Nb 5+. CNGG exhibited appreciably different thermal-expansion coefficients at room temperature and above 400°C. The dielectric constant measured parallel to the [001] direction for CNGG starts to increase non-linearly at 400°C, but exhibits no peak up to 600°C.

Growth, morphology and superconducting properties of TmBa 2Cu 3O 7− x single crystals

Nearly twin-free TmBa 2Cu 3O 7− x single crystals of high crystalline perfection were grown from nonstoichiometric melts in the system Tm 2O 3BaO(BaO 2CuO(Cu 2O) in air atmosphere. The superconducting transition temperatures and widths depend on crystallization conditions and are T c = 89–91 K and ΔT c = 0.2 K for the best samples. Morphology of the crystals and growth mechanisms of different faces were studied and are discussed.

YBa2Cu3O6+δ microcrystals by extraction of a nonstoichiometric melt

YBa2Cu3O6+δ microcrystals are synthesized by extraction of a nonstoichiometric melt similar to that employed for crystal growth of this compound. This method uses, first, initial compositions corresponding to the field YBa2Cu3O6+δ plus liquid, and second, the interaction properties of the flux with an appropriate substrate, i.e., Y2O3. Using other substrates like alumina produces 123 microcrystals, but embedded in the residual flux; in contrast yttrium oxide reacts with this one, and it is essential for its extraction. Choosing proper conditions makes it possible to obtain relatively pure YBa2Cu3O6+δ samples made of regular pellets (size: 10 to 20 μm thickness: 3 to 6 μm). The process is discussed using the phase diagram and the concept of dissolution-reprecipitation often involved in liquid phase sintering.

Double superconducting transition in the specific heat of non-stoichiometric UPt 3+ x

We have studied the double superconducting transition of polycrystalline UPt 3+ x prepared from a non-stoichiometric melt (−0.06 < x < 0.06). Samples prepared with a Pt deficit show the sharpest double transition, suggesting the binding of oxygen by excess uranium. However, microprobe investigations reveal that impurity phases (UPt 2 and UPt 5) are formed and that the actual deviation from stoichiometry (−0.01 < x < 0.01) is much smaller than the nominal value.

Superconductivity and morphological studies on Bi2Sr2CaCu2O8 single crystals grown from stoichiometric and nonstoichiometric melts

Single crystals of Bi2Sr2CaCu2O8 (2212) have been grown by self-flux technique using stoichiometric and non-stoichiometric melts of excess CuO and Bi2O3. Single-crystal and powder X-ray diffraction studies have been made on the grown crystals to confirm their single crystallinity and structure respectively. Resistivity and susceptibility measurements provide information on the superconducting nature of the crystals. The effects of fluxing agents and starting composition on surface morphology and superconducting properties have been discussed.

A new approach to crystal growth of Hg 1− xCd xTe by the travelling heater method (THM)

Crystal growth by the travelling heater method (THM) is reported using a source material preparation process that is different from all methods used before. Non-stoichiometric (Hg, Cd)Te melts were homogenized and quenched to prevent macroscopic segregation effects. Inclusions of excess Te were removed during a first THM pass, resulting in stoichiometric solid alloys with a shift of the mole fraction towards higher CdTe contents. The amount of the shift, dependent on the Te excess and on the equilibrium temperature of the first THM run, was calculated and taken into account in the preparation of x=0.22 and x=0.30 Hg 1- x Cd x Te single crystals. Source material ingots, as well as THM single crystals, were characterized with special emphasis of the compositional homogeneity. Radial as well as axial homogeneity are comparable with the best results on THM crystals reported so far. The described method can be used in growing all materials for which THM is possible. However, quantitative calculation requires the exact knowledge of the particular ternary phase diagram.

Growth of high-temperature superconductor crystals from flux

Crystallization of high-temperature superconductors was studied in La-Sr-Cu-O, Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O systems. Platelet crystals YBa2Cu3O6.5+x were obtained by spontaneous crystallization from homogeneous nonstoichiometric melts enriched in barium and copper oxide. La2−x Sr x CuO4 was prepared by slow cooling of melts enriched in copper oxide. Bi2(Sr,Ca) n + 1Cu n O y , (n=1; 2) was obtained by melting zone travelling. The crystals show transition to superconducting state atT=93 K, ΔT 0.2–0.5 K (Y, Ba cuprate),T=87 K, ΔT 2K (Bi, Sr, Ca-cuprate). La, Sr-cuprate single crystals obtained by Czochralski method did not show transition to superconducting state. For flux-grown crystalsT c was 5–26 K depending on the composition, growth and heat treatment. The short characterization of some accessory phases (Ba3Y2Cu3PtO10, Ca1.75Sr1.5Cu0.75PtO6, BaCuO2, Ba41Cu44O84Cl2) is reported.

Properties of photorefractive nonstoichiometric bismuth silicon oxide, BixSiO_15x_+2

Single crystals of bismuth silicon oxide (BSO) were Czochralski grown nonstoichiometrically from high-purity starting materials. Several series of three crystals were grown from nonstoichiometric melts starting with a bismuth oxide–rich melt. The intrinsic absorption shoulder of BSO was modified in amplitude with growth from nonstoichiometric melts. Two-beam coupling was used to determine photorefractive material parameters of the nonstoichiometric crystals. The data were analyzed with the two-beam coupling, two-carrier single multivalent ion model of the photorefractive effect. Parameters determined include the effective trap density, the hole–electron competition parameter, and the ratio of hole-to-electron absorption. Higher absorption correspond to a higher effective trap density and gain coefficient. The two-beam coupling gains, measured in the diffusion regime, are believed to be the largest reported for BSO, 0.70 cm−1, and are correlated to nonstoichiometric-growth-induced changes in the native defect concentrations.

Relationship between crystal growth and physical properties in the high temperature superconductor YBa 2Cu 3O 7-δ

The availability of single crystals of superconducting cuprates has made measurements of a number of physical properties of these highly anisotropic systems possible. However, in spite of major developments in crystallization techniques and a massive literature on the growth process, there is generally considerable variation in the critical temperature obtained from transport, magnetic or optical measurement. Certain persistent problems arising during the crystal growth process are examined and analyzed in an attempt to isolate factors influencing physical properties. In this context we review the growth of YBa 2 Cu 3 O 7-δ (123) crystals from nonstoichiometric melts, emphasizing impurity effects and complexities of the oxygenation process, using as an example the substitution of Ca in the 123 single crystals. Finally we look critically at the measurements themselves as a means of evaluating crystal quality.

Crystal growth and properties of the high-temperature superconductors of the RBa2Cu3O7−y and La2CuO4-types and bahaviour of these compounds at high temperatures

The RBa2Cu3O7−y compounds (R = Y, La to Lu) are found to melt incongruently within the region from 1080°C for R = La to 870°C for R = Lu. All they crystallize with large undercooling and with the formation of metastable intermediate phases. Single crystals of the RBa2Cu3O7−y compounds with R = Y, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and La2−xSrxCuO4 are grown from the nonstoichiometric melts, and their growth mechanisms and physical properties are studied.

Crystal growth of the new superconducting compound Pb0.6Bi3Sr2CaCu3Ox by the flux method

Single crystals of a new superconducting compound Pb 0.6 Bi 3 Sr 2 CaCu 3 O x were grown by the flux method from nonstoichiometric melts of Pb-Bi-Sr-Ca-Cu-O. Crystals with size up to 10 x 4 x 3 mm 3 were separated mechanically. Measurements of electrical resistivity and AC susceptibility showed T c ( onset) = 93 K and T c0 = 82 K. Powder X-ray diffraction patterns revealed that the crystals have orthorhombic structure with cell parameters a = 5.375 A ̊ , b = 12.202 A ̊ and c = 30.550 A ̊ .

Structural and electrical properties ofn-type bulk gallium arsenide grown from non-stoichiometric melts

The pulling of Si-doped GaAs from melts either Ga- or As-rich resulted in crystals having substantially different properties. The crystals grown from As-rich melts exhibited higher electron mobility with respect to those grown under Ga-rich conditions. These higher mobilities have been ascribed to more pronounced incorporation of silicon in donor sites and to lower density of acceptor-like complexes involving Si. The Ga-rich crystals were characterized by a lower concentration of dislocations. Such dislocation reduction is explained in terms of reduction of point defects (As interstitials) which can generate microloops and subsequently dislocations. An extensive TEM investigation further supports the idea of Asi as the point defect which more largely influences the structural properties of LEC GaAs. It is shown that the concentration and size of microdefects is drastically reduced when pulling from Ga-rich melts.

Preparation of single crystals of YBa2Cu3O7−x from nonstoichiometric melts

Studies on the single crystal growth of YBa2Cu3O7−x show that the growth conditions have not been optimised yet and they vary in many ways. Here we report the growth of single crystals of YBCO in the size range 0·5–1·2 mm from nonstoichiometric melts. We have made systematic variations in the flux composition (constituting CuO and BaCO3) in order to arrive at an optimum composition for consistently getting single crystals of size 0·5–1·2 mm. The tetragonal phase was confirmed by X-ray diffraction and single crystallinity by the Laue technique. Superconductivity was confirmed in oxygenannealed crystals.