Showing papers on "Crystal growth published in 1995"

Solution-Liquid-Solid Growth of Crystalline III-V Semiconductors: An Analogy to Vapor-Liquid-Solid Growth

Abstract: Until now, micrometer-scale or larger crystals of the III-V semiconductors have not been grown at low temperatures for lack of suitable crystallization mechanisms for highly covalent nonmolecular solids. A solution-liquid-solid mechanism for the growth of InP, InAs, and GaAs is described that uses simple, low-temperature (≤203°C), solution-phase reactions. The materials are produced as polycrystalline fibers or near-single-crystal whiskers having widths of 10 to 150 nanometers and lengths of up to several micrometers. This mechanism shows that processes analogous to vapor-liquid-solid growth can operate at low temperatures; similar synthesis routes for other covalent solids may be possible.

Understanding and control of nucleation, growth, habit, dissolution and structure of two‐ and three‐dimensional crystals using `tailor‐made' auxiliaries

Abstract: Tailor-made auxiliaries for the control of nucleation and growth of molecular crystals may be classified into two broad categories: inhibitors and promoters. Tailor-made inhibitors of crystal growth can be used for a variety of purposes, which include morphological engineering and etching, reduction of crystal symmetry, assignment of absolute structure of chiral molecules and polar crystals, elucidation of the effect of solvent on crystal growth, and crystallization of a desired polymorph. As for crystal growth promoters, monolayers of amphiphilic molecules on water have been used to induce the growth of a variety of three-dimensional crystals at the monolayer-solution interface by means of structural match, molecular complementarity or electrostatic interaction. A particular focus is made on the induced nucleation of ice by monolayers of water-insoluble aliphatic alcohols. The two-dimensional crystalline structures of such monolayers have been studied by grazing incidence X-ray diffraction. It has become possible to monitor, by this method, the growth, dissolution and structure of self-aggregated crystalline monolayers, and indeed multilayers, affected by the interaction of solvent molecules in the aqueous subphase with the amphiphilic headgroups, and by the use of tailor-made amphiphilic additives

Laser‐induced crystallization phenomena in GeTe‐based alloys. I. Characterization of nucleation and growth

Abstract: The laser‐induced crystallization behavior of GeTe‐based amorphous alloy thin films has been quantitatively studied by local reflection measurements with a focused 780 nm laser. The use of multiple laser pulse sequences enables the nucleation rate and crystal‐growth speed to be separately deduced, allowing the compositional variation of both these processes to be followed. This not only gives detailed information on the crystallization mechanism, but also allows the fine tuning of phase change alloy compositions for use in erasable optical recording. The differences between the as‐deposited and melt‐quenched amorphous phases are also discussed. In particular, it is shown that the crystallization speed of the as‐deposited layer can differ by over an order of magnitude from that of the melt‐quenched amorphous layer. The as‐deposited state can, however, be transformed into a modified amorphous state equivalent to that obtained by melt quenching a previously crystalline layer. This allows the determination of...

Prediction of Crystal-Growth Morphology Based on Structural-Analysis of the Solid-Fluid Interface

Abstract: PREDICTING the shape of growing crystals is important for industrial crystallization processes. The equilibrium form of a crystal can be determined unambiguously from a consideration of the surface free energies of the various crystallographic faces {hkl}1, but the growth morphology is determined by kinetic factors which are harder to predict. This morphology depends on the relative growth rates R rel hkl of the crystal faces. Several theories have been advanced2,3 to relate R rel hkl to geometric or energetic characteristics of the surfaces {hkl}, but these have met with limited success in predicting the crystal morphologies observed. Here we present a theoretical approach to the problem in which R rel hkl is determined by quantities that are accessible either from kinetic models or from computer simulations of the solid-fluid interface. The important parameters controlling the growth rate are the energy required to create a step at the crystal surface and the free-energy barrier for an adsorbed solute molecule to be incorporated into the crystal. Both can be related to the mole fraction of adsorbed solute molecules in dynamic equilibrium with those in the crystal surface. When this approach is applied to the case of urea crystals grown from aqueous solution, we predict a needle-like shape which is consistent with experimental observations.

Science and technology of crystal growth

Abstract: Science and Technology of Crystal Growth: An Introduction J. P. van der Eerden, O. S. L. Bruinsma. 1: Classical and Statistical Thermodynamics. 1.1. Thermodynamics and Phase Diagrams - Fundamentals and Tools for Crystal Growth G. Krabbes. 1.2. Atomic Models for Crystal Growth J. P. van der Eerden. 1.3. The Syncrystallization of Thianaphthene and Naphthalene, an Exercise in Thermodynamic Phase Diagram Analysis H. A. J. Oonk. 1.4. From Thermoelasticity to Surface Melting T. H. M. van den Berg, J. P. van der Eerden. 2: Crystallization Concepts. 2.1. Nucleation D. Kashchiev. 2.2. Topics in Crystal Growth Kinetics A. A. Chernov, H. Komatsu. 2.3. Lattice Growth Models J. I. D. Alexander. 2.4. Macroscopic Transport Processes during the Growth of Single Crystals from the Melt J. J. Derby. 3: Single Crystals and Epitaxy. 3.1. Large-Scale Numerical Modeling of the Bulk Crystal Growth from the Melt and Solution J. J. Derby, S. Kuppurao, Q. Xiao, A. Yeckel, Y. Zhou. 3.2. Vapour Growth G. Krabbes. 3.3. Advanced Epitaxial Growth Techniques for III-V Materials I. Moerman, P. Demeester. 4: Crystal Shape. 4.1. Morphology of Crystals: Past and Future P. Bennema. 4.2. Modulated and Quasicrystals H. Meekes. 4.3. Modelling the Habit Modification of Molecular Crystals by the Action of 'Tailor-Made' Additives G. Clydesdale, K. J. Roberts. 4.4. Morphological Instability: Dendrites, Seaweed and Fractals K. Kassner. 5: Mass Crystallization. 5.1. Mass Crystallization, Number Balances and Size Distributions J. Garside. 5.2. Crystallizers G. Hofmann. 5.3. Melt Suspension Crystallization M. Matsuoka. 5.4. Melt Layer Crystallization J. Ulrich, J. Bierwirth. 5.5. Secondary Nucleation G. M. van Rosmalen, A. E. van der Heyden. 6: Crystals Grown from Large Growth Units. 6.1. Crystallization in Colloidal Suspensions J. S. van Duijneveldt, H. N. W. Lekkerkerker. 6.2. Polytopism and Inorganic Crystal Growth and Reactivity A. Baronnet. 6.3. Polymer Crystallization G. Goldbeck-Wood. 6.4. Principles of Crystal Growth in Protein Crystallization A. A. Chernov, H. Komatsu. 7: Surface Structure. 7.1. Some Common Pathologies in Step Growth: Impurities and Surface Reconstruction W. J. P. van Enckevort. 7.2. Characterization of Crystal Growth Processes Using Synchrotron X-Ray Techniques K. J. Roberts. 7.3. Optical and Scanning Probe Microscopy K. Tsukamoto. Subject Index.

Substrate holder for MOCVD

Abstract: A substrate holder employed for MOCVD and supporting a wafer on which crystal growth proceeds includes a molybdenum holder body, a GaAs polycrystalline film with a flat surface grown on a part of the surface of the molybdenum holder body where the wafer is absent, and an InP polycrystalline film grown on the GaAs polycrystalline film. Each of the polycrystalline films is grown to a thickness of 0.3 μm or more at a temperature higher than the epitaxial growth temperature of 575° C. During the MOCVD process, the emissivity of the molybdenum substrate holder is stable at a value near the emissivity of the wafer on the substrate holder and, therefore, the decomposition ratio of PH3 gas on the substrate holder is stable at a value near the decomposition ratio on the wafer, whereby any variation of the incorporation ratio of P atoms in the grown InGaAsP, i.e., a variation of the composition of the InGaAsP, is reduced and run-to-run variations of the composition of the grown crystal are reduced.

The crystal structure of the high-temperature polymorph of α–hexathienyl (α–6T/HT)

Abstract: α-hexathienyl (α–6T) is a highly promising material for application in thin film transistor devices. Recently, record high mobilities, together with record high current on/off ratios, have been reported.1 Thus far, structural information on this exciting material is sketchy. The crystal structures of several such hexamers have been investigated, but only with powder samples, since the crystal growth has proven exceedingly difficult.2-5 Powder Rietveld refinements on these materials are severely hampered by the large number of overlapping reflections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of α–6T (α–6T/HT), as determined from a single-crystal structure analysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Extended Huckel theory (EHT) band structure calculations show that α–6T/HT is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at Γ. The conduction and valence bands both show a remarkable degree of dispersion along X and Y for a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic superconductors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as κ−(ET)2 Cu(NCS)2.

Mechanisms of growth for protein and virus crystals

Abstract: The growth of six protein and virus crystals was investigated in situ using atomic force microscopy. Most of the crystals grew principally on steps generated by two-dimensional nucleation on surfaces though some grew by development of spiral dislocations. Apoferritin grew by a rarely encountered mechanism, normal growth, usually associated only with melt or vapour phase crystallization. Cubic crystals of satellite tobacco mosaic virus (STMV) grew, at moderate to high levels of supersaturation, by the direct addition of three-dimensional nuclei followed by their rapid normal growth and lateral expansion, a mechanism not previously described to promote controlled and reproducible crystal growth from solutions. Biological macromolecules apparently utilize a more diverse range of growth mechanisms in their crystallization than any previously studied materials.

Surfactant mediated crystal growth of semiconductors.

Abstract: The influence of surfactants on semiconductor thin film growth is studied by means of a mesoscopic model combined with first principles calculations. We introduce a new kinetic mechanism that explains how surfactants induce layer-by-layer growth. The experimentally observed high density of 2D islands is a natural consequence of the chemical passivation of step edges, as well as flat surfaces, by the surfactant. In heteroepitaxial growth, we take strain effects into account, which leads to layer-by-layer growth at low temperatures and three-dimensional growth at high temperatures, in agreement with experiments.

Laser‐induced crystallization phenomena in GeTe‐based alloys. II. Composition dependence of nucleation and growth

Abstract: The laser‐induced crystallization behavior of GeTe‐based amorphous alloys has been measured with a novel multipulse laser technique. This enables the composition dependence of the nucleation rate and crystal growth speed to be independently followed. Two types of crystallization are investigated. The first involves single‐phase crystallization of quaternary alloys based on Ge39Sb9Te52, in which the composition dependence of nucleation and growth is followed as Se, S, Sn, and Si are included. Both the nucleation rate and crystal‐growth speed vary exponentially with the composition, and a correlation is found between crystallization behavior and bond strengths. The second involves multiphase crystallization in the GeSbTe ternary system. It is shown that the observed variations in crystallization behavior primarily arise from the composition dependence of nucleation rather than crystal growth. The implications of this finding for the importance of long range diffusion during crystallization in the GeSbTe system are discussed.

Molecular‐dynamics study of the synthesis and characterization of a fully dense, three‐dimensional nanocrystalline material

Abstract: A molecular‐dynamics simulation method is described that permits space‐filling, fully dense three‐dimensional nanocrystalline materials to be grown by crystallization from the melt. The method is illustrated by computer synthesis of an eight‐grain polycrystal of Cu with a grain size of 43 A. At the beginning of the simulation, eight small pre‐oriented single‐crystal seeds are embedded in the melt which is subsequently cooled below the melting point to enable crystal growth under an applied external pressure. The fully relaxed nanocrystalline material contains large perfect‐crystal regions separated by well‐defined grain boundaries, most of which have approximately the same width and energy. In spite of the rather small number of grains in the simulation cell, the thermal expansion of the material is practically isotropic, and almost identical to that of the perfect crystal. The elastic moduli are also almost isotropic and are somewhat lower than in the coarse‐grained material. The material exhibits a low‐temperature anomaly in the specific heat.

Investigation of virus crystal growth mechanisms by in situ atomic force microscopy

Abstract: For the first time, virus crystal growth dynamics and morphology have been investigated in real time on the nanometer scale. Individual monomers on the (111) face of cubic satellite tobacco mosaic virus (STMV) crystals were resolved and used to determine crystal packing. Growth of STMV proceeded by two- and three-dimensional nucleation to formed ``stacks'' of islands. No dislocations were observed. Small islands provided an estimate of critical radius size and the free energy of the step edge, \ensuremath{\alpha}. Step advancement rates were used to determinate the kinetic coefficient \ensuremath{\beta}. Images illustrate mechanisms for defect incorporation and suggest factors that limit growth rate and uniformity.

Shape instability during precipitate growth in coherent solids

Abstract: The effect of coherent elastic strain on shape instabilities during growth of a single precipitate in an elastically anisotropic cubic system is examined. A general phenomenological field approach to phase transformation kinetics is employed. Emphasis has been given to understanding the formation of concave interfaces of a coherent γ′ particle in the Ni-based superalloys. It is shown by a two-dimensional computer simulation that the infinite-range and highly anisotropic strain-induced interaction results in a shape transition from circle to faceted square with 10 habits and rounded corners at the early stage of growth. Then the divergence of solute atom supply at the corners enhances their growth which results in a concave morphology. Vanishing of the supersaturation (transition to the coarsening stage) causes a diffusional relaxation of the concave interfaces back into flat ones, indicating that the concave square is a nonequilibrium shape. A semi-quantitative criterion for the formation of concave shapes is derived. The concentration profile inside the misfitting particle is found to be nonuniform during its growth and coarsening.

Mechanisms of protein crystal growth: An atomic force microscopy study of canavalin crystallization.

Abstract: The evolution of surface morphology and step dynamics during growth of single crystals of the protein Canavalin and of the cubic satellite tobacco mosaic virus crystals (STMV) have been investigated by in situ atomic force microscopy. These two crystals were observed to grow by very different mechanisms. Growth of Canavalin occurs on complex vicinal hillocks formed by multiple, independently acting screw dislocations. Small cluster were observed on the terraces. STMV on the other hand, was observed to grow by 2D nucleation of islands. No dislocations were found on the crystal. The results are used to determine the growth mechanisms and estimate fundamental materials parameters. The images also illustrate the important mechanism of defect incorporation and provide insight to the processes that limit the growth rate and uniformity of these crystals.

Controlled growth of microporous crystals nucleated in reverse micelles

Abstract: THE development of new methods for nucleating and growing microporous crystals has made available new framework structures and morphologies for these technologically important materials1. These approaches have included solution-based synthesis2 and the use of simple organic structure-directing agents3 and complex organic assemblies such as liquid-crystal phases4,5. Here we show that the growth of microporous zincophosphate6–8 with the sodalite structure can be controlled by preparing the crystals from reactants included within the interior aqueous phase of reverse micelles dispersed in an organic solvent. The growth of inorganic phases in reverse micelles has been exploited previously for the preparation of monodisperse oxide9, semiconductor10 and metal particles11. In this study, we introduce the two inorganic components—zinc and phosphate ions—in separate micelles, so that crystallization is controlled by the collision and exchange kinetics of the surfactant structures. Moreover, the surfactant–water interface provides the site for crystal nucleation, favouring initial nucleation at the (111) and/or (110) crystal faces and subsequent growth of the zinco-phosphate crystals by deposition along the {100} faces. The growth process is ultimately interrupted by sedimentation when the crystals grow large enough, producing a precipitate of micro-crystals several hundred nanometres in size. Our results indicate that this approach can provide a means of controlling the morphology as well as the size of growing crystals.

Protein purification by bulk crystallization: the recovery of ovalbumin.

Abstract: Crystallization is used industrially for the recovery and purification of many inorganic and organic materials. However, very little is reported on the application of bulk crystallization for proteins. In this work, ovalbumin was selected as a model protein to investigate the feasibility of using bulk crystallization for the recovery and purification of proteins. A stirred 1-L seeded batch crystallizer was used to obtain the crystal growth kinetics of ovalbumin in ammonium sulfate solutions at 30 degrees C. The width of the metastable region, in which crystal growth can occur without any nucleation, is equivalent to a relative supersaturation of about 20. The bulk crystallizations were undertaken within this range (using initial relative supersaturations less than 10) and nucleation was not observed. The ovalbumin concentration in solution was measured by UV absorbance and checked by crystal content measurement. Crystal size distributions were measured both by using a Malvern Mastersizer and by counting crystals through a microscope. The crystal growth rate was found to have a second-order dependence upon the ovalbumin supersaturation. While there is no discernible effect of ammonium sulfate concentration at pH 4.90, there is a slight effect at higher pH values. Overall the effect of ammonium sulfate concentration is small compared to the effect of pH, for which there is a 10-fold increase in the growth rate constant, k(Gsigma) over the range pH 4.6-5.4. To demonstrate the degree of purification which can be achieved by bulk crystallization, ovalbumin was crystallized from a solution containing conalbumin (80,000 Da) and lysozyme (14, 600 Da). After one crystallization and a crystal wash, ovalbumin crystals were produced with a protein purity greater than 99%. No contamination by the other proteins was observed when using overloaded sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) stained with Coomassie blue stain and only trace amounts of lysozyme were observed using a silver stain. The presence of these other proteins in solution did not effect the crystal growth rate constant, k(Gsigma). The study demonstrates the feasibility of using bulk crystallization for the recovery and purification of ovalbumin. It should be readily applicable to other protein systems.

Nucleation and anisotropic crystalline growth of polyethylene under shear

Abstract: The crystallization of polyethylene was observed during shear experiments in isothermal conditions. The nucleation and the crystalline growth rates were measured from a microscopic observation of the growing morphologies. An unusual formation and development of row nuclei was observed throughout the experiment, followed by anisotropic growth. Three main growth rates were measured with respect to the main directions of the process. All these growth rates are enhanced differently by the shear rate. This result is interpreted first by an increase of the equilibrium melting temperature emanating from an entropy loss due to the chain orientation, second the anisotropy of growth is discussed as an effect of chain orientation with respect to the growth front of lamellae and of a local flow different from macroscopic shear-flow. An overall kinetic equation with shear effect has been proposed for this polyethylene and tested in a stimulation of crystallization during injection moulding of a polyethylene disc. The shear effect on the crystallization is necessary to predict the crystallization temperature and the thickness of polymer crystallized during the filling stage.

Biologically Induced Reduction in Symmetry: A Study of Crystal Texture of Calcitic Sponge Spicules

Abstract: Organisms can exert a remark- able degree of control over crystal growth. One way of achieving this is by the ad- sorption of specialized macromolecules on specific planes of the growing crystals. With continued growth of the crystal, the macromolecules are incorporated inside the crystal bulk. Their presence does not change the crystal structure, but creates discontinuities in the perfect lattice. Here we study in detail three unusual cases of reduction in symmetry at the level of crys- tal domain shapes, induced by this con- trolled intercalation. We examined sponge spicules, which are single crystals of Mg- bearing calcite. They were specifically chosen for this study, because their mor- phologies do not reflect the hexagonal symmetry of calcite. Their crystal textures (coherence lengths and angular spreads) were characterized by high-resolution X- ray diffraction with well-collimated syn- chrotron radiation. The results are com- pared to analogous studies of synthetic calcite and Mg-bearing calcite. In all the selected spicules reduction in symmetry is observed in the coherence lengths among symmetry-related crystallographic direc- tions. The reconstructed shapes of the do- mains of perfect structure closely match the specific spicule morphologies. The synthetic crystals show no such reduction in symmetry. Although the manner by which such exquisite control is achieved is not known, we envisage it involving a combination of oriented nucleation with either physical or stereochemically driven adsorption.