Growth Rate Anisotropy Research Articles

Study on anisotropy of growth rate of ice crystal in supercooled water

The purpose of this research was to investigate the ice growth of a single crystal in three dimensions. Three-dimensional pattern of ice crystal growth in supercooled water was observed using Mach–Zehnder spectro-interferometer. Temperature was varied from −0.3 to −1.6 °C. It was found that the ice crystal began to grow as a single crystal at the tip of the capillary tube and propagated freely in supercooled water. Time variation of the shape of dendrite on a–c plane was obtained. It was found that half parabola fits the shape very closely, and the coefficient of squared term, a, of a quadratic function was calculated. The coefficient, a varied in time but at quasi steady state it was found to be depending mostly upon the degree of supercooling. Furthermore, the growth velocity in c-axis at the flat surface was calculated from the thickness measured. It was found that the velocity in c-axis is independent of the degree of supercooling but depends upon time, in other words, the thickness in c-axis.

Angular dependence of lateral growth rate on the InP (1 1 1)A,B surface by liquid-phase epitaxy

The angular dependences of the lateral growth rate are determined as a function of growth temperature in liquid-phase epitaxy (LPE) of (1 1 1)A,B InP at T g=450–490°C. From the observations of the deformation of artificially made tables after epitaxy, the lateral growth rate has shown a maximum in the 〈1 1 2〉 direction steps on (1 1 1)B at T g=450°C. This indicated that the kink density was high in the 〈1 1 2〉 direction on the InP (1 1 1)B surface. It is also shown that the anisotropy of lateral growth rate decreases as the growth temperature becomes high on (1 1 1)B. And isotropic lateral growth rate was observed on the (1 1 1)A surface at T g=450°C. From these results, the kink-step structures during LPE on the {1 1 1} InP surface were discussed.

Transmission electron microscopy study of a defected zone in GaN on a SiC substrate grown by hydride vapor phase epitaxy

A defected zone (DZ) in the hydride vapor phase epitaxy (HVPE)-deposited GaN located near an interface with a SiC substrate was investigated by transmission electron microscopy for both plan-view and cross-section specimens. Predominant defects in the DZ are dislocations and stacking fault-type interfaces. Analysis of the defects by a moiré contrast and high-resolution imaging has suggested that the interfaces resulted from the process of coalescence and overgrowth of three-dimensional nucleated islands. The islands differ by a translation with respect to the reference (substrate) lattice, and therefore their coalescence results in the formation of domains separated by stacking fault-type boundaries. For the HVPE process used in depositing the specimens studied, we infer that the islands adopted the shape of {112̄l}-faceted truncated pyramids. Continued coalescence and overgrowth of the nonequivalent by translation domains result in a substructure of connected (0001) and {112̄0} stacking faults and threading dislocations. The density of these defects decreases with continued coalescence as the growth of GaN progresses, and thus determines the effective thickness of the DZ. We anticipate that the extent of such defected zones depends on the nucleation frequency and anisotropic growth rate of different crystallographic facets.

Anisotropy of lateral growth rate in liquid phase epitaxy of {0 0 1} InP

The angular dependences of the lateral growth rate are determined as the functions of growth temperature and growth time in liquid phase epitaxy of (0 0 1) InP at low growth temperature of 330–450 °C. From the deformation of artificially made tables after epitaxy, it is shown that the lateral growth rate has a strong anisotropy in [1 1 0] direction especially at low growth temperature ( T g<400 °C). This indicated that kink density was high in [1 1 0] direction on InP(0 0 1) surface. It is also shown that the anisotropy of lateral growth rate decreases as the growth temperature becomes high.

Computer simulations of the Stranski–Krastanov growth of heteroepitaxial films with elastic anisotropy

A three-dimensional finite element method is developed to simulate the surface morphological evolution during the Stranski–Krastanov heteroepitaxial growth. In the formulation, the surface evolves through surface diffusion driven by the gradient of the surface chemical potential, which includes the elastic strain energy, elastic anisotropy and surface energy. Surface condensation rate is assumed to depend on the difference between the surface chemical potential and the chemical potential of the vapor phase. Our simulations reveal that the self-assembly of quantum dots are strongly dependent on the variation of growth rate and elastic anisotropy strength. With appropriate choice of growth rate and elastic anisotropy strength, a relatively more uniform and regular quantum dot array can be obtained.

Strain in GaP/GaAs and GaAs/GaP resonant tunnelling heterostructures

We studied the morphology of GaP/(0 0 1)GaAs and GaAs/(0 0 1)GaP heterostructures grown by metal-organic vapour-phase epitaxy and found wire-like surface undulations elongated in the [1 1 0] direction. We attribute this elongation to anisotropic lateral growth rates in the [1 1 0] and [1 1 ̄ 0] directions, which are due to a different roughness of monolayer surface steps. In III–V materials grown by molecular beam epitaxy, such surface corrugations are usually elongated in [1 1 ̄ 0]. We explain this difference by the two growth methods having inverted ratios of lateral growth rates in [1 1 0] and [1 1 ̄ 0]. Resonant tunnelling diodes fabricated from the GaP/GaAs heterostructures showed very symmetric I– V characteristics. Their peak-to-valley ratio was limited to 2, most probably due to the corrugation of the GaP barriers.

Self-Assembly of Amphoteric Azopyridine Carboxylic Acids II: Aspect Ratio Control of Anisotropic Self-Assembled Fibers By Tuning the π–π Stacking Interaction

Abstract Two ways to control the macroscopic morphology of fibrous organizates of amphoteric azopyridine carboxylic acids by tuning the strength of π–π stacking among the component molecules are presented. The self-organization of the azopyridine carboxylic acids (1–5) from aqueous solutions is governed not only by carboxyl/pyridyl hydrogen bondings, but also by dipole–dipole and π–π stacking interactions. The level of the latter could be tuned by the substituent structure at the phenyl ring. As a result, whereas leaflet crystals were formed from non-substituted derivative 1, 3 with a propyl substituent gave fibrous assemblages, and needle-like assemblages were obtained from 2, 4, and 5 with methyl, s-butyl, and ethoxy substituents, respectively. The propyl-substitution is likely to enhance an anisotropic growth rate of the intermolecular hydrogen bonds due to efficient suppression of π–π stacking among aromatic cores, leading to the appearance of microfibers with the highest aspect ratio. The second way to control the organization morphology is based on “supramolecular copolymerization”, which is attained by mixing 1 and 3 to modulate the strength of the π–π stacking. The aspect ratio of fibrous materials was significantly influenced by the mixing ratio.

Localized Corrosion Growth Kinetics in AA2024 Alloys

The kinetics of localized corrosion in AA2024 alloys were studied using the foil penetration technique. The growth kinetics for localized corrosion in AA2024-T3 exhibited a strong anisotropy, being slower in the short transverse or plate through-thickness direction than in either the longitudinal or long transverse directions. This growth rate anisotropy depended on the grain size and aspect ratio, and was different for different forms of AA2024-T3. The attack was primarily intergranular corrosion (IGC) at potentials of −610 mV SCE and higher in 1 M NaCl. The IGC growth rate was almost independent of potential over the range from −610 to +500 mV SCE, which suggests that only a small amount of the anodic dissolution current at the active IGC tip escapes from the IGC crevice. At lower potentials the attack was pit-like, and the rate of growth decreased with decreasing potential. Artificial aging of AA2024 to the T8 temper changed the attack from sharp IGC to selective grain dissolution, but the growth kinetics at high potentials were not strongly affected. The addition of nitrate increased the breakdown potential, and therefore decreased the growth kinetics in that region. At high potential, the nitrate had no effect. Sulfate addition had no significant effect at any potential studied. © 2002 The Electrochemical Society. All rights reserved.

Comparative study on texture development of MgO and YSZ films grown by inclined substrate deposition technique

Texture development of MgO and YSZ films prepared by the inclined substrate deposition (ISD) technique was investigated. A similar fiber texture tilted toward the vapor mass transport flux was observed in common at the early stage of the ISD-grown films. The crystallographic plane to form the fiber texture was (1 0 0) for MgO and (1 1 0) for YSZ. The tilt angles agree well with the calculated values in view of maximum vertical growth rate after the so-called evolutionary selection in which the faster grains overgrow the slower ones. This simple rule was extended to consideration of the in-plane growth rate anisotropy based on the crystal growth shape and geometric configuration. The texture development of both MgO and YSZ films was explained well using the same mechanism.

Determination of angular dependence of lateral growth rate in liquid phase epitaxy of (0 0 1) InP

The angular dependences of the lateral growth rate are determined as the functions of growth temperature and growth time in liquid phase epitaxy of (0 0 1) InP at low growth temperature of 330–450°C. From the observations of the deformation of artificially made tables after epitaxy, the lateral growth rate has a strong anisotropy in [1 1 0] direction especially at low growth temperature ( T g<400°C). This indicated that kink density was high in [1 1 0] direction on InP (0 0 1) surface. It is also shown that the anisotropy of lateral growth rate decreases as the growth temperature becomes high.

Simulation of Anisotropic Particle Shape Development during 2D Transformation

Simulations were conducted of transformations involving anisotropic particles growing into a matrix of simultaneously growing isotropic particles. The ultimate shape development of the anisotropic particles, present in dilute concentration, was determined. The faster growing directions are found to impinge neighboring grains earlier in the transformation, thus creating a final particle morphology that is less eccentric than the shape would be if grown in isolation. The extent of this effect was determined as a function of intrinsic growth rate anisotropy and compared with analytical predictions.

Statistical model for intergranular corrosion growth kinetics

A statistical model has been developed to predict the anisotropy of intergranular corrosion (IGC) kinetics and effect of microstructure on IGC kinetics in high strength AA2024-T3 alloys. The methodology is to estimate the statistical distribution of minimum IGC path length that will be found through the thickness of a given thickness ST sample using the principle of order statistics. A brick-wall model is adopted for wrought Al alloy microstructure and different distributions of grain dimension are assumed for the purpose of the estimation of total IGC path length. Numerical stimulation using a gamma distribution model stimulated the anisotropy of localized corrosion in AA2024-T3 alloys, which quite agrees with the growth rate anisotropy experimentally determined by the foil penetration technique. Some limitations regarding the statistical model are discussed. The statistical model provides a new approach to predicting and quantifying localized corrosion kinetics on basis of the alloy microstructure.

Anisotropic threshold stress intensity factor, KIH and crack growth rate in delayed hydride cracking of Zr-2.5Nb pressure tubes

Anisotropic threshold stress intensity factor, KIH and crack growth rate in delayed hydride cracking of Zr-2.5Nb pressure tubes

Observation of the early stage of insulin crystallization by confocal laser scanning microscopy

It is demonstrated that high resolution confocal laser scanning microscopy (CLSM) is a powerful tool for in situ observation and analysis of protein crystal growth. CLSM is used to study the early crystallization stage of Des-ThrB30 human insulin in aqueous solution, under conditions known to lead to monoclinic crystals. A modified batch crystallization method for CLSM purposes is applied which allows the growth behavior of crystallites to be studied in reflected light. A few hours after the start of the experiment, microcrystallites of characteristic shapes (mainly prismatic and pyramidal) are observed, the number of which strongly depends on the concentration of higher insulin aggregates in the initial solution. From direct observation as well as from model calculations we conclude that for solute concentrations up to about 3.5-times the saturation value, growth starts from few active insulin precipitate particles while 3D nucleation is neglegible for observation times up to 24 h. The anisotropic growth rates of monoclinic, prismatic crystallites are measured along the long edge of the cover face and perpendicular to the latter. A simultaneous crossover to signifcantly higher growth rates is found when the crystallite size reaches about 2 μm. The higher growth rates are connected with the appearence of striations. We argue that this growth rate crossover is caused by an increased 2D nucleation rate at the edges and corners, which finally results in bunching of steps simultaneously spreading over adjacent crystallite faces.

Investigation of texture development on MgO films prepared by inclined substrate deposition with electron-beam evaporation

Texture development of MgO films prepared by the electron-beam evaporation process using the so-called inclined substrate deposition technique was investigated. Thickness dependence on the degree of texturing showed that the in-plane alignment was developed faster with increase of the inclination angle, which was correlated with MgO(1 0 0) tilt angles. The mechanism of the in-plane alignment of the MgO films was discussed considering a directional flow of adatoms on the growing (1 0 0) surface of MgO, in-plane growth rate anisotropy, and three-dimensional overgrowth. A growth model for in-plane alignment was proposed, which is based on Schweobel effect, two-dimensional nucleation growth, and geometric configuration. The model could explain the experimental results at least qualitatively.

Control of the anisotropic growth rates of oxide single crystals in floating zone growth

We have designed an anisotropic heating floating zone (AHFZ) method to control the grown shape of some crystals, in which facets are observed due to their anisotropic growth rate. The ellipsoidal cross-section of a cylindrical crystal grown under an isotropic lamp power was found to change to a circular one when the crystals were grown under an anisotropic lamp power. This effect was observed for both a congruent melting compound, CuGeO 3, and an incongruent melting compound, Sr 14Cu 24O 41. When the crystals of CuGeO 3 were grown under a highly anisotropic lamp power, the facet plane changed to another crystallographic plane. These results suggest that the AHFZ method is useful in controlling the facets of the grown crystals.

Crystalline growth rate and microstructure in YBaCuO thin films

The influence of the anisotropy of the crystalline growth rate on the microstructure and morphology of YBaCuO thin film deposited by laser ablation is studied by means of transmission electron microscopy. The results obtained from high resolution lattice imaging and large angle convergent beam diffraction, together with the investigations of the thin film surface morphology show the strong influence of the different crystalline growth rates on both the internal stress present in the YBaCuO film and the roughness of the outer surface. These results should be of prime importance for the superconducting properties. This is an essential in tailoring these thin films for device applications.

Preparation and properties of MnSi 1.7 on Si(001)

The most rich silicon silicides of manganese, the group of higher manganese silicides (HMS), have the composition MnSi x with x in the range from 1.67 to 1.75. This material group with a tetragonal crystal structure shows semiconducting electronic properties, with promising application in thermoelectric devices. This paper reports the structural and morphological properties of HMS prepared by a reactive deposition process on Si(001) under UHV conditions. X-ray diffraction shows, for all samples grown at substrate temperatures ranging from 400 to 750°C, the growth of HMS only. Scanning electron microscopy and Rutherford backscattering spectrometry show a transition from film growth to island growth with increasing substrate temperature. A detailed analysis of the XRD spectra shows a change of the texture of HMS at the transition of the sample morphology. The results are discussed on the basis of anisotropic growth rates for differently oriented grains of HMS.

Autodeformation bending of gypsum crystals grown under the conditions of counterdiffusion

The dynamics and kinetics of growth-induced bending of gypsum crystals grown from solution have been studied. Crystallization was performed by the method of chemical reaction under the conditions of component counterdiffusion. It is established that autodeformation bending occurs in the [001] direction at the growth front and is caused by cationic impurities. The crystal curvature depends on the anisotropy of growth rate and increases at lower supersaturations. The mechanism of growth-induced crystal bending is suggested which takes into account the heterometry stresses providing the appearance of a bending moment at the growth front.

Observation of anisotropic initial growth nucleation in liquid phase epitaxy of InP

Liquid-phase epitaxial growth of InP was performed at constant growth temperature by the temperature difference method under controlled vapor pressure on InP substrates. Growth temperature was kept constant from 550°C to 280°C. The as-grown surface morphology was investigated by Nomarski interference optical microscope and electron microscope. The anisotropic initial growth nuclei are investigated on {0 0 1},{1 1 1}A,B and {1 1 0} oriented InP substrates. This anisotropic behavior is discussed in view of the anisotropy of lateral growth rate due to the preferential surface migration and sticking at specific kinks and steps on the surface.