Oriented growth of whiskers of AIIIBV compounds by VLS-mechanism

The oriented growth of GaAs, GaP, InAs and GaInAs whiskers on the same (GaAs, GaP) or different (InAs/GaAs, GaInAs/GaAs) substrates was studied. A detailed morphological study of GaAs whiskers on polar A(III), B(111 ) and non-polar (001), (011) substrates was performed. The growth conditions for ordered (perpendicular to substrate) growth on the A(111) and B(111 ) faces were determined. There were found discrete spectra of whisker systems on all substrates with the preferential growth of “arsenic” B{111 } faces. The dependence of the growth rate on the whisker diameter is typical for the vapour-liquid-solid (VLS) mechanism and is used for the determination of kinetic coefficients for polar faces. There was observed a periodic instability in growth of InAs and GaInAs whiskers.     

Crystal growth in c direction and crystallographic polarity in ZnO crystals

The c-axis polarity of ZnO crystals growing in the c direction was investigated for crystals grown by chemical reaction in the vapor, by VLS mechanism and by chemical vapor deposition. In the initial growth stage ZnO crystals nucleate both in the +c and -c directions in each type of growth. In growth by chemical reaction in the vapor, a higher growth rate of the crystal in the +c direction results in dominant growth of large crystals in this direction. The difference in the growth rate is attributed to a higher effective surface energy of the Zn atom surface than that of the O atom surface. On the other hand, in VLS growth long whiskers and needles grow in both c directions. This fact indicates that the difference in the growth rates between the two c directions decreases in VLS growth.     

A Calorimetric Measurement of Phase Transformation Kinetics in Solid Octaphenylcyclotetrasiloxane: Evidence for Nucleation as the Rate Determining Process

The kinetics of transformation from the metastable (at high temperature) solid 3-phase of octaphenylcyclotetrasiloxane to the stable 2-phase have been measured by a calorimetric technique. The results of the kinetic study and several related experiments are consistent with the view that nucleation is the rate determining process. Transformation time constants for large and small crystallites were found; the temperature dependence of each is in accord with the Turnbull-Fisher nucleation theory. From that theory γ, the interfacial energy between the 3- and 2-phases, has been found to be -3 ergs/cm², an order of magnitude smaller than that for typical liquid-solid interfaces. A study of the dependence of the transformation kinetics on crystallite size suggests that the phase change is surface nucleated.     

Growth mechanism of copper whiskers by the hydrogen reduction of cuprous iodide

A small quantity of cuprous iodide (less than 30 mg) was reduced by hydrogen at 650 or 700°C for various periods of time (15 sec to 4 min) and quenched to interrupt whisker growth. This was done in order to examine the nature of the growth of copper whiskers. Scanning electron microscopic observation and X-ray microanalysis revealed that almost all the whiskers quenched after the growth for 30 sec or less had cuprous iodide droplets at their tips and also the lateral surfaces were covered by thin cuprous iodide layers. The manner in which the copper crystals grow is similar to growth by the droplet mechanism or by the VLS mechanism in that the nutrient is supplied by a liquid drop on the tip of the whisker. Whiskers thicken during and after the growth in length. The whiskers quenched after the growth for 1 min or more had well-developed lateral surfaces and did not have the droplets at the tips. Preferential condensation and reduction were found to occur at the whisker edges. Based on the observed facts, a growth mechanism is presented. Secondary whisker growth on the whisker tips was also investigated.     

Growth of hollow Se–Te whisker crystals

On the basis of micro- and macro-morphological studies the mechanism responsible for the growth of Se–Te whiskers has been suggested. It has been shown that Sc–Te whiskers grow by layer growth mechanism in which screw dislocation is not the source of step but the growth proceeds by two-dimensional nucleation. This has been supported by the supersaturation data. The plausible mechanism for the growth of hollow whiskers has also been suggested.     

Hollow rhombohedral calcite crystals encompassing CO2 microcavities nucleated in solution

A new way for heterogeneous nucleation of crystals from the solution is described: 3D crystalline structure can nucleate and grow encompassing gaseous cavities, formed on solid substrates in the aqueous solution, so giving rise to crystal bubbles. The double surface of a crystal bubble is made, in this case, by the faces of the cleavage calcite rhombohedron separating the solution from the enclosed gas cavity. The faces thickness and the edges length amount to a few nanometers and to about 5 μm, respectively. Then, the surface-to-volume ratio is the highest obtained for 3D crystals, amidst the consulted literature. Bubbles’ shape is explained through the equilibrium properties of crystal–cavity and crystal–solution interfaces and in terms of growth mechanisms operating on the separate interfaces. The nucleation of the crystals around gas cavities can provide new insight on those mineralization phenomena where the gas phase participate in crystal growth. Potential applications are also envisaged concerning the nucleation of the gas cavities on peculiar templates which impose 1D (or 2D) periodicity to crystal bubbles, in order to obtain 1D and/or 2D photonic crystals.     

Evidence of VLS growth in CdTe film preparation by NH4Cl closed-tube CVD

By thermodynamical calculations of the vapour phase composition expected to occur in the closed — tube CVD of CdTe films on CdTe substrates, it is shown that a VLS growth mechanism should be active in the growth process when use is made of the thermal decomposition of NH₄Cl for producing the proper amount of HCl as a transport agent. Responsible of this VLS mechanism is the liquid tellurium, which is always present when the NH₄Cl concentration reaches values sufficiently high for the epitaxial deposition of CdTe films.     

Wachstum von Graphitkristallen in kohlenstoffhaltigen Eisenschmelzen bei höheren Überschreitungen

The rates of growth for the basic and the prismatic faces of graphite crystals are calculated in dependence of the supersaturation on the assumption that the faces at high supersaturations grow by twodimensional nucleation from an iron-carbon melt. Near the thermodynamic equilibrium prismatic faces grow more quickly than basic faces. At high supersaturations the ratio of the growth rate is reversed. The results are suitable to explain the different kinds of graphite crystals in cast iron.     

Vapor-phase growth of CdF<Subscript>2</Subscript> whiskers in the CdF<Subscript>2</Subscript>-GaF<Subscript>3</Subscript> system

Insulating CdF₂ whiskers of helical shape with a diameter from 10 to 30 μm and a length exceeding 7 mm have been grown for the first time. The conditions of their growth suggest that they grow through the classical vapor-liquid-solid mechanism.     

VLS Growth of ITO Whiskers Prepared by the Electron Shower Method

Indium tin oxide (ITO) whiskers were grown by VLS (vapour-liquid-solid) mechanism, using the electron shower method. The whiskers were grown above 200 °C, and the deposition rate was above 0.6 nm/s. The electron shower controlled the size of the whiskers, and the size was 30 nm in diameter and 600 nm in length. The whiskers grew along the substrate at t < 300 s, but grew in a direction perpendicular to the substrate at t > 300 s. When the ITO whiskers grown along the substrate were used as NO₂ gas sensor, the sensitivity was 340, and about 300 times higher than those of the whiskers grown in a direction perpendicular to the substrate and plate-like ITO crystallites.     

Crystallization-in-emulsion process of a melted organic compound: In situ optical monitoring and simultaneous droplet and particle size measurements

The crystallization-in-emulsion process allows the production of solid particles exhibiting specific features. Here, the batch crystallization process carried out by cooling a melted oil dispersed as an oil-in-water emulsion was studied. Two experimental set-ups allowing the in situ visualization of the nucleation and growth phenomena occurring in the dispersed liquid phase were developed. Observations in quiescent medium of motionless droplets having a diameter of few tens of micrometers showed that primary nucleation started on the inner surface of the droplets. The fast growth of the crystals consumed all the liquid contained within each droplet and was confined within each droplet by the oil–water interface. Solid polycrystalline particles similar in size to the parent droplets were produced. Dynamic tracking of the transient evolution of the size distributions of the two populations of droplets and solid particles during the cooling process in a stirred vessel was carried out using an in situ optical probe. It was shown that the droplets crystallized very progressively during cooling, starting with the largest droplets and ending with the smaller size droplets since the induction time of primary nucleation was dependent on droplet volume. In dilute conditions (1% wt% of dispersed phase) each droplet was converted into a single solid particle. Secondary nucleation based on inter-droplet collisions was not observed in these conditions.     

Direct observation of nucleation and early stages of growth of GaN nanowires

We report direct observations of the nucleation and early stages of growth of GaN nanowires. The nanowires were formed by exposing Au+Ga droplets to ammonia. The formation process was observed in situ, and controlled in real time using an environmental transmission electron microscope. Observations show that nuclei forms only at thin region of the Au+Ga, following only the VLS mechanism. The droplet–nanowire interface in the initial stages is multi-faceted, and become planar during the growth. Chemical and structural analysis shows that the resulting nuclei are GaN with the wurtzite structure.     

Lu5Ir4Si10 whiskers: Morphology,crystal structure,superconducting and charge density wave transition studies

Highly perfect single crystal whiskers of Lu₅Ir₄Si₁₀ were successfully grown out of the melt. Details of the surface and morphology of the whiskers are presented. X-ray diffraction data confirmed that the whisker structure has the same tetragonal P4/mbm space group symmetry as bulk single crystals with lattice parameters a=12.484(1) and c=4.190(2) Å. By means of field emission scanning electron microscopy, the morphology of the whiskers has been studied. Using a 4-circle X-ray diffractometer we found that whiskers grow along the c-axis direction and all side faces are oriented along the [1 1 0] direction. The mosaicity has been measured and is found to be almost perfect: below 0.15° along the c-axis. According to our transport measurements performed along the c-axis, the whiskers present a sharp superconducting transition at T_c=4.1 K and show a charge density wave (CDW) transition at 77 K. From the hysteresis of the temperature dependance of the electrical resistivity study, the CDW transition is found to be of first order.     

Morphological change from copper/α-copper-zinc whiskers to dendrites in zinc reduction growth

Cu whiskers grown by Zn reduction of CuCl where acetylene black is added show remarkable secondary growth. As a result, whiskers change their shape into dendrites. The change is a successive process of the three following stages: (I) Thickening of whiskers. (II) Formation of helical or “bellows-like” crystals. (III) Dendritic growth where the growth axis of the branches is 〈100〉. Regardless of the marked morphological change, crystals remain single crystals of Cu and α-Cu-Zn alloys. A VLS process of morphological change where a liquid covers the surface is described.     

Effect of organic impurities on the surface morphology and growth mechanism of KBP crystals (C8H5O4K)

The surface morphology of the (010) face of potassium biphthalate (KBP) crystals grown from aqueous solutions under the supersaturation ranging within 0.029–0.04 has been studied by the methods of optical and electron microscopies. It was revealed that the (010) surface has polygonal growth macrohills of the dislocation nature, small hillocks developing by the mechanism of successive two-dimensional nucleation, and numerous two-dimensional nuclei. The density of small hillocks (10⁴–10⁵ cm^?2) exceeds the dislocation density in KBP crystals by one to two orders of magnitude. It is shown that at low supersaturations, the (010) face grows simultaneously by the dislocation mechanism and the mechanism of successive two-dimensional nucleation. It is also established that the tangential velocity of growth-step motion on the (010) face increases in the presence of organic impurities. This effect can be used as one of the factors increasing the growth rates of crystal faces at low impurity concentrations (the so-called catalytic effect of impurities).     

Über die Züchtung von ZnS-, ZnSe- und ZnTe-Einkristallen unter Anwendung der chemischen Transportreaktion

In connection with investigations on the synthesis of ZnS, ZnSe and ZnTe single crystals by chemical transport using iodine as transport agent in sealed tubes the mechanism of mass-transport and the relationship between material-transport and growth process were studied. In this way the optimum conditions for growing single crystals were found. The dependence of the transport rate on the undercooling ΔT, on the diameter of the ampoule and on the pressure in the system is described. Relative large crystals in good quality could be propared by influencing the nucleation and the growth process by help of well defined transport rates. In all cases crystals with sphalerite structure only were obtained. The habit with the most common faces (110), (111) and (211) was found to be predominant. The dominant habit is responsive to variations in the experimental parameters. The crystal perfection have been determined by etch patterns and by X-ray topography.     

Ordered arrays of epitaxial silicon nanowires produced by nanosphere lithography and chemical vapor deposition

Gold dot arrays on (1 1 1) Si substrates obtained through nanosphere lithography (NSL) combined with sputtering and annealing in Ar at 1000 °C are used to catalyze vapor liquid solid (VLS) epitaxial growth of silicon nanowires (Si NWs) using chemical vapor deposition (CVD) with SiH₄ in Ar. The NWs grow primarily epitaxially on the underlying (1 1 1) Si wafer following the four independent 〈1 1 1〉 directions. The diameter distribution of the wires reflects the diameter distribution of the catalyst gold dot arrays and is therefore predictable. The wire length depends on the size of the gold catalyst for the same CVD parameters. The wire position is foreseeable within the limits of the pattern geometrical quality, but one-to-one growth of NWs to gold dots is not always observed, probably due to (very locally) the remaining presence of silicon oxide. Overall, this inexpensive patterning method for obtaining high-quality crystalline VLS Si NWs by CVD fulfills the requirements of many device applications, where patterning control, quality and reproducibility of the nanostructures are crucial.     

Investigation of 3C-SiC growth on Si(111) by vapor–liquid–solid transport using a SiGe liquid phase

The crystal growth of 3C-SiC onto silicon substrate by Vapor–Liquid–Solid (VLS) transport, where a SiGe liquid phase is fed with propane, has been investigated. Three sample configurations were used. In a preliminary approach, the VLS growth of SiC was conducted directly onto Si substrate using a Ge film as liquid catalyst. It led to the growth of a thick continuous SiC polycrystalline layer which was floating over a SiGe alloy located between the silicon substrate and the topping SiC layer. In the second configuration, a thin seeding layer of 3C-SiC grown by chemical vapor deposition (CVD) was used and the VLS growth was localized using a SiO₂ mask. The liquid phase was a CVD deposited SiGe alloy. The growth of a few hundred nanometers thick 3C-SiC epitaxial layer was demonstrated but the process was apparently affected by the presence of the oxide which was dramatically etched at the end. In the last configuration, the silicon substrate was patterned down to 10 μm and a thin seeding layer of 3C-SiC was grown by CVD onto this patterned substrate. The liquid phase was again a CVD deposited SiGe alloy. In this last configuration, the presence of epitaxial SiC was evidenced but it grew as trapezoidal islands instead of an uniform layer.     

Fluctuations in growth rate of gypsum crystals

Growth of needlelike gypsum crystals from highly supersaturated aqueous solutions has been studied by the methods of the optical and electron microscopies. The average growth rates of the end faces and their fluctuations are determined. It is shown that the fluctuations in the growth rates of end faces are described by the Fokker-Planck equation and that the end faces of gypsum crystals grow by the layer mechanism with the layer nucleation in the vicinity of one of the crystal vertices.