Lateral Growth Rate Research Articles

First Direct Observation of Impurity Effects on the Growth Rate of Tetragonal Lysozyme Crystals under Microgravity as Measured by Interferometry

The normal growth rates R and apparent step velocities (lateral growth rates of a spiral hillock) V of tetragonal hen egg-white lysozyme (HEWL) crystals were for the first time measured by Michelson interferometry in the international space station (as part of the NanoStep project) using commercialized HEWL samples containing 1.5% impurities. A significant increase in V under microgravity was confirmed compared to step velocities Vstep on the ground, while a decrease in R was also confirmed compared to that in the purified solution under microgravity as expected. Because of exact measurement of growth rates, kinetic analyses of R were conducted as a function of supersaturation, σ (σ ≡ ln(C/Ce), where C is the concentration; Ce is the solubility), using a spiral growth model and a two-dimensional (2D) nucleation growth model. For both models over a wide range of σ, R in the impure solution was significantly lower than that in the purified solution. The degree of the suppression of impurity effects was also...

Te-doping of self-catalyzed GaAs nanowires

Tellurium (Te)-doping of self-catalyzed GaAs nanowires (NWs) grown by molecular beam epitaxy is reported. The effect of Te-doping on the morphological and crystal structure of the NWs is investigated by scanning electron microscopy and high-resolution transmission electron microscopy. The study reveals that the lateral growth rate increases and axial growth rate decreases with increasing Te doping level. The changes in the NW morphology can be reverted to some extent by changing the growth temperature. At high doping levels, formation of twinning superlattice is observed alongside with the {111}-facetted sidewalls. Finally, the incorporation of Te is confirmed by Raman spectroscopy.

Study of the inhibition effect of two polymers on calcium carbonate formation by fast controlled precipitation method and quartz crystal microbalance

In this paper, the inhibition efficiency of two inhibitors, namely poly(acrylic acid-co-maleic acid) and polyaspartic acid, towards calcium carbonate scaling was evaluated using fast controlled precipitation (FCP) method and electrochemical quartz crystal microbalance (EQCM). FCP method gave some insight to the calcium carbonate precipitation in solution, whereas EQCM was used to study the calcium carbonate formation on a metallic substrate. It has been shown that these polymers were efficient to delay or to prevent nucleation/growth process, depending on their concentration. Moreover they significantly decreased the crystal growth rate. The FCP method showed that these inhibitors were very efficient at low concentrations (4mgL−1) when no precipitation occurred. In addition, EQCM showed that the surface coverage of deposits on a substrate was reduced by the presence of these inhibitors at very low concentration (4mgL−1). Scanning electronic microscopy and X-ray diffraction showed that the presence of these polymers modified the morphology of calcium carbonate crystal. In order to model nucleation/growth process of calcium carbonate on surface, mass–time transients were interpreted using a 3D model based on a nucleation following a Poisson law associated to vertical and lateral growth rates.

Hydrate growth at the interface between water and pure or mixed CO2/CH4 gases: Influence of pressure, temperature, gas composition and water-soluble surfactants

The morphology and growth of gas hydrate at the interface between an aqueous solution and gaseous mixtures of CO2 and CH4 are observed by means of a simple experimental procedure, in which hydrate formation is triggered at the top of a sessile water drop by contact with another piece of gas hydrate and the ensuing hydrate growth is video-monitored. The aqueous solution is either pure water or a solution of a nonionic or anionic surfactant at low concentration (in the 100–1000ppmw range). In agreement with previously published data, hydrates formed from pure water and aqueous solutions of non-ionic surfactant grow rapidly as a low-permeable polycrystalline crust along the water/gas interface, which then inhibits further growth in a direction perpendicular to the interface. Lateral growth rates increase strongly with subcooling and CO2 content in the gas mixture. Similar lateral growth rates, but varying morphologies, are observed with the non-ionic surfactants tested. In contrast, the two anionic surfactants tested, sodium dodecyl sulfate (SDS) and dioctyl sodium sulfosuccinate (AOT), promote in the presence of CH4 (but not in the presence of CO2) a rapid and full conversion of the water drop into hydrate through a ‘capillary-driven’ growth process. Insights are given into this process, which is observed with AOT for an unprecedented low concentration of 100ppmw.

Influence of atmospheric stability on wind-turbine wakes: A large-eddy simulation study

In this study, large-eddy simulation is combined with a turbine model to investigate the influence of atmospheric thermal stability on wind-turbine wakes. The simulation results show that atmospheric stability has a significant effect on the spatial distribution of the mean velocity deficit and turbulence statistics in the wake region as well as the wake meandering characteristics downwind of the turbine. In particular, the enhanced turbulence level associated with positive buoyancy under the convective condition leads to a relatively larger flow entrainment and, thus, a faster wake recovery. For the particular cases considered in this study, the growth rate of the wake is about 2.4 times larger for the convective case than for the stable one. Consistent with this result, for a given distance downwind of the turbine, wake meandering is also stronger under the convective condition compared with the neutral and stable cases. It is also shown that, for all the stability cases, the growth rate of the wake and wake meandering in the vertical direction is smaller compared with the ones in the lateral direction. This is mainly related to the different turbulence levels of the incoming wind in the different directions, together with the anisotropy imposed by the presence of the ground. It is also found that the wake velocity deficit is well characterized by a modified version of a recently proposed analytical model that is based on mass and momentum conservation and the assumption of a self-similar Gaussian distribution of the velocity deficit. Specifically, using a two-dimensional elliptical (instead of axisymmetric) Gaussian distribution allows to account for the different lateral and vertical growth rates, particularly in the convective case, where the non-axisymmetry of the wake is stronger. Detailed analysis of the resolved turbulent kinetic energy budget in the wake reveals also that thermal stratification considerably affects the magnitude and spatial distribution of the turbulence production, dissipation, and transport terms.

Effects of bath composition on the morphology of electroless-plated Cu electrodes for hetero-junctions with intrinsic thin layer solar cell

The morphology of an electroless-plated Cu electrode was investigated as a function of bath composition. To enhance the selectivity of Cu electrode deposition on the surface of an indium tin oxide layer, a Ti/Cu multi-layer was deposited as a Cu electrode seed layer by physical vapor deposition, and then electroless plating was performed using various complexing agents and a surfactant. The degree of selectivity was effectively influenced by the type of complexing agent. The electroless plating solution containing N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine (THPED) as complexing agent showed excellent selective growth of the Cu electrode as compared to the solution containing ethylenediaminetetraacetic acid. Even though THPED led to better selective growth of the electroless-plated Cu electrode, the aspect ratio of electrode lateral growth was about 2.7 times that of vertical growth. By adding a nonionic surfactant, the ratio between vertical growth rate and lateral growth rate was improved about 4.6 times. The Cu–THPED electroless plating with nonionic surfactant provided a drastic decrease in lateral growth rate, compared with the Cu–THPED electroless plating bath excluding nonionic surfactant. The Cu–THPED solution including nonionic surfactant is a promising composition of electroless plating solution for the clear selective plating of Cu electrodes on hetero-junctions with intrinsic thin layer solar cells.

A criterion for cracking during solidification

Cracking during solidification is a serious defect in castings and welds. When solidification shrinkage and thermal contraction of the semisolid and its surrounding solid are obstructed, tensile deformation can be induced in the semisolid to cause cracking along grain boundaries that are not fed with sufficient liquid. A criterion for cracking was derived, focusing on events occurring at the grain boundary including separation of grains from each other, lateral growth of grains toward each other, and liquid feeding between grains. An index for the susceptibility of an alloy to cracking during solidification was also proposed, that is, |dT/d(fS1/2)| near (fS)1/2=1, where T is temperature and fS the fraction solid in the semisolid. The index affects: (a) the lateral growth rate of two neighboring grains toward each other to bond together to resist cracking, and (b) the length of the grain-boundary liquid channel through which feeding has to occur to resist cracking. The index was verified with experimental data in casting and welding of Al alloys.

Factors controlling hydrate film growth at water/oil interfaces

The methane hydrate film growth kinetics on the surface of water droplets suspended in n-octane was investigated. When the water droplet was suspended in n-octane unsaturated with methane, the hydrate formation behavior on its surface showed a hydrate curtain caused by the continuous forming, moving, and dissociating of hydrate crystals. When n-octane was saturated with methane, the lateral growth rates of the hydrate film were measured under different pressures. The mass transfer of methane molecules near the film front was suggested as the control step of lateral growth. For hydrate film growth in thickness, massive apophyses occurred and developed in the outer surface of the hydrate film, indicating an outward water transfer through the film. The sinking of the hydrate film caused by leaking of the inner water was observed. These two phenomena indicated the contribution of the outward growth of water through the hydrate film in thickening growth.

Selective area growth of Ga‐polar GaN nanowire arrays by continuous‐flow MOVPE: A systematic study on the effect of growth conditions on the array properties

Site‐controlled growth of GaN nanowires (NWs) on GaN‐on‐sapphire templates with a patterned SiN mask has been carried out by metalorganic vapor phase epitaxy using a continuous‐flow growth mode. A low V/III ratio compared to that used for GaN layer growth, combined with low precursor flow rates for both Ga and N precursors, has been used to promote the nanowire growth on Ga‐polar substrates. The lateral growth rate, that is, perpendicular to the c‐axis, could be further controlled using appropriate growth temperatures and H2/N2 ratios. Besides, the influence of the pattern geometry on the nanowire aspect ratio and size homogeneity has been addressed.

Understanding of the mechanism of pulsed NH3 growth in metalorganic chemical vapor deposition

We have studied the mechanism of pulsed NH3 growth in metalorganic chemical vapor deposition by investigating the influence of interruption duration of NH3, growth temperature, pressure and NH3 flow rate on the growth behavior of GaN selective area growth (SAG). The essential mechanism of pulsed NH3 is to create a short-term metal-rich growth condition, thus facilitating the growth of {101¯1} facets. Optimized pulse duration, growth temperature, pressure and NH3 flow rate balance the amount of Ga atoms on the surface and create an appropriate metal-rich condition, resulting in an enhanced lateral growth rate by dramatically increasing the growth rate of {101¯1} facet. We have achieved hexagonal GaN plates with flat c-plane top surfaces and {101¯0} vertical sidewalls on substrates with a fill factor of about 0.1%. The understanding of pulsed NH3 growth technique will significantly promote the preparation of crystals by SAG or epitaxial lateral overgrowth, especially on substrates with a very low fill factor.

Metallic Nanofilms on Single Crystal Silicon: Growth, Properties and Applications

The metal–silicon thin−film system is not isostructural and furthermore exhibits pronounced interdiffusion and chemical reactions. Therefore the growth of metallic films on silicon leads to a high concentration of defects in the film, especially at its substrate interface. The material also contains stress and a transition layer consisting of melts or compounds (silicides). We have considered theoretical viewpoints and reviewed experimental data on the growth and properties of metallic nanofilms (including multilayered ones) on silicon, and also provided a brief review of their applications. The films consist either of atomic−sized, quabquantum sized and quantum sized layers. We have suggested a low temperature film growth technology based on freezing growing layers during deposition by maintaining a low temperature of the substrate and using an atomic beam with a reduced heat power. The technology uses a specially shaped deposition system in which the distance between the source and the substrate is comparable to their size or smaller. Furthermore, we use a special time sequence of deposition that provides for a reduced substrate surface temperature due to greater intervals between deposition pulses. This growth method of atomically thin films and multilayered nanofilms excludes interdiffusion between the layers, reduces three−dimensional growth rate and relatively increases lateral layer growth rate.

Macroscopic Approach to the Nucleation and Propagation of Foreign Polytype Inclusions during Seeded Sublimation Growth of Silicon Carbide

The nucleation and propagation of foreign polytypes during seeded sublimation growth of silicon carbide is addressed on a macroscopic footing, using a coupled experimental and numerical simulation approach. Experiments are conducted in a contactless growth geometry for different seed substrates with varying polarity, miscut angle, polytype, and crystal shape (concave and convex). The interface shape evolution is tracked by periodic nitrogen marking, which also allows the measurement of lateral and normal growth rates at any time during growth. Classical 2D nucleation theory is included in modeling the full process, and this is compared to experiments. We demonstrate that the occurrence of a foreign polytype is linked to the combined effects of (i) the presence of the natural {0001} facets and (ii) the minimization of 2D nucleation energy. Areas with a high probability of foreign polytype nucleation can thus be predicted. Once formed, the progress or disappearance of foreign inclusions is directly related ...

Selective area growth of GaN nanostructures: A key to produce high quality (11–20) a-plane pseudo-substrates

Selective area growth of GaN nanostructures was performed on (11-20) a-plane GaN/sapphire templates. The dominant lateral growth rate along the in-plane c-direction produces the coalescence of the individual nanostructures into a continuous film. Photoluminescence measurements show the appearance of donor-bound and free exciton emissions in individual nanostructures, pointing towards an improvement of the material quality as compared to the original template. Upon nanostructures coalescence a decrease of the full width half maximum value, down to 2 meV, is observed. These results reveal the high quality of the coalesced film, opening the way to fabricate high quality, non-polar GaN pseudo-substrates.

Position-Controlled Hydrothermal Growth of Periodic Individual ZnO Nanorod Arrays on Indium Tin Oxide Substrate

Periodic individual ZnO nanorod arrays were grown on indium tin oxide (ITO) conductive substrates buffered with a ZnO seed layer. Electron beam lithography was used to pattern the pre-coated poly(methyl methacrylate) (PMMA) layer into a periodic aperture template. Because of the constraint effects of this template, the ZnO nanorod arrays were strictly vertical to the substrate with a controlled pitch. To avoid damage to the electrical properties of conductive substrate, a low temperature (90 °C) hydrothermal route was adopted for the ZnO nanorod synthesis. The nutrient concentration played an important role in obtaining individual ZnO nanorod arrays. Several thin ZnO nanorods could merge into an individual ZnO nanorod at each aperture due to the increased lateral growth rate in ultra-high concentration (0.15 M) nutrient solution. Under the coaction of PMMA aperture template and hydrothermal growth, periodic individual ZnO nanorod arrays on nonsingle crystalline ITO substrates were successfully fabricated. The electrical properties of these periodic ZnO nanorods also were investigated, the current–voltage curve indicated that a barrier-free Ohmic contact formed between ZnO nanorod and ITO interface. The adjustable pitch and high conductance features of the periodic ZnO nanorod arrays made them high performance photoelectric devices.

Structural evolution of platinum thin films grown by atomic layer deposition

The structural properties of Pt films grown by atomic layer deposition (ALD) are investigated with synchrotron based x-ray scattering and x-ray diffraction techniques. Using grazing incidence small angle scattering, we measure the lateral growth rate of the Pt islands to be 1.0 Å/cycle. High resolution x-ray diffraction reveals that the in-plane strain of the Pt lattice undergoes a transition from compressive strain to tensile strain when the individual islands coalescence into a continuous film. This transition to tensile strain is attributed to the lateral expansion that occurs when neighboring islands merge to reduce their surface energy. Using 2D grazing incidence x-ray diffraction, we show that the lattice orientation becomes more (111) oriented during deposition, with a sharp transition occurring during coalescence. Pt ALD performed at a lower deposition temperature (250 °C) is shown to result in significantly more randomly oriented grains.

Effects of growth interruption, As and Ga fluxes, and nitrogen plasma irradiation on the molecular beam epitaxial growth of GaAs/GaAsN core–shell nanowires on Si(111)

We investigate fundamental issues on the growth of GaAs/GaAsN core–shell heterostructure nanowires (NWs) by plasma-assisted molecular beam epitaxy. A Ga catalyst crystallizes during growth interruption at a high As pressure, and afterwards the growth dominantly progresses mainly increasing the NW diameter, thereby forming a wire shell. The shell diameter increases linearly depending on growth time and group III flux, similarly to the growth mechanism of planar layers. The lateral growth rate is 0.19 times lower than the growth rate of planar GaAs on a (100) substrate. At a substrate temperature 570 °C, nitrogen incorporation is inefficient in the shell layer. At a substrate temperature of 430 °C, the nitrogen is effectively introduced under continuous plasma irradiation during the growth of the GaAsN shell, resulting in the introduction of nitrogen within the shell estimated up to about 0.5%.

Growth response of an early successional assemblage of coralline algae and benthic diatoms to ocean acidification

The sustained absorption of anthropogenically released atmospheric CO2 by the oceans is modifying seawater carbonate chemistry, a process termed ocean acidification (OA). By the year 2100, the worst case scenario is a decline in the average oceanic surface seawater pH by 0.3 units to 7.75. The changing seawater carbonate chemistry is predicted to negatively affect many marine species, particularly calcifying organisms such as coralline algae, while species such as diatoms and fleshy seaweed are predicted to be little affected or may even benefit from OA. It has been hypothesized in previous work that the direct negative effects imposed on coralline algae, and the direct positive effects on fleshy seaweeds and diatoms under a future high CO2 ocean could result in a reduced ability of corallines to compete with diatoms and fleshy seaweed for space in the future. In a 6-week laboratory experiment, we examined the effect of pH 7.60 (pH predicted to occur due to ocean acidification just beyond the year 2100) compared to pH 8.05 (present day) on the lateral growth rates of an early successional, cold-temperate species assemblage dominated by crustose coralline algae and benthic diatoms. Crustose coralline algae and benthic diatoms maintained positive growth rates in both pH treatments. The growth rates of coralline algae were three times lower at pH 7.60, and a non-significant decline in diatom growth meant that proportions of the two functional groups remained similar over the course of the experiment. Our results do not support our hypothesis that benthic diatoms will outcompete crustose coralline algae under future pH conditions. However, while crustose coralline algae were able to maintain their presence in this benthic rocky reef species assemblage, the reduced growth rates suggest that they will be less capable of recolonizing after disturbance events, which could result in reduced coralline cover under OA conditions.

Analysis of crystal orientation in AlN layers grown on m-plane sapphire

Our study reports on the microstructure of AlN layers grown on m-plane sapphire by metal organic vapor phase epitaxy. We have found that AlN can nucleate with three different orientations on the m-plane sapphire surface: semipolar (112̄2) and (11̄03) as well as m-plane (11̄00). Depending on the growth conditions, i.e. V/III ratio, the differently oriented crystallites exhibit different lateral and vertical growth rates. At a low V/III ratio of 626 the vertical growth rate of semipolar (112̄2) AlN regions is much lower than that of the (11̄03) and (11̄00) oriented grains, which results in an almost complete lateral overgrowth of the (112̄2) AlN oriented regions. In contrast, a high V/III ratio of 1043 leads to the formation of uniform semipolar (112̄2) AlN layers. Nevertheless, the formation of differently oriented AlN crystallites could not be suppressed completely. These randomly appearing crystallites still show a high vertical growth rate and lead to a deterioration of the surface morphology.

Growth kinetics and mass transport mechanisms of GaN columns by selective area metal organic vapor phase epitaxy

Three-dimensional GaN columns recently have attracted a lot of attention as the potential basis for core-shell light emitting diodes for future solid state lighting. In this study, the fundamental insights into growth kinetics and mass transport mechanisms of N-polar GaN columns during selective area metal organic vapor phase epitaxy on patterned SiOx/sapphire templates are systematically investigated using various pitch of apertures, growth time, and silane flow. Species impingement fluxes on the top surface of columns Jtop and on their sidewall Jsw, as well as, the diffusion flux from the substrate Jsub contribute to the growth of the GaN columns. The vertical and lateral growth rates devoted by Jtop, Jsw and Jsub are estimated quantitatively. The diffusion length of species on the SiOx mask surface λsub as well as on the sidewall surfaces of the 3D columns λsw are determined. The influences of silane on the growth kinetics are discussed. A growth model is developed for this selective area metal organic vapor phase epitaxy processing.

Study of Surface Morphologies of On-Axis 6H-SiC Wafer after High-Temperature Etching and Epitaxial Growth

Gas etching and homoepitaxial growth on a nominally on-axis 2-inch 6H-SiC (0001) Si-face were studied. Regular steps with one unit cell height and complex pattern with facets and steps were observed after gas etching in the central region and edge region, respectively. The homoepitaxial growth shows that the complex (facets & steps) pattern expands and merges during the growth to bring on a rough epi-layer surface in the edge region. The steps with one unit cell height on the substrate split into steps with bilayers on the epilayer. The different lateral growth rates of <11-20>- and <1-100>-orientated steps make the width of steps orientated to <11-20> much larger than the ones orientated to <1-100>.