2D Nucleation Research Articles

On the origin of basal stacking faults in nonpolar wurtzite films epitaxially grown on sapphire substrates

The microstructure of nonpolar heteroepitaxial wurtzite films (GaN and ZnO-based) is dominated by the presence of planar basal stacking faults (BSFs). In this paper, transmission electron microscopy studies of both GaN and ZnO nonpolar films oriented either (11–20) or (1–100) and grown on sapphire substrates, permit to propose and evaluate different mechanisms of BSFs formation. The main mechanism of formation of BSFs results from a Volmer Weber growth mode. The first stage of the growth is a 3D nucleation. The 3D nuclei are relaxed at least along one in-plane orientation. BSFs are created in the coalescence boundaries in order to compensate translations between neighbouring islands. BSFs are well adapted to compensate in-plane translations in the case of nonpolar films. In fact, their plane is perpendicular to the substrate surface and this orientation is similar to the orientation of the coalescence boundaries. Moreover, their displacement vector has a component parallel to the translation between islands and their formation energy is low. On the other hand, in the case of the polar growth, BSFs are not adapted to compensate in-plane translation as their plane is perpendicular to the coalescence boundaries.

Leidenfrost temperature related CVD-like growth mechanism in ZnO-TFTs deposited by pulsed spray pyrolysis

Zinc oxide thin films were prepared by pulsed spray pyrolysis (SP) from aqueous zinc acetate precursor solution. The deposition behavior of the solution with regard to the pool boiling curve of the solvent was investigated to gain insights of the deposition mechanism which leads to homogeneous and reproducible ZnO layers. The Leidenfrost effect was found to play an important role since the Leidenfrost temperature has to be exceeded to form high quality layers. It is concluded that 3D nucleation of ZnO nano-crystals takes place out of the gaseous phase by a CVD-like process on ITO as well as on SiO2 substrates. Crystal orientation and surface roughness of the zinc oxide layer are found to depend strongly on the substrate. An increasing grain size with film thickness is observed. These findings were utilized to fabricate and investigate the semiconducting properties of the films in ZnO thin film transistors (TFTs). The mobility exceeded 12cm2V−1s−1, the on-set was at 1V and the on–off current ratio was found to be higher than 108. Hence the morphology and the electrical parameters of the ZnO films deposited by pulsed spray pyrolysis from non-toxic aqueous zinc acetate solution above the Leidenfrost point show excellent properties for electronic applications.

Single photon emission from impurity centers in AlGaAs epilayers on Ge and Si substrates

We show that the epitaxial growth of thin layers of AlGaAs on Ge and Si substrates allows to obtain single photon sources by exploiting the sparse and unintentional contamination with acceptors of the AlGaAs. Very bright and sharp single photoluminescence lines are observed in confocal microscopy. These lines behave very much as single excitons in quantum dots, but their implementation is by far much easier, since it does not require 3D nucleation. The photon antibunching is demonstrated by time resolved Hanbury Brown and Twiss measurements.

Electrocrystallization mechanism of iron phosphate coatings onto mild steel electrode surfaces

Samples of mild steel AISI-SAE 1018, 1045 and 1080 were electrochemically phosphatised, both potentiodynamically and potentiostatically, in a concentrated H3PO4 aqueous solution. The conversion coating layers formed were characterised by means of scanning electron microscopy and energy-dispersive x-ray analysis. The kinetics and mechanisms of the electrocrystallization process of FePO4·2H2O(c) films were studied using the potential step technique. It was found that the mechanism governing the formation of FePO4·2H2O(c) anodic film was common for all the steel samples, namely, the overall electrodic process comprises four different simultaneous contributions to the overall current (j(t)). The individual contributions were: j ad(t), the current density contribution due to H3PO4 adsorption; j d(t), the current density involved during electrodissolution of iron of the steel electrode in the zones which were not covered yet by the FePO4·2H2O(c) film; j g(t), the current density contribution due to 2D nucleation and growth of an insoluble FePO4·2H2O(c) conducting adlayer; and j f(t), the current due to the growth of the passive layer induced by electrodissolution of iron metal and transport of the resulting Fe(II) ions through the FePO4·2H2O(c) film. However, each stage contributed to the overall process in a different manner, depending upon the carbon contents. The results from voltammetry indicated that it is possible to establish the potential at which the FePO4·2H2O(c) film begins to form and that this was also a function of the carbon contents.

Solubility and metastable zone width of DL‐tartaric acid in aqueous solution

AbstractSolubility and metastable zone width (MSZW) of DL‐tartaric acid (DL‐TA) in aqueous solution have been determined. Solubility of DL‐TA was measured in the temperature range from 0 to 50 °C at atmospheric pressure by means of the conventional polythermal method using Turbidity Monitoring Technique, which was verified by a gravimetric method. The dissolution enthalpy and entropy of DL‐TA were then calculated from the solubility data using van't Hoff equation. Two approaches was used to estimate the nucleation kinetics from the measured metastable zone width data, the self‐consistent approach and the approach based on 3D nucleation. In addition, the metastable zone width slightly decreases with increasing agitation rate and was independent of working volume.

Formation and growth mechanism of TiC terraces during self-propagating high-temperature synthesis from a FeTiC system

TiC terraces were prepared in situ through self-propagating high-temperature synthesis (SHS) reaction with 10wt.% FeTiC elemental powder mixtures. The formation and growth mechanism of TiC during the SHS process were explored. The results of combustion wave quenching experiment showed that the formation mechanism of TiC could be ascribed to the dissolution of C into FeTi melt and the precipitation of TiC from the saturated melt. The X-ray diffraction, field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analyses revealed that TiC terraces grew through the layer-by-layer mechanism along [100] direction, while thin TiC monolayer was formed by two-dimension (2D) nucleation growth mode.

Investigation on the crystal growth and characterisation of an organometallic non linear optical crystal – Tetrathiourea mercury tetrathiocyanato manganate

► Optimization of solvent, pH and temperature was carried out. ► Good transparent single crystal of TMTM of dimension 2.7 cm × 0.5 cm × 0.4 cm was grown. ► The layer growth pattern reveals the 2D growth mechanism. ► The stiffness constant shows that the bond strength in the complex is good. An organometallic complex tetrathiourea mercury tetrathiocyanato manganate (TMTM) was synthesised by two step chemical route. The influence of pH on the solubility, stability of the growth solution and growth of TMTM single crystal was studied. The bulk crystal of dimension 2.7 cm × 0.5 cm × 0.4 cm was grown at an optimized pH value of 3.0 by solvent evaporation method using water as a solvent. The structure and the crystallinity of the grown crystal were confirmed from single crystal XRD and powder XRD studies. The presence of thiourea and thiocyanate ligand and their coordination with the metal ions are confirmed from the IR studies. The grown single crystal of TMTM shows 69% transparency over the entire range of visible region with a cut-off wavelength of 362 nm. The SHG efficiency of TMTM was found to be 0.08 time that of MMTC. The complex exhibits good thermal stability up to 205 °C and it is non-hygroscopic in nature. The surface analysis was carried out by etching studies which shows a layer growth pattern revealing the 2D nucleation growth mechanism. From the Vickers hardness studies, the work hardening coefficient and the elastic stiffness constant were calculated.

Nucleation and Growth of Bismuth Electrodeposition from Alkaline Electrolyte

The early stages of bismuth (Bi) electrodeposition on glass carbon electrode from alkaline electrolyte were studied by cyclic voltammetry, chronoamperometry, scanning electron microscopy, atomic force microscopy and X-ray diffraction. The CV analysis showed that the electrodeposition of Bi was determined to be quasireversible process with diffusion controlled. The current transients for Bi electrodeposition were analyzed according to the Scharifker-Hills model and the Heerman-Tarallo model. It can be concluded that the nucleation and growth mechanism was carried out under a 3D instantaneous nucleation, which was confirmed by SEM analysis. The kinetic growth parameters were obtained through a nonlinear fitting. In addition, the Bi film obtaining at -0.86 V for 1 hour was of compact and uniform surface with good smoothness, small roughness and a very high purity. The Bi film were indexed to rhombohedral crystal structure with preferred orientation of (0 1 2) planes to growth.

An Unconventional Mode of Protein Crystal Growth: Case Study Xylanase

We report on the violation of the succession of crystal growth modes as a function of supersaturation. Typically, a crystal transitions from spiral growth to two-dimensional (2D) nucleation mediate...

Effects of a Forced Solution Flow on the Step Advancement on {110} Faces of Tetragonal Lysozyme Crystals: Direct Visualization of Individual Steps under a Forced Solution Flow

To grow protein crystals of better quality, there still exists an open question whether a solution flow should be suppressed or intentionally introduced. To obtain a comprehensive understanding of the effects of a solution flow, we directly measured the velocities of individual elementary steps (on {110} faces of tetragonal lysozyme crystals) under a forced solution flow, for the first time, by laser confocal microscopy combined with differential interference contrast microscopy. When we used crystals grown by a two-dimensional (2D) nucleation growth mechanism in a solution of commercial lysozyme (98.5% purity, from Seikagaku Co.), while increasing the solution flow rate, the step velocity decreased monotonically. We confirmed that this decrease in the step velocity with flow rate was due to the enhancement of the mass transfer of impurity (mainly covalently bonded dimer), by the observation using a lysozyme further purified (dimer was removed). In contrast, when we used crystals grown by a spiral growth mechanism in a commercial lysozyme solution, with increasing the flow rate, the step velocity increased and had the maximum at the flow rate of 10 μm/s, and then decreased monotonically. Also, the step velocity was 2–4 times higher than in the case of the 2D nucleation growth. These results demonstrate that the growth of spiral steps is less affected by impurities because the density of spiral steps is much higher than that of 2D island ones.

Dynamics, anisotropy, and stability of silicon-on-insulator dewetting fronts

We report on the anisotropy of solid state dewetting of straight fronts of Si(001)/SiO${}_{2}$ obtained by electron beam lithography. The $\ensuremath{\langle}110\ensuremath{\rangle}$ front is stable. It recedes with formation of a faceted rim that thickens in a layer-by-layer mode kinetically limited by 2D nucleation on the top facet of the rim. The front position and the height of the rim respectively obey $\ensuremath{\sim}{t}^{0.37\ifmmode\pm\else\textpm\fi{}0.03}$ and $\ensuremath{\sim}{t}^{0.38\ifmmode\pm\else\textpm\fi{}0.02}$ power laws. The $\ensuremath{\langle}100\ensuremath{\rangle}$ front is unstable. It breaks down then recedes at constant velocity. The $\ensuremath{\langle}100\ensuremath{\rangle}$ front instability is characterized by the formation of void fingers whose tips retract at constant shape with a kinetics limited by the mass transfer from the void tip to the Si fingers. The conditions of stability of a straight front are discussed in the light of recent theoretical models. New perspectives for controlling solid dewetting are opened.

Spiral-Mediated Growth Can Lead to Crystals of Higher Purity

The morphology and step kinetics of 2D islands and spiral hillocks of lysozyme crystals growing from purified and contaminated solutions were determined and compared. It was found that the morphology and step dynamics of spiral hillocks of lysozyme crystals are less affected by the presence of impurities in the growth solution as compared to steps generated by 2D nucleation. These observations could be satisfactorily explained considering the terrace exposure time of spiral hillocks (τsp) and 2D islands (τ2D) and the characteristic impurity adsorption time (τi). For lysozyme, overlapping time scales of terrace exposure and impurity adsorption exist and τi ≈ τsp < τ2D. Hence, when crystal growth is dominated by spiral hillocks, less impurities are adsorbed onto the crystal surface and a more pure crystal lattice is formed. Although spiral hillocks reduce the effect of impurities, they do play a significant role in the mechanism of step bunching.

Spiral Growth without Dislocations: Molecular Beam Epitaxy of the Topological InsulatorBi2Se3on Epitaxial Graphene/SiC(0001)

We report a new mechanism that does not require the formation of interfacial dislocations to mediate spiral growth during molecular beam epitaxy of Bi2Se3. Based on in situ scanning tunneling microscopy observations, we find that Bi2Se3 growth on epitaxial graphene/SiC(0001) initiates with two-dimensional (2D) nucleation, and that the spiral growth ensues with the pinning of the 2D growth fronts at jagged steps of the substrate or at domain boundaries created during the coalescence of the 2D islands. Winding of the as-created growth fronts around these pinning centers leads to spirals. The mechanism can be broadly applied to the growth of other van der Waals materials on weakly interacting substrates. We further confirm, using scanning tunneling spectroscopy, that the one-dimensional helical mode of a line defect is not supported in strong topological insulators such as Bi2Se3.

Growth kinetics on silicon facets during low-temperature crystallization from indium solution

Large-grained Si layers are attractive for cost-effective thin film solar cells. Our experiments using a low-temperature steady-state solution growth procedure for growing Si crystallites on glass show the dominance of Si{111} facets. Recesses observed in the centers of these facets suggest that growth starts at {100} vertices and/or {110} edges and continues rather by spreading of monatomic steps on the {111} facets than by nucleation on these planes. The aim of this paper is to explain the observed morphology by calculation and to find out if the growth can be improved by nucleation on {111} facets. Thermodynamic analysis shows that {100} facets are rough under our growth conditions. Therefore, the calculation of two-dimensional (2D) nucleation is focused on the flat {110}, and {111} facets. The dissolution enthalpy of silicon in indium is used to evaluate the ledge free energy of Wulff-shaped 2D nuclei on both planes. It results, that 2D nucleation takes place on {110} planes already at low degrees of supersaturation. In contrast, the saturated solution would have to be undercooled by 34.2K to explain observed size of grown crystallites by 2D nucleation on {111} planes. Such high degree of supersaturation does not occur in the experiments. Hence, the calculations support the assumed growth model.

Influence of organic additives on electrodeposition of Co–Cu alloys from sulphate bath

In this work, the authors report on the influence of additives on the onset of deposition, the current efficiency (CE) and the nucleation growth mechanism of Co–Cu alloys electrodeposited on n-Si(100) substrate from sulphate solution with an addition of sodium citrate (SC) and citric acid (CA). The study was carried out by means of cyclic voltammetry, chronoamperometry methods using the Scharifker–Hills model for the determination of nucleation and growth mechanism and some kinetic parameters for nucleation. The CV curves indicate that the deposition potential of Cu(II) is shifted to more negative potentials while additive anion is added in the solution. Also, the results show that the additives do not improve the CE. For all baths, electrodeposited Co–Cu alloy follows instantaneous nucleation and three-dimensional (3D) diffusion limited growth. The nucleation density in the solutions without additive and with SC increases exponentially with the potential whereas in solution containing CA additive, it is no longer possible to consider exponential increase, indicating the existence of a reaction in addition to the 3D nucleation mechanism in the Co–Cu electrodeposition process.

Epitaxial Ag(001) grown on MgO(001) and TiN(001): Twinning, surface morphology, and electron surface scattering

Epitaxial Ag(001) layers were deposited on MgO(001) in order to study electron surface scattering. X-ray reflection indicates 3D layer nucleation with a high rms surface roughness of 1.0 nm for a layer thickness d = 3.5 nm. X-ray diffraction shows that {111} twins form at d &amp;lt; 11 nm, followed by 2nd generation twinning for 11 nm &amp;lt; d &amp;lt; 120 nm. Increasing the growth temperature from 25 to 150 °C suppresses 2nd generation twinning and reduces the twin density by 2 orders of magnitude. In situ deposition of epitaxial 2.5-nm-thick TiN(001) underlayers prior to Ag deposition results in twin-free single-crystal Ag(001) with 10 × smoother surfaces for d = 3.5 nm. This is attributed to a better wetting on the higher energy TiN(001) than MgO(001) surface, resulting in the absence of 3D nuclei with exposed {111} facets, which facilitate twin nucleation. The twinned Ag/MgO layers have a higher resistivity ρ than the single crystal Ag/TiN layers at both 298 and 77 K, due to electron scattering at grain and twin boundaries. The ρ for single-crystal Ag layers increases with decreasing d, which is well explained with known surface scattering models and provides specularity parameters for the Ag-vacuum and the Ag-air interfaces of p = 0.8 ± 0.1 and 0.4 ± 0.1, respectively. A comparison with corresponding epitaxial Cu(001) layers shows that ρAg &amp;lt; ρCu for d &amp;gt; 50 nm, consistent with known bulk values. However, ρAg &amp;gt; ρCu for d &amp;lt; 40 nm. This is attributed to the larger electron mean free path for electron-phonon scattering and a correspondingly higher resistivity contribution from surface scattering in Ag than Cu. In contrast, air exposure causes ρAg &amp;lt; ρCu for all d, due to diffuse scattering at the oxidized Cu surface and the correspondingly higher Cu resistivity.

Thermodynamical analysis of polytype stability during PVT growth of SiC using 2D nucleation theory

We studied dependence of process parameters, such as temperature of a seed, pressure in a furnace and surface polarity of a substrate, on polytypes of SiC in a process of physical vapor transport. The analysis was based on a classical thermodynamic nucleation theory in conjunction with numerical results obtained from a global model. We investigated which polytype was more stable in the nucleation stage by a comparison of nucleation energies of each polytype. The results show that the formation of 4H-SiC was more stable than that of 6H-SiC when we used C-face SiC as a seed. Furthermore, the most stable polytype could change from 4H-SiC to 6H-SiC in a condition of higher supersaturation, with a condition of higher temperature of a seed and lower pressure in a furnace. Meanwhile, the formation of 6H-SiC was more stable than 4H-SiC when Si-face of a seed was used.

Electrodeposition of Ni, Sn and Ni–Sn Alloy Coatings from Pyrophosphate-Glycine Bath

In this work the electrodeposition of Ni, Sn and Ni–Sn alloy from the solution containing pyrophosphate and/or glycine has been investigated by cyclic voltammmetry (CV), potentiostatic pulse and polarization curve measurements on two substrates, Ni and GC. It has been shown that the process of Sn electrodeposition in pure pyrophosphate solution commences at the potential of about −0.90 V on both substrates being characterized by the formation of isolated 3D crystals and their further growth by the reduction of [Sn(Pyr)2]6− complex. On the GC surface Sn 3D crystals remain isolated, following 3D nucleation and growth mechanism which does not fit any of the theoretically predicted models. Ni–Sn alloy deposition in the solution containing both cations (Sn2+, Ni2+) and both anions (pyrophosphate and glycine) occurs by the same growth mechanism as pure Sn deposition by simultaneous reduction of [Sn(Pyr)2]6−, [Ni(Pyr)2]6−and/or [Ni(Pyr)3]− complexes at pH 8.0. Depending on the current density/potential of the Ni–Sn alloy coating deposition onto Ni electrode the composition of the flat and compact coatings varies in the range from 66 to 50 atom% Ni, i.e. 34 to 50 atom% Sn.

Electrodeposition of Zinc from Choline Chloride-Ethylene Glycol Deep Eutectic Solvent: Effect of the Tartrate Ion

The use of deep eutectic solvents (DES) for metal electrodeposition has become an area of interest in the recent years however the use of additives on the electrodeposition from deep eutectic solvents is still an unexplored area. In this study we describe the influence of the tartrate ion on the deposition mechanism of zinc and on the resultant morphology of the deposits. Electrochemical techniques were used to characterize the deposition process and scanning electron microscopy was used to study the deposit morphology. It is shown that the presence of potassium hydrogen tartrate in ethaline does not alter the generic voltammetric profile of zinc in solution but in contrast the presence of tartaric acid cause major modifications. From the analysis of the chronoamperometric transients a 3D progressive nucleation mechanism is proposed for the deposition of zinc in absence and in the presence of potassium hydrogen tartrate. In the presence of tartaric acid a 2D progressive nucleation mechanism is proposed for the initial part of the deposition then changing to a 3D progressive mechanism. The SEM images reveal that both additives change the morphological characteristics of the deposits. However the use of tartaric acid introduces a more dramatic change.

Copper electrodeposition on glassy carbon and highly oriented pyrolytic graphite substrates from perchlorate solutions

In present work, we analyzed the copper electrodeposition onto GCE (System I) and HOPGE (System II) from perchlorate solutions. The current density transients obtained from system I and II were well described through a kinetic mechanism that involves four different contributions: (a) a Langmuir type adsorption process, b) an electron transfer from Cu2+→Cu+, (c) a 3D nucleation limited by a mass transfer reaction and (d) a proton reduction process. It was observed that the values of the nucleation rate, the number of active nucleation sites were increased with the overpotential and they are bigger onto GCE in comparison with HOPGE.