Nucleation Of Two-dimensional Islands Research Articles

High-performance infrared photodetector based on InAs/GaAs submonolayer quantum dots grown at high temperature with a (2×4) surface reconstruction

We studied the impact of surface reconstruction on the performance of infrared photodetectors containing InAs/GaAs submonolayer quantum dots (SMLQDs) grown by molecular beam epitaxy. Adjusting the substrate temperature before InAs deposition allowed us to move between the conventional c(4×4) reconstruction of the GaAs(001) surface, observed at low growth temperatures, and the (2×4) reconstruction which can only be stabilized at higher sample temperatures than those commonly used to deposit InAs. Photodetectors based on SMLQDs grown on such a (2×4) surface reconstruction outperformed those grown on a c(4×4) reconstruction and provided specific detectivities as high as 8.3×1011 cm Hz1/2 W−1 at 12 K. This increase in performance is due to the nucleation of true two-dimensional InAs islands which are the building blocks of SMLQDs and can only form on a (2×4)-reconstructed GaAs(001) surface.

2D island nucleation controlled by nanocluster diffusion during Si and Ge epitaxy on Si(1 1 1)-(7 × 7) surface at elevated temperatures

We have studied two-dimensional (2D) island nucleation at the initial stages of Si and Ge epitaxial growth near monatomic steps and on wide (up to 10 μm) terraces on the Si(1 1 1)-(7 × 7) surface in 500–750 °C interval by in situ ultrahigh vacuum reflection electron microscopy (UHV REM) and ex situ atomic force microscopy (AFM). Concentration of 2D islands nucleated on the terraces and depletion zone width near the steps have been measured as functions of deposition rate and substrate temperature. We have shown that GeSi and Si nanoclusters are dominating diffusion species providing surface mass transport on the Si(1 1 1)-(7 × 7) surface during Ge and Si epitaxial growth at temperatures above 500 °C and 600 °C, respectively; the activation energy of GeSi nanocluster diffusion has been first estimated at 1.3–1.4 eV. The kinetics of 2D island nucleation and growth near the steps (both for Si and Ge epitaxy) and on the wide terraces (for Ge epitaxy) is limited by surface diffusion only, and critical nucleus consists of i ≫ 1 nanoclusters.

On the performance of vertical MoS2 nanoflakes as a gas sensor

Despite their potential applications, a limited number of studies for synthesizing vertical MoS2 nanoflakes especially via CVD have been reported so far, which generally involve tedious complex- and/or multi-step growth processes. In this study, direct synthesis of vertical MoS2 nanoflakes grown on the SiO2/Si substrate during a rapid sulfidation process by CVD method has been reported. Material characterization was performed using Raman spectroscopy, XRD and FE-SEM. The XRD results indicated the dominant phase of 2H–MoS2 within the synthesized layers. The characteristic distance between the two dominant peaks of E12g and A1g in the Raman spectra confirms the multi-layered structure for grown nanoflakes. Based on the experimental results, the growth mechanism has been explained considering nucleation and growth of two-dimensional islands, followed by coalescence of these islands. Subsequently, in the final stage, standing nanoflakes grow vertically. The vertical MoS2 nanoflakes film forms n-channel for back-gated FET gas sensor, of which gas sensing performance towards ethanol and methanol vapors have been studied. These structures with an increased number of edge sites presented high and fast responses to ethanol and methanol at 1 and 10 ppm concentrations and showed significant potential and promising application as a gas sensor transistor.

Evolution of Micropits on Large Terraces of the Si(111) Surface during High-Temperature Annealing

The transformation of micropits on large terraces of the Si(111) surface containing no vicinal atomic steps has been investigated by in situ ultrahigh-vacuum reflection electron microscopy upon thermal annealing of the substrate in the range of 1200–1400°C. A procedure for the formation of micropits on large terraces of the Si(111) surface with the application of focused-ion-beam (Ga+) technology has been proposed. It has been found that the micropit decay kinetics varies upon reaching the critical radius Rcrit, which is caused by the activation of nucleation of two-dimensional vacancy islands on the micropit bottom. A theoretical model describing variations in the lateral sizes of the micropit both before and after reaching Rcrit has been proposed. Based on analysis of the found temperature dependence of the nucleation frequencies of two-dimensional vacancy pits on the micropit bottom, the effective energy of nucleation of a vacancy island has been determined to be 4.1 ± 0.1 eV.

Step Kinetics Dependent on the Kink Generation Mechanism in Colloidal Crystal Growth

Nucleation and growth of two-dimensional (2D) islands on a terrace are the dominant growth mechanisms of colloidal crystals whose particle interaction is attractive. The step velocity, vstep, of the 2D islands at various area fractions, ϕarea, and polymer concentrations, Cp, has been investigated. At low Cp (weak attractive interaction between particles), there is a nearly linear relationship between vstep and ϕarea, whereas it is parabolic for high Cp (strong attraction). Depending on the Cp, two manners of kink generation at a step are observed: 1D nucleation under strong attraction and association of mound formation under weak attraction. As a result of mound formation, abundant kinks are created at steps, resulting in a linear relationship between vstep and ϕarea, whereas the relationship is parabolic for step propagation associated with 1D nucleation. Though the kink site is the most favored site for particles to be incorporated into crystals, weak attractive interaction makes the step front site an ...

Refinement of Nucleation Theory for Vapor–Liquid–Solid Nanowires

Features of nucleation of two-dimensional islands in vapor–liquid–solid nanowires determine the nanowire growth rates, morphologies, and crystal phases and thus are central for controlling the physical properties of such structures. Herein, we present a development of nucleation theory that accurately accounts for the droplet depletion effect on the island formation energy. We show that the latter modifies itself to a curve with a maximum at a critical size and a minimum at a stationary size, which determines the size to which the island can actually grow without a refill from vapor. This effect is general and should occur in any nucleation from an isolated nanoparticle. For III–V nanowires, the depletion is extremely sensitive to the concentration of the group V atoms in the droplet and is more pronounced for smaller nanowire radii and lower concentrations. Whenever the stationary size is smaller than that of the facet, the droplet depletion decreases the amplitude of the chemical potential oscillations....

Nucleation of two-dimensional islands on Si (111) during high-temperature epitaxial growth

The process of two-dimensional island nucleation at the surface of ultra large Si (111) during hightemperature epitaxial growth is studied by in situ ultrahigh-vacuum reflection electron microscopy. The critical terrace size D crit, at which a two-dimensional island is nucleated in the center, is measured in the temperature range 900–1180°C at different silicon fluxes onto the surface. It is found that the parameter D crit 2 is a power function of the frequency of island nucleation, with the exponent χ = 0.9 ± 0.05 in the entire temperature range under study. It is established that the kinetics of nucleus formation is defined by the diffusion of adsorbed silicon atoms at temperatures of up to 1180°C and the minimum critical nucleus size corresponds to 12 silicon atoms.

Evolution of planar defects during homoepitaxial growth of β-Ga2O3 layers on (100) substrates—A quantitative model

We study the homoepitaxial growth of β-Ga2O3 (100) grown by metal-organic vapour phase as dependent on miscut-angle vs. the c direction. Atomic force microscopy of layers grown on substrates with miscut-angles smaller than 2° reveals the growth proceeding through nucleation and growth of two-dimensional islands. With increasing miscut-angle, step meandering and finally step flow growth take place. While step-flow growth results in layers with high crystalline perfection, independent nucleation of two-dimensional islands causes double positioning on the (100) plane, resulting in twin lamellae and stacking mismatch boundaries. Applying nucleation theory in the mean field approach for vicinal surfaces, we can fit experimentally found values for the density of twin lamellae in epitaxial layers as dependent on the miscut-angle. The model yields a diffusion coefficient for Ga adatoms of D = 7 × 10−9 cm2 s−1 at a growth temperature of 850 °C, two orders of magnitude lower than the values published for GaAs.

Miscut dependent surface evolution in the process of N-polar [formula omitted] growth under N-rich condition

The evolution of surface morphology during the growth of N-polar (0001¯) GaN under N-rich conditions is studied by kinetic Monte Carlo (kMC) simulations for two substrates miscuts 2° and 4°. The results are compared with experimentally observed surface morphologies of (0001¯) GaN layers grown by plasma-assisted molecular beam epitaxy. The proposed kMC two-component model of GaN(0001¯) surface where both types of atoms, nitrogen and gallium, attach to the surface and diffuse independently shows that at relatively high rates of the step flow (miscut angle <2°) the low mobility of gallium adatoms causes surface instabilities and leads to experimentally observed roughening while for low rates of the step flow (miscut 4°), smooth surface can be obtained. In the presence of almost immobile nitrogen atoms under N-rich conditions crystal growth is realized by the process of two-dimensional island nucleation and coalescence. Larger crystal miscut, lower growth rate or higher temperature results in similar effect of the surface smoothening. We show that the surface also smoothens for the growth conditions with very high N-excess. In the presence of large number of nitrogen atoms the mobility of gallium atoms changes locally thus providing easier coalescence of separated island.

Advacancy-mediated atomic steps kinetics and two-dimensional negative island nucleation on ultra-flat Si(111) surface

We have investigated Si(111) surface morphology transformations during high-temperature sublimation and oxygen treatments by means of in situ ultrahigh vacuum reflection electron microscopy. By analyzing atomic steps kinetics and two-dimensional negative (vacancy) islands nucleation on ultra-flat Si(111) surface with extremely wide (up to 120μm in size) terraces we have estimated the activation energy associated with the surface-bulk vacancy exchange processes. We show that atomic steps motion and negative islands nucleation kinetics at temperatures above 1180°C can be described by the step-flow model of Burton, Cabrera and Frank taking into account advacancies formation. By comparing experimental results with predictions of model we conclude that the surface mass transport at temperatures above 1180°C is governed by surface vacancies nucleation and interaction with atomic steps rather than via adatoms surface diffusion.

Nucleation of two-dimensional Si islands near a monatomic step on an atomically clean Si(111)-(7×7) surface

The process of nucleation of 2D islands near a monatomic step at the initial stage of growing of a silicon film on the Si(111)-(7 × 7) surface is studied by means of in situ ultrahighvacuum reflection electron microscopy. The dependence of the depletion region width W near the step, where no islands are formed, on the deposition rate R is described by the expression W 2 ∝ R -χ with the exponent χ = 1.18 and χ = 0.63 at temperatures of 650 and 680 °C, respectively. It is demonstrated that the change in χ is associated with the step structure, which provides the transformation from the growth kinetics limited by attachment of adatoms to the step to that limited by diffusion of adatoms. A competition of the processes of nucleation and attachment to the step leads to an increase in the critical size of the island nucleus from i = 1 far from the step to i = 3–5 near the step and to i = 6–8 on the terrace of critical width for 2D nucleation.

Formation Two Dimensional Negative Islands At High Rate Cooling Of Ultra-Flat Surface Si(111)

In situ ultrahigh vacuum reflection electron and ex situ atomic force microscopy have been applied to investigate morphology transformations of the ultra-flat stepped Si(111) surface with wide (20–50 µm in diameter) singular terraces during sublimation and quenching from elevated temperatures. The formation of two dimensional negative (vacancy) islands has been observed on the wide terraces after the quenching from temperatures above 1 200°C. The increasing of the critical terrace size for the two-dimensional negative island nucleation has been explained by the changing of the atomic mechanism of mass transport on silicon surface.

Formation of Two-Dimensional Islands on Si(111) Surface During Homoepitaxial Growth

The nucleation of two-dimensional Si islands has been studied by in situ ultrahigh vacuum reflection electron microscopy on extra-large (~ 10–100 μm) atomically flat terraces of Si(111) surface. The dependence of two-dimensional island concentration N2D on substrate temperature T and silicon deposition rate R is found to obey relation N2D ן Rχ exp(E2D/kT) with χ≈0.58 or 0.82 and E2D ≈ 1.77 eV or 1.02 eV on the Si(111) surface with (7×7) or (1×1) structure, respectively. The critical nucleus during the growth on the extra-large terraces is found to consist of i = 1 particle at T ~ 700°С, and the critical nucleus size increases to i = 7–10 on terraces with smaller width, which is caused by the competition between the 2D island nucleation and the interaction of adatoms with steps bordering the critical terrace

Electrochemical and in-situ stm study of the surface alloy formation in the system Au(100)/Cd

The Cd UPD process in the system Au(100)/Cd2+, SO42− and the involved Au-Cd surface alloy formation have been investigated, using conventional electrochemical techniques and in-situ STM analysis. The voltammetric experiences have shown three adsorption/desorption steps in the extended UPD range 0≤ΔE/mV≤800. The obtained desorption spectra indicate the formation of a surface alloy at low underpotentials and long polarization times. STM images have shown that the Cd deposition becomes evident at low underpotentials, with the nucleation of two-dimensional islands of monatomic height, which grow and coalesce up to a complete monolayer. At lower underpotentials, new islands nucleate on the monolayer previously formed, and grow acquiring a quadratic structure. The subsequent stripping of this film promotes morphological changes that are related to the dissolution of a Cd-Au surface alloy, which is consistent with the electrochemical experiences.

Nucleation and growth kinetics of RaxBa1−xSO4 solid solution in NaCl aqueous solutions

Co-precipitation of Ra and Ba in barite (i.e., the formation of a RaxBa1−xSO4 solid solution) has long been established as an important process that has the potential to control Ra concentration. This process is commonly described by a distribution model. Ample studies have shown that the key parameter of this model, the partition coefficient, varies in the range of 1–2 as a function of temperature, salinity and precipitation kinetics of the RaxBa1−xSO4 solid solution. This roughly twofold change in the partition coefficient may lead to large differences in the concentration of dissolved Ra.The present study systematically investigated the co-precipitation kinetics of the RaxBa1−xSO4 solid solution from aqueous solutions up to 5.9 molkgH2O-1 NaCl, circum-neutral pH and at ambient temperature. Laboratory batch experiments designed to follow the nucleation of the RaxBa1−xSO4 solid solution and the co-precipitation kinetics of Ba and Ra from aqueous solutions which were initially supersaturated with respect to barite (degree of supersaturation, βbarite=20±2).The following empirical law describes the dependence of the activity-based partition coefficient, KD,barite″, on the degree of supersaturation, βbarite:KD,barite″=(1.99±0.05)-(0.58±0.06)·log(βbarite).This empirical law is in good agreement with other literature data. The outcomes of the empirical law are compared to the prediction of a model for the nucleation of two-dimensional islands.

Nucleation of in-grown stacking faults and dislocation half-loops in 4H-SiC epitaxy

Ultraviolet photoluminescence, transmission electron microscopy and KOH etching were used to characterize extended defects in 4H-SiC epilayers grown at high growth rates (18 μm/h). Layers exhibited high densities of in-grown stacking faults and dislocation half-loops. The stacking faults were 8H Shockley-type faults. The Burgers vector of the dislocation half-loops was in the (0001) basal plane. Both defects nucleate within the epilayer at early stages of growth. Defect nucleation is directly correlated with high initial growth rate and is not related to any defects/heterogeneities in the substrate or epilayer. Epilayer growth by nucleation of two-dimensional islands is proposed as a possible mechanism for the formation of both defects, through nucleation of faulted Si-C bilayers.

Kinetics of two-dimensional island nucleation on reconstructed surfaces

Nucleation of two-dimensional (2D) islands on reconstructed surfaces might involve formation of metastable nonepitaxial clusters as precursors for 2D islands. Being a thermally activated process the transformation of nonepitaxial clusters to epitaxial 2D islands might become a rate limiting step of the nucleation process at low deposition temperatures leading to substantial accumulation of the depositing atoms in nonepitaxial clusters. In the present paper, we study how slow cluster transformation influences the nucleation kinetics. For a simple model case of metastable nonepitaxial dimers and irreversible adatom incorporation into the epitaxial 2D islands, we analyze possible nucleation regimes and derive scaling relations for the density of stable 2D islands. We show that the 2D island density depends strongly on the cluster transformation rate as well as on the atomic mechanism of the dimer transformation. In particular, a steady-state nucleation regime different from the standard diffusion-mediated aggregation is possible if the transformation of a nonepitaxial dimer to the smallest epitaxial 2D island is triggered by attachment of an additional adatom. In this regime, the 2D island density obeys a nonstandard scaling $N\ensuremath{\sim}{(F/D)}^{1/4}$. Deposition conditions where this nonstandard scaling could be observed are determined.

Epitaxial growth of atomically flat gadolinia-doped ceria thin films by pulsed laser deposition

Epitaxial growth of Ce0.8Gd0.2O2(CGO) films on (001) TiO2-terminated SrTiO3 substrates by pulsed laser deposition was investigated using in situ reflective high energy electron diffraction. The initial film growth shows a Stransky–Krastanov growth mode. However, this three-dimensional island formation is replaced by a two-dimensional island nucleation during further deposition, which results in atomically smooth CGO films. The obtained high-quality CGO films may be attractive for the electrolyte of solid-oxide fuel cells operating at low temperature.

Nucleation and growth of Si on Pb monolayer covered Si(111) surfaces

With a scanning tunneling microscope (STM), we study the initial stage of nucleation and growth of Si on Pb monolayer covered Si(111) surfaces. The Pb monolayer can work as a good surfactant for growth of smooth Si thin films on the Si(111) substrate. We have found that nucleation of two-dimensional (2D) Pb-covered Si islands occurs only when the substrate temperature is high enough and the Si deposition coverage is above a certain coverage. At low deposition coverages or low substrate temperatures, deposited Si atoms tend to self-assemble into a certain type of Si atomic wires, which are immobile and stable against annealing to ~200°C. The Si atomic wires always appear as a double bright-line structure with a separation of ~9Å between the two lines. After annealing to ~200°C for a period of time, some sections of Si atomic wires may decompose, meanwhile the existing 2D Pb-covered Si islands grow laterally in size. The self-assembly of Si atomic wires indicate that single Si adatoms are mobile at the Pb-covered Si(111) surface even at room temperature. Further study of this system may reveal the detailed atomic mechanism in surfactant-mediated epitaxy.

Monosodium urate monohydrate crystallization

The crystallization of monosodium urate monohydrate (MSU) in the synovial fluid has long been associated with the joint disease gout, which is characterized by recurring attacks of pain and inflammation within the joints. In this work, in situ atomic force microscopy (AFM) on single crystal (010) MSU surfaces and dynamic light scattering were used complementarily to investigate the growth of MSU in 2–10 mM urate solutions under model physiological conditions (37 °C, pH = 7.4, 150 mM NaCl). In this solution regime, crystal growth rates normal to the (010) surface followed the square of supersaturation, which is consistent with a two-dimensional island nucleation and spread mechanism. Islands observed under in situ AFM imaging conditions had sizes consistent with dynamic light scattering measurements but exhibited unique faceting not typically observed in fully developed crystals or predicted by previous theoretical work. Preferred alignment of the islands' long axis and the underlying ±c axis of the MSU (010) was observed, as was island overgrowth over time.