Coalescence Of Islands Research Articles

Unstable kinetic roughening during the island coalescence stage of sputtered tantalum films

Kinetic roughening of tantalum films during the initial growth stages has been studied by atomic force microscopy, scanning electron microscopy, and dynamic scaling theory. Different from the time-independent scaling behavior for continuous film growth, an intriguing unstable kinetic roughening occurs during island coalescence. In such case, roughness exponent α increases with growth time, accompanied by lower growth exponent β and higher coarsening exponent η. Detailed analysis of film surface morphology and simple phenomenological models suggests that this unstable behavior is related to the pronounced lateral growth of surface islands, which arises from the combined effect of the formation of grain boundary and the covering of heterogeneous substrate surface.

Nucleation and Growth of Copper on Ru-Based Substrates: I. The Effect of the Inorganic Components

The effect of the inorganic components, namely copper sulfate (CuSO4), sulfuric acid (H2SO4) and hydrochloric acid (HCl) on the nucleation and growth of copper on Ru-based substrate is reported in the absence and the presence of a suppressor molecule, polyethylene glycol (PEG). In the absence of PEG it was that found the island density, Np was the highest when CuSO4 and HCl concentrations were low while H2SO4 concentration was high. The effect of each component remained similar when all components were mixed together. In the presence of PEG, the effect of CuSO4 and H2SO4 remained similar. However, for the HCl, an optimal concentration was found where Np reached a maximum value. The study illustrates the importance of an optimal bath composition toward faster coalescence of the Cu islands into a continuous Cu film for direct plating applications.

Extraordinary epitaxial alignment of graphene islands on Au(111)

Pristine, single-crystalline graphene displays a unique collection of remarkable electronic properties that arise from its two-dimensional, honeycomb structure. Using in situ low-energy electron microscopy, we show that when deposited on the (111) surface of Au carbon forms such a structure. The resulting monolayer, epitaxial film is formed by the coalescence of dendritic graphene islands that nucleate at a high density. Over 95% of these islands can be identically aligned with respect to each other and to the Au substrate. Remarkably, the dominant island orientation is not the better lattice-matched 30° rotated orientation but instead one in which the graphene [01] and Au [011] in-plane directions are parallel. The epitaxial graphene film is only weakly coupled to the Au surface, which maintains its reconstruction under the slightly p-type doped graphene. The linear electronic dispersion characteristic of free-standing graphene is retained regardless of orientation. That a weakly interacting, non-lattice matched substrate is able to lock graphene into a particular orientation is surprising. This ability, however, makes Au(111) a promising substrate for the growth of single crystalline graphene films.

FAST MAGNETIC RECONNECTION AND PARTICLE ACCELERATION IN RELATIVISTIC LOW-DENSITY ELECTRON-POSITRON PLASMAS WITHOUT GUIDE FIELD

Magnetic reconnection and particle acceleration in relativistic Harris sheets in low-density electron-positron plasmas with no guide field have been studied by means of two-dimensional particle-in-cell simulations. Reconnection rates are of the order of one when the background density in a Harris sheet is of the order of 1% of the density in the current sheet, which is consistent with previous results in the non-relativistic regime. It has been demonstrated that the increase of the Lorentz factors of accelerated particles significantly enhances the collisionless resistivity needed to sustain a large reconnection electric field. It is shown analytically and numerically that the energy spectrum of accelerated particles near the X-line is the product of a power law and an exponential function of energy, γ–1/4exp (– aγ1/2), where γ is the Lorentz factor and a is a constant. However, in the low-density regime, while the most energetic particles are produced near X-lines, many more particles are energized within magnetic islands. Particles are energized in contracting islands by multiple reflection, but the mechanism is different from Fermi acceleration in magnetic islands for magnetized particles in the presence of a guide field. In magnetic islands, strong core fields are generated and plasma beta values are reduced. As a consequence, the fire-hose instability condition is not satisfied in most of the island region, and island contraction and particle acceleration can continue. In island coalescence, reconnection between two islands can accelerate some particles, however, many particles are decelerated and cooled, which is contrary to what has been discussed in the literature on particle acceleration due to reconnection in non-relativistic hydrogen plasmas.

GaP collector development for SiGe heterojunction bipolar transistor performance increase: A heterostructure growth study

To develop a III/V wide bandgap collector concept for future SiGe heterobipolar transistor performance increase, a heterostructure growth study of GaP on pseudomorphic 4° off-oriented Si0.8Ge0.2/Si(001) substrates was performed. For pseudomorphic GaP/Si0.8Ge0.2/Si(001) heterostructure growth, critical thickness of GaP on Si and maximum thermal budget for GaP deposition were evaluated. A detailed structure and defect characterization study by x-ray diffraction, atomic force microscopy, and transmission electron microscopy is reported on single crystalline 170 nm GaP/20 nm Si0.8Ge0.2/Si(001). Results show that 20 nm Si0.8Ge0.2/Si(001) can be overgrown by 170 nm GaP without affecting the pseudomorphism of the Si0.8Ge0.2/Si(001) layer. The GaP layer grows however partially relaxed, mainly due to defect nucleation at the GaP/Si0.8Ge0.2 interface during initial island coalescence. The achievement of 2D GaP growth conditions on Si0.8Ge0.2/Si(001) systems is thus a crucial step for achieving fully pseudomorphic heterostructures. Anti-phase domain-free GaP growth is observed for film thicknesses beyond 70 nm.

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.

Surface evolution of NaCl-type cubic AlN films on MgO (100) substrates deposited by laser molecular beam epitaxy

Highly oriented NaCl-type cubic AlN (c-AlN) films were successfully deposited on MgO (100) substrates by laser molecular beam epitaxy (LMBE) technique, and the time-dependent evolution of film surface was characterized. Based on the topography images and roughness analysis, c-AlN surfaces display the Stranski–Krastanov growth mode. In the initial stage, AlN adatoms prefer to settle along the substrate steps and the surface appears to be slightly rough. The following deposition causes an undulate surface, and the nucleation and early growth of 3D islands are observed. During the islands growth, a rapid increase of roughness occurs, and large and deep gaps on the top of islands are striking. A smooth surface with a roughness of 1.06nm is obtained at the growth time of 2h owing to the spread and coalescence of islands.

Using the kinetic Wulff plot to design and control nonpolar and semipolar GaN heteroepitaxy

For nonpolar and semipolar orientations of GaN heteroepitaxially grown on sapphire substrates, the development of growth procedures to improve surface morphology and microstructure has been driven in a largely empirical way. This work attempts to comprehensively link the intrinsic properties of GaN faceted growth, across orientations, in order to understand, design and control growth methods for nonpolar (1 1 2 0) GaN and semipolar (1 1 2 2) GaN on foreign substrates. This is done by constructing a comprehensive series of kinetic Wulff plots (or v-plots) by monitoring the advances of convex and concave facets in selective area growth. A methodology is developed to apply the experimentally determined v-plots to the interpretation and design of evolution dynamics in nucleation and island coalescence. This methodology offers a cohesive and rational model for GaN heteroepitaxy along polar, nonpolar and semipolar orientations, and is broadly extensible to the heteroepitaxy of other materials. We demonstrate furthermore that the control of morphological evolution, based on invoking a detailed knowledge of the v-plots, holds a key to the reduction of microstructural defects through effective bending of dislocations and blocking of stacking faults. The status and outlook of semipolar and nonpolar GaN growth on sapphire substrates will be presented.

Growth of Sub-Nanometer Thin Continuous TiN Films by Atomic Layer Deposition

This work reports on the initial growth of atomic layer deposited titanium nitride thin films on SiO2 substrate at temperatures of 350–425°C and process pressures of 2.6–3.2 × 10−2 mbar. We used spectroscopic ellipsometry for in situ monitoring the growth, atomic force microscopy and electrical measurements for further film characterization. We demonstrate that the growth obeys Stranski – Krastanov mode with an initial 2D growth followed by a 3D island formation. The growth of the islands eventually leads to coalescence. We found the 2D–3D transition to be independent of temperature whereas the coalescence of the 3D islands is strongly affected by temperature. The former takes place as the film thickness reaches 0.69 ± 0.1 nm, which is equivalent to 3 monolayers of TiN. The latter occurs at a thickness of 2.5 ± 0.1 nm for 350°C and at a thickness of 3.5 ± 0.1 nm for 425°C.

Nucleation and growth mechanisms of Fe on Au(111) in the sub-monolayer regime

The growth of Fe on Au(111) at 300K in the sub-monolayer regime has been investigated using scanning tunneling microscopy, focusing on the mechanisms of nucleation, coalescence and interlayer diffusion. Below a coverage of 0.1 ML, Fe growth proceeds in a well-ordered fashion producing regular arrays of islands, while approaching the island coalescence threshold (above 0.35–0.4 ML), we observed a consistent increasing of random island nucleation. These observations have been interpreted through rate equation models for the island densities in the presence of preferred nucleation sites. The evolution of the second layer fraction has also been interpreted in a rate equation scheme. Our results show that the ordered to random growth transition can be explained by including in the model bond breaking mechanisms due to finite Fe–Fe bond energy. A moderate interlayer diffusion has been inferred from data analysis between the second and the first layer, which has been used to estimate the energy barrier of the adatoms descending process.

Unravelling the peculiarities of island life: vicariance, dispersal and the diversification of the extinct and extant giant Galápagos tortoises

In isolated oceanic islands, colonization patterns are often interpreted as resulting from dispersal rather than vicariant events. Such inferences may not be appropriate when island associations change over time and new islands do not form in a simple linear trend. Further complexity in the phylogeography of ocean islands arises when dealing with endangered taxa as extinctions, uncertainty on the number of evolutionary 'units', and human activities can obscure the progression of colonization events. Here, we address these issues through a reconstruction of the evolutionary history of giant Galápagos tortoises, integrating DNA data from extinct and extant species with information on recent human activities and newly available geological data. Our results show that only three of the five extinct or nearly extinct species should be considered independent evolutionary units. Dispersal from mainland South America started at approximately 3.2 Ma after the emergence of the two oldest islands of San Cristobal and Española. Dispersal from older to younger islands began approximately 1.74 Ma and was followed by multiple colonizations from different sources within the archipelago. Vicariant events, spurred by island formation, coalescence, and separation, contributed to lineage diversifications on Pinzón and Floreana dating from 1.26 and 0.85 Ma. This work provides an example of how to reconstruct the history of endangered taxa in spite of extinctions and human-mediated dispersal events and highlights the need to take into account both vicariance and dispersal when dealing with organisms from islands whose associations are not simply explained by a linear emergence model.

Strong Nonlinear Effects in the Dynamics of Tearing Modes Controlled by Electron Cyclotron Current Drive

The 2-D time-dependent response of a tearing mode magnetic island to a radio frequency (RF)-driven current may lead to strong nonlinear effects. An extended magnetohydrodynamics model shows first the prompt appearance of current sheets tending to shield the RF current effect and, subsequently, a nonlinear filamentation of the island structure that eventually proceeds in the coalescence of secondary islands.

Surface structure, morphology, and growth mechanism of Fe3O4/ZnO thin films

We have investigated the growth mechanism and surface structure of magnetite (Fe3O4) films grown on semiconducting ZnO substrates. Growth proceeds in a wetting layer plus island-mode and a later coalescence of islands, as was verified by electron diffraction, topography measurements of the surface, and the investigation of the microstructure. The typical (111) surface diffraction pattern magnetite is observed already at the early island growth stage. Due to the island-like growth mode, domain boundaries form upon coalescence of the islands. Twin boundaries have been evidenced between neighboring domains. Island growth enables partial relaxation of the misfit strain.

Depth-dependent etch pit density in Ge epilayer on Si substrate with a self-patterned Ge coalescence island template

Ge epilayer with low dislocation density is prepared on a low temperature self-patterned Ge coalescence island template on Si substrate by ultra-high vacuum chemical vapor deposition. The depth profile of dislocation density in the Ge epilayer measured by etch-pit counting indicates that the dislocation density decreases drastically when the thickness of Ge layer is larger than 270 nm, and then depends weakly on the further increase of Ge thickness. The reduction of dislocations is ascribed to the Ge islands coalescence and lateral growth during deposition of Ge at higher temperature. 1.05-μm-thick Ge epilayer on Si with a dislocation density of the order of 10 6 cm − 2 and root-mean-square surface roughness of 0.45 nm are achieved without any additional thermal treatments.

Understanding and controlling heteroepitaxy with the kinetic Wulff plot: A case study with GaN

This work represents a comprehensive attempt to correlate the heteroepitaxial dynamics in experiments with fundamental principles in crystal growth using the kinetic Wulff plot (or v-plot). Selective area growth is employed to monitor the advances of convex and concave facets toward the construction of a comprehensive v-plot as a guidepost for GaN heteroepitaxy. A procedure is developed to apply the experimentally determined kinetic Wulff plots to the interpretation and the design of evolution dynamics in nucleation and island coalescence. This procedure offers a cohesive and rational model for GaN heteroepitaxy on polar, nonpolar, and semipolar orientations and is broadly extensible to other heteroepitaxial material systems. We demonstrate furthermore that the control of morphological evolution, based on invoking a detailed knowledge of the v-plots, holds a key to the reduction of microstructural defects through effective bending of dislocations and geometrical blocking of stacking faults, paving a way to device-quality heteroepitaxial nonpolar and semipolar GaN materials.

Dislocation structure of GaN films grown on planar and nano-patterned sapphire

Plane view and cross-section transmission electron microscopy (TEM) images were used to compare the density, character, and curvature of dislocations developed during metalorganic vapor phase epitaxy (MOVPE) of GaN on planar c-plane sapphire with those developed during growth on nano-patterned c-plane sapphire. Scanning electron microscopy (SEM) characterization of GaN films at different stages of growth for both types of substrates complemented the TEM investigation. GaN growth on wafers patterned with an array of submicron sapphire bumps exhibited relatively uniform nucleation and initial growth, as well as early island coalescence. It is suggested that this coalescence results in a relatively small fraction of dislocations with partial screw character at the surface of the films grown on the patterned substrate, and that this may be responsible for the improvements in carrier lifetime and device efficiency seen in earlier studies on similar sapphire substrates.

Surface striation, anisotropic in-plane strain, and degree of polarization in nonpolar m-plane GaN grown on SiC

We report the roles of island coalescence rate and strain relaxation in the development of anisotropic in-plane strains, striation feature, and subsequent degree of polarization in NH3-flow-rate-dependent m-plane GaN. In the high-NH3-flow-rate samples, the results of cathodoluminescence, polarized Raman and in situ optical reflectance measurements reveal that a slower coalescence and unrelieved lattice misfit strain lead to larger anisotropic in-plane strains, striated surface and luminescence patterns, and a lower density of basal-plane stacking fault (BSF) and prismatic stacking fault (PSF). In contrast, a lower NH3 flow rate leads to more rapid island coalescence and fully relaxed lattice misfit strain such that relaxed in-plane strains, a reduced striation surface, and a higher density of BSF and PSF are observed. It is suggested that the anisotropic in-plane strains, striation feature, and BSF and PSF density are consequences of how rapidly coalescence occurs and the degree of relaxation of lattice misfit strain. In addition, the simulation results of the k · p perturbation approach confirm a larger anisotropic strain results in a smaller degree of polarization. The research results provide important information for optimized growth of nonpolar III-nitrides.

ICP 표면 처리된 Si 기판 위에 성장된 Ge 층의 초기 성장 상태 연구

(Received June 27, 2011)(Revised July 13, 2011)(Accepted July 15, 2011)Abstract We have investigated the effect of inductively coupled plasma (ICP) treatment on the early growth stage ofheteroepitaxial Ge layers grown on Si(100) substrates using low pressure chemical vapor deposition (LPCVD). The Si(100)substrates were treated by ICP process with various source and bias powers, followed by the Ge deposition. The ICPtreatment led to the enhancement in the coalescence of Ge islands. The growth rate of Ge on Si(100) with ICP surfacetreatment is about 5 times higher than that without ICP surface treatment. A missing dimer caused by the ICP surfacetreatment can act as a nucleation site for Ge adatoms, which could be responsible for the improvement in growth behaviorof Ge on Si(100) substrates.Key wordsICP, SEM, Island, Coalescence, LPCVD, Germanium, Silicon, missing dimer

Mismatch relaxation by stacking fault formation of AlN islands in AlGaN/GaN structures on m-plane GaN substrates

We study the mismatch relaxation of 2-5 nm thin elongated AlN islands formed during growth of AlGaN on bulk m-plane GaN by molecular beam epitaxy. The relaxation of these m-plane AlN layers is anisotropic and occurs through the introduction of stacking faults in [0001] planes during island coalescence, while no relaxation is observed along the perpendicular [112¯0] direction. This anisotropy in the mismatch relaxation and the formation of stacking faults in the AlN islands are explained by the growth mode of the AlN platelets and their coalescence along the [0001] direction.

Flux Pileup in Collisionless Magnetic Reconnection: Bursty Interaction of Large Flux Ropes

Using fully kinetic simulations of the island coalescence problem for a range of system sizes greatly exceeding kinetic scales, the phenomenon of flux pileup in the collisionless regime is demonstrated. While small islands on the scale of λ ≤ 5 ion inertial length (d(i)) coalesce rapidly and do not support significant flux pileup, coalescence of larger islands is characterized by large flux pileup and a weaker time averaged reconnection rate that scales as √(d(i)/λ) while the peak rate remains nearly independent of island size. For the largest islands (λ = 100d(i)), reconnection is bursty and nearly shuts off after the first bounce, reconnecting ~20% of the available flux.