Island Formation Process Research Articles

Two-step unconventional protocol for epitaxial growth in one dimension with hindered reactions.

We study the effect of hindered aggregation and/or nucleation on the island formation process in a two-step growth protocol. In the proposed model, the attachment of monomers to islands and/or other monomers is hindered by additional energy barriers which decrease the hopping rate of the monomers to the occupied sites of the lattice. For zero and weak barriers, the attachment is limited by diffusion while for strong barriers it is limited by reaction. We describe the time evolution of the system in terms of the monomer and island densities, N_{1} and N. We also calculate the gap length, the capture zone and the island distributions. For all the sets of barriers considered, the results given by the proposed analytical model are compared with those from kinetic Monte Carlo simulations. We found that the behavior of the system depends on the ratio of the nucleation barrier to the aggregation barrier. The two-step growth protocol allows more control and understanding on the island formation mechanism because it intrinsically separates the nucleation and aggregation processes in different time regimes.

Island size distribution with hindered aggregation.

We study the effect of hindered aggregation on the island formation processes for a one-dimensional model of epitaxial growth with arbitrary nucleus size i. In the proposed model, the attachment of monomers to islands is hindered by an aggregation barrier, ε_{a}, which decreases the hopping rate of monomers to the islands. As ε_{a} increases, the system exhibits a crossover between two different regimes; namely, from diffusion-limited aggregation to attachment-limited aggregation. The island size distribution, P(s), is calculated for different values of ε_{a} by a self-consistent approach involving the nucleation and aggregation capture kernels. The results given by the analytical model are compared with those from kinetic Monte Carlo simulations, finding a close agreement between both sets of data for all considered values of i and ε_{a}. As the aggregation barrier increases, the spatial effect of fluctuations on the density of monomers can be neglected and P(s) smoothly approximates to the limit distribution P(s)=δ_{s,i+1}. In the crossover regime the system features a complex and rich behavior, which can be explained in terms of the characteristic timescales of different microscopic processes.

Sponges as a complement of sedimentary facies analysis in island deposits of Upper Paraná River

This paper shows the importance of the sponge spicules as complement of sedimentary facies analysis in order to reconstruct the hydrach stages involved in island formation in the Upper Paraná River, Brazil. River in the study reach is anabranching with islands of different sizes covered by typical alluvial forest. We noted that the sponge spicules communities vary according to the changes in the environments involved in the island formation processes. The sponges were identified by their microscleres and gemoscleres in optical microscope as Metania spinata, Oncosclera navicella, Oncosclera jewelli, and possibly the genus Corvoheteromeyenia sp. (Ezcurra de Drago, 1979). By correlating the information coming from the sponges and sedimentary facies, it was possible to identify five phases of the island construction and their respective hydrach stages: 1) bar-island channel (Eupotamic stage), 2) blind channel (Parapotamic stage), 3) lake, 4) swamp (both Paleopotamic stage), and 5) forested island (Terrestrial stage). Using 14C dating and rate of sedimentation, we observed that the development of these five phases took ∼900–1000 years. The data also supported the idea that the forest begin to be formed 134–160 years ago. We concluded that sponge is a strong tool on paleoenvironmetal reconstruction when used with another indicators (in this case the facies analysis) and can be applied other fluvial studies as river management especially for long-term impacted systems (by dams) as those of the Paraná River Basin.

Fragmentation approach to the point-island model with hindered aggregation: Accessing the barrier energy.

We study the effect of hindered aggregation on the island formation process in a one- (1D) and two-dimensional (2D) point-island model for epitaxial growth with arbitrary critical nucleus size i. In our model, the attachment of monomers to preexisting islands is hindered by an additional attachment barrier, characterized by length l_{a}. For l_{a}=0 the islands behave as perfect sinks while for l_{a}→∞ they behave as reflecting boundaries. For intermediate values of l_{a}, the system exhibits a crossover between two different kinds of processes, diffusion-limited aggregation and attachment-limited aggregation. We calculate the growth exponents of the density of islands and monomers for the low coverage and aggregation regimes. The capture-zone (CZ) distributions are also calculated for different values of i and l_{a}. In order to obtain a good spatial description of the nucleation process, we propose a fragmentation model, which is based on an approximate description of nucleation inside of the gaps for 1D and the CZs for 2D. In both cases, the nucleation is described by using two different physically rooted probabilities, which are related with the microscopic parameters of the model (i and l_{a}). We test our analytical model with extensive numerical simulations and previously established results. The proposed model describes excellently the statistical behavior of the system for arbitrary values of l_{a} and i=1, 2, and 3.

Island nucleation inside a one-dimensional gap with hindered aggregation

The island formation process inside a one-dimensional gap with length L is studied. In the model i + 1 monomers are required to form an stable island. The aggregation of monomers to the islands at the ends of the gap is hindered by an additional attachment barrier Ea which reduces the hopping rate of the monomers to the island. Depending on Ea there are different regimes: zero and weak barriers, large but finite barriers and infinite barriers. The probability of nucleation at position n inside a gap with length L, , and the nucleation rate ω are studied. Both, and ω, are calculated by using two different analytical methods: exact master equation for the time evolution of i + 1 monomers deposited simultaneously and mean field approximation. In all cases the results are compared with Kinetic Monte Carlo simulations to explore the range of applicability of both methods.

Anurans of the sandy coastal plains of the Lagamar Paulista, state of São Paulo, Brazil

Insular habitats are considered ideal for biogeographic studies, as they represent almost closed habitats with known formation timelines. Although São Paulo state in Brazil has a large number of continental and oceanic islands, few studies have examined the composition of the fauna in each locality. These studies are crucial to determine the community structure and to make biogeographic and phylogenetic inferences. From February 2007 to January 2009, we surveyed the anuran community of sandy coastal plains in four municipalities of the Lagamar Paulista: Cananéia, Ilha Comprida, Iguape and Pariquera-Açu, in the state of São Paulo, Brazil. This region is located in the Cananéia-Iguape estuary complex, comprising three continental islands: Ilha de Cananéia, Ilha Comprida, and Ilha do Cardoso, and the continental portions adjacent to these islands in the municipalities of Iguape and Pariquera-Açu. Through the use of pitfall traps and active search, we registered a total of 32 anuran species: 17 in Ilha Comprida, 21 in Ilha de Cananéia, 26 in Ilha do Cardoso, 27 in Iguape, and 23 in Pariquera-Açu. The analysis of the richness indices of the study sites revealed higher values in continental areas. This result is very common in comparative studies between islands and continents. This may be caused by an island effect and may reveal the historical process of island formation in the Lagamar. Our findings provide important information for identifying patterns of amphibian distribution in insular areas of the São Paulo coast, and for examining how events of marine transgression and regression have affected the local amphibian communities.

In situ Control of Si/Ge Growth on Stripe-Patterned Substrates Using Reflection High-Energy Electron Diffraction and Scanning Tunneling Microscopy

Si and Ge growth on the stripe-patterned Si (001) substrates is studied using in situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). During Si buffer growth, the evolution of RHEED patterns reveals a rapid change of the stripe morphology from a multifaceted “U” to a single-faceted “V” geometry with {119} sidewall facets. This allows to control the pattern morphology and to stop Si buffer growth once a well-defined stripe geometry is formed. Subsequent Ge growth on “V”-shaped stripes was performed at two different temperatures of 520 and 600°C. At low temperature of 520°C, pronounced sidewall ripples are formed at a critical coverage of 4.1 monolayers as revealed by the appearance of splitted diffraction streaks in RHEED. At 600°C, the ripple onset is shifted toward higher coverages, and at 5.2 monolayers dome islands are formed at the bottom of the stripes. These observations are in excellent agreement with STM images recorded at different Ge coverages. Therefore, RHEED is an efficient tool for in situ control of the growth process on stripe-patterned substrate templates. The comparison of the results obtained at different temperature reveals the importance of kinetics on the island formation process on patterned substrates.

1806 エピタキシャル薄膜のひずみ誘起島形態形成Phase-fieldシミュレーション(J06-1 マイクロ・ナノ構造と変形解析,J06 計算マイクロ・ナノメカニクス)

The morphological evolution of a strained heteroepitaxial thin film of Si_<1-x>Ge_x/Si during deposition has been investigated by two-dimensional phase-field simulations. The developed phase-field model can create the facet morphologies which are modeled using the generalized gradient correction coefficient for a crystal with a high anisotropy of surface energy, and can simulate the sequence of shape transitions which is well known in Si_<1-x>Ge_x/Si(001) system: 2D wetting layer, faceted 3D pyramids, and multifaceted domes. The effects of Ge composition, mobility for phase field and deposition rate on the island formation process and the island evolutions have been studied here. Some typical and important phenomena observed, in previous experimental studies, during Si_<1-x>Ge_x/Si(001) heteroepitaxy have been able to simulate by the developed phase-field model.

Defects and morphological changes in nanothin Cu films on Mo(100) studied by thermal helium desorption spectrometry

Thermal helium desorption spectrometry has been used for a comprehensive experimental investigation of vacancy-related defects and thermal stability of nanothin Cu films (10–300Å) deposited on a Mo(100) substrate at 300K in UHV. In the helium desorption spectra (300–2000K), a peak is observed that shows that the Cu films are highly unstable. Postdeposition annealing treatments show that this peak is due to the thermally-induced island formation of the copper films. This process is strongly dependent on film thickness; its temperature ranging from 429K for a 10-Å film to 1312K for a 300-Å film, which is not simply due to the considerable change of stress with film thickness. Between 70 and 75Å, there is a distinct narrowing of the peak shape, indicating a change in the kinetics. For a 100-Å film lying in the large thickness range, an activation energy E=1.2±0.1eV was determined. The island-formation process is defect-mediated, as suggested by the fact that the temperature is lowered by an increase of the helium implantation energy and fluence. The island formation during deposition at an elevated substrate temperature occurs at a lower temperature than during postdeposition annealing. The helium released from monovacancies (E=1.8±0.2eV) and vacancy clusters in the Cu films and during high-temperature evaporation of the Cu films from the Mo substrate (Esubl=3.5±0.5eV) is also identified. The x-ray diffraction measurements on 60 and 200-Å films show that they are both fcc Cu(100) films with identical macrostrains; the 200-Å film has a sharper texture than the 60-Å film. The orientation relation between the film and the substrate was found to be (100)[100]fccCu‖(100)[110]bccMo.

Stable and uniform arrays of self-assembled nanocrystalline islands

The stability of islands against coarsening during the self-assembly process of the Stranski-Krastanow systems was explored by an energy analysis and by numerical simulation for the morphological evolution of the islands dominated by surface diffusion. The results demonstrate that the Stranski-Krastanow systems can develop islands stable against coarsening when subject to a large film-substrate interaction and a large mismatch strain. The stable systems can be adopted to grow uniform and regular island arrays by adjusting the length scale in the island formation process and by introducing a regular pattern of small amplitude on the film surface.

Kinetics of the initial stage of coherent island formation in heteroepitaxial systems

The stress-driven formation of coherent islands in heteroepitaxial systems is studied within the frame of the kinetic theory of nucleation under dynamical conditions. The kinetic model for a description of the time evolution of the island size distribution and wetting layer thickness in Stranski-Krastanow growth mode is developed. The time scale hierarchy of the nucleation stage, the size relaxation stage, and the stage of critical wetting layer formation enable us to construct analytical solutions to the model kinetic equations. The time dependence of the lateral size distribution, wetting layer thickness, and other characteristics of the island formation process are calculated. Analytical expressions for the substrate temperature and growth rate dependence of the mean lateral size and surface density of islands are obtained and analyzed.

Influence of strain, surface diffusion and Ostwald ripening on the evolution of nanostructures for erbium on Si(001)

This study explores the evolution of nanoscale islands and wire structures during deposition and surface ripening. Ultraviolet photoelectron emission microscopy has been employed to study the real time growth process of individual erbium silicide nanostructures on Si(001) surfaces at temperatures up to 1050 °C. During the initial island formation process compact islands form and some undergo a shape transition to elongated islands oriented along the 〈110〉 directions of the Si substrate. The initial island formation is driven by the surface and interface energies of the silicide/Si structure. The widths of the growing islands remain essentially constant while the lengths increase. The observed elongated islands are ∼150 nm wide, which is larger than the width of prior reported erbium silicide nanowire structures. We propose that the ∼150 nm elongated islands are partially relaxed, possibly through the formation of misfit dislocations. The results indicate a temperature regime where island growth is mainly governed by surface diffusion of the deposited Er adatoms and a higher temperature regime where Ostwald ripening contributes to the island morphology.

Ultrathin Cu Films on Mo(110) Characterized by Helium Implantation

ABSTRACTPoint defects and thermal stability of ultrathin Cu films (3–200 Å) deposited on Mo(110) substrate at 300 K have been characterized using thermal helium desorption spectrometry (THDS). Implantation of the samples (10–100 Å Cu/Mo) with 1000 eV He+ aided in detecting He release from monovacancy in Cu films and desorption of the films (above 1200 K) from Mo substrate. A thickness dependent peak is identified in the helium desorption spectra, which is shown to be due to the process of island formation in the Cu films, using pre-implantation annealing treatments for the 40 Å film. Native defects in the films (3–200 Å) were probed using 75 eV He+ implantation.

Thermally assisted formation of silicon islands on a silicon-on-insulator substrate

We report the self-formation of nanometer-size silicon islands on a silicon-on-insulator (SOI) substrate that is associated with simple thermal treatment in the range of 500–900 °C. We study the island formation process versus the temperature of the thermal annealing, the thickness of the top silicon layer, and the presence of a native oxide on this top layer. The island size distribution is also studied. To follow the chemical evolution of the top layer, we used in situ Auger electron spectroscopy in an ultrahigh vacuum chamber. The island morphology is studied using ex situ atomic force microscopy (AFM). The formation temperature increases with the thickness of the top silicon layer and can be explained by thermal stress induced at the Si/SiO2 interface. From a technological point of view, this study shows the limitation of a SOI substrate with a thin silicon top layer under thermal treatment. On the other hand, it opens up an easy way in which to build silicon dots on an insulator. Finally, we present preliminary data that show the possibility of charging these nanocrystallites with an AFM tip.

Kinetic roughening during rare-gas homoepitaxy

Using an optical-reflectivity-difference technique, we monitored the growth of multilayer Xe films on a commensurate monolayer of Xe on Ni(111) from 35 to 60 K. A transition occurs near 40 K from rough growth at low temperature to quasi-layer-by-layer growth characterized by persistent oscillations in the reflectivity difference. We discuss this transition in terms of changes in the island formation process and the onset of second-layer nucleation. The Xe sticking coefficient at 40 K is obtained from the period of the oscillations in the reflectivity difference. We find that the sticking coefficient decreases with increasing film thickness at fixed Xe pressure.

Rotational epitaxy of a ‘soft’ metal overlayer on Si(111)

The observation of rotational epitaxy of an incommensurate ‘soft’ Tl metal overlayer on a pseudomorphic Si(111)1×1-Tl surface by STM is reported. The structural ‘softness’ of this two-dimensional overlayer is destroyed by additional metal atom deposition, which induces condensation into three-dimensional island structures as a result of the quenching of a strain relaxation mechanism. It is suggested that the ‘soft’ Tl metal adlayer represents a meta-stage in the three-dimensional island formation process of Stranski–Krastanov growth. It is interesting that this metastable second Tl layer collapses on further growth and becomes incorporated into the three-dimensional islands once these have nucleated.

A histopathological study of lymphoepithelial island formation in labial salivary glands in patients with primary Sjögren's syndrome.

The proliferative status of lymphoepithelial islands in the labial salivary glands of primary Sjögren's syndrome (pSS) patients was investigated by counting the number of argyrophilic nucleolar organizer regions (AgNORs) in epithelial cells constituting the islands. The islands were classified into four groups and evaluated in terms of total area and three discrete zones of the islands. In each pSS group, the mean AgNOR number per total island epithelial cell nucleus was significantly higher than in control ductal epithelial cells. The zonal AgNOR number fluctuated during the process of island formation but became more uniform as the islands developed. Furthermore, statistically significant trends among the four pSS groups were observed in the ratio of T lymphocytes, B lymphocytes and plasma cells surrounding the islands. The results indicated that the islands are highly proliferative once island formation begins and that zonal island cell proliferation may be associated with the inflammatory cells.

Kinetic Roughening During Rare-Gas Homoepitaxy

AbstractUsing an optical reflectivity difference technique, we monitored the growth of multilayer Xe films on a commensurate monolayer of Xe on Ni(111), from 35 to 60K. A transition occurs near 40K, from rough growth at low temperature to quasi-layer-by-layer growth characterized by persistent oscillations in the reflectivity difference. We discuss this transition in terms of changes in the island formation process and the onset of second layer nucleation. The Xe sticking coefficient at 40K is obtained from the period of the oscillations in the reflectivity difference. We find that the sticking coefficient decreases with increasing fihn thickness at fixed Xe pressure.

Three Dimensional Finite Element Simulations of Vertical Dot Correlation in Quantum Dot Superlattice

ABSTRACTA three dimensional finite element method is used to simulate the kinetic process of island formation in quantum dot superlattices. Depending on the thickness of spacer layers and interruption time, top layer islands can be fully vertically aligned with the same morphology as the buried islands, or can be partially vertically aligned with increasingly less uniform and regular arrangement, or can be alternately misaligned with increasingly uniform and regular arrangement.

Island formation in Ge/Si epitaxy

We review current understanding of the island formation process, and outline some of the remaining problems. The initial island formation is coherent, with islands stabilised by the elastic relaxation associated with deformation of the substrate. We use finite element analysis to demonstrate the degree of relaxation associated with these deformations. The islands then relax further by introduction of dislocations. The first dislocations introduced in the large coherent islands are shown to be inclined burgers vectors (about 60°) adopting a nearly semicircular path around the island perimeter. The outstanding anomaly in Ge/Si growth is the temperature dependence of islanding, with slower relaxation rates and greater metastability associated with higher temperatures. We describe various possible explanations for this problem.