Self-assembly of three-dimensional metal islands: nonstrained versus strained islands

A theoretical model for the Volmer-Weber growth of three-dimensional metal islands is proposed, with a dipolar island edge-edge interaction. The existence of such an island edge effect makes the island shape dependent on island size. Furthermore, it induces a stable island size against coarsening, leading to self-assembled islands of uniform size. The dependence of the stable island size on total film coverage is shown to be different for nonstrained versus strained islands, in the regime of strong island-island interaction.     

Strongly-driven coarsening of height-selected Pb islands on Si(1 1 1)

A theoretical model is proposed to describe the rapid coarsening observed for Pb islands on a Si(1 1 1) surface where classical kinetics breaks down. In this system, quantum size effects produce mesa-like Pb islands with chemical potentials depending strongly on their heights, in addition to the usual dependence on the step curvature. Furthermore, a dense wetting layer enables fast mass transport between islands. Incorporating these features, our theoretical model predicts evolution of the island height distribution in good agreement with experiments.     

Nucleation, coarsening and kinetic scaling of two-dimensional islands on GaSb(0 0 1)

Using a combination of molecular beam epitaxy and in situ surface X-ray diffraction, we investigate the nucleation and coarsening of monolayer high islands on GaSb(0 0 1) during deposition in real time. We find an activation energy for island nucleation of 1.55 ± 0.16 eV, indicating a stable nucleus size larger than two atoms. For intermediate temperatures where GaSb homoepitaxy is stable, the lateral coarsening of the islands after deposition is described by Ostwald ripening. The average island sizes during coarsening are isotropic, although with different size distributions in different directions. The size distributions do not change during coarsening, implying kinetic scaling.     

Attractive migration and coalescence: a significant process in the coarsening of TiSi2 islands on the Si(111) surface

The dynamics and coarsening of TiSi2 islands on Si(111) surfaces are studied in real time with photoelectron emission microscopy. A significant fraction of events are observed in which nearby islands move attractively toward each other and subsequently coalesce. It is proposed that attractive island migration is due to the growth-decay flow of the island edges driven by a nonuniform surface concentration around the islands. The local surface concentration is induced by the neighboring islands. This coarsening mechanism should significantly affect the evolution of the island distribution.     

Influence of quantum size effects on island coarsening

Surface x-ray scattering and scanning-tunneling microscopy experiments reveal novel coarsening behavior of Pb nanocrystals grown on Si(111)-(7 x 7). It is found that quantum size effects lead to the breakdown of the classical Gibbs-Thomson analysis. This is manifested by the lack of scaling of the island densities. In addition, island decay times tau are orders of magnitude faster than expected from the classical analysis and have an unusual dependence on the growth flux F (i.e., tau is approximately 1/F). As a result, a highly monodispersed 7-layer island height distribution is found after coarsening if the islands are grown at high rather than low flux rates. These results have important implications, especially at low temperatures, for the controlled growth and self-organization of nanostructures.     

Kinetic evolution and equilibrium morphology of strained islands

Self-assembled SiGe islands grown on Si(001) leave behind characteristic "footprints" that reveal that small islands shrink, losing material to nearby larger islands. The critical size, dividing shrinking from growing islands, corresponds to the pyramid-to-dome shape transition, consistent with "anomalous coarsening" While shrinking, {105}-faceted pyramids transform into truncated pyramids and ultimately into unfaceted mounds. The similarity to behavior during island growth indicates that island shape and facet formation are thermodynamically determined.     

Coarsening in Sintering: Grain Shape Distribution,Grain Size Distribution,and Grain Growth Kinetics in Solid-Pore Systems

Sintering occurs when packed particles are heated to a temperature where there is sufficient atomic motion to grow bonds between the particles. The conditions that induce sintering depend on the material, its melting temperature, particle size, and a host of processing variables. It is common for sintering to produce a dimensional change, typically shrinkage, where the powder compact densifies, leading to significant strengthening. Microstructure coarsening is inherent to sintering, most evident as grain growth, but it is common for pore growth to occur as density increases. During coarsening, the grain structure converges to a self-similar character seen in both the grain shape distribution and grain size distribution. Coarsening behavior during sintering conforms to classic grain growth kinetics, modified to reflect the evolving microstructure. These modifications involve the grain boundary coverage due to pores, liquid films, or second phases and the altered grain boundary mobility due to these phases. The mass transport rates associated with each of these interfaces are different, with different temperature and composition dependencies. Hence, the coarsening rate during sintering is not constant, but changes with the evolving microstructure. Core aspects treated in this review include models for coarsening, grain shape, grain size distribution, and how pores, liquids, dispersoids, and other phases determine microstructure coarsening during sintering.     

Cooperative growth of nano-islands on Si(1 0 0) surface

By setting up two low temperature regions on a terrace of a vicinal Si(1 0 0)-2 × 1 surface, we have studied growth of nano-islands in the two regions using a kinetic Monte Carlo simulation. At first two islands are formed and grow independently without any supply of atoms from the outside. As the growth proceeds further, two islands are connected with each other by forming a bridge region. After the connection, the growth changes dramatically showing a competitive mode in one stage and a cooperative mode in the other. Two islands grow cooperatively in a sense that a larger island ceases to grow and waits until the size of the other smaller island becomes similar to that of the larger one. When two islands become similar in size, one of the islands grows faster than the other competitively, by accumulating atoms from then smaller one. The origin of the growth mode is analyzed.     

The crossover from collective motion to periphery diffusion for two-dimensional adatom-islands on Cu(111)

The diffusion of two-dimensional adatom-islands (up to 100 atoms) on Cu(111) has been studied, using the self-learning kinetic Monte Carlo method (Trushin et?al 2005 Phys. Rev. B 72 115401). A variety of multiple-?and single-atom processes are revealed in the simulations, and the size dependences of the diffusion coefficients and effective diffusion barriers are calculated for each. From the tabulated frequencies of events found in the simulation, we show a crossover from diffusion due to the collective motion of the island to a regime in which the island diffuses through periphery-dominated mass transport. This crossover occurs for island sizes between 13 and 19 atoms. For islands containing 19-100 atoms the scaling exponent is 1.5, which is in good agreement with previous work. The diffusion of islands containing 2-13 atoms can be explained primarily on the basis of a linear increase of the barrier for the collective motion with the size of the island.     

薄膜生长中的表面动力学(Ⅱ)

本文较全面地从理论上研究了薄膜生长过程中原子在表面上的各种动力学表现，涉及的内容包括亚单层生长时，原子在表面上的扩散，粘接，成核，以及已经形成的原子岛之间的相互作用，兼并，失稳，退化等一系列过程。在第一部分(即0—6章，刊登在《物理学进展》23卷1期上)介绍了薄膜生长动力学的基础之后，从第7章开始我们侧重研究一个在向同性和各向异性表面都普遍成立的原子岛尺寸大小及密度分布的标度定律；建立了一套研究在各向同性和各向异性表面上二维原子岛退化过程的广义动力学标度理论。基于对层间质量扩散通道的研究，本文提出了两种不同的原子下跃机制，既任意跃迁机制和选择跃迁机制，并做了进一步的实验验证。利用这一新的层间质量扩散机制，我们成功地解释了实验上观测到的三维原子岛的退化规律。更加有趣的是，本文讨论了应力对岛的形状和各种动力学规律的影响。在最后我们还提出了一个利用凝聚能转化来控制二维原子岛生长的方法，其目的是希望能够找到一种人为有效地在表面上制备低维量子结构的方法。     

薄膜生长中的表面动力学(Ⅱ)

本文较全面地从理论上研究了薄膜生长过程中原子在表面上的各种动力学表现 ,涉及的内容包括亚单层生长时 ,原子在表面上的扩散 ,粘接 ,成核 ,以及已经形成的原子岛之间的相互作用 ,兼并 ,失稳 ,退化等一系列过程。在第一部分 (即 0～ 6章 ,刊登在《物理学进展》2 3卷 1期上 )介绍了薄膜生长动力学的基础之后 ,从第 7章开始我们侧重研究一个在向同性和各向异性表面都普遍成立的原子岛尺寸大小及密度分布的标度定律 ;建立了一套研究在各向同性和各向异性表面上二维原子岛退化过程的广义动力学标度理论。基于对层间质量扩散通道的研究 ,本文提出了两种不同的原子下跃机制 ,既任意跃迁机制和选择跃迁机制 ,并做了进一步的实验验证。利用这一新的层间质量扩散机制 ,我们成功地解释了实验上观测到的三维原子岛的退化规律。更加有趣的是 ,本文讨论了应力对岛的形状和各种动力学规律的影响。在最后我们还提出了一个利用凝聚能转化来控制二维原子岛生长的方法 ,其目的是希望能够找到一种人为有效地在表面上制备低维量子结构的方法。     

Self-assembled magnetic nanostructures: Epitaxial Ni nanodots on TiN/Si (001) surface

Systems containing single domain magnetic particles are of great interest in view of their possible applications in ultrahigh-density data storage and magnetoelectronic devices. The focus of this work is plan-view STEM Z-contrast imaging study of the self-assembly growth of magnetic nickel nanostructures by domain matching epitaxy under Volmer–Weber (V–W) mode. The growth was carried out using pulsed laser deposition (PLD) technique with epitaxial titanium nitride film as the template, which was in turn grown on silicon (001) substrate via domain matching epitaxy. Our results show that the base of nickel islands is rectangular with the two principal edges parallel to two orthogonal 〈110〉 directions, which is [110] and [] for [001] oriented growth. The size distribution of the islands is relatively narrow, comparable to that obtained from self-assembled islands grown under Stranski–Krastanow (S–K) mode. A certain degree of self-organization was also found in the lateral distribution of islands: island chains were observed along the directions close to 〈011〉, which are also the edge directions. The interaction between neighboring islands through the island edge-induced strain field is believed to be responsible for the size uniformity and the lateral ordering.An erratum to this article can be found at     

Kinetics of facile bilayer island formation at low temperature: Ag/NiAl(110)

Facile nucleation and growth of bilayer Ag(110) islands on NiAl(110) is observed by STM for Ag deposition at temperatures as low as 127 K. Density functional theory analysis for supported Ag films determines adatom adsorption energies (which favor bilayer islands), interaction energies, and diffusion barriers. Analysis of an atomistic lattice-gas model incorporating these energies elucidates the role of strongly anisotropic interactions in enabling the upward mass transport needed for bilayer island formation.     

Logarithmic islanding in submonolayer epitaxial growth

We investigate submonolayer epitaxial growth with a fixed monomer flux and irreversible aggregation of adatom islands due to an effective island diffusion, with a diffusivity for an mass k island proportional to . For , there is a steady state, while for , continuously evolving logarithmic islanding occurs in which the island density grows extremely slowly, as . In the latter regime, the island size distribution exhibits complex, but universal, multiple-scale mass dependence which we account for theoretically. Received: 3 June 1998 / Accepted: 13 July 1998     

A numerical study on the formation of diffusion flame islands in a turbulent hydrogen jet lifted flame

This paper presents a numerical study on the formation of diffusion flame islands in a hydrogen jet lifted flame. A real size hydrogen jet lifted flame is numerically simulated by the DNS approach over a period of about 0.5 ms. The diameter of hydrogen injector is 2 mm, and the injection velocity is 680 m/s. The lifted flame is composed of a stable leading edge flame, a vigorously turbulent inner rich premixed flame, and a number of outer diffusion flame islands. The relatively long-term observation makes it possible to understand in detail the time-dependent flame behavior in rather large time scales, which are as large as the time scale of the leading edge flame unsteadiness. From the observation, the following three findings are obtained concerning the formation of diffusion flame islands. (1) A thin oxygen diffusion layer is developed along the outer boundary of the lifted flame, where the diffusion flame islands burn in a rather flat shape. (2) When a diffusion flame island comes into contact with the fluctuating inner rich premixed flame, combustion is intensified due to an increase in the hydrogen supply by molecular diffusion. This process also works for the production of the diffusion flame islands in the oxygen diffusion layer. (3) When a large unburned gas volume penetrates into the leading edge flame, the structure of the leading edge flame changes. In this transformation process, a diffusion flame island comes near the leading edge flame. The local deficiency of oxygen plays an important role in this production process.     

Island-growth of SiCGe films on SiC

SiCGe ternary alloys have been grown on SiC by hot-wall low-pressure chemical vapour deposition. It has been found that the samples cxhibit an island configuration, and the island growth of SiCGe epilayer depends on the processing parameters such as the growth temperature. When the growth temperature is comparatively low, the epilayer has two types of islands： onc is spherical island; another is cascading triangular island. With the increase of the growth temperature, the islands change from spherical to cascading triangular mode. The size and density of the islands depend on the growth duration and GeH4 flow-rate. A longer growth time and a larger GeH4 flow-rate can increase the size and density of the island in thc initial stage of the epitaxy. In our case, The optimal growth for a high density of uniform islands occurred at a growth temperature of 1100℃ for l-minute growth, with 10 SCCM GeH4, resulting in a narrow size distribution （about 30nm diameter） and high density （about 3.5 ×10^10 dots/cm2）. The growth follows Stranski- Krastanov modc （2D to 3D modc）, both of the islands and the 2D growth layer have face-centred cubic structure, and the critical thickness of the 2D growth layer is only 2.5 nm.[第一段]     

Enhancing the photoluminescence intensity of silicon-rich nitride film by localized surface plasmon enhanced photo-excitation

The photoluminescence of silicon-rich nitride (SRN) film was coupled with the surface plasmon (SP) of Ag island film. It shows that the photoluminescence (PL) enhancement or quenching is strongly dependent on the excitation wavelength. When the excitation wavelength is near the SP resonance spectral region, the Ag islands act as a photo antenna, leading to the enhancement of the excitation cross-sections and therefore the photoemission enhancement. Furthermore, it is demonstrated that the metal island size also has an influence on the emission enhancement, but the enhancement is much more decided by the excitation wavelength than by the Ag island radiative scattering.     

Optimal conditions for submonolayer nanoisland growth in ion beam assisted deposition

We study the optimal conditions for nanoisland growth in ion beam assisted deposition (IBAD). This situation occurs when adatom islands remain small enough to prevent the onset of three-dimensional growth, while at the same time preventing ion-induced surface erosion. To this end, we develop a rate equation model of IBAD, which embodies continuous deposition of adatoms and creation of vacancies, recombination of vacancies at adatom island edges, as well as recombination of adatoms at vacancy island edges. These rate equations are solved by numerical simulations based on the particle coalescence method. To determine the optimal growth condition, we find the largest mean size of the vacancy islands leading to their survival. We show that at this onset between the rough and smooth layer-by-layer growth regimes there is a simple exponential relation between the largest size of the vacancy islands and the external control parameters of the growth.     

Ripening of surface phases coupled with oscillatory dynamics and self-induced spatial chaos through surface roughening

Some pattern formation processes on single-crystal catalytic surfaces involve transitions between alternative surface phases coupled with oscillatory reaction dynamics. We describe a two-tier symmetry-breaking model of this process, based on nanoscale boundary dynamics interacting with oscillations of adsorbate coverage on microscale. The surface phase distribution oscillates together with adsorbate coverage, and, in addition, undergoes a slow coarsening process due to the curvature dependence of the drift velocity of interphase boundaries. The coarsening is studied both statistically, assuming a circular shape of islands of the minority phase, and through detailed Lagrangian modeling of boundary dynamics. Direct simulation of boundary dynamics allows us to take into account processes of surface reconstruction, leading to self-induced surface roughening. As a result, the surface becomes inhomogeneous, and the coarsening process is arrested way before the thermodynamic limit is reached, leaving a chaotic distribution of surface phases. (c) 1999 American Institute of Physics.     

Dewetting of a solid monolayer

We report on the dewetting of a monolayer on a solid substrate, where mass transport occurs via surface diffusion. For a wide range of parameters, a labyrinthine pattern of bilayer islands is formed. An irreversible regime and a thermodynamic regime are identified. In both regimes, the velocity of a dewetting front, the wavelength of the bilayer island pattern, and the rate of nucleation of dewetted zones are obtained. We also point out the existence of a scaling behavior, which is analyzed by means of a geometrical model.