Time evolution of adatom and vacancy clusters on Ag(110)

The time evolution of adatom and vacancy islands on an Ag(110) surface is studied by using a variable-temperature ultrahigh vacuum scanning tunneling microscope. The islands are created by Ar⁺ ion sputtering at low temperatures and at very low fluxes (0.004 ML s⁻¹). The area of both kinds of island decreases linearly as a function of time, at least for small islands. The experimental results are compared with kinetic Monte Carlo simulations.     

Magnetization reversal of individual Co nanoislands

We investigate the magnetization reversal of individual Co islands on Cu(111) in the size range of N=700 to 18,000 atoms by spin-polarized scanning tunneling microscopy at 8 K. The switching field H(sw) changes with island size in a nonmonotonic manner: it increases with island size and reaches a maximum value of 2.4 T at N=5500 atoms, and it decreases for larger islands. We extract the energy barrier for magnetization reversal as a function of island size. The maximum H(sw) corresponds to an energy barrier of 1 eV. Our results elucidate a crossover of the magnetization reversal from an exchange-spring behavior to domain wall formation with increasing size at around 7500 atoms.     

Structure of gold island films

Structure of island gold films on carbon and glass substrates is investigated in the present work. The films were deposited by means of vacuum evaporation at constant pressure, temperature and deposition rate. Shapes, diameters, number densities and statistical distributions of island sizes are investigated in the region of the proper growth of island films. Experimental values are compared with the latest considerations on the coalescence theory. In the region of thicknesses where the number density of islands increases, their sizes do not change and the islands have the circular symmetry. In the region with the decreasing number density the island sizes increase, they have approximately the elliptical symmetry and the shapes of the statistical distributions of island sizes are different. It follows from the above facts that the mechanism of the origin of the two types of islands is different.     

薄膜外延生长的计算机模拟

以Cu膜为例，用Monte-Carlo算法模拟了薄膜生长的随机过程，并提出了更加完善的模型.在合理选择原子间相互作用计算方法的基础上，考虑了原子的吸附、在生长表面的迁移及迁移所引起的近邻原子连带效应、从生长表面的脱附等过程.模拟计算了薄膜的早期成核情况以及表面粗糙度和相对密度.结果表明，随着衬底温度的升高或入射率的降低，沉积在衬底上的原子逐步由离散型分布向聚集状态过渡形成一些岛核，并且逐步由二维岛核向三维岛核过渡.在一定的原子入射率下，存在三个优化温度，成核率最高时的最大成核温度Tn、薄膜的表面粗糙度最低 关键词： Monte-Carlo算法 计算机模拟 薄膜生长     

Shape, facet evolution and photoluminescence of Ge islands capped with Si at different temperatures

We investigate the embedding of Ge islands in a Si matrix by means of atomic force microscopy and photoluminescence (PL) spectroscopy. The Ge islands were grown between 360°C and 840°C and subsequently capped with Si at different temperatures. For the highest Ge growth temperature (840°C), we show that the surface flattens at high Si capping temperatures while new facets can be identified at the island base for intermediate capping temperatures (650–450°C). At low capping temperatures (300–350°C), the island morphology is preserved. In contrast to the observed island shape changes, the decreasing Si capping temperature causes only a small redshift of the island related PL signal for islands grown on high temperatures. This redshift increases for Ge islands grown at lower temperatures due to an increased Ge content in the islands. By applying low-temperature capping (300°C) on the different island types, we show that the emission wavelength can be extended up to 2.06 μm for hut clusters grown at 400°C. Further decreasing of the island growth temperature to 360°C leads to a PL blueshift, which is explained by charge carrier confinement in Ge quantum dots.     

Oscillations and island evolution in radiating diffusion flames

We show how an island (isola) evolves out of the usual S-curve of steady states of diffusion flames when radiation losses are accounted for and how it eventually disappears when radiation increases further. At small activation temperatures there are never any islands. We show that stable oscillations evolve first out of perturbations of steady states on the S-curve at large Damköhler numbers. Only if the activation temperature is large enough do they also appear on the islands. The region of the stable oscillations grows larger as activation temperature decreases.     

Probing the statistics of transport in the Hénon Map

The phase space of an area-preserving map typically contains infinitely many elliptic islands embedded in a chaotic sea. Orbits near the boundary of a chaotic region have been observed to stick for long times, strongly influencing their transport properties. The boundary is composed of invariant “boundary circles.” We briefly report recent results of the distribution of rotation numbers of boundary circles for the Hénon quadratic map and show that the probability of occurrence of small integer entries of their continued fraction expansions is larger than would be expected for a number chosen at random. However, large integer entries occur with probabilities distributed proportionally to the random case. The probability distributions of ratios of fluxes through island chains is reported as well. These island chains are neighbours in the sense of the Meiss-Ott Markov-tree model. Two distinct universality families are found. The distributions of the ratio between the flux and orbital period are also presented. All of these results have implications for models of transport in mixed phase space.     

Observation of "Ghost" islands and surfactant effect of surface gallium atoms during GaN growth by molecular beam epitaxy

We observe "ghost" islands formed on terraces during homoepitaxial nucleation of GaN. We attribute the ghost islands to intermediate nucleation states, which can be driven into "normal" islands by scanning tunneling microscopy. The formation of ghost islands is related to excess Ga atoms on the surface. The excess Ga also affect island number density: by increasing Ga coverage, the island density first decreases, reaching a minimum at about 1 monolayer (ML) Ga and then increases rapidly for coverages above 1 ML. This nonmonotonic behavior points to a surfactant effect of the Ga atoms.     

Fast dimer diffusion near the island: A computational study

Performing atomic scale simulations, we study the diffusion of small Co clusters on Cu(111) surface in the presence of Co islands. It is revealed that mesoscopic relaxations at the island-substrate interface play an important role during the diffusion events of the clusters. The diffusion barrier of small Co clusters near the islands increases with increasing cluster size. Especially, we find that dimer diffusion near the island with B step is as fast as monomer’s, which might be responsible for the formation of dendritic shaped islands at low temperature.     

Simulation of multilayer homoepitaxial growth on Cu （100） surface

The processes of multilayer thin Cu films grown on Cu (100) surfaces at elevated temperature (250--400\,K) are simulated by mean of kinetic Monte Carlo (KMC) method, where the realistic growth model and physical parameters are used. The effects of small island (dimer and trimer) diffusion, edge diffusion along the islands, exchange of the adatom with an atom in the existing island, as well as mass transport between interlayers are included in the simulation model. Emphasis is placed on revealing the influence of the Ehrlich--Schwoebel (ES) barrier on growth mode and morphology during multilayer thin film growth. We present numerical evidence that the ES barrier does exist for the Cu/Cu(100) system and an ES barrier $E_{\rm B} >0.125$\,eV is estimated from a comparison of the KMC simulation with the realistic experimental images. The transitions of growth modes with growth conditions and the influence of exchange barrier on growth mode are also investigated.     

Effect of self-assembled Ge nanostructures on Si surface electronic properties

Incorporating self-assembled Ge islands on Si surfaces into electronic devices has been suggested as a means of forming small features without fine-scale litho- graphy. For use in electronic devices, the electrical properties of the deposited Ge and their relation to the underlying Si substrate must be known. This report presents the results of a surface photovoltage investigation of the surface energy barrier as increasing amounts of Ge are added to a Si surface by chemical vapor deposition. The results are interpreted in terms of band discontinuities and surface states. The surface barrier increases as a wetting layer is deposited and continues to increase as defect-free islands form. It saturates as the islands grow. As the amount of Ge continues increasing, defects form, and the surface barrier decreases because of the resulting allowed states at the Ge/Si interface. Qualitatively similar behavior is found for Si(001) and Si(111). Covering the Ge with Si reduces the surface-state density and possibly modifies the wetting layer, decreasing the barrier to one more characteristic of Si. Initial hydrogen termination of the surface decreases the active surface-state density. As the H desorbs, the surface barrier increases until it stabilizes as the surface oxidizes. The behavior is briefly correlated with scanning-tunneling spectroscopy data. Received: 13 November 2000 / Accepted: 14 November 2000 / Published online: 23 May 2001     

Growth of Ag on Pb Island with Si(111) Substrate

We study the growth of Ag on Pb island surface with low temperature scanning tunnelling microscopy.Two growth modes,the subsurface island mode and the surface alloy mode,are observed on tie substrate at room temperature and at 100 K,respectively.In the surface alloy mode,the perfect alloy AgPb_2 is formed on the Pb island surface after annealing.The two growth modes at different substrate temperatures are attributed to the existence of an exchange barrier of Ag atoms on the Pb island surface.The modulation of the exchange barrier by the quantum well states is also observed on the Pb island.     

STUDY OF THE MECHANISM OF GaAs(001) MOLECULAR-BEAM EPITAXY

In this paper the nucleation and growth processes of GaAs(001) molecular-beam epitaxy were studied by Monte Carlo simulation, The density of islands and the density of isolated Ga adatoms were obtained for different growth temperatures, and the island size distribution at low coverage, as well as the correlation function between atoms and its relation with the temperature were studied in detail.     

Scaling of submonolayer island growth with reversible adatom exchange in surfactant-mediated epitaxy

Surfactant-mediated epitaxial growth is studied with a realistic model, which includes three main kinetic processes: diffusion of adatoms on the surfactant terrace, exchange of adatoms with their underneath surfactant atoms, and reexchange in which an exchanged adatom resurfaces to the top of the surfactant layer. The scaling behavior of nucleus density and island size distributions in the initial stage of growth is investigated by using kinetic Monte Carlo simulations. The results show that the temperature dependence of nucleus density and island size distributions governed by the reexchanging-controlled nucleation at high temperatures exhibits similar scaling behavior to that obtained by the standard diffusion-mediated nucleation at low temperatures. However, at intermediate temperatures, the exchanging-controlled nucleation leads to an increase of nucleus density with temperature, while the island size distribution scales to a monotonically decreasing function, showing nonstandard scaling behavior.     

Nanowedge island formation on Mo(1 1 0)

The deposition of ultrathin Fe films on the Mo(1 1 0) surface at elevated temperatures results in the formation of distinctive nanowedge islands. The model of island formation presented in this work is based on both experiment and DFT calculations of Fe adatom hopping barriers. Also, a number of classical molecular dynamics simulations were carried out to illustrate fragments of the model. The islands are formed during a transition from a nanostripe morphology at around 2 ML coverage through a Bales-Zangwill type instability. Islands nucleate when the meandering step fronts are sufficiently roughened to produce a substantial overlap between adjacent steps. The islands propagate along the substrate [0 0 1] direction due to anisotropic diffusion/capture processes along the island edges. It was found that the substrate steps limit adatom diffusion and provide heterogeneous nucleation sites, resulting in a higher density of islands on a vicinal surface. As the islands can be several layers thick at their thinnest end, we propose that adatoms entering the islands undertake a so-called “vertical climb” along the sides of the island. This is facilitated by the presence of mismatch-induced dislocations that thread to the sides of the islands and produce local maxima of compressive strain. Dislocation lines also trigger initial nucleation on the surface with 2-3 ML Fe coverage. The sides of the nanowedge islands typically form along low-index crystallographic directions but can also form along dislocation lines or the substrate miscut direction.     

Ag growth on Si(111) with an Sb surfactant investigated by scanning tunneling microscopy

We have investigated a room-temperature growth mode of ultrathin Ag films on a Si(111) surface with an Sb surfactant using STM in a UHV system. On the Sb-passivated Si surface, small sized islands were formed up to 1.1 ML. Flat Ag islands were dominant at 2.1 ML, coalescing into larger islands at 3.2 ML. Although the initial growth mode of Ag films on the Sb-terminated Si(111) surface was Volmer-Weber (island growth), the films were much more uniform than Ag growth on clean (Si(111) at the higher coverages. From the analysis of STM images of Ag films grown with and without an Sb surfactant, the uniform growth of Ag films using an Sb surfactant appears to be caused by the kinetic effects of Ag on the preadsorbed Sb layer. Our STM results indicated that Sb suppresses the surface diffusion of Ag atoms and increases the Ag-island density. The increased island density is believed to cause coalescence of Ag islands at higher coverages of Ag, resulting in the growth of atomically flat and uniform Ag islands on the Sb surfactant layer.     

反应限制聚集模型的动力学行为的研究

根据反应限制聚集（reaction limited aggregation, RLA）模型,研究表面活性剂存在时 的薄膜外延生长动力学过程. 研究结果表明,在二维岛的生长初期,分形岛与紧致岛具有不 同的岛密度和“死”原子密度（岛的相对总面积）增长方式：分形岛密度随覆盖率生长指数 小于1,紧致岛密度的生长指数大于1;分形岛相对总面积随覆盖率线性增长,紧致岛相对总 面积随覆盖率非线性增长. 关键词： 反应限制聚集模型 岛密度 “死”原子密度     

Dendrochronology of strain-relaxed islands

We report on the observation and study of tree-ring structures below dislocated SiGe islands (superdomes) grown on Si(001) substrates. Analogous to the study of tree rings (dendrochronology), these footprints enable us to gain unambiguous information on the growth and evolution of superdomes and their neighboring islands. The temperature dependence of the critical volume for dislocation introduction is measured and related to the composition of the islands. We show clearly that island coalescence is the dominant pathway towards dislocation nucleation at low temperatures, while at higher temperatures anomalous coarsening is effective and leads to the formation of a depletion region around superdomes.     

On the Statistics of Nucleation of 3D Coherent Islands in Heteroepitaxy from a Molecular Beam

Within the framework of the cell method, the statistics of nucleation on the vertex of a 2D island is analyzed considering the influence of elastic stresses on the bond energy of an atom in a disruption. It is shown that elastic stresses at high temperatures may decrease the nucleation time as the temperature increases and the growth rate decreases. The results obtained agree qualitatively with the experimental data on the formation of 3D coherent islands in heteroepitaxy of semiconductors in molecular-beam epitaxy systems.     

Temperature dependent low energy electron microscopy study of Ge growth on Si(1 1 3)

We investigated the initial Ge nucleation and Ge island growth on a Si(1 1 3) surface using low energy electron microscopy and low energy electron diffraction. The sample temperature was varied systematically between 380 °C and 590 °C. In this range, a strong temperature dependence of the island shape is observed. With increasing temperature the Ge islands are elongated in the direction. Simultaneously, the average island size increases while their density decreases. From the Arrhenius-like behaviour of the island density, a Ge adatom diffusion barrier height of about 0.53 eV is deduced.