NOVEL FORMATION AND DECAY MECHANISMS OF NANOSTRUCTURES ON THE SURFACE

For decades the research on thin-film growth has attracted considerable attention as these kinds of materials have the potential for a new generation of device application. It is known that the nuclei at the initial stage of the islands are more stable than others and certain atoms are inert while others are active. In this paper, by using kinetic Monte Carlo simulations, we will show that, when a surfactant layer is used to mediate the growth, a counter-intuitive fractal-to-compact island shape transition can be induced by increasing deposition flux or decreasing growth temperature. Specifically, we introduce a reaction-limited aggregation (RLA) theory, where the physical process controlling the island shape transition is the shielding effect of adatoms stuck to the stable islands on the incoming adatoms. Moreover, the origin of a transition from triangular to hexagonal and then to inverted triangular as well as the decay characteristics of three-dimensional islands on the surface and relations of our unique predictions with recent experiments will be discussed. Furthermore, we will present a novel idea to make use of the condensation energy of adatoms to control the island evolution along a special direction.     

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.     

Growth mechanisms for wire-like epitaxial gold silicide islands on Si(1 1 0) surfaces

Epitaxial islands grown on various substrates are usually strained because of differences in lattice constants of the materials of the island and the substrate. Shape transition in the growth of strained islands has been proposed as a mechanism for strain relief and a way to form self-organized quantum wires. Shape transition usually leads to an elongated island growth. However, an elongated island growth may also be due to an anisotropic diffusion of material, the anisotropy being imposed by the symmetry of the substrate surface. In the present example, growth of gold silicide wire-like nanostructures on a Si(1 1 0) surface has been investigated by photoemission electron microscopy (PEEM). Growth of elongated unidirectional gold silicide islands, with an aspect ratio as large as 12:1, has been observed by PEEM following gold deposition on the Si substrate and subsequent annealing at the Au-Si eutectic temperature. Distribution of the width and the length of the gold silicide islands as a function of island area shows a feature similar to that for the shape transition. However, detailed investigations reveal that the elongated growth of gold silicide islands is rather mainly due to anisotropic diffusion of gold due to the twofold symmetry of the (1 1 0) surface of the Si substrate.     

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。     

Bistability in the shape transition of strained islands

The equilibrium shape of a monatomic strained island on a substrate depends on the step free energies and the difference in surface stress between the island and the substrate. For small island sizes the step free energies dominate, resulting in compact islands. Beyond a critical island size, however, the strain energy becomes dominant and the island maximizes its perimeter, resulting in elongated islands. Here we show that for strained islands with force monopoles pointing in opposing directions at neighboring steps, a regime exists near the critical island size where both compact and elongated shapes can coexist.     

石墨上纳米尺度金岛的形成

用扫描隧道显微镜研究了真空蒸镀在高定向热解石墨上金岛的形成和形状．随着蒸镀量的增加及时间的推移，金原子在表面通过扩散而逐渐合并成越来越大的原子团，以至岛，甚至岛群．虽然几个纳米大小的原子团仍十分可动，但在蒸镀量大于20单层时的岛或岛群已十分稳定．研究发现，同一蒸镀量下，各个金岛具有非常接近的宽度和高度，用薄膜成核的圆柱状模型计算岛的宽度与高度之比表明，金岛非常接近热力学平衡状态．这些形状各异的通过生长而自组装形成的纳米尺度金岛可用来进行介观物理的研究 关键词：     

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     

Metal-semiconductor nanocontacts: silicon nanowires

Strain evolution of coherent Ge islands on Si(001) is measured using a newly developed transmission electron microscopy technique based on two-beam dark-field strain imaging. The strain measurements show that a metastable Ge island shape is involved in the shape transition between pyramids and domes; this shape is more readily observed for growth at 550 than 600 degrees C because of the slower rate at which islands cross the kinetic barrier between shapes. The strain relaxation changes discontinuously between pyramids and domes, indicating that the underlying shape transition is first order.     

Monte carlo simulation of transitions related to growth on (001) faces of Si and Ge

Based on Monte Carlo simulations for molecular beam epitaxy, three types of growth related transitions on the Si(001) and Ge(001) surfaces have been studied. In the thermal roughening simulations on a Ge(001) surface, a different type of transition from the Kosterlitz and Thouless type is obtained. The simulated result is consistent with the experimental x-ray diffraction data, at least qualitatively. In the growth simulations, a transition in the shape of growing islands is shown at the very initial stage of the homoepitaxial growth on a Si(001)-2x1 flat surface. During the transition, the step density variations as a function time show different behaviors at various temperatures. In the homoepitaxial growth on Si(001)-2x1 vicinal surfaces, the growth mode transition from two-dimensional island formation to the step-flow mode is reproduced by increasing the system temperature, which agrees qualitatively with the observed results. At the intermediate temperature, a transient growth mode is obtained, in which the two-dimensional island formation and the step flow growth modes coexist on two types of terraces on the surface.     

薄膜生长中的表面动力学(Ⅰ)

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和种动力学表现，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，作为研究的基础。在本文的第一部分（即0-6章）中，我们首先介绍了目前这方面理论研究中所主要使用的各种方法。例如，第一性原理计算，分子动力学模拟。蒙特卡罗模拟。速率方程和过渡态（TST）理论等。基于这些研究。我们介绍给读者为什么原子成岛时在低温下选择分形状，而在高温时则选择紧致状。这一过程可以用经典的扩散子限制集聚理论（Diffusion-Limited Aggregation,DLA)。然而当有表面活性剂存在时形核的规律安全相反，由此提出了一个反应限制集聚理论（Reaction-Limited Aggregation,RLA)，这两个理论目前可以很好的解释亚单层生长时的一般形核规律。接下来我们讨论了长程相互作用对生长初期原子形核的影响。并进一步得出了相应的标度理论。在第6章我们系统地研究了分了吸附对二维原子岛形状的控制性，从而提出了边-角原子扩散的对称破缺模型。     

薄膜生长中的表面动力学(Ⅰ)

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。作为研究的基础 ,在本文的第一部分 (即0～ 6章 )中 ,我们首先介绍了目前这方面理论研究中所主要使用的各种方法。例如 ,第一性原理计算 ,分子动力学模拟 ,蒙特卡罗模拟 ,速率方程和过渡态 (TST)理论等。基于这些研究 ,我们介绍给读者为什么原子成岛时在低温下选择分形状 ,而在高温时则选择紧致状。这一过程可以用经典的扩散子限制集聚理论 (Diffusion_LimitedAggregation ,DLA)。然而当有表面活性剂存在时形核的规律完全相反 ,由此提出了一个反应限制集聚理论 (Reaction_LimitedAggregation,RLA)。这两个理论目前可以很好地解释亚单层生长时的一般形核规律。接下来我们讨论了长程相互作用对生长初期原子形核的影响 ,并进一步得出了相应的标度理论。在第 6章我们系统地研究了分了吸附对二维原子岛形状的控制性 ,从而提出了边 角原子扩散的对称破缺模型。     

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.     

Formation of quasi-one-dimensional Cu2O structures by in situ oxidation of Cu(100)

Epitaxial Cu2O islands on a Cu(100) surface formed through oxidizing Cu(100) films at 600 degrees C in an ultrahigh vacuum transmission electron microscope were observed to undergo a shape transition from initially square shaped islands to elongated islands at a critical size of approximately 110 nm. Our experimental data on the elongation of Cu2O islands agree with the energetic calculations based on the balance between surface and interface energies and the elastic stress relaxation in the three-dimensional islands. We developed a kinetic model based on oxygen surface diffusion that fits well with the observed volume evolution of the Cu2O islands.     

Au/Cu(001)异质外延岛演化的分子动力学研究

利用分子动力学弛豫方法模拟了Au/Cu(001)异质外延生长初期Au异质外延岛的形貌演化，分析了Au外延岛演化过程中的局域应力及与基体结合能随表面岛尺寸的变化. 研究结果表明：当异质外延岛小于7×7时，外延岛原子分布呈现赝Cu点阵形貌；当外延岛达到8×8后，外延岛内开始出现失配位错，失配位错数量随外延岛尺寸的增加而增加. 局域压力分析指出，外延岛上原子之间的近邻环境不同导致了所受应力的差异，而外延岛的形变则是由外延岛原子的应力分布所决定. 研究还发现，失配位错的产生导致错位原子与基体原子之间的结合强度减弱，但相对增加了非错位原子与基体原子之间的结合强度. 关键词： 异质外延 表面形貌 局域压力 分子动力学模拟     

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.     

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.     

Structure and magnetic properties of epitaxial cobalt islands

Growth, structural and magnetic properties of cobalt island structures grown by molecular beam epitaxy on 4 nm thick island shaped Ru buffers are studied. The islands are characterized by a narrow distribution of sizes, and are separated on average by spacing of around one hundred nanometers. Growth of the islands into continuous films is found to proceed in three well defined stages, and corresponding magnetic behaviors are identified. The particles formed in the first stage of growth appear to be similar in shape and size, and their magnetic behavior suggest the presence of superparamagnetism. Particles in the second stage of growth are ferromagnetic and display a reorientation transition of their magnetization out of the film plane, consistent with that seen in patterned magnetic dot arrays. Particles coalesce in the last stage of growth and magnetic properties of continuous films are observed. Received 24 August 2001     

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.     

Unified phase-field model for epitaxial growth of multi-scale three-dimensional islands on insulating surfaces

In this paper we present a unified phase-field model for non-equilibrium growths of various three-dimensional metal islands on insulating surfaces. We introduce a phase-field variable to distinguish the island from the non-island regions and substrate and a density variable to describe local density of deposited adatoms. Two partial differential equations with appropriate boundary conditions, as the governing equations, are used to describe the evolution of the three-dimensional metal islands and the diffusion of adatoms. We solve the equations by using an adaptive mesh refinement method so that we can simulate the non-equilibrium growth of three-dimensional metal islands from tens of nanometers to several micrometers. We investigate the dependence of simulated results on the model parameters and experimental conditions. Equilibrium shape of such islands can be obtained through sufficient post-deposition relaxation. Experimental trends of island size and shape on various scales are obtained with reasonable parameters. This method should be a good approach to non-equilibrium growths of multi-scale three-dimensional metal islands.