Molecular dynamics simulation of nanoscale surface diffusion of heterogeneous adatoms clusters

Molecular dynamics simulation employing the embedded atom method potential is utilized to investigate nanoscale surface diffusion mechanisms of binary heterogeneous adatoms clusters at 300 K, 500 K, and 700 K. Surface diffusion of heterogeneous adatoms clusters can be vital for the binary island growth on the surface and can be useful for the formation of alloy-based thin film surface through atomic exchange process. The results of the diffusion process show that at 300 K, the diffusion of small adatoms clusters shows hopping, sliding, and shear motion; whereas for large adatoms clusters(hexamer and above), the diffusion is negligible. At 500 K, small adatoms clusters, i.e., dimer, show almost all possible diffusion mechanisms including the atomic exchange process; however no such exchange is observed for adatoms clusters greater than dimer. At 700 K, the exchange mechanism dominates for all types of clusters, where Zr adatoms show maximum tendency and Ag adatoms show minimum or no tendency toward the exchange process. Separation and recombination of one or more adatoms are also observed at 500 K and 700 K. The Ag adatoms also occupy pop-up positions over the adatoms clusters for short intervals. At 700 K, the vacancies are also generated in the vicinity of the adatoms cluster,vacancy formation, filling, and shifting can be observed from the results.     

沉积粒子能量对薄膜早期生长过程的影响

利用Monte Carlo(MC)模型研究了能量粒子对薄膜生长的初始阶段岛膜的形貌和岛的尺寸的影响,沉积粒子的能量范围为:0—0.7eV.在模型中考虑了原子沉积、吸附原子扩散和蒸发等过程,并详细考虑了临近和次临近原子的影响.结果表明,在所采用的参量范围内不同的基底温度情况下,能量粒子的影响有很大的区别.低基底温度情况下,沉积粒子强烈地影响着薄膜的生长过程中,岛膜的形貌、数量和尺寸随能量粒子的能量增加而有很大的变化.分析表明,这些变化都是由于能量粒子的介入使得表面吸附粒子的扩散能力增强所致 关键词： 薄膜生长 Monte Carlo方法 扩散     

Multilayer growth of BaTiO3 thin films via pulsed laser deposition: An energy-dependent kinetic Monte Carlo simulation

An energy-dependent kinetic Monte Carlo approach was proposed to simulate the multilayer growth of BaTiO₃ thin films via pulsed laser deposition, in which the four steps, such as the deposition of atoms, the diffusion of adatoms, the bonding of adatoms, and the surface migration of adatoms, were considered. Distinguishing with the traditional solid-on-solid (SOS) model, the adatom bonding and the overhanging of atoms, according to the perovskite structure, were specially adopted to describe the ferroelectric thin film growth. The activation energy was considered from the interactions between the ions, which were calculated by Born-Mayer-Huggins (BMH) potential. From the simulation the relative curves of the each layer coverage and roughness vs total coverage were obtained by varying the parameter values of the incident kinetic energy, laser repetition rate and mean deposition rate. The relationship between growth modes and the different parameters was also acquired.     

Adatom kinetics on and below the surface: the existence of a new diffusion channel

Employing density-functional theory in combination with scanning tunneling microscopy, we demonstrate that a thin metallic film on a semiconductor surface may open an efficient and hitherto not expected diffusion channel for lateral adatom transport: adatoms may prefer diffusion within this metallic layer rather than on top of the surface. Based on this concept, we interpret recent experiments: We explain why and when In acts as a surfactant on GaN surfaces, why Ga acts as an autosurfactant, and how this mechanism can be used to optimize group-III nitride growth.     

Atomic-level investigation of Al and Ni thin film growth on Ni(1 1 1) surface: Molecular dynamics simulation

Using molecular dynamics (MD) simulation, the structural characteristics of Al and Ni thin film growth on Ni(1 1 1) substrate according to the incident energy of adatoms were investigated. In case of Al on Ni(1 1 1), Al adatoms were grown basically through the layer-by-layer growth mode. On the other hand, Ni thin films on Ni(1 1 1) surface at low incident energy were shown to favor island growth. The steering effect due to atomic attraction, which results in rougher surface, was significantly observed at low incident energy. The growth mode of Ni film was, however, changed to follow layer-by-layer growth mode for the incident energy of 6 eV. The different aspects of surface morphology between Al and Ni deposition on Ni(1 1 1) surface could be successfully explained by the surface diffusion and impact cascade diffusion.     

非均匀基底上三维薄膜生长的模拟研究

考虑原子在基底表面的扩散、沿岛周界的扩散和不同层间的扩散以及非均匀基底上表面吸附能分布的各向异性，建立起非均匀基底表面上原子扩散和三维薄膜生长的动力学蒙特卡罗模型.模拟得到在不同生长条件下出现的层状生长、岛状生长和混合生长三种生长模式和相应的多层薄膜生长形貌图.通过统计三维薄膜中原子在各层的分布，计算薄膜的表面粗糙度，得到薄膜生长模式与生长条件之间的关系. 关键词： 薄膜生长 非均匀基底 动力学蒙特卡罗模拟     

Growth mechanism and morphology of Ge on Pb covered Si(111) surfaces

We study the mechanism and surface morphology in epitaxial growth of Ge on Pb covered Si(111) using a scanning tunneling microscope (STM). We find that Ge adatoms have a very large diffusion length at room temperature. The growth is close to perfect layer-by-layer for the first two bilayers. Surface roughness increases gradually with the film thickness, but no 3D islands are found at room temperature. For growth at 200°C, 3D Ge islands appear after completion of the second bilayer. At room temperature, we believe, the Pb layer enhances surface diffusion and the descending-step motion of Ge adatoms, but the ascending-step motion is hindered and thus 3D island growth is suppressed.     

Non-monotonous dependence of surface roughness on factors influencing energy of adatoms during thin island film growth

Many experimental results show that surface roughness of thin films can increase, decrease, stay constant or pass through the minimum with the change in substrate temperature, energy of arriving atoms or assisted beam (electrons, photons, ions), depending on material and interval of variation of those parameters. The aim of this paper is to explain and analyze this non-monotonous behavior of surface roughness by proposed kinetic model. The model is based on rate equations and includes processes of surface diffusion of adatoms, nucleation, growth and coalescence of islands in the case of thin films growth in Volmer-Weber mode. It is shown by modeling that non-monotonous dependence of surface roughness on the factors influencing energy of adatoms (e.g. temperature, assisted beam irradiation, accelerating voltage) occurs as a result of interplay between diffusion length of adatoms and size of islands, because both parameters depend on energy of adatoms. Variation of island size and diffusion length results in atomic jumps from islands forming rougher or smoother surface. The functions of surface roughness, island size, island density on diffusion length of adatoms and on other parameters are calculated and analyzed in this work.     

Diffusion-corrected simultaneous multilayer growth model

A diffusion-corrected simultaneous multilayer (DCSM) model was developed, taking into account up-step and down-step diffusion of adatoms and enabling rapid identification of different film growth modes. This DCSM model was applied to the initial growth of Cr on (100) SrTiO3 and (0001) alpha-Al2O3 surfaces, monitoring the deposition process by in situ Auger electron spectroscopy. We conclude with general remarks on the usefulness of the DCSM model for exploiting solid state wetting processes of thin metal films on different substrates.     

Surface Diffusion Induced Growth of Solid Solution over a Free Crystal Surface

It has been shown that interdiffusion along a free vicinal crystal surface can lead, at comparably low temperatures, to layered growth of solid solution over the surface without the involvement of bulk diffusion. The alloy concentration distribution along the surface, as well as the normal rate of solution growth has been calculated. The formation of a layer of the solid solution has been experimentally observed on a vicinal (111) surface of Ni single crystal as a result of surface interdiffusion between Ni and a thin film of Au deposited on a part of the Ni surface. The surface diffusion coefficients, as well as other parameters responsible for the exchange rate between the adatoms and kinks of elementary steps have been measured in the temperature range 550–700°C.     

Simulation of multilayer Cu/Pd（100） heteroepitaxial growth by pulse laser deposition

The heteroepitaxial growth of multilayer Cu/Pd(100) thin film via pulse laser deposition (PLD) at room temperature is simulated by using kinetic Monte Carlo (KMC) method with realistic physical parameters. The effects of mass transport between interlayers, edge diffusion of adatoms along the islands and instantaneous deposition are considered in the simulation model. Emphasis is placed on revealing the details of multilayer Cu/Pd(100) thin film growth and estimating the Ehrlich--Schwoebel (ES) barrier. It is shown that the instantaneous deposition in the PLD growth gives rise to the layer-by-layer growth mode, persisting up to about 9 monolayers (ML) of Cu/Pd(100). The ES barriers of The heteroepitaxial growth of multilayer Cu/Pd（100） thin film via pulse laser deposition （PLD） at room temperature is simulated by using kinetic Monte Carlo （KMC） method with realistic physical parameters. The effects of mass transport between interlayers, edge diffusion of adatoms along the islands and instantaneous deposition are considered in the simulation model, Emphasis is placed on revealing the details of multilayer Cu/Pd（100） thin film growth and estimating the Ehrlich-Schwoebel （ES） barrier. It is shown that the instantaneous deposition in the PLD growth gives rise to the layer-by-layer growth mode, persisting up to about 9 monolayers （ML） of Cu/Pd（100）. The ES barriers of 0.08 ± 0.01 eV is estimated by comparing the KMC simulation results with the real scanning tunnelling microscopy （STM） measurements,     

扩散理论对RLA模型中交换作用的研究

以扩散理论为基础，建立以“基本微观过程” 为核心的新模型，引入交换比的概念，对存在表面活化剂时薄膜生长的微观过程进行Kinetic Monte Carlo模拟.模拟发现，活化层原子和沉积原子都会发生跨层间的扩散，跨层扩散主要是单个原子的扩散，层间扩散的原子数目随着温度的升高或沉积厚度的增加而增多.RLA模型中的“交换作用”只是若干个“基本微观过程”的组合，大多数交换不是位置的“完全交换”，交换比也并非恒为1.     

Neural network approach to diffusion of B and N adatoms on the Pt(111) surface

Artificial neural network boasts an outstanding class in terms of its performance and efficiency and is being spread widely in most fields of physics. We report our investigation of the diffusions of boron and nitrogen adatoms on the Pt(111) surface by performing molecular dynamics simulations equipped with machine-learned potentials. Platinum is commonly used as a substrate for the growth of hexagonal boron nitride (h-BN) thin films, and the diffusion of B and N atoms on the substrate, which are decomposed from the precursor molecules, plays important roles in the initial stages of h-BN growth. The two-dimensional potential energy surfaces and the trajectories of the B and N adatoms are consistent with the DFT calculation. The Arrhenius plots of the diffusion coefficients produce the diffusion barriers of the B and N adatoms on Pt(111), which agree well with the DFT barriers.     

扩散理论对RLA模型中交换作用的研究

以扩散理论为基础,建立以“基本微观过程” 为核心的新模型,引入交换比的概念,对存在表面活化剂时薄膜生长的微观过程进行Kinetic Monte Carlo模拟.模拟发现,活化层原子和沉积原子都会发生跨层间的扩散,跨层扩散主要是单个原子的扩散,层间扩散的原子数目随着温度的升高或沉积厚度的增加而增多.RLA模型中的“交换作用”只是若干个“基本微观过程”的组合,大多数交换不是位置的“完全交换”,交换比也并非恒为1. 关键词： 扩散理论 薄膜生长 交换作用 RLA     

Glancing-angle ion enhanced surface diffusion on gaAs(001) during molecular beam epitaxy

We describe the effects of glancing incidence 3-4 keV Ar ion bombardment on homoepitaxial growth on vicinal GaAs(001). The average adatom lifetime on surface terraces, measured during growth using specular ion scattering, decreased monotonically with increasing ion current density. The results indicated that surface diffusivity was increased by the ions. The ion beam also suppressed growth oscillations and decreased the film surface roughness. This indicates a change from two-dimensional island nucleation to step-flow growth due to increased adatom surface diffusivity. A simple model, involving direct momentum transfer from ions to adatoms, is shown to be consistent with the measured enhanced diffusion.     

Kinetic Monte Carlo simulations of heteroepitaxial growth with an atomistic model of elasticity

We have implemented Kinetic Monte Carlo (KMC) simulations of growth of heteroepitaxial thin films. A simple cubic Solid-on-Solid (SOS) model is used to describe the atomic configurations and nearest neighbor bonds are used to describe the energetics. Elastic effects are modeled using harmonic springs between atoms displaced from their lattice positions. The misfit strain is a consequence of different equilibrium spring lengths for the substrate and film. The consistency of this elastic model with continuum theories for strained surfaces has been shown by performing elastic energy calculations for various morphologies. KMC simulations for submonolayer deposition show scaling behavior in the island size distribution. The resulting island shapes are predominantly square and do not show any shape transitions in the physically relevant range of conditions. This method gives a detailed understanding of elastic interactions and their interplay with surface diffusion in heteroepitaxial systems.     

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

The growth of Fe on Au(111) at 300 K 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.     

Pseudo-epitaxial C60 films prepared by a hot-wall method

We have used both reflection-geometry and grazing-incidence-geometry X-ray scattering to study thin films of C₆₀ evaporated onto mica substrates via a hot-wall technique. The growth mode yields close-packed C₆₀ planes, which are parallel to the substrate surface and which exhibit out-of-plane correlation lengths of 850 Å. In the film plane the C₆₀ is at best pseudo-epitaxial, with a 0.9° distribution of crystallite orientations, a 450 Å in-plane correlation length, and a 3.7% lattice mismatch, better than obtained by other thin film techniques but far from the accepted definition of single crystal thin film epitaxy.     

Organic chemistry at interfaces studied by optical second-harmonic and IR-vis sum-frequency generation

Three selected systems are presented which demonstrate the versatility of second-order nonlinear techniques for interface analysis and thin film monitoring. With second-harmonic generation processes at buried interfaces can be monitored in real time as is shown by thiol adsorption on polycrystalline gold from solution. The time dependent second-harmonic signal shows a strong wavelength dependence due to resonances. In situ monitoring of thin-film formation by second-harmonic generation can differentiate between the reaction at the interface and the subsequent film growth. The chemical vapor deposition of polyamic acid is presented as example. Hydrogen adsorption on diamond C(111) is analysed by sum-frequency generation. It is demonstrated that sum-frequency generation has the sensitivity and specificity to trace chemical reactions at surfaces and to infer on the orientation of chemical bonds.     

Revision of single atom local density and capture number varying with coverage in uniform depletion approximation and its effect on coalescence and number of stable clusters

In vapour deposition,single atoms (adatoms) on the substrate surface are the main source of growth.The change in its density plays a decisive role in the growth of thin films and quantum size islands.In the nucleation and cluster coalescence stages of vapour deposition,the growth of stable clusters occurs on the substrate surface covered by stable clusters.Nucleation occurs in the non-covered part,while the total area covered by stable clusters on the substrate surface will gradually increase.Carefully taking into account the coverage effect,a revised single atom density rate equation is given for the famous and widely used thin-film rate equation theory,but the work of solving the revised equation has not been done.In this paper,we solve the equation and obtain the single-atom density and capture number by using a uniform depletion approximation.We determine that the single atom density is much lower than that evaluated from the single atom density rate equation in the traditional rate equation theory when the stable cluster coverage fraction is large,and it goes down very fast with an increase in the coverage fraction.The revised equation gives a higher value for the ’average’ capture number than the present equation.It also increases with increasing coverage.That makes the preparation of single crystalline thin film materials difficult and the size control of quantum size islands complicated.We also discuss the effect of the revision on coalescence and the number of stable clusters in vapour deposition.