扫描隧道显微镜针尖调制的薄膜表面的原子扩散

采用原子尺度的模拟方法,探讨了在零偏压下扫描隧道显微镜(STM)针尖调制的金属表面岛上原子运动及岛边的层间质量输运. 研究结果显示STM的移动对岛上及岛边的原子扩散有重要的影响. 针尖与吸附原子的交互作用及岛和基体中强的形状变化影响了岛上吸附原子的跳跃扩散及岛边的跳下扩散和交换扩散过程. 研究发现,通过调节针尖与基体的垂直距离及针尖与吸附原子的水平距离,可以降低岛上吸附原子的跳跃扩散能垒及岛边的跳下扩散和交换扩散能垒,从而实现薄膜由三维生长模式向二维生长模式的转变. 关键词： 扫描隧道显微镜 原子运动 质量输运     

Cu dimer diffusion on strained Cu(0 0 1)

Cu dimer diffusion energy barrier on strained Cu(0 0 1) surfaces has been studied with nudged elastic band method (NEB) and embedded atom method (EAM). Dimer exchange and hopping mechanisms are chosen as the initial diffusion paths in the NEB method. It is shown here that the dimer exchange is dominant on tensile surfaces and the dimer hopping is dominant on compressive surfaces. For most strain conditions Cu dimer diffusion energy barrier is lower than Cu monomer diffusion barrier. The concerted movement of the remaining adatom toward the hopping adatom lowers the dimer hopping barrier. The adsorption induced relaxation makes the dimer exchange barriers lower than the monomer exchange barriers on tensile surfaces. Transition state theory is used to calculate the diffusion frequencies as a function of temperature. No surface crowdion is observed on the shear strained surfaces for the dimer diffusion.     

The effect of embedded Pb on Cu diffusion on Pb/Cu(1 1 1) surface alloys

We have used scanning tunneling microscopy and low-energy electron microscopy to measure the thermal decay of two-dimensional Cu, Pb-overlayer, and Pb-Cu alloy islands on Pb-Cu(1 1 1) surface alloys. Decay rates covering 6-7 orders of magnitude are accessible by applying the two techniques to the same system. We find that Cu adatom diffusion across the surface alloy is rate-limiting for the decay of both Pb and Pb-Cu islands on the surface alloy and that this rate decreases monotonically with increasing Pb concentration in the alloy. The decrease is attributed to repulsive interactions between Cu adatoms and embedded Pb atoms in the surface alloy. The measured temperature dependences of island decay rates are consistent with first-principles calculations of the Cu binding and diffusion energies related to this “site-blocking” effect.     

Atomic diffusion inside a STM junction: simulations by kinetic Monte Carlo coupled to tunneling current calculations

The influence of a static scanning tunneling microscope (STM) tip on the diffusion of xenon atoms adsorbed on a Cu(1 1 0) stepped surface is studied. Semi-empirical potentials for the Xe-surface interaction and a N-body energy based method for the Xe-tip contribution are used to calculate the adsorption energy of adsorbates in the STM junction. First, we analyse the variation of this energy when the adatom is placed near a step edge and for different tip positions. When the tip is situated in the neighbourhood of the step edge, the Ehrlich-Schwoebel barrier experienced by the adatom is lowered. This opens a specific diffusion channel, allowing a possible crossing of the step edge. Second, through a kinetic Monte Carlo approach coupled to the elastic scattering quantum chemistry method, the noisy tunneling current created by the random motion of diffusing atoms in the vicinity of the tip can be analyzed. We show that, by counting the number of diffusion events, we can determine effective barriers related to the most dominant processes contributing to the diffusion at a particular temperature. We also demonstrate that the interaction mode of the tip (attractive or imaging) greatly modifies the diffusion processes.     

Study of the effects of an adatom Sn on the Cu surface electromigration using a first principles method

It is well known that the doped Sn can effectively suppress the electromigration of Cu thin-film interconnects in integrated circuits. In this paper, the first-principles method was introduced to investigate the suppression mechanism. All the calculations were performed on Sn or Cu adatom/Cu (0 0 1), (1 1 0) and (1 1 1) surface systems within local density approximation. As a Sn adatom was attached to the Cu surface, stable CuSn bonds were formed. The energy calculations show that the Sn/Cu system was more stable than Cu/Cu system with the same structure. Analysis of density of states shows that the nearest neighbor Cu atoms were stabilized by CuSn bond relative to the Cu atoms which are far away from the adatom. The diffusion barrier energies were calculated and found to be consistent with the experimental results. Also, bond population analysis shows that stronger covalent bonds were formed between Sn and Cu relative to that between Cu and Cu. All the results indicate that the CuSn bond plays an important role in the suppression of Cu electromigration.     

分子动力学模拟Gd原子在Cu(110)表面的扩散过程

为了分析Gd吸附原子在Cu(110)表面的扩散机理，用分子动力学对该扩散过程进行模拟.模拟 结果表明在［1 1 0］方向Gd原子通过跳跃机理扩散，而且多步跳跃频率很高.而在［0 0 1］方向则通过交换机理扩散.吸附原子在［1 1 0］方向的扩散能力要比［0 0 1］ 方向强.通过对扩散频率的拟合，发现两种扩散机理都符合Arrhenius公式，从而确定了跳跃 机理的扩散势垒为0.097eV，交换机理的扩散势垒为0.33eV.另外还用能量弛豫的方法确定了 跳跃机理的扩散势垒. 关键词： 分子动力学 表面扩散 跳跃机理 交换机理 扩散势垒     

Monolayered adatom aggregation induced by metallofullerene molecules on Cu(100)

The adsorption and self-assembly of Gd@C₈₂ molecules on Cu(100) surface have been investigated using scanning tunneling microscopy (STM). The metallofullerene molecules in the assemblies showed two characteristic apparent heights in the STM images. STM manipulation and spectroscopy was performed and revealed the formation of Cu adatom islands underneath the Gd@C₈₂ molecules. The monolayered Cu aggregates were resulted from the adatom–molecule complexation, which is supported by density functional theory (DFT) calculations that show charge transfer and electrostatic interactions between Gd@C₈₂ and adatoms. In addition, sub-molecularly resolved STM images demonstrated the structural and orientational ordering of Gd@C₈₂ assemblies upon thermal annealing. DFT calculations demonstrated that Gd atom located at the lower part of the carbon cage is a favored adsorption configuration for Gd@C₈₂ molecules adsorbed on Cu(100).     

异质扩散过程中ES势垒的计算

利用分子动力学中的静态结构计算方法对Pd，Ag及Cu原子在面心立方铜的台阶表面扩散过程中的Ehrlich-Schwoebel(ES)势垒进行了模拟计算，研究了各种台阶表面情况下增原子扩散过程中的ES势垒；讨论了与衬底互溶的金属和与衬底不互溶的金属增原子扩散的ES势垒的异同，并将模拟结果与同质情况的研究结果进行了对比. 结果表明： 1)在同质和异质扩散过程中ES势垒随着台阶高度的变化关系是相似的，即随着台阶高度的增加，ES势垒逐渐增加；当台阶高度达到某一高度时ES势垒将趋于定值. 2)在跳跃机理下，与Cu互溶的金属(Pd)在Cu表面台阶上扩散的ES势垒最大，其次是Cu，最小的是与Cu不互溶的金属 (Ag)；而在交换机理下，与Cu不互溶的金属(Ag)在Cu表面台阶上扩散的ES势垒最大，其次是Cu，最小的是与Cu互溶的金属(Pd). 3)对大多数台阶的情况，交换机理支配着原子在台阶边缘的扩散行为；且表面台阶高度对交换扩散过程影响较大.     

STM-mediated atom motion: a Co atom and mixed CoCu<Subscript>n</Subscript> chains on a Cu(111) surface

Performing atomic scale simulations, we study the effect of the scanning tunneling microscopy tip on atom motion on a metal surface at zero bias voltage. We concentrate on a Co atom and mixed CoCu _n (n ? 68) chains on a Cu(111) surface. It is revealed that the atom motion can be tuned by adjusting the tip-substrate distance. The change in the potential landscape induced by the tip is found to depend on the tip height. In the presence of the tip, the Co atom can freely jump from the fcc site to the hcp site or vice versa when putting the tip above the adatom at a certain height. For the mixed CoCu _n chains on the Cu(111) surface, the diffusion barrier of the end Co atom from the fcc site to the nearby hcp site increases with the increasing chain length and reaches the limit when the chain length is beyond CoCu₇ without the tip. Especially, the short chains can perform a collective motion with the help of the tip. The importance of the relaxation induced by the tip-adatom interaction is demonstrated.     

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.     

Excitation of frustrated translation and nonadiabatic adatom hopping induced by inelastic tunneling

The dynamics of lateral manipulation for cobalt/Cu(111) has been investigated combining the model of vibrational heating and first-principles density functional calculations. The frustrated translational mode responsible for lateral excitation is identified as a vibrational resonance involving a concerted motion between the adatom and surface phonons. The calculated frequency shows good agreement with the onset energy for adatom hopping induced by inelastic tunneling. Simulation of the power law, compared with experiment, suggests that the atom hopping overcomes a nonadiabatic barrier due to the nonequilibrium local heating of the translational mode.     

Ab-initio calculations of interactions between Cu adatoms on Cu(1 1 0): Sensitivity of strong multi-site interactions to adatom relaxations

We have parameterized the various interactions between Cu adatoms on Cu(1 1 0) using density-functional theory based ab-initio calculations. Our results indicate that in addition to pair interactions, 3-adatom and 4-adatom interactions of significant strengths are present in this system. This further stresses the importance of multi-site interactions in constructing a complete lattice–gas picture. Even though adding these multi-site interactions leads to good convergence in interaction energies, we find that some multi-site interactions are very sensitive to adatom relaxations. This makes the application of a simple lattice–gas picture inadequate for such surfaces. We also parameterize adatom interactions on this surface using the recently developed connector model. The connector model parameterization is as efficient as the parameterization using lattice–gas model. Further, we present diffusion barriers for nearest-neighbor (NN) and next-nearest-neighbor (NNN) hops on this surface.     

异质原子在Cu（001）表面扩散的分子动力学模拟

采用分子动力学方法模拟单个增原子Ag,Pd和Cu在Cu（001）表面上的扩散过程.通过对自扩散和异质扩散过程中扩散机制的观察,统计三种不同的增原子在不同温度下的扩散频率,拟合给出扩散势垒和扩散频率的指前因子,并与扩散势垒的静力学计算结果进行比较.结果表明：在800 K以下时,三种增原子均以简单跳跃机制为主扩散,与衬底不互溶的Ag增原子的跳跃频率最大,与衬底互溶的Pd增原子的跳跃频率最小.同质增原子与异质增原子的扩散频率和温度的关系均较好地符合Arrhenius公式,由Arrhenius公式拟合给出的三种不同增原子的扩散势垒与表面结构和增原子表面结合能有关.Pd和Cu增原子从跳跃机制为主向交换机制为主的转换温度分别在825和937 K左右. 关键词： 表面扩散 分子动力学模拟     

表面Cu原子间相互作用对Cu(001)表面跳跃扩散行为的影响

采用嵌入原子方法的原子间相互作用势，利用准静态分子动力学模拟研究了Cu原子在Cu(001)表面吸附所导致的基体晶格畸变以及对其附近的另一个吸附原子自扩散行为的影响.研究结果表明，吸附原子的存在可以导致多达10层的Cu基体晶格产生畸变.两个吸附原子所产生的晶格畸变应力场之间的相互作用，可以导致吸附原子运动活性的增加.通过比较同一路径上往返跳跃扩散势垒的差异发现，在原子间相互作用势的有效距离之外，两个吸附原子的扩散行为可以认为是存在晶格畸变应力场相互作用的两个独立吸附原子的扩散；在原子间相互作用势的有效距离之 关键词： 表面吸附原子 晶格畸变 表面二聚体 扩散     

Reliable lateral manipulation of a single Ag adatom on a Ag(1 1 1) surface with a trimer-apex tip

We study the reliability of the lateral manipulation of a single Ag adatom on a Ag(1 1 1) surface with the single-atom and trimer-apex tips based on molecular statics simulations using surface embedded-atom-method potential. The dependence of the manipulation reliability on tip height and orientation is investigated. For the single-atom tip the manipulation reliability increases monotonically with decreasing tip height, which is owing to the strengthened lateral tip-adatom interaction as the tip height lowers. For the trimer-apex tip, the manipulation reliability is sensitive to the tip orientation in the lower tip-height range, while in the higher tip-height range the manipulation reliability is independent of the tip orientation and moreover can be greatly improved due to the strong vertical attraction of the tip on the adatom as compared to the single-atom tip. We also compare these results to those for manipulating single Cu adatoms on the Cu(1 1 1) surface, reveal the underlying physics, and propose the method to improve the manipulation reliability for different systems.     

Link between adatom resonances and the Cu(111) Shockley surface state

Low-temperature scanning tunneling microscopy and spectroscopy at 7 K was used to assemble and characterize native adatom islands of successive size on the Cu(111) surface. Starting from the single adatom we observe the formation of a series of quantum states which merge into the well known two-dimensional Shockley surface state in the limit of large islands. Our experiments reveal a natural physical link between this fundamental surface property and the sp(z) hybrid resonance associated with the single Cu/Cu(111) adatom.     

Formation and diffusion of S-decorated Cu clusters on Cu(111)

S-decorated Cu trimers are a likely agent of S-enhanced Cu transport between islands on Cu(111). According to ab initio calculations, excellent S bonding to trimer-Cu dangling valence results in an ad- Cu(3)S(3) formation energy of only approximately 0.28 eV, compared to 0.79 eV for a self-adsorbed Cu atom, and a diffusion barrier < or =0. 35 eV.     

A multi-scale Monte Carlo study of oxide structures on the Cu(1 0 0) surface

We present a multi-scale Monte Carlo study of the oxidation of the Cu(1 0 0) surface based on the Bortz-Kalos-Lebowitz model with the equilibrium energetics obtained from ab initio calculations. The radial and island size distribution functions are examined and Cu-O structures are analyzed at different temperatures and coverages. We concentrate on the coverages of 0.3 monolayer O or less, with variable sub-monolayer coverages of Cu. The results show that even though the ab initio calculations yield a higher barrier for O than for Cu adatom diffusion on Cu(1 0 0), the stability of Cu structures causes the O adatoms to be more mobile on the Cu(1 0 0) surface than the Cu adatoms. We are able to reproduce the c(2 × 2)-O domains seen in the experiments. However, we give an alternative explanation based on the repulsive interactions of O that, on one hand, cause the local ordering and, on the other hand, prohibits large well-ordered domains. We also give interpretation on the formation of the R45°-O reconstruction of Cu(1 0 0) above the O coverages of 0.3 monolayer based on the ab initio energetics.     

Cu(111)三维表面岛对表面原子扩散影响的分子动力学研究

采用嵌入原子方法的原子间相互作用势，利用分子动力学方法计算了同质外延生长中不同层数的三维Cu(111)表面岛上表面原子扩散激活能，分析了三维表面岛的层数对表面原子交换扩散和跳跃扩散势垒的影响. 研究结果表明，二维Enrilich-Schwoebel(ES)势垒小于三维ES势垒，且三维ES势垒不随表面岛层数的增加而显著变化. 对于侧向表面为(100)的表面岛，表面原子沿〈011〉方向上的扩散行为，随表面岛层数增加而逐渐变化；在表面岛层数达到3层时，扩散路径上的势垒变化趋于稳定，表面原子扩散以下坡扩散为主. 对于侧面取向为(111)的表面岛，当表面岛层数大于3层后，开始呈现上坡扩散的可能. 关键词： 表面原子 扩散 分子动力学模拟